X /<^. /^.^:S^ Digitized by the Internet Archive in 2009 with funding from Boston Library Consortium IVIember Libraries http://www.archive.org/details/titanotheresofan01osbo Department of the Interior Ray Lyman Wilbur, Secretary S Oi U. S. GEOLOGICAL SURVEY George Otis Smith, Director Monograph 55 THE TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA BY HENRY FAIRFIELD OSBORN VOLUME 1 \ ) i DocuMmrs department RECEIVED JAM 2 m? Wilbui Cross Library Univeisity ci Connecticut UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON; 1929 Note. — Monograph 55 is issued in two volumes. Volume 1 contains Chapters I- VII and Plates I-XLII; volume 2 contains Chapters VIII-XI, Plates XLIII- CCXXXVI, an appendix, and the index to both volumes. xi^ii CONTENTS Page Letter of transmittal xix Preface i xxi Vertebrate paleontology in the national surveys xxi Preparation of the present monograph xxi Work by the author, 1878-1919 xxii Research and collaboration xxii Cooperation of museums xxiii Work on text and illustrations xxiii Summary of geologic and anatomic principles xxiii Chapter I. Introduction to mammalian paleontology 1 Section 1. Exploration and research made in the preparation of this monograph 1 Section 2. Preliminary survey of the monograph and the conclusions presented 2 Range of the titan otheres in geologic time 2 Hay den's subdivisions of the Eocene and the Oligocene 5 Discovery of the titanotheres of the plains 6 Discovery of the mountain-basin environment of the titanotheres 6 Discovery and delimitation of periods of sedimentation and of life zones 8 Principle of local and continental adaptive radiation 10 Comparison of the four life phases of Europe and North America during Eocene and early Oligocene time 12 Old and new systems of classification 13 Old terminology retained 13 Linnaean methods of defining species, genera, and phyla of titanotheres 14 Recognition of many lines of descent; polyphyly the key to interpretation of the family 14 Relation of the phylogenetic classification to the Linnaean classification 15 Comparison between zoologic and paleontologic species 18 Proportions of the skull in bears and in titanotheres 19 Features distinguishing phyla of titanotheres - 19 Mutations of Waagen 19 Zoologic and paleontologic nomenclature 20 Summary of differences between old and new systems 22 Study of the evolution of single characters 22 Phylogeny of the nine typical families of the Perissodactyla 23 Wide geographic distribution of the Perissodactyla 24 Causes of evolution 27 Adaptive evolution and overs volution of the form of skull, tooth, and foot 27 Phyletic divergence in the evolution of new proportions in horses and in titanotheres 28 Evolution of the limbs and feet of the titanotheres 33 Origin of new characters as distinguished from changes in proportion 34 Velocity in the development of characters and in phylogeny 39 Summary of the evolution of the titanotheres 41 Section 3. Bibliography of literature cited or consulted in the preparation of Chapter I 42 Chapter II. Environment of the titanotheres and effect of adaptive radiation on their variation 43 Section 1 . Geology and geography 43 Correlation of early Tertiary events in the Rocky Mountain region with those in western Europe 43 Late Cretaceous and early Tertiary climates 45 Eocene geography of western North America and its relation to f aunal migrations 47 Geographic divisions and their bearing on migration 47 Character of the mountain-basin, plateau, and plains regions 51 Eocene topography in the Rocky Mountain region 51 Contrast in phj-siographic conditions east and west of the Rocky Mountain Front Range 53 Lateral and main river systems in the mountain-basin region 54 Section 2. Eocene and lower Oligocene formations and f aunal zones 56 First f aunal phase (basal Eocene) 56 Seventeen life zones 56 Basal Eocene time in Montana and New Mexico 60 Summary of faunal events of basal Eocene time 60 Basal Eocene faunal zones 63 Zones 1 and 2: Ectoconus and Polymastodon zones (Puerco fauna; part of Thanetian of Europe) 63 Zones 3 and 4: Deltatherium and Pantolamhda zones (Torrejon and Fort Union faunas; part of Thanetian of Europe) 64 IV CONTENTS Chaptek II — Continued. Section 2 — Continued. Page Second f aunal phase (lower Eocene) 64 Transitional basal Eocene faunas 64 Zone 5: Phenacodus-Nothodectes-Coryphodon zone (base of Wasatch formation of Big Horn Basin, first Wasatch Ufe zone, Big Horn A; Cernaysian of Europe) 64 Early Eocene time 65 Lower Eocene faunal zones 68 Zone 6: Eohippus-Coryphodon zone (second Wasatch life zone, Big Horn B; lower Sparnacian of Europe) 68 Zone 7: Systemodon-Coryphodon-Eohippus zone (third Wasatch life zone, Big Horn C; upper Sparnacian of Europe) 69 Zone 8: Heptodon-Coryphodon-Eohippics zone (fourth Wasatch life zpne, Big Horn D and Wind River A; lower Ypresian of Europe) 69 Zone 9: Lambdotherium-Eotitanops-Coryphodon zc*ne (fifth Wasatch life zone. Big Horn E, Wind River B, and Huerfano A; upper Ypresian of Europe) 69 Transitional lower to middle Eocene deposits 74 Huerfano formation of Colorado 74 Wind River beds and their fauna 74 Third faunal phase (middle and upper Eocene) 77 Correlation of American zones with European stages 77 Typical Bridger formation 78 Zone 10: Eometarhinus-Trogosus-Palaeosyops fontinalis zone (Bridger A and Huerfano B; lower Lutetian of Europe) ^ 82 Zone 11: Palaeosyops paludosus-Orohippus zone (Bridger B; upper Lutetian of Europe) 84 Zone 12: Uintatherium-Manteoceras-Mesatirhinus zone (Bridger C and D, Washakie A, and Uinta A; part of Bartonian of Europe) 84 Washakie Basin, Wyo 85 Stratigraphy of the basin 85 Zones 13 and 14: Metarlmius zone and Eobasileus-Dolichorhinus zone (Uinta B 1 and Washakie B 1; Uinta B 2) 89 Uinta Basin, Utah 91 Physiographic, climatic, and volcanic conditions in the Uinta Basin during middle (?) and later Eocene time 91 Geologic horizons in the Uinta Basin 91 Uinta B 1 (Metar/iinus zone = zone 13) 94 Uinta B 2 {Eobasileus-Dolichorhinus zone = zone 14) 94 Zone 15: Diplacodon-Protiianotherium-Epihippus zone (Uinta C 1; Ludian of Europe) 94 Summary of faunas of Uinta B and C 97 Adaptive radiation of the titanotheres in the Uinta Basin 97 Genera and species represented 97 Adaptive radiation of phyla 98 Fauna unrepresented 99 Zone 16: Theoretic Uinta C 2 99 Composite Eocene and lower Oligocene section at Beaver Divide 99 Fourth faunal phase (lower Oligocene) 101 Lower Oligocene mammals 101 Correlation of European and American forms 101 Zone 17: Titanotheriuni-Mesohippus zone (Chadron A, B, and C; Sannoisian of Europe) 101 Oligocene flood-plain sedimentation in the western Great Plains region 103 Conditions of deposition 103 South Dakota in Titanotherium time : 106 Rapid fluviatile sedimentation in the Cypress Hills, Saskatchewan 109 Slow sedimentation in South Dakota 109 Geographic distribution of the Chadron formation 110 Comparison of basins in western United States with the flood plain of the Nile 112 Faunal divisions in the Chadron formation 113 Three f aunistic levels determined 113 Stratigraphic distribution of species of Oligocene titanotheres 113 Hatcher's coUections, 1886-1888 115 Sources of error in determining stratigraphic levels 116 Mammalian life of the lower Oligocene Titanotherium zone 117 Notes on the habitat of the fauna of the clay and sandstone as a whole 120 Section 3. Adaptive radiation, primary and secondary, through change of environment a cause of diversification of the titanotheres 121 Habitat of the ungulates 121 Polyphyly among hoofed mammals ' ; 121 The titanotheres and other extinct forms 121 The existing African antelopes 124 CONTENTS V Chapter II — Continued. Page Section 3 — Continued. Continental adaptive radiation of the African antelopes 125 Adaptive radiation in the feeding habits of antelopes 126 Causes of variation and polyphyly among quadrupeds 127 Habits of the rhinoceroses parallel to those of the Oligocene titanotheres 128 Habits of the existing tapirs parallel to those of the Eocene titanotheres 128 Vertical geographic range of quadrupeds -- 129 Vertical geographic range of the titanotheres 129 Ten chief habitat zones of mammals _ 129 Conclusions as to habitats of the titanotheres 132 Section 4. Bibliography for Chapter II 132 Chapter III. Discovery of the titanotheres and original descriptions of the types 141 Section 1. History of discovery 141 The Oligocene titanotheres 141 The pioneer period: Prout, Owen, Evans, Leidy (1846-1873).-^ 141 Taxonomic arrangement and comparison 144 Work of Marsh and Cope (1870-1887) 144 Summary of Marsh's contributions 145 Summary of Cope's contributions 146 Reinterpretation and phylogenetio study (Osborn, 1887-1919) 146 Study of certain features 146 Geologic levels and succession of t3'pes (Hatcher, 1886-1893) 147 First European notice (Toula, 1892) 148 Distinctions of sex (Osborn and Wortman, 1895) 148 Monoph3'Ietic interpretation (Osborn, 1896) 148 Polyphyletic interpretation (Osborn, 1902-1919) 148 Recent discoveries by Lull, Lambe, and others 149 The Eocene titanotheres 149 Pioneer discoveries 149 Work in the Bridger, Washakie, and Uinta Basins by Leidy, Marsh, Cope, Scott, Osborn, and others (1870-1889) 149 Discovery in Hungary 150 Princeton and Cope-Wortman expeditions 150 First systematic and evolutionary revision (Earle, 1889-1891) 150 American Museum and other explorations of the Eocene basins (1891-1895) 151 Investigations and explorations made in preparation for the present monograph (1900-1919) 152 Section 2. Original descriptions of types of Eocene titanotheres 153 Five rules for determining the names of titanotheres 153 The genera and species of Eocene titanotheres 155 Descriptions of the species- 157 Section 3. Original descriptions of types of Oligocene titanotheres 201 List of genera and species 201 Prout's descriptions of a fragmentary lower jaw, the first titano there made known to science 202 Pomel's genus Menodus, based on Prout's description and figure 204 Early notices by Leidy and others, 1850-1870 205 Species described by Marsh and Cope in 1873-1876 209 First notice of Canadian titanotheres by Cope, 1886 219 Species described by Scott and Osborn in 1887 219 Species described by Marsh in 1887 222 Canadian species described by Cope in 1889 225 Last five species described by Marsh, 1890-91 227 Last species described by Cope, 1891 229 First European Oligocene species, described by Toula, 1892 230 Species described by Osborn in 1896 and 1902 231 Species described by Lull in 1905 234 Species described by Osborn in 1908 235 Canadian species described by Lambe in 1908 235 Second European Oligocene species, described by Kiernik, 1913 240 Final Oligocene species described by Osborn in 1916-1919 241 Chapter IV. Systematic classification of the titanotheres 243 Section 1. Phyletic versus Linnaean S3'stem of classification 243 Neo-Linnaean systematic divisions (zoologic) and evolutionary phyla (paleontologic) 243 Superfamily names proposed by Osborn (1898) and Hay (1902) 243 Family names proposed or adopted by Marsh (1873), Flower (1875), Cope (1879-1889), and Osborn (1889) 243 Subfamily names and phsda proposed by Steinmann and Doderlein (1890), Earle (1892), and Riggs (1912) 245 Division of the Oligocene titanotheres into four contemporary phyla, Osborn (1902) 245 Reclassification of the Eocene and Oligocene subfamilies by Osborn (1914) 246 Species wrongly referred to the titanotheres 246 Section 2. Classification of the titanotheres adopted in this monograph 247 VI CONTENTS Page Chapter V. Evolution of the skull and teeth of Eocene titanotheres 251 Section 1. General principles of the study of the characters of the skuU and teeth 251 Proportion characters and tendencies of evolution distinguished by analysis and synthesis 251 Distinctions between proportion characters and new rectigradation characters 251 Steps in transformation of characters 252 Proportion and flexures of the skull 254 Summary as to craniometrj' 255 Changing proportions of the cranium and face 256 Cyptocephaly , or f aciocranial flexure 256 Dolichocephaly, brachycephaly, and correlation 257 Zygomatic cephalic indices in the titanotheres and other perissodactyls 259 Relative values of indices 259 Indices of skulls of Eocene and Oligocene titanotheres 259 Differences in terminology of skull proportions in titanotheres and in man 260 Contrast in features of brachy cephaUc and dolichocephalic skulls and teeth 261 List of abbreviations used in illustrations of skulls -262 Terminology of the upper molar teeth 263 Section 2. Introduction to the anatomy of the skuU and teeth of the Eocene titanotheres ._- 264 Tj-pes of skull of Eocene titanotheres 264 Feeding habits of broad-headed and long-headed titanotheres 264 Origin and structure of the "horns" in titanotheres 266 Proportion and rectigradation in the grinding teeth of Eocene titanotheres 267 Mechanism of the titanothere grinding teeth 269 Molarization of the premolars 270 Correlation of dimensions of upper and lower teeth 272 Geologic succession and geographic distribution of the Eocene titanotheres 272 Section 3. The lower Eocene titanotheres 273 Ancestral titanotheres of the Lambdolherium zone of Wyoming at the end of lower Eocene time 273 Physiographic environment at the end of lower Eocene time 273 Contrasts and resemblances between Lambdotherium and Eotitanops 276 Explorations and discoveries 279 Systematic descriptions of the lower Eocene titanotheres 279 Section 4. The middle and upper Eocene titanotheres 297 Phyla distinguished 297 Species of Palaeosj'opinae and Dolichorhininae from the upper Huerfano {Trogosus zone) 297 S3'stematic descriptions of the middle and upper Eocene titanotheres 297 The palaeosy opine group 297 The Manteoceras-DolichorJiinus group 357 Successors to the Manteoceras-Dolichorhinus group 434 Chapter VI. Evolution of the skull and dentition of OHgocene titanotheres 443 Section 1. Review of the environment, geologic succession, and geographic distribution of the lower Oligocene titano- theres 443 Section 2. Introduction to the anatomy of the skull and the dentition of the Oligocene titanotheres 444 Horns : transformation, elongation 444 Nasals : expansion, abbreviation 446 Zygomatic arches: expansion, buccal plates 446 Occipital pillars : auditory meatus 446 Sexual characters common to all phyla 448 Teeth : distinctive features and evolution 448 Development of the skull and dentition 451 Summary of the replacement of the teeth in OUgocene titanotheres 455 Stages of wear of the adult grinding teeth 456 Age and other characters common to both sexes of titanotheres of all stratigraphic levels 456 Section 3. Division of the Oligocene titanotheres into groups and subfamilies 457 Characters of the skuU and teeth of the menodontine and brontotheriine groups 457 Characters and relations of the subfamilies 465 Possible Eocene ancestors of the brontotheriine group — 468 Section 4. Oligocene genera accepted as vahd in this monograph 469 Section 5. The menodontine group 470 Subfamily Brontopinae, including the phyla Manteoceras, Protilanotherium, Teleodus, Brontops, and Diploclonus... 470 Stratigraphic level and distinguishing features 470 Subfamily characters of Teleodus, Brontops, and Diploclonus 471 Comparisons and contrasts 471 Conspectus of characters of the subfamily ' 477 Conspectus of characters of species 478 Measurements of the Brontops series 479 Systematic descriptions of genera and species in the Brontops-Diploclonus phylum ; 481 CONTENTS VII Chaptjsr VI — Continued. Section 5 — Continued. Page Subfamily Menodontinae 505 Systematic descriptions of genera and species in the Alloys phylum 506 The Menodus monophjdum 518 Systematic descriptions of genera and species in the Menodus phylum 522 Section 6. The brontotheriine group 538 Group characters 538 Sexual characters 540 Subfamily Megaceropinae 540 Systematic descriptions of genera and species in the Megacerops phjdum 541 Subfamily Brontotheriinae 550 Systematic descriptions of genera and species in the Broniotherium phylum 555 Chapter VII. Evolution of the skeleton of Eocene and Oligocene titanotheres 583 Section 1. Methods by which the titanothere skeleton has been studied 583 Principles of the evolution of the limbs of hoofed animals 583 Size and proportions of Eocene titanotheres 584 Divergence and convergence in the skeleton of polyphyletic series 586 Diverse adaptive types of limb structure ^ 586 Terms used in describing the skeleton of the titanotheres 588 Section 2. The postcranial skeleton of lower Eocene titanotheres 590 Subfamily Lambdotheriinae 590 Subfamily Eotitanopinae 59 1 Section 3. Middle Eocene groups and phyla J : 598 Double parallelism in the palaeosyopine and Manteoceras-DoKchorhinus groups 598 Family and subfamily characters of skeletal parts in middle Eocene titanotheres 599 Systematic descriptions of middle Eocene titanotheres 612 Subfamily Palaeosyopinae 612 Subfamily Manteoceratinae 631 Section 4. The postcranial skeleton of upper Eocene titanotheres 636 Subfamily Dolichorhininae 636 Subfamilies Telmatheriinae, Brontopinae?, and Diplacodontinae 652 Section 5. The postcranial skeleton of Oligocene titanotheres 662 Subfamily Brontopinae 664 Subfamily Menodontinae 678 Subfamily Brontotheriinae 689 Bibliography for Chapters III- VII 698 ILLUSTRATIONS Plate Frontispiece. Herd of Brontotherium platyceras. Page I. .4, Eruption of the crater of Taal, Philippine Islands; B, Flooded area 140 II. A, Qjo Alamo, San Juan County, N. M.ex., looking north; B, Base of Puerco formation resting on eroded surface of Ojo Alamo sandstone 140 III. A, Upper Torrejon beds, Torrejon Arroyo, Sandoval County, N. Mex.; B, Exposures of Puerco formation east of Ojo Alamo, N. Mex 140 IV. A, Eohippus-Coryphodon zone. Little Sand Coulee, Clark Fork Basin, Wyo.; B, Phenacodus-Nothodecles- Coryphodon zone, Clark Fork Basin, north of Ralston, Wyo 140 V. A, Typical "Lysite" locality, at Cottonwood Draw, north of Lost Cabin, Wyo.; B, Typical "Gray Bull" locality, south of Otto, Big Horn Basin, Wyo 140 VI. A, A typical Huerfano locality, west of Gardner, Huerfano Basin, Colo.; B, A typical "Lost Cabin" locality, east of Lost Cabin, Wind River Basin, Wyo 140 VII. A, Henrys Fork Table, looking northward across Henrys Fork, Bridger Basin, Wyo.; B, Grizzly Buttes, south of Mountain View, Uinta County, Wyo 140 VIII. A, Northwest point of Haystack Mountain, head of Bitter Creek, Sweetwater County, Wyo.; B, View southeastward from Laclede station, Sweetwater County, Wyo 140 IX. A, Columnar sandstones, LTinta A, White River Canyon, Uinta Basin, Utah; B, Panoramic view. White River Canyon below Wagonhound Bend, Uinta Basin, Utah 140 X. A, Northern boundary of Coyote Basin, Uinta Basin, Utah; B, Divide between White River Canyon and Coyote Basin, Uinta Basin, Utah 140 XL A, North face of Beaver Divide, Wind River Basin, Wyo.; B, Exposures at Wagonbed Spring, Beaver Divide, Fremont County, Wyo 140 XII. A, Contact between Titanotherium zone and Pierre shale. Cedar Creek, Big Badlands, S. Dak.; B, Badlands south of White River, Utah, showing the Diplacodon zone 140 XIII. " Mauvaises Terres" or Big Badlands of South Dakota 140 XIV. Exposures at Quinn Draw, Big Badlands, S. Dak., showing summit of Chadron formation 140 XV. A, South end of Sheep Mountain, near head of Corral Draw, Big Badlands, S. Dak.; B, Cedar Creek, Big Badlands, S. Dak., showing the Oreodon zone overlying the Titanotherium zone 140 VIII CONTENTS Plate Page XVI. The region of the horn swelling in Palaeosyops, Manteoceras, and Telmatherium 266 XVII. The region of the horn swelling in Manteoceras, Mesatirhinus, and Dolichorhinus 267 XVIII. Restorations of the heads of four genera of Oligocene titanotheres 582 XIX. Incisors and canines of Brontotherium and Teleodus 582 XX. Upper and lower canines of Oligocene titanotheres 582 XXI. Left upper premolars of Oligocene titanotheres 582 XXII. Third left lower molar in Menodus and Brontotherium 582 XXIII. Juvenile jaw referred by Marsh to Brontops 582 XXIV. Juvenile jaws and teeth of Oligocene titanotheres 582 XXV. Superior deciduous and permanent grinding teeth of Menodus giganteus 582 XXVI. Type skeleton of Eotitanops princeps 702 XXVII. Mounted skeleton of Palaeosyops leidyi 702 XXVIII. Restoration of Palaeosyops of the Bridger Basin, Wyo 702 XXIX. Restoration of Manteoceras and Dolichorhinus of the Uinta Basin, Utah 702 XXX. Restoration of the skeleton of Dolichorhinus longiceps 702 XXXI. Manus and pes of Dolichorhinus longiceps 702 XXXII. Skeleton of Dolichorhinus longiceps 702 XXXIII. Mounted skeletons of Brontops dispar and Brontops robustus (type) 702 XXXIV. Mounted skeleton of Brontops robustus (type) , oblique front and side views 702 XXXV. Mounted skeleton referred to Brontops robustus? 702 XXXVI. Vertebral column of Brontop srobustus • 702 XXXVII. Manus and hind limb of Diploclonus tyleri . 702 XXXVIII. Mounted skeleton of Allops marshi L 702 XXXIX. Mounted skeleton of Brontotherium hatcheri, left side view ___ 702 XL. Mounted skeleton of Brontotherium hatcheri, right side view 702 XLI. Mounted skeleton of Brontotherium hatcheri, front view 702 XLII. Mounted skeleton of Brontotherium hatcheri, back view 702 Figure 1. "Fragment of the inferior maxillary of the left side" of Front's "gigantic Palaeotherium" 1 2. Type of Palaeotherium? proutii 1 3. Geologic ages and orogenic periods in North America 2 4. Successive and overlapping Oligocene and early Eocene formations of the Rocky Mountains 3 5. Map showing areas throughout the world in which remains of titanotheres have been found and areas in which titano- theres were probably in migration during Eocene and Oligocene time 4 6. The Meek and Hayden Tertiary section of 1862 5 7. Panoramic section of the Big Badlands of South Dakota, looking southeastward across Cheyenne and White Rivers to Porcupine Butte 6 8. Map showing the type locality of the Titanotherium zone on Bear Creek, S. Dak 7 9. Map showing cluster of typical lower, middle, and upper Eocene sedimentary basins in the Rocky Mountain region. _ 8 10. Restorations of Eotitanops borealis and Brontotherium platyceras 10 1 1 . Ambly poda : Skeletons and restorations of an ancestral and a specialized form 11 12. Diagram showing the gradual extinction of archaic mammals and their replacement by modernized mammals 14 13. Phenacodus and Coryphodon drawn to the same scale 15 14. Contrast between the Linnaean and phylogenetic systems of classification 16 15. The family tree of the titanotheres 17 16. Theoretic descent of existing members of the dog family from a common ancestor 19 17. Successive invasion of nine families of perissodactyls in North America and western Europe 23 18. Outlines of the body form of the perissodactyls, drawn to the same scale 25 19. The family tree of the Perissodactyla 26 20. Periods of expansion and extinction of the perissodactyls and contemporary forms 27 21. Phyletie divergence in the evolution of new proportions in horses and in titanotheres 29 22. Contours of the head and of parts of the mouth in browsing and grazing perissodactyls 30 23. Heads of lower Eocene and modern perissodactyls, showing changes of proportion and of the lip structure 31 24. Restorations of the heads of some of the principal types of titanotheres 32 25. Lower jaws of the first and the last of the titanotheres 33 26. Structure of the feet in extinct and living odd-toed ungulates 34 27. Restorations of nine species of titanotheres 35 28. Evolution of the skeleton of the titanotheres 36 29. Evolution of the skull and molar teeth in the titanotheres 37 30. Adaptive radiation in the evolution of the upper molar teeth in the perissodactyls 38 31. Three types of teeth of members of nine typical famihes of perissodactyls 39 32. The family tree of the perissodactyls, showing adaptive radiation of the nine families and thirty-five subfamilies 40 33. Outlines of the bodies of titanotheres at different stages of evolution 44 34. Map showing the known areas and the hypothetical areas of titanothere migration and habitat 45 35. General geologic sketch map of the Rocky Mountain region, showing existing topography and drainage areas and their relation to areas of Eocene and lower Oligocene sedimentation 46 36. Map of western North America showing supposed routes of migration of animals 49 37. Map showing the orogeny of the western mountain and plateau region 50 CONTENTS IX Figure Page 38. Geologic map of the Uinta Range, showing the Tertiary sediments of the Bridger Basin, Wye, at the north, and of the Uinta Basin, Utah, at the south 52 39. Chronologic relations of formations in the mountain-basin region 54 40. Section of deposits near Barrel Springs, Washakie Basin, Wyo 55 41. Eocene and lower Oligocene mammalian life zones in eleven typical correlated areas in New Mexico, Colorado, Utah, Wyoming, South Dakota, and Montana 59 42. Section of Upper Cretaceous and basal Eocene (Fort Union) deposits in Sweet Grass County, Mont 61 43. Section of Eocene deposits in the San Juan Basin, N. Mex 62 44. Columnar section of Cretaceous and Eocene sediments exposed along Bear River, Wyo., showing the typical Wasatch group of Hayden 66 45. Generalized section through Upper Cretaceous and basal and lower Eocene deposits near Pumpkin Buttes, Powder River Valley, Wyo 68 46. Composite section of the Eocene deposits of the Big Horn and Clark Fork Basins, Wyo 70 47. A typical "Lost Cabin" locality. Alkali Creek, Wind River Basin, Wyo 71 48. Section through the Wind River formation (lower Eocene) near Lost Cabin, Wyo 72 49. Map showing cluster of lower, middle, and upper Eocene sedimentary basins in southwestern Wyoming and northern Utah, exhibiting parts of areas of the Wasatch, Wind River, Bridger, and Uinta formations 73 50. Sketch map of the region of the Huerfano and Cuchara formations in southern Colorado 74 51. Section of the Huerfano formation in southeastern Colorado 75 52. Section of exposures from lower Eocene to lower Oligocene at Green Cove, on Beaver Divide, Wind River Basin, Wyo_- 76 53. Section across Wind River Basin, Wyo., from Hudson to top of Beaver Divide 77 54. Map showing the Eocene sediments encircling the Uinta Mountains of southwestern Wyoming and northern Utah 78 55. Geologic section of the Bridger formation in the Bridger Basin, Wyo 80 56. Map of the Bridger Basin, Wyo., and section of the Bridger formation 82 57. Section of the lower part of the Bridger formation in the Bridger Basin, Wyo 83 58. Section of the upper part of the Bridger formation in the Bridger Basin, Wyo 86 59. Section of deposits near Barrel Springs, Washakie Basin, southern Wyoming 87 60. Section of the Washakie Basin, Wyo., from north to south 88 61. Sketch map of the Washakie Basin region, Wyo 88 62. Columnar section of Washakie Basin, Wyo., showing life zones 90 63. Section of the Uinta formation exposed in the north wall of White River Canyon, Utah 91 64. Section of the Uinta formation from Kennedy's Basin to White River Canyon, Utah 1 92 65. Section of the Eobasileus-Dolichorhinus and Metarhinus zones in the Uinta Basin, Utah 93 66. Badlands near mouth of White River, Uinta Basin, Utah 95 67. Section of deposits at Green Cove, Beaver Divide, Wyo 100 68. Section across the Wind River Basin, Wyo., from Hudson to top of Beaver Divide 101 69. Map showing exposures originally described as the "White River group" by Meek and Hayden 102 70. Facsimile of the Meek and Hayden Tertiary section of 1862 103 71. Map showing tributaries of Chej'enne River, S. Dak., and the type locality of the " Titanotherium beds" of Hayden. _ 104 72. Type locality of the " Titanotherium beds" of Hayden, on Bear Creek, S. Dak 105 73. Panoramic section of the Big Badlands of South Dakota 106 74. Section of the Big Badlands of South Dakota, showing the chief faunal zones of the OUgocene (White River group) and the Miocene 107 75. Map showing principal exposures of the Chadron formation 108 76. Section showing the results of stratigraphic leveling in the Chadron formation (Titanotherium zone) in the badlands of White River, S. Dak ■ 115 77. The family tree of the Perissodactyla 116 78. Geographic cross section showing the nature of the habitats of the larger existing ungulates and of the titanotheres as illustrating adaptive radiation 122 79. Original radiation of the unguligrade Herbivora, Carnivora, and Insectivora, with adaptations to environment 123 80. Adaptations in the structure of the skull and teeth of Herbivora to diverse habits of feeding 125 81. Convergent adaptations in the structure of the limbs and feet of ungulates 125 82. Adaptive radiation in the feeding habits of antelopes 126 83. Mauvaises Terres, Nebraska 142 84. "Vertical view of the posterior tooth belonging to the lower jaw of Mr. Front's Palaeotherium" 143 85. Original figures of Front's "gigantic Palaeotherium" 143 86. Osborn's first restoration of Palaeosyops paludosus Leidy 151 87. Four stages in the origin and evolution of the horns in titanotheres 152 88. Leidy's cotypes of Palaeosyops paludosus 157 89. Leidy's type (holotype) of Palaeosyops major 158 90. Leidy's type of Palaeosyops humilis 159 91. Leidy's cotypes of Palaeosyops Junius 159 92. Marsh's type of Palaeosyops laticeps 160 93. Marsh's type of Telmatherium validus 161 94. Marsh's type of Limnohyus robustus 161 95. Cope's cotypes of Palaeosyops vallidens 162 96. Cope's cotypes of Limnohyops laevidens 163 97. Cope's type (holotype) of Limnohyus fontinalis ■ — 164 X CONTENTS Figure Page 98. Cope's type (holotype) o{ Palaeosyops diaconus 1(55 99. Marsh's type of Diylacndon elatus 166 100. Type (holotype) lower jaw of Brachydiastematherium transilvanicum 167 101. T3'pe (holot^'pe) oi Leurocephalus cultridens 168 102. Type (holotype) of Palaeosyops borealis 168 103. Type (holotype) of Lambdotherium popoagicum 169 104. Cope's type of Lambdotherium brownianum 1 170 105. Type (holotype) of Palaeosyops hyognathus 170 106. Type (holotype) of skull of Palaeosyops megarhinus 171 107. Earle's cotypes of Palaeosyops minor 172 108. Earle's type of Palaeosyops longirostris 173 109. Type (holotype) of Telmatotherium diploconum 173 110. Type (holotype) of Telmatotherium cornutum 174 111. Type (holotype) of Sphenocoelus uintensis 175 112. T\'pe (holotype) of Diplacodon emarginatus 176 113. Cotypes of Manleoceras manieoceras {Telmatotherium vallidens) 179' 114. T^'pe (holotype) of Lambdotherium primaevum 180 115. Type (holotype) of Limnohyops prisons 180 116. Type (holotype) skull oi Limnohyops matthewi 180 117. Type (holotype) skull oi Limnohyops 7nonoconus _' 180 118. Type (holotype) skull of Palaeosyops leidyi 181 119. Type (holotype) of Palaeosyops grangeri 181 120. Type (holotype) of Palaeosyops copei 182 121. Type (holotype) skull of Manteoceras washakiensis 182 122. Type (holotype) skull of Mesatirhinus petersoni 183 123. Type (holotype) skull of Metarhinus fltwiatilis 183 124. Type (holotype) skull of Metarhinus earlei 183 125. Type (holotype) skull oi Dolichorhinus intermedius 184 126. Type (holotype) skull of Telmatherium ullimum 184 127. Type (holotype) of Telmatherium? altidens 185 128. Type (holotype) of Protitanotherium superbum 185 129. Type (holotype) skull of Telmatherium? incisivum 186 130. Type (holotype) of Telmatherium? incisivum -- 187 131. Type (holotype) skull of Manieoceras uintensis -- 187 132. Type (holotype) of Manteoceras uintensis 187 133. Type (holotype) skull of Dolichorhinus heterodon 188 134. Type (holotype) of Dolichorhinus heterodon 188 135. Type (holot.ype) skull of Dolichorhinus longiceps 188 136. Type (holotype) of Dolichorhinus longiceps 189 137. Tj'pe (holotype) skull of Mesatirhinus superior 190 138. Type (holotype) skull of Metarhinus riparius 191 139. Type (holotype) skull of Metarhinus cristatus 191 140. Type (holot3'pe) skull oi Dolichorhinus fluminalis . 192 141. Type (holotype) skull oi Rhadinorhinus abbolti 193 142. Type (holotype) teeth of Eotitanops gregoryi 193 143. Lower jaws of Lambdotherium and Eotitanops 194 144. Type (holotype) of Eotitanops princeps 195 145. Type (holotype) of Eotitanops major 195 146. Type (holotype) of Lambdotherium priscum 195 147. Type (holotype) of Lambdotherium progressum 196 148. Type of Diploceras oshorni 196 149. Type of Diploceras osborni 197 1.50. Type (holotype) skeleton of H eterotitanops parvus 198 151. Type (holotj'pe) skull of H eterotitanops parvus 198 152. Type (holotype) of H eterotitanops parvus 198 153. Cotypes of Telmatherium? birmanicum : 198 154. Type (holotype) of Lambdotherium magnum 199 155. Type (holotype) of Eotitanops minimus 199 156. Type (holotype) skull of Eometarhinus huerfanensis 200 157. "Vertical view of the posterior tooth belonging to the lower jaw of Mr. Prout's Palaeotherium" 203 158. Original figures of Prout's "gigantic Palaeotherium" 203 159. Type of Menodus giganteus 204 160. Owen's specimens of Palaeotherium? proutii ,-- 205 161. Type (holotype) of Palaeotherium maximum 206 162. Cotypes of Rhinoceros americanus 206 163. Cotypes of Palaeotherium giganteum 207 164. Type (holotype) of Megacerops coloradensis. 208 165. Type (lectotype) of Brontotherium gigas 210 CONTENTS XI Figure Page 166. Type (lectotype) jaw of Symborodon ioruus ^ 211 167. Type (holotype) skull of Megaceratops acer 212 168. Type (holotype) skull of Megaceratops heloceras 213 169. Type (lectotj'pe) skull of Symborodon bucco 214 170. Type skulls of Symborodon altirostris, S. bucco, and Megaceratops acer 215 171. Type (holotype) skull of Symborodon altirostris 216 172. Type (holotype) skull of Symborodon trigonoceras 217 173. Type (holotype) skull of Brontolherium ingens 218 174. Type (lectotype) of Symborodon hypoceras 218 175. Type (holotype) of Anisacodon montanus 219 176. Cope's cotypes of Menodus angustigenis 220 177. Anterior part of skulls of " Megacerops coloradensis," Menodus iichoceras, and Menodus dolichoceras 221 178. Type (holotype) horns of Menodus platyceras 222 179. Type (holotype) skeleton of Brontops robustus 222 180. Type (holotype) lower jaw of Brontops dispar 223 181. Type (holotype) skull of Menops varians 223 182. Type (holotype) skull of Titanops curtus 224 183. Type (holotype) skull of Titanops elatus 224 184. Type (holotype) skull of Allops serotinus 225 185. Type of Menodus selwynianus 225 186. Type of Menodus syceras 226 187. Type skull of Diploclonus amplus 227 188. Type of Teleodus avus , 228 189. Type skull of Allops crassicornis 229 190. Type (holotype) skull of Brontops validus 230 191. Type (holotype) skull of Titanops medius ■ 231 192. Type (holotype) nasofrontal shield of Menodus peltoceras 232 193. Cotypes of Menodus? rumelicus 232 194. Type (holotype) skull of Titanotherium ramosum 232 195. Type skull of Megacerops hrachycephalus 233 196. Type (holotype) skull and lower jaw of Megacerops bicornutus 234 197. Type skull of Megacerops marshi 234 198. Type (holotype) skull of Brontolherium leidyi 235 199. Upper premolars of type skull of Brontolherium leidyi 235 200. Type (holotype) skull of Megacerops lyleri 236 201. Right manus and right hind limb of the type of Megacerops iyleri 237 202. Type (holotype) skull of Brontolherium haicheri 238 203. Type (holotype) skull of Symborodon copei 238 204. Type (holotype) jaw of Megacerops primilivus 239 205. Type (holotype) jaw of Megacerops assiniboiensis 239 206. Type of Titanotherium bohemicum 240 207. Type (holotype) skull of Allops walcotli 241 208. Type (holotype) jaw of Megacerops riggsi 242 209. Characteristic basal sections of horns of Oligocene titanotheres 245 210. Skulls showing different numerical and proportional characters in five separate phyla of titanotheres 253 211. Standard measurements of Eocene titanothere skulls 255 212. Unequal elongation of face and cranium in titanotheres and horses 256 213. Faciocranial flexure, or cyptocephaly 256 214. Faciocranial flexure in Patoeosyops and Dolichorhinus 256 215. Cranial proportions of Eocene titanotheres — Palaeosyops, Manteoceras, and Dolichorhinus 257 216. Cranial proportions in man and in the titanotheres 258 217. Natural and artificial brachycephaly and dolichocephaly ■ 258 218. Contra.sting forms of upper teeth in Eocene titanotheres 264 219. Skulls of Eocene titanotheres of the principal genera 265 220. Heads of Eocene titanotheres of four phyla 266 221. Upper and lower molars of bunoselenodont pattern 268 222. Upper and lower molar patterns of Hyracotherium 268 223. Bunoselenodont patterns of upper and lower molars in Tertiary perissodactyls 268 224. Relations of upper and lower molars in Telmatherium cultridens 269 225. Dental mechanism of titanotheres 269 226. Grinding teeth of a titanothere and an insectivore 270 227. Contrast of braohyodont and semihypsodont molars in titanotheres 270 228. Cross sections through second upper and lower molars of Lambdotherium and Menodus 270 229. Upper premolar-molar teeth of the earliest and latest known titanotheres 271 230. Reconstructed skeletons and restorations of Lambdotherium popoagicum and Eotitanops borealis 277 231. Lower jaws of Lambdotherium, Eotitanops, and Tapirus 278 232. Restored contours of skulls of Lambdotherium and Eotitanops . 278 233. Skull of Lambdotherium popoagicum, reconstructed , 281 XII CONTENTS Figure Page 234. Lower premolars of three "species" or mutations of Lambdotherium 282 235. Upper and lower grinding teeth of Lambdotherium 283 236. Lower jaws and teeth of Lambdotherium popoagicum 284 237. Lower jaws and teeth of Lambdotherium popoagicum, side view 285 238. Front part of type lower jaw of Lambdotherium priscum 286 239. Incomplete lower jaw of Lambdotherium priscum 286 240. Jaws and teeth of Lambdotherium priscum and L. magnum 287 241. Lower jaw and teeth of Lambdotherium progressum • 288 242. Upper teeth of Lambdotherium progressum 288 243. Restoration of Eotitanops borealis 289 244. Skulls of the oldest known titanotheres, Lambdotherium popoagicum and Eotitanops borealis 290 245. Model of skull of Eotitanops gregoryi 291 246. Lower premolars and molars of Eotitanops 291 247. Lower jaws of Eotitanops gregoryi and E. brownianus 292 248. Lower jaw of Eotitanops borealis 293 249. Lower teeth of Eotitanops borealis 294 250. Skull of Eotitanops borealis, palatal and side views 294 251. Skull of Eotitanops borealis, top and occipital views 294 252. Lower jaw of Eotitanops princeps 296 253. Lower grinding teeth of three species of Eotitanops from the upper Huerfano formation 296 254. Skull sections of brachycephalic and dolichocephalic Eocene titanotheres 299 255. Cross sections of the skull m middle Eocene titanotheres 300 256. Three skulls typical of the palaeosyopine group 301 257. Distribution of Palaeosyops and associated fauna in the Bridger formation 301 258. Anterior part of skull of Limnohyops laevidens 306 259. Skull of Limnohyops priscus 307 260. Back part of skull of Limnohyops priscus 308 261. SkuUs of three species of Limnohyops 309 262. SkuU of Limnohyops matthemi 309 263. Skull of Limnohyops monoconus 310 264. Skull of Limnohyops laticeps 311 265. Third right upper molar of Limnohyops laticeps 311 266. Lower jaws of Limnohyops and Palaeosyops 314 267. Lower jaws of Palaeosyops 314 268. Lower jaws of three species of Palaeosyops 316 269. Young skull of Palaeosyops fontinalis 317 270. Upper molars of Palaeosyops fontinalis 318 271. Teeth of Palaeosyops fontinalis 1 318 272. Skull of Palaeosyops major 319 273. SkuU and head of Palaeosyops leidyi 324 274. Incisors and canines ot Limnohyops a.nd Palaeosyops 325 275. Skull of Palaeosyops leidyi 326 276. Type skull of Palaeosyops leidyi 327 277. Type skull of Palaeosyops leidyi, top view 328 278. Type skull of Palaeosyops leidyi, palatal view 328 279. Skulls of Palaeosyops major and P. leidyi 329 280. Lower jaws of Palaeosyops leidyi : 330 281. Skulls of Palaeosyops leidyi and P. copei? (aff. P. robustus) 331 282. Jaws and deciduous teeth of Palaeosyops leidyi? 332 283. Deciduous cheek teeth of Palaeosyops leidyi? 332 284. Fragments of jaws of Palaeosyops 333 285. Skull of Palaeosyops robustus 333 286. Hyperbrachycephalic old male skull of Palaeosyops robustus 334 287. Basicranial region of Palaeosyops robustus 334 288. Nasals of Palaeosyops robustus 335 289. Progressive hypsodonty of the molars in Telmatherium 341 290. Upper jaw of Telmatherium cuUridens 342 291. Upper and lower teeth of Telmatherium cultridens 343 292. Upper and lower teeth of Telmatherium cultridens, interlocked 343 293. Lower jaw of Telmatherium cultridens 344 294. Type skull and lower jaw of Telmatherium ultimum 346 295. Type skull of Telmatherium ultimum, side, front, and occipital views 347 296. Type skull of Telmatherium ultimum, palatal and top views 348 297. Paratype skull of Telmatherium ultimum 349 298. Lower jaw of Telmatherium ultimum 350 299. Hypothetical reconstruction of the skull of Telmatherium altidens 352 300. Lower jaws of Telmatherium ultimum and T. altidens 353 301. Type skull of Sthenodectes incisivus . — 356 CONTENTS XIII Figure Page 302. Skulls of titanotheres of the Manteoceras-DoKchorhinus group 359 303. Skulls of Manteoceras manieoceras 363 304. Type skull of Manieoceras manteoceras 366 305. Skulls of Manteoceras manteoceras and Palaeosyops leidyi 367 306. Skulls of Manteoceras manteoceras and M. washakiensis 367 307. Skull of Manteoceras manteoceras, side view 368 308. Skull of Manteoceras manteoceras, anterior half 368 309. Incisors and canines of Manteoceras manteoceras 369 310. Lower jaw of Manteoceras 370 311. Skulls of Manteoceras manteoceras and M. washakiensis 371 312. Type skull of Manteoceras uintensis •- 373 313. Upper canines and incisors of Manteoceras uintensis 374 314. Restoration of Protitanotherium emarginatum 374 315. Lower jaws of Protitanotherium and Brachydiastematherium 375 316. Type skull of Protitanotherium emarginatum; reconstruction, side view 376 317. Type skull of Protitanotherium emarginatum, front and side views 376 318. Nasal region in three specimens of Protitanotherium 377 319. Sections of the nasals and horns of Protitanotherium emarginatum 377 320. Lower jaw of Protitanotherium emarginatum 378 321. Lower jaw of Protitanotherium superbum 381 322. Phylogenetic relations of the species of Metarhinus, Mesatirhinus, Dolichorhinus, and Rhadinorhinus 383 323. Top view of the skull in the Manteoceras-Dolichorhinus group 385 324. Palatal view of the skull in the Manteoceras-Dolichorhinus group 385 325. Leidy's cotypes of Palaeosyops Junius 386 326. Type skull of Mesatirhinus megarhinus 389 327. Type skull of Mesatirhinus petersoni 390 328. Skull of Mesatirhinus petersoni, side, top, and palatal views 391 329. Skulls of Mesatirhinus petersoni, front and occipital views 392 330. Incisors, canines, and premaxillae of Mesatirhinus 392 331 . Lower jaws of Mesatirhinus 394 332. Lower jaw of Mesatirhinus sp. with deciduous dentition 395 333. Imperfect cranium of Mesatirhinus petersoni? 396 334. Geologic section of the Bridger formation in the Washakie Basin 397 335. Restoration of Dolichorhinus longiceps ■ 398 336. Skull and lower jaw of Dolichorhinus hyognathus 398 337. Skulls of Dolichorhinus hyognathus and modern horse 399 338. Geologic section of the Eobasileus-Dolichorhinus and Metarhinus zones in the Uinta Basin 400 339. Skulls showing progressive dolichoceplialy in the Mesatirhinus-Dolichorhinus phylum, side view 401 340. Skulls showing progressive dolichocephaly in the Mesatirhinus-Dolichorhinus phylum, top and palatal views 402 341. Upper premolars of Mesatirhinus, Dolichorhinus, and Metarhinus 403 342. Skull of Dolichorhinus intermedins 406 343. Skulls of Dolichorhinus intermedins, D. heterodon, and D. longiceps 408 344. Skull referred to Dolichorhinus longiceps? 409 345. Hyoid apparatus of Dolichorhinus longiceps? compared with that of a modern tapir 410 346. Skulls of Dolichorhinus 411 347. Skull of Dolichorhinus hyognathus, palatal view 412 348. Skulls of Dolichorhinus hyognathus, front and occipital views 413 349. Skull of Dolichorhinus hyognathus, side view 413 350. Upper incisors and canines of Dolichorhinus hyognathus : 414 351. Lower incisors and canines of Dolichorhinus hyognathus 414 352. Left upper canine of Dolichorhinus hyognathus 414 353. Lower jaws of Dolichorhinus 415 354. Skull of Sphenocoelus uintensis 418 355. Type skull of Eometarhinus huerfanensis 419 356. Skull of Metarhinus fluviatilis 423 357. Right lower premolars of Metarhinus fluviatilis 424 358. Lower jaws of Metarhinus 425 359. liOwer jaw o{ Metarhinus? (Rhadinorhinus?) sp 426 360. Skull and deciduous teeth of type of lieterotitanops parvus 426 361. Type skull of Metarhinus earlei ; 427 362. Type skull of Rhadinorhinus diploconus, side and top views 432 363. Type skull of Rhadinorhinus diploconus, top and palatal views 433 364. Type skull of Rhadinorhinus diploconus, side, front, and occipital views 434 365. Skulls of Eotitanotherium osborni , 436 366. Nasals and horn swellings of Eotitanotherium osborni 437 367. Two upper raolara of Eotitanotherium (" Diploceras") osborni 438 368. Type skull of Diplacodon elatus, partial reconstruction, palatal view 439 369. Type skull of Diplacodon elatus, upper jaw and zygoma 440 XIV CONTENTS Figure Page 370. Third and fourth upper premolars of Diplacodon elatus 440 371. Upper molars of Diplacodon and Proiitanotherium compared 441 372. Facial region of Eotitanoiherium osborni and Bronlotherium leidyi 441 373. Map showing areas in which remains of titanotheres have been found 443 374. Comparison of upper Eocene and lower OUgocene titanotheres 444 375. Sections at base of horn in the six chief generic types of Oligocene titanotheres 445 376. Position of the standard sections and contours of Oligocene titanotheres skulls 445 377. Male and female skulls of Bronlotherium gigas 446 378. Occipital view of skulls in different phyla of OHgocene titanotheres 447 379. Influence of progressive brachycephaly on the auditory region of perissodactyls 447 380. Inferior aspect of chin in Manleoceras • 449 381. Upper molars of Menodus giganteus and Allops marshi 450 382. Extreme dolichocephaUc and brachycephahc types of upper premolar-molar series in Oligocene titanotheres 450 383. Third left lower molar of Bronlotherium leidyi 451 384. Development of jaws and teeth, stage 4 452 385. Development of jaws and teeth, stage 6 453 386. Occiput of young skull of Brontops? brachycephalus 454 387. Stages of wear in the adult upper grinding teeth of Ohgooene titanotheres 455 388. Skull contours showing extreme divergence between Menodus giganteus and Bronlotherium platyceras 456 389. Skulls of the menodontine group, side view 459 390. Skulls of the bronototheriine group, side view 460 391. Skulls of the menodontine group, top view 461 392. Skulls of the brontotheriine group, top view 462 393. Skulls of the menodontine and brontotheriine groups, palatal view 463 394. Skulls of the menodontine and brontotheriine groups, front view 464 395. Lower jaws of the Bronlotherium phylum 465 396. Lower jaws of the Brontops and Menodus phyla 466 397. Lower jaws of the Diplodonus and Allops phyla 466 398. Heads of Oligocene titanotheres 466 399. Sections at base of horn in five principal lower Ohgocene phyla of titanotheres 468 400. Restorations of lower OUgocene titanotheres of the four principal genera 469 401. Skulls of Rhadinorhinus and Bronlotherium, palatal view 470 402. Skulls of Rhadinorhinus and Bronlotherium, side view 471 403. Skulls of Rhadinorhinus and Bronlotherium, top view 472 404. Lower jaws of Metarhinus fluvialilis and Bronlotherium halcheri 473 405. Progressive evolution of the upper premolars in Bronlotherium. and its predecessors 474 406. Progressive evolution of the upper premolars in Menodus and Brontops and their predecessors 474 407. Progressive evolution of the lower premolars in Bronlotherium and its predecessors 475 408. Progressive evolution of the lower premolars in Brontops and its predecessors 475 409. Phyla of the Brontopinae and Menodontinae 477 410. Evolution of the horns in the Brontops phylum 477 411. Basal section of the horns in the Brontops phylum 477 412. Progressive broadening of the nasals in the Brontops phylum 478 413. Lower jaws of Teleodus primitivus, Brontops brachycephalus, and Allops walcotti? _ 478 414. Sections and contours of skulls of Brontops brachycephalus 483 415. Upper canines and incisors of Brontops brachycephalus 484 416. Reconstruction of crushed skull of Brontops brachycephalus 485 417. Left upper grinding teeth of Brontops brachycephalus 486 418. Skull and horn region of Brontops brachycephalus? 486 419. Sections and contours of skulls of Brontops brachycephalus and B. dispar 487 420. Restoration of Brontops robuslus 492 421. Sections and contours of skull of Brontops robuslus 493 422. Skull of Brontops robuslus 495 423. Lovi'er jaws of Brontops dispar and B. robuslus 496 424. Sections and contours of skull of Brontops sp 497 425. Hyoid bones of Brontops compared with those of the tapir, black rhinoceros, and horse 497 426. Sections and contours of skulls of Diplodonus hicornutus and D. tyleri 498 427. Sections and contours of skull of Diplodonus ampins 499 428. Lower jaws of Diplodonus bicornutus and D. tyleri 503 429. Sections and contours of skulls of Allops walcotti and A. marshi 610 430. Upper teeth of Allops walcotti 511 431. Skull of Allops marshi 513 432. Lower jaws of Allops marshi and Allops? sp 514 433. Sections and contours of skulls of Allops serotinus and A. crassicornis 516 434. Coossified nasals and proximal part of horns of Allops? serotinus? 517 435. Sections and contours of skulls of Menodus heloceras and M. trigonoceras 519 436. Skull of Menodus heloceras 526 437. Lower jaws of Menodus (Symhorodon) lorvus and M. trigonoceras 527 CONTENTS XV Figure Page 438. Upper teeth of Menodus ■proutii 528 439. Skull of Menodus trigonoceras 629 440. Skull of Menodus trigonoceras belonging with the mounted skeleton in the Colorado Museum, Denver 530 441. Restoration of Menodus giganteus 531 442. Skull of Menodus giganteus, front view 532 443. Skull of Menodus giganteus, palatal view 532 444. Sections and contours of skulls of Menodus giganteus and M. various 534 445. Sections and contours of skull of Menodus giganteus 535 446. Lower jaws of Menodus giganteus 536 447. Teeth and nasals of Menodus montanus i 538 448. Sections and contours of nasals and horns of Megacerops coloradensis 544 449. Sections and contours of skull of Megacerops bucco 545 450. Sections and contours of skulls of Megacerops copei and M. acer 546 451. Upper part of occiput of Megacerops acer 547 452. Skull of Megacerops acer, side and top views 548 453. Skull of Megacerops acer, palatal view 549 454. Restoration of Megacerops copei 549 455. Lower jaws of Megacerops assiniboiensis and M. riggsi 550 456. Sections and contours of skull of Megacerops? syceras 550 457. Composite sections showing the evolution of the horns and reduction of the free nasals in the Brontotherium phylum. _ 551 458. Basal sections of the horns in the Brontotherium phylum 552 459. Skulls of male and female brontotheres 552 460. Contrast in contour of horns and nasals between male and female brontotheres 552 461. Sections and contours of skulls of Brontotherium leidyi and B. hypoceras 558 462. Lower jaws of Brontotherium leidyi ' 559 463. Two lower molars and symphyseal region of Brontotherium? rumelicum 560 464. Lower jaws of Brontotherium rumelicum? and B. leidyi 561 465. Sections and contours of skull of Brontotherium? hatcheri 563 466. Skull of Brontotherium hatcheri, side view j 564 467. Skull of Brontotherium hatcheri, front view 564 468. Lower jaws of Brontotherium hatcheri and B. gigas 566 469. Sections and contours of skull of Brontotherium? tichoceras 567 470. Sections and contours of skulls of Brontotherium hatcheri and B. gigas 568 471. Lower jaws of Brontotherium gigas and B. medium 569 472. Sections and contours of skull of Brontotherium gigas? 572 473. Sections and contours of skull of Brontotherium dolichoceras 572 474. Skull of Brontotherium dolichoceras 572 475. Sections and contours of skulls of Brontotherium medium and B. curtum 674 476. Horns of Brontotherium curtum 576 477. Sections and contours of skull of Brontotherium curtum 576 478. Left horn and nasals of Brontotherium curtum? 577 479. Sections and contours of skull of Brontotherium ramosum 578 480. Restoration of Brontotherium platyceras 579 481. Sections and contours of skull of Brontotherium platyceras 580 482. Evolution of the skeleton in titanotheres 584 483. Estimated height at shoulder of Eocene and Oligocene titanotheres and tapir 585 484. The phyla of Eocene titanotheres, as represented by the manus 587 485. Progressive broadening of the magnum in Eocene titanotheres 587 486. Reconstructed skeleton and restoration of Lambdotherium popoagicum 591 487. Atlas and scapula of Lambdotherium popoagicum 591 488. Fore limb of Lambdotherium popoagicum 592 489. Forearm and manus of Lambdotherium popoagicum 592 490. Left manus of Lambdotherium and Eotitanops 592 491. Astragalus of Lambdotherium popoagicum 593 492. Restorations of Lambdotherium popoagicum, Eotitanops princeps, and E. gregoryi 593 493. Metatarsals of Eotitanops 593 494. Reconstructed skeleton and restoration of Eotitanops borealis 594 495. Atlas of Eotitanops borealis 595 496. Vertebrae of Eotitanops princeps 595 497. Radius of Eotitanops borealis 595 498. Lunars of Eotitanops 595 499. Manus of Eotitanops princeps 595 500. Humerus and femur of Eotitanops princeps 596 501. Pelvis of Eotitanops borealis -• 596 502. Left pes of cursorial and subeursorial Eocene Perissodactyla 597 503. Astragalus and calcaneum of cursorial and submediportal Eocene Perissodactyla 598 504. Astragalocalcaneal facets in lower Eocene Perissodactyla , 598 505. Left astragalus and calcaneum of Eotitanops sp 599 XVI CONTENTS Figure Page 506. Metatarsal and tibia of Eotitanops major 599 507. Restoration of Eoiitmiops horealis 600 608. Atlas of Eocene titanotheres 601 509. Types of scapula in middle Eocene titanotheres 602 510. Tj'pes of fore limb in Eocene and Oligocene titanotheres 603 511. Characteristic details of radius and ulna in middle and upper Eocene titanotheres 604 512. Manus of lower and middle Eocene titanotheres 605 513. Comparison of the riglit scaphoid in middle Eocene titanotheres 605 514. Terminal phalanges of the manus in middle Eocene titanotheres and amyuodonts 605 515. Progressive graviportal adaptation in the pelvis of Eocene and Oligocene titanotheres ■ 606 516. Femora and tibiae of middle Eocene titanotheres 609 517. Distal end of the femur in Manteoceras and Amynodon 610 518. Angulation of the knee joint: relation of patellar facet to long axis of femur * 611 519. Inner side view of left fibula of Palaeosyops, Limnohyops, and Brontotherium 611 520. Comparison of pes in four species of middle Eocene titanotheres 613 521. Astragali of Eocene titanotheres 614 622. Calcanea of Eocene titanotheres 615 523. Left ectocuneiform tarsi of lower and middle Eocene titanotheres j 615 524. Principal measurements of the carpus and tarsus 615 525. Humerus, radius, and ulna of Limnohyops monoconus? 615 526. Left manus, radius, and ulna of Mesatirhinus petersoni 616 527. Manus, radius, and ulna of Limnohyops monoconus 616 528. Right scaphoid of Palaeosyops sp. and Limnohyops monoconus 617 529. Left hind limb of Limnohyops monoconus 618 530. Right pes of Limnohyops monoconus? 618 631. Ventral surface of sacrum of Limnohyops laticeps 618 632. Right OS innominatum of Limnohyops laticeps 619 533. Pelvis of Palaeosyops major 619 534. Right femur and tibia of Palaeosyops major 620 635. Astragalus and calcaneum of Palaeosyops major 620 536. Composite mounted skeleton of Palaeosyops leidyi 621 537. Manus of Palaeosyops leidyi 622 538. Pelvis of Limnohyops 624 539. Pelvis of Palaeosyops cf. P. leidyi 624 640. Left pes of Palaeosyops leidyi 626 641. Relations of facets of the astragalus and calcaneum in Palaeosyops 626 542. Atlas of Palaeosyops robustus 627 543. Atlas and axis of Palaeosyops leidyi? 627 544. Cervicals and dorsals of Palaeosyops robustus 627 645. Left scapula of Palaeosyops robustus 627 546. Bones of forearm of Palaeosyops 628 647. Left astragalus of Palaeosyops copei? 629 648. Fore limb of Palaeosyops copei? 629 649. Left manus of Palaeosyops copei? 629 550. Right hind limbs of Palaeosyops major and P. copei? 630 551. Atlas of Manteoceras manteoceras 632 562. Seventh cervical vertebra of Manteoceras manteoceras compared with that of Palaeosyops leidyi 633 553. Left humerus of Manteoceras manteoceras 633 554. Right manus of Manteoceras manteoceras 633 655. Pelvis of Manteoceras? 1 634 556. Femora and tibiae of Manteoceras manteoceras 635 557. Left astragalus of Manteoceras manteoceras 635 558. Restoration of the skeleton of Mesatirhinus petersoni 637 559. Restorations of Mesatirhinus petersoni and Palaeosyops leidyi 637 660. Atlas of Mesatirhinus megarhinus * 638 561. Humerus of Mesatirhinus megarhinus 638 562. Radius and ulna of Mesatirhinus petersoni 638 563. Left forearm and manus of Mesatirhinus petersoni? 639 564. Right manus and fragments of radius and ulna of Mesatirhinus petersoni 639 565. Left manus, radius, and ulna of Mesatirhinus petersoni? 639 566. Right scaphoid of Mesatirhinus and Manteoceras 639 567. Right manus of Mesatirhinus petersoni? 641 568. Left femur and tibia of Mesatirhinus petersoni? 642 569. Left pes of Mesatirhinus petersoni? 642 570. Left astragali of Mesatirhinus petersoni? 642 571. Left entocuneiform tarsi of Palaeosyops and Mesatirhinus 642 572. Pes referred to Mesatirhinus 644 573 Pes of Meiarhinus cf. M. earlei 644 CONTENTS XVII Figure i'ags 574. Astragalus, calcaneum, and navicular of Metarhinus cf. M. earlei 644 575. Astragalus of Metarhinus cf. M. earlei 644 576. Left scapula of Metarhinus? sp 645 577. Left radius and ulna of Metarhinus earlei'? 645 578. Skeleton of a newly born animal, provisionally identified as Metarhinus sp 646 579. Provisional restoration of the skeleton of Dolichorhinus hyognathus 646 580. Vertebral column of Dolichorhinus hyognathus 647 581. Atlas referred to Dolichorhinus sp 647 582. Left scapula of Dolichorhinus? hyognathus 649 583. Humerus of Dolichorhinus hyognathus 649 584. Radius and ulna of Dolichorhinus hyognathus 649 585. Metatarsals of Dolichorhinus hyognathus 649 586. Manus of AmynodoJi and Mesatirhinus compared 650 587. Left fore limb of the amphibious rhinoceros Amynodon intermediusf 650 588. Left astragali of Dolichorhinus and allied types 651 589. Cervical vertebrae of Dolichorhinus longiceps? 651 590. Right fore limb of Dolichorhinus longiceps? 652 591. Manus of Dolichorhinus longiceps? 652 592. Hind limb referred to Telmatherium ultimum 653 593. Pes of Tehnatheriumf ultimum? 653 594. Vertebrae and fore limb of Diplacodon or Protitanotherium 654 595. Astragalus and calcaneum of Diplacodon or Protitanotherium 655 596. Left astragalus of Protitanotherium superbu7n 655 597. Incomplete ilium and ischium of Diplacodon elatus 656 598. Atlas and axis of Eotitanotherium osborni : 656 599. Vertebrae of Eotitanotherium osborni 657 600. Scapula of Eotitanotherium osborni 657 601. Humerus, radius, and ulna of Eotitanotherium osborni , 657 602. Manus of Eotitanotherium osborni 658 603. Femur, tibia, and pelvis of Eotitanotherium osborni 658 604. Pes of Eotitanotherium osborni 658 605. Restoration of skeleton of Eotitanotherium osborni 659 606. Mounted skeleton of Brontops ^ 670 607. Three views of mounted skeleton of Brontops -. 671 608. Scapulae of Oligocene titanotheres 673 609. Manus of Brontops? sp. and B. dispar? 674 610. Mounted skeleton of Brontops brachyeephalus? 676 611. Mounted skeleton of Brontops brachyeephalus?, oblique front view 677 612. Parts of skeleton of Allops crassicornis? 680 613. Pes of Menodus trigonoceras, referred, and M. heloceras 681 614. Manus of Menodus trigonoceras? 682 615. Restorations of Menodus trigonoceras and Allops marshi 683 616. Mounted skeletons of Brontops dispar? and Menodus trigonoceras. 684 617. Left astragalus of Menodus giganteus - 685 618. Cervical and first four dorsal vertebrae of Brontops robustus and Menodus giganteus 686 619. Manus referred to Menodus giganteus 687 620. Restorations of Brontotherium leidyi and B. platyceras 688 621. Atlas and axis of Brontotherium leidyi 689 622. Vertebrae of Brontops robustus and Brontotherium gigas 689 623. Scapulae of Oligocene titanotheres 690 624. Humeri of Brontops robustus and Brontotherium leidyi 691 625. Humeri of Megacerops? acer? and Brontotherium gigas? 691 626. Radii of Brontops robustus, Brontotherium leidyi, and Brontotherium gigas 691 627. Radius and ulna of Brontotherium 692 628. Ulnae of Brontops robustus, Brontotherium leidyi, and Brontotherium gigas 692 629. Olecrana of Brontotherium and Megacerops? 692 630. Manus of Oligocene titanotheres . 693 631. Manus and pes referred to Brontotherium gigas hatcheri 694 632. Manus and pes referred to Brontotherium hatcheri? 695 633. Manus referred to Brontotherium gigas, as restored 695 634. Pelvis of Brontotherium gigas hatcheri 696 635. Femora of Brontops robustus and Brontotherium leidyi 696 636. Tibiae of Brontops robustus and Brontotherium leidyi 696 637. Tibia and fibula of Brontotherium leidyi 696 638. Femora of Megacerops? and Brontotherium? 696 639. Pes of Oligocene titanotheres 697 101959— 29— VOL 1 2 LETTER OF TRANSMITTAL Dr. George Otis Smith, Director United States Geological Survey, Washington, D. C. Dear Sir: I have the honor to transmit herewith a monograph on the evolution of a pecuharly American family of quadrupeds known as the titanotheres. This designation was given to them in 1852 by Joseph Leidy while he was employed as vertebrate paleon- tologist in David Dale Owen's survey of a part of the ancient territory of Nebraska. This family is one of a group of vertebrate animals whose fossil remains, found in the western United States, were long studied by Othniel Charles Marsh, my distin- guished predecessor in this work in the United States Geological Survey. Early in the eighties Professor Marsh projected a monograph on the Brontotheridae (here called the titanotheres), and subsequently he made the largest and most valuable contributions to our knowledge of this family and of its evolution. He planned the monumental field work of John Bell Hatcher, by which the great collection for the United States National Museum was made, and he super- vised the preparation of sixty lithographic plates, which are here reproduced. Unfortunately he died before he had even begun to prepare the manuscript. The duty of continuing his work was intrusted to me June 30, 1900, by your predecessor, Charles D. Wal- cott. During this period of nearly 20 years I have supervised the preparation of the monograph on the Ceratopsia by Hatcher and Lull and have half com- pleted the monograph on the Sauropoda. The mono- graph on the Stegosauria has not yet been prepared. The task of preparing the present monograph has been long and difficult. First, it proved necessary to reexplore the entire Eocene and lower Oligocene series of rocks in Wyoming, Colorado, and South Dakota, where the fossilized remains of titanotheres are found, both to determine precisely their geologic succession and to close up gaps in the stages of evolu- tion; second, it proved necessary to examine and com- pare the titanotheres of these geologic epochs in all the museums of this country and in several museums abroad; third, it pi-oved necessary, in order thoroughly to understand the titanotheres, to discover and to follow many side lines of investigation that have not hitherto been followed in vertebrate paleontology. This work has been done with the aid of many specialists, foremost among whom is my junior col- league Prof. William K. Gregory, without whose in- telligent and unremitting cooperation the monograph could never have been completed. It is perhaps not too much to say that this work has transformed our knowledge of the early Tertiary geology of the Rocky Mountain basin region. First, the six life periods recognized by Marsh and his no less distinguished contemporary Edward Drinker Cope may now be replaced by sixteen life periods, which may be clearly defined and separated and certain of which may be more or less precisely correlated with life periods established for western Europe. Second, a much clearer notion has been gained of the changing geographic, physiographic, climatic, and volcanic con- ditions in Wyoming and Dakota and of their influence on the migration and succession of forms of life. Third, the whole method of attack on problems of vertebrate paleontology has been developed; we seek to know the entire living animal, its musculature, its mode of locomotion, and its feeding habits, in order to insure the complete restoration of the body. Fourth, the study of the many branches of this group has given the most convincing demonstration that evolution, even in any one geographic region, seldom moves along a single line of descent ; more frequently it moves along many lines — it is polyphyletic; in other words, it radiates, following the principles of local adaptive radiation. Finally, the history of the titanothere family in its evolution from very small and relatively weak forms into titanic quadrupeds, second in size only to the elephants, has afforded us a unique oppor- tunity to enlarge our previous knowledge of the actual modes of evolution as well as to revise our theories as to the causes of evolution and of extinction. I desire to express my appreciation of the support given by the Geological Survey under your direction in the completion and publication of this work. With the aid of many coworkers I have endeavored to set a new standard of broad, thorough, and ex- haustive research in vertebrate paleontology which shall be worthy of the great geologic traditions of our national Geological Survey. I trust that this mono- graph, like Leidy's classic memoir of 1869, may ex- ercise a permanent influence upon future studies of the geologic history of the great West. Henry Fairfield Osborn, Vertebrate Paleontologist. American Museum of Natural History, December 19, 1919. PREFACE VERTEBRATE PALEONTOLOGY IN THE NATIONAL SURVEYS Joseph Leidy, Edward Drinker Cope, and Othniel Charles Marsh, who successively served as members of United States Government surveys of the West, were the founders of American vertebrate paleontology. Leidy's memoir of 1869, entitled "The extinct mam- malian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America," marked the end of the first period of exploration. Cope's great memoir of 1885, entitled "The Vertebrata of the Tertiary formations of the West," marked the end of the second period of explor- ation. Meanwhile the subject had become too broad to be comprehended in a single work. Accordingly Marsh, as vertebrate paleontologist, planned a series of ex- haustive monographs on special groups of extinct birds, reptiles, and mammals, which should treat in great detail the anatomical structure and form the basis of a systematic classification. For these mono- graphs he carried out the most intensive field explora- tions known to science and published a large number of preliminary papers, which fairly revolutionized our knowledge of these and many other groups. In 1880 the Fortieth Parallel Survey published his monograph on the Odontornithes, an extinct group of birds of North America. In 1883 the United States Geological Sur- vey published his paper entitled "Birds with teeth," and in 1886 his monograph on the Dinocerata, an extinct order of gigantic mammals. This was the first of the series of five monographs projected for pub- lication by the United States Geological Survey on the Dinocerata, the Stegosauria, the Sauropoda, the Ceratopsia, the Brontotheridae. The monograph last indicated has developed into the present monograph on the titanotheres, which covers a much broader field than that contemplated by Marsh for the monograph on the Bronototheridae. For the monographs on the Ceratopsia and on the Brontotheridae exploration on an unprecedented scale was begun by the United States Geological Survey under the direction of Marsh. For the four mono- graphs on the Stegosauria, Sauropoda, Ceratopsia, and Brontotheridae 204 superb lithographic plates were completed under Marsh's direction. Altogether he had been engaged on this work nearly 17 years when death interrupted his monumental labors on March 18, 1899. PREPARATION OF THE PRESENT MONOGRAPH The first important step taken by Marsh in his series of contributions to our knowledge of this extinct fam- ily was the publication of his paper on "The structure and affinities of the Brontotheridae," published in 1874, based on the collections at Yale University. The second was his paper entitled "Principal charac- ters of the Brontotheridae," published in 1876. In the meantime he had made a geologic excursion to White River in South Dakota, in the vicinity of the Red Cloud Agency. This visit marks an interesting epoch in the history of paleontologic exploration for the titanotheres. Late in the autumn of 1875 Marsh, accompanied by an escort from Fort Laramie to the Red Cloud Agency, went to the Badlands of Nebraska and Dakota. The consent of the Indians was deemed necessary to permit safe search for fossil bones in their country. This con- sent was obtained with difficulty, and after it had been obtained the Indians withheld their assistance. An account of Marsh's visit is given in a manuscript en- titled "Sketches of the life of Red Cloud," by Capt. James H. Cook, of Agate, Nebr., at that time serving as a scout for the United States Army. Captain Cook writes: It was in the autumn of 1875 that I visited the Red Cloud Agency, which was at that time located on the White River, in the northwestern part of Nebraska, the agency buildings stand- ing about 2 miles up the river from the place where the city of Crawford is now situated. The chief of the Sioux, Red Cloud, made me welcome to his lodge. It was on this visit that I first learned of the petrified bones of strange creatures that had once occupied the lands to the eastward of the agency. Two of Red Cloud's subchiefs, American Horse and Little Wound, took me to the lodge of Afraid of Horses, where I was shown a piece of bone, perfectly petrified, containing a molar tooth .3 inches or more in diameter. American Horse explained that the tooth had belonged to a "Thunder Horse" that had lived "away back" and that then this creature would sometimes come down to earth in thunder- storms and chase and kill buffalo. His old people told stories of how on one occasion man_v, many j^ears back, this big Thunder Horse had driven a herd of buffalo right into a camp of Lacota people during a bad thunderstorm, when these people were about to starve, and that they had killed many of these buffalo with their lances and arrows. The "Great Spirit" had sent the Thunder Horse to help them get food when it was needed most badly. This story was handed down from the time when the Indians had no horses; While I was the guest of Red Cloud on this occasion, Prof. O. C. Marsh, of the Smithsonian Institution and Yale Uni- versity, came over from Fort Laramie to Camp Robinson and the Red Cloud Agency to get permission to collect fossils in XXI XXII the Sioux country. The Sioux, however, did not take kindly to this proposition, thinking it was yellow lead (gold) that the white chief wanted, not stone bones. I met Professor Marsh at that time and talked with him. I showed him the tooth the Indians had shown me. When I returned to Red Cloud's lodge I told Red Cloud that Professor Marsh was a friend of the "Great Father" (the President) at Washington, and that I thought if he were allowed to hunt for stone bones in the Sioux country he would be a good friend to the Sioux people. Red Cloud said that if Professor Marsh was a good man he would help the Sioux people to get rid of the agent that was then in charge of the agency, whom the.y disliked very much. This being brought to the attention of Professor Marsh, he took the matter in hand, and an investi- gation of affairs took place at the Red Cloud Agency, the re- sult of which was at least pleasing to the Indians concerned, as the agent was removed. Professor Marsh was allowed to collect with his field parties unmolested from that time on. He was named by Red Cloud "Wicasa Pahi Hohu" (pronounced we-ch5-shJl pa-he ho-hii), Man-that-Pioks-Up-Bones. The professor and Red Cloud became friends to the extent that Red Cloud was entertained at the home of the professor in New Haven, Conn., and the two were photographed there with clasped hands and the "peace pipe" between them. The first collections made for this monograph were those brought together from Colorado and South Dakota, part of them under the direction of Marsh, for the Peabody Museum of Yale University. By far the greatest collection was that brought together by John Bell Hatcher for the Geological Survey, now preserved in the United States National Museum. Between 1870 and 1891 Marsh published 14 papers on these collections. These papers relate more or less directly to the Brontotheridae; the last appeared in 1891 and contained descriptions of three new types from South Dakota — AUops crassicornis, Brontops dispar, and Brontotherium medium. WORK BY THE AUTHOR, 1878-1919 In the meantime the present author made his first contribution to the history of this family in 1878 in a paper on the results of the Princeton collections of 1877 and 1878 in the Bridger Basin. His second contribution was made in 1887 in a paper entitled "Preliminary report on the vertebrate fossils of the Uinta formation collected by the Princeton expedition of 1886." His third and fourth contributions were made in 1890, in the two papers entitled, respectively, "Preliminary account of the fossil mammals from the White River and Loup Fork formations," which related to a collection made in South Dakota by Dr. S. Garman for the Harvard University Museum, and "The MammaUa of the Uinta formation," Parts III and IV, on the Perissodactyla. These have been followed by 38 papers by the author, based chiefly on his paleontologic and geologic expeditions in the field for the American Museum of Natural History, planned by the author and ably directed by Dr. J. L. Wortman, Mr. O. A. Peterson, and Mr. Walter Granger. To these indefatigable field explorers science is indebted for the wonderful series of Eocene titano- theres which have enabled us to trace the ancestry of the Oligocene titanotheres and to establish all the early phases in the history of this family. To Peter- son, Earl Douglass, and Elmer S. Riggs in the Uinta, and especially to Granger in the entire series from the basal Eocene to the base of the Uinta, is due the remarkable precision of the geologic records by which the faunal life zones of the Eocene have been deter- mined. The stratigraphic succession of the Eocene and of the lower Oligocene mammal life has been determined chiefly by the field observations and collections of Granger in the Eocene and of John Bell Hatcher in the lower Oligocene and by the systematic examina- tions of species by Dr. William Diller Matthew and by the author. RESEARCH AND COLLABORATION Prof. William K. Gregory has been in the closest cooperation with the author in all the details of the preparation of the monograph since the beginning of the work in the year 1900. Words are inadequate to express the author's sense of indebtedness to his former student and present colleague in the American Museum and in Columbia University. The author desires also to acknowledge his special indebtedness to Mr. Granger for his valuable notes and his cooperation in the preparation of the text and the geologic sections, as presented in Chapter II, on the Eocene and Oligocene formations of the Rocky Mountains, as well as to Prof. William J. Sinclair for his work on the volcanic nature of the middle Eocene deposits and to Mr. Albert Johannsen of the Geological Survey for his analyses of the material of these deposits. It is hoped that that chapter will furnish a key to future exploration of this mountain- basin region as well as to the Oligocene sections of the Great Plains. Matthew, by means of the rich col- lections in the American Museum, has furnished critical determinations for the discrimination of mam- malian species in the sixteen life zones and has cooper- ated with the author in the preparation of "Cenozoic mammal horizons of western North America," pub- lished by the Geological Survey in 1909 as its Bulletin 361, which forms the foundation of the more de- tailed life-zone work whose results are presented in Chapter II. Details of the history of the collections at home and abroad are presented in Chapter III under the head- ing "History of explorations and discoveries and original descriptions of the Eocene and Oligocene titanotheres." Every known significant specimen is referred to, its species and its sex are determined, and its principal characters are described. This monograph will furnish a much desired key to the present and future collections and surveys in Wyo- ming, Nebraska, Colorado, the Dakotas, and Assin- iboia. XXIII COOPERATION OF MUSEUMS To the museums of the United States, Great Britain, and Bavaria, where titanothere remains are preserved, the author is indebted for cordial coopera- tion in furnishing materials for study and in affording every possible facility for measurements and illustra- tions. The author would mention especially Prof. Charles Schuchert and Prof. Richard S. Lull, of the Yale University Museum, present custodians of the great Marsh collections, as well as their assistant, Mr. Thomas A. Bostwick, who is in charge of all the field records of Marsh. In connection with the superb Hatcher collection in the United States National Museum, which far surpasses any other in existence. Dr. Charles W. Gilmore and Dr. James W. Gidley have rendered every possible assistance. The author is especially indebted to the director of the Carnegie Museum at Pittsburgh, Dr. W. J. Holland, and to Mr. O. A. Peterson of that museum for the liberal use of collections of the Uinta titanotheres; also to Mr. Earl Douglass of the same institution for his invaluable field notes and observations on the Uinta section. The systematic location of species in the great Uinta section is due to the precise field work of Mr. Elmer S. Riggs of the Field Museum of Natural History, Chicago, an institution that is especially rich in remains from the horizon known as Uinta B. To his former colleague Prof. William B. Scott of Princeton University, as well as to his colleague Prof. William J. Sinclair, the author is indebted for the liberal use of valuable collections, including many types from several levels of the Bridger and from the uppermost Eocene horizon, known as Uinta C. From 1846, when the earliest remains of titanotheres were found, until 1918 almost every year has added one or more stages or types to the history of this great family. The stages still to be discovered are in the unknown interval between the uppermost Eocene horizon, known as Uinta C, and the lowermost Oligocene horizon, known as Chadron A. WORK ON TEXT AND ILLUSTRATIONS The great task of preparing the bulk of the manu- script — a task performed between 1902 and 1918 — fell upon Miss H. Ernestine Ripley, the work being done chiefly from the dictation and notes of thp author. The preparation of the bibliography and the first revision of the entire manuscript were also undertaken by Miss Ripley with interest and per- formed with precision. The author warmly appreci- ates this invaluable service to paleontology. The final arrangement of the illustrations together with the preparation of the accompanying legends, was undertaken, under the author's general direction, by Doctor Gregory with the cooperation of Miss Chris- tine D. Matthew, Mr. Erwin S. Christman, and Mrs. Lindsey Morris Sterling. The preliminary editorial work has been performed with celerity and skill by Miss Mabel Rice Percy, of the American Museum. The final arrangement and verification of illustra- tions and captions were the work of Miss Christine D. Matthew. The final editorial work and preparation of the text for the printers were accomplished by Messrs. George M. Wood and Bernard H. Lane, Mr. Wood continuing the work as a member of the American Museum staff after his retirement from the Geological Survey. The illustrations, which are taken from many sources, date back to the early lithographic figures of Leidy. They include the unpublished lithographic plates prepared under the direction of Cope, and especially the superb lithographic drawings made for the United States Geological Survey by Mr. F. Berger under Marsh's direction. These lithographic plates are supplemented by numerous plates based upon photographs taken chiefly by Mr. A. E. Ander- son of the American Museum staff. The text and plates are adorned with reproductions of the fine series of drawings from the pen and brush of Mr. Christman and from the numerous pen draw- ings of Mrs. Sterling. The geologic sections in Chapter II are the work of Mr. William E. Belanske. To Mrs. Sterling, Mr. Christman, and Mr. C. A. Weck- erly of the Geological Survey were assigned the final preparation for the photoengraver of all the illustra- tions for the monograph, which, including those in the Appendix, consist of 797 figures and 236 plates. This review affords a partial explanation of the great length of the period of time occupied by the author in the preparation of this monograph. The work has involved repeated explorations in the West in search of the remains of all the ancestors of the family and in establishing the full chronology. It has necessitated repeated journeys to all the museums of the country and long and painstaking research. The greatest effort, however, has been expended on the solution of the series of related problems in stratig- raphy, in adaptation, in the origin of new characters, in the mechanics of locomotion, in the modes of evolution, and in the causes of evolution and of extinction that presented themselves as essential to the exposition of the life history of a long extinct family. To restore the living and the lifeless environ- ment of the Rocky Mountain region and to present the titanotheres as living forms has been the persistent purpose of this monograph. SUMMARY OF GEOLOGIC AND ANATOMIC PRINCIPLES The following is a brief statement of the principles developed and discriminated in this monograph : 1. The principle of the division and correlation of geologic formations in Eocene and lower Oligocene time by mamma- lian life zones and bv the subdivision of these zones. XXIV 2. The principle of the correlation of local physiographic diversity with the adaptive radiation, local and continental, of titanotheres and other ungulates. 3. The principle of adaptive radiation as expressed in adap- tations to aquatic, forest, savanna, and plains life at different altitudes. 4. The principle of multiple lines of descent in the same regions, of polyphyly and of polyphyletic evolution as more common among ungulates than monophyletic evolution. 5. The principle of distinguishing phyla by contrasting pro- portions of the head (dolichocephaly and brachycephaly) , of the Umbs (dohchomely and brachj'mely) , of the feet (dolicho- pody and brachypody), and of the teeth (hypsodonty and brachyodonty) . 6. The principles of the lengthening and shortening of the limb segments in harmony, respectively, with adaptation to speed and to weight. 7. The principles of evolution by rectigradation (origination of new characters) and by allometry (changes of proportion) as effecting the chief changes in the hard parts. 8. The principles of continuity and of orthogenesis — the direct continuation of animal form estabUshed in adaptation to environment and of the evolution of new types irrespective of external influences. ' The theoretic causes underlying these principles of evolution are briefly stated in Chapter I, and the con- clusions reached are summarized in Chapter XI. THE TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA By Henry Fairfield Osborn CHAPTER I INTRODUCTION TO MAMMALIAN PALEONTOLOGY SECTION 1. EXPLORATION AND RESEARCH MADE IN THE PREPARATION OF THIS MONOGRAPH The preparation of this monograph was actually begun in 1846, when a part of a jawbone of a titano- there was found in the region now known as South Dakota and sent first to Dr. Hiram A. Prout of St. Louis and then to Dr. Joseph Leidy of Phila- delphia for description. This bit of bone gave the first hint to science of the wonderful deposits of vertebrate fossils in the Rocky Mountain region that have revo- lutionized vertebrate paleontology. The de- tails of this epoch-mak- ing discovery are given in Chapter III. The original fragment bears the generic name Meno- dus, which was assigned to it by the keen system- atic paleontologist of France, Nicolas Auguste Pomel,who gave it the specific name giganteus. Menodus giganteus is thus the first titanothere known to science, and it is a representative of the most imposing family of mammals that was evolved in ancient North America. Figure 1. — "Fragment of the inferior maxillary of the left side' Front's "gigantic Palaeotherium," the first titanothere discovered After Prout (1847). One-fourth natural size. of America — Joseph Leidy, Edward Drinker Cope, Othniel Charles Marsh, John Bell Hatcher — up to the time when the whole long and difficult study of family history, of geologic succession, and of environ- ment was intrusted to the present author. From the first it seemed desirable that this study should encompass more than a dry, systematic description — that these animals and their envi- ronment should, so far as possible through pale- ontology, be made to live again as the domi- nant animals of a long and very interesting epoch in the history of North America — the first third of the Terti- ary period. The field explorations made in the prosecution of this research should, more- over, sustain the guiding principles of the union of paleontology and geology established by the pioneers of our national surveys, as seen especially in the com- bined work of the geologist, Frederick V. Hayden, and the paleontologists, Charles A.White and Joseph Leidy,^ whose reports are still fundamental standards of Terti- FiGURE 2. — Tj'pe of Palaeotherium ? proutii Owen's specimen, Nat. Mus. 113. After Leidy (1852). One-third natural size. This was one of the specimens referred to by Leidy (1852.1)1 in proposing the name Titanolherium. This family, from its earliest known beginnings in the Wind River Mountains of the present State of Wyoming to the height of its development on the plains of the ancient Dakota- Nebraska -Colorado region, attracted the attention of the leading vertebrate paleontologists 1 The figures in parentheses refer to entries i chapter. , the bibliography at the end of this ary geologic and paleontologic history. Subsequent works have surpassed these in specialization and in number and variety of animal forms, and the geologic areas and life zones have been greatly increased by subsequent discovery, but none have surpassed them ' See reports of Hayden and White (1867-73.1, 1868.1), based on surveys begin- ning in 1802, and Leidy's great memoh (1869.1). TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA in scientific method — in the constant union of paleon- tologic with geologic evidence in the reconstruction of the slow succession of events in the wonderful history of this western resion. ROCKY MOUNTAIN LARAMIDE SIERRA NEVADA APPALACHIAN 7/ , , , , ■ ,, / r^y /// /// ,,. /PENNSYLVANjAN / ' MISSfeSIPPlKN^ / .''//•> / ^, / /'/// ? PRE-CAMBRIAN ? ARCHEAN FiGtTRE 3. — Geologic ages and orogenic periods in North America Age of mammals, stipple; age of reptiles, vertical lines; age of amphibians and fishes, oblique lines. The peri- ods of the birth and elevation of the chief American mountain systems, notably the Rocky Mountains (including the Laramide revolution), are indicated approximately by incisions on the right. Modified from diagram by Henry Shaler Williams. The present monograph is made up of this introduc- tory chapter and of ten other chapters, covering the following six main lines of exploration and research that have been followed in order to restore, at least in part, the life and times of the titanotheres : 1. Geologic, physiographic, climatic, and faunal environmental conditions of the titanothere epoch — • the Eocene and lower Oligocene divisions of the Tertiary. Principles of adaptive radiation in animals as explaining the variation of the titanotheres. (Chap. II.) 2. History of the discoveries of the remains of titanotheres, the original published descriptions, and the previous and present classification of genera and species. (Chaps. Ill and IV.) 3. Systematic study of the titanotheres: Eocene and lower Oligocene subfamilies, genera, and species. Characters of the skull, dentition, and postcranial skeleton. (Chaps. V, VI, and VII.) 4. Muscular anatomy of the titanotheres: Princi- ples of locomotion and evolution of limb structure in the hoofed mammals (Ungulata) in relation to habits. (Chaps. VIII and IX.) 5. Origin, ancestry, and adaptive radiations of the titanotheres and other odd-toed ungulates. (Chap. X.) 6. Evolution and extinction of the titanotheres: Evidence regarding modes and causes of evolutionary development and decline in mammals. (Chap. XI.) SECTION 2. PRELIMINARY SURVEY OF THE MONO- GRAPH AND OF THE CONCLUSIONS PRESENTED RANGE OF THE TITANOTHERES IN GEOLOGIC TIME Geographic distribution. — The earliest known titano- theres lived near the end of early Eocene time, after the appearance in the Rocky Mountain region of three kinds of quadrupeds — the horses, the related forest-living tapirs, and the more remotely related rhinoceroses, which still exist elsewhere. The successive immigrations of related odd-toed ungulates are recorded in the Eocene deposits of the region now included in the State of Wyoming, which during Eocene time was a fertile land inhabited by an abundant fauna. The Eocene titanothere epoch in northern Utah, south of the great Uinta Mountain range, which, according to Powell, rose to majestic heights, ended in late Eocene time. In lower Oligocene time the titanotheres had seemingly become the largest mammals in North America. They were second in size to the existing elephants only, but recent paleontologic evidence indicates the existence in Oligocene time in India of mammals that exceeded in size both the titanotheres and the elephants. In 1913 Mr. C. Forster-Cooper (1913. 1) described a new genus of perissodactyls from the upper Oligocene deposits of the Bugti Hills of Baluchistan, BalucTiiiherium (Thaumastotherium) os- borni, an animal of proportions so gigantic that it dwarfs the largest known titanothere. Sedimentary divisions and faunal life zones. — The lower Eocene to lower Oligocene sediments in which titanothere remains have been found occur here and INTRODUCTION TO MAMMALIAN PALEONTOLOGY there in several of the ancient river drainage basins of Wyoming. While the remains of the animals and plants of the period were accumulating in these sedi- ments the titanotheres and other herbivorous quadru- peds and the carnivores that preyed upon them, as well as the other mammals and invertebrates of the land, of the water, and of the air, were constantly evolving, appearing and disappearing through mi- gration and extinction. Thus where the sediments of Front's "gigantic PalaeotJierium" (Menodus gigan- teus) in 1846 to the present time, it has been found that the lower division of this zone is distinguished by the presence of 85 species of vertebrates. The names of the dominant form or forms of each zone are used to designate the several life zones. For the designa- tion of the Titanotherium zone the name of this single genus Titanotherium (Menodus) is used, for it is the most distinctive form in that zone. swEETGRAss co.jNTRODUCTION OF ANCESTORS (MONT.) 2 ^I'aunal Period g::=a ARCHAIC MAMMALS ONLY 1-1/ ^^Faunal Period CorypTiodon - Ayriblypods and. Eohippjis -^vrstSorses W$m wm MAMMALS Pan.toZcmibda -^-mhlypods Polynhastodan. Figure 4. — Successive and overlapping Oligocene and early Eocene formations of the Rocky Mountains The duration of the titanothere epoch is indicated by the arrow. are very rich in fossils of all kinds — mammals, reptiles, iishes, and rarely birds — we are able to restore the life that was distinctive of certain more or less con- tinuous phases of geologic sedimentation. These time divisions are designated life zones, as distin- guished from the sedimentary divisions of groups and formations. After an exploration of the Titanotherium zone that covered a period of over 70 years, from the discovery Many genera persist through several successive life zones. Two genera, the large-hoofed Coryphodon and the small primitive horse EoMppus, persist through four lower Eocene geologic phases or life zones, during which a succession of other species, as well as migra- tions, extinctions, etc., may be clearly observed. It may therefore be necessary to select more than one genus, perhaps as many as three genera, in order to define clearly a certain life zone. For example, the TITANOTHEHES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA amblypod Coryphodon, the horse EoMppus, the tapir Systemodon unite to define the Systemodon-Coryphodon- EoMppus life zone of the lower Eocene. It is through these zonal resemblances in the mammalian life, and more rarely in the plant life, that relatively sure estimates are made of the time during which the sediments containing certain fossils were deposited, irrespective of such geologic data as whether the sediments are thick or thin, whether they are products of erosion or of volcanic eruption, whether they were deposited in still water or in rapidly moving water, or whether they are composed of clay, sand, gravel, conglomerate, or tuff. The life zone, when adequately defined, is an absolutely reliable means of time correlation as distinguished from other means — physiographic, geologic, or lithologic. Similar sediments. — It is true that in the Rocky Mountain region there prevailed at times over wide mentation in one region (for example, the Cypress Hills, Saskatchewan) and with excessively slow sedi- mentation on river flood plains in another region (Chey- enne and White Rivers, S. Dak.), or with a fall of volcanic ash in still another region (Beaver Divide, Wyo.). Evolution of mammals a stable process. — ^By com- paring all the events in the history of the American continent for which the records afforded by geology and paleontology harmonize with others afforded by paleontology alone we reach the conclusion that one of the most uniform, the most stable geographically, and the most widespread is the evolution of mammalian life. This evolution proceeds more or less uniformly in Europe, in Asia, and in North and South America. The apparently sensitive protoplasm (body substance) and germ plasm (hereditary substance) are far more stable and far more uniform in their progressive evolution Former land orea^ Former migration areas Known fossil areas Figure 5. — Map showing areas throughout the world in which remains of titanotheres have been found (solid black) and areas in which titanotheres were probably in migration during Eocene and Oligocene time (oblique lines) Titanotheres have been found in the northwestern United States, the Gobi Desert (Mongolia), Burma, and southeastern Europe. areas similar physiographic, climatic, and eruptive volcanic conditions, as, for example, during what we designate Fort Union time, Wasatch time, upper Bridger time. During such periods of uniform con- ditions the geologic evidence is concordant or harmoni- ous with the paleontologic evidence afforded by life zones, and doubtless any paleobotanic evidence that may be found must also be concordant. In basal Eocene (Fort Union) time, for instance, the forests, the mammals, the reptiles, the climate, the physiogra- phy of the chief areas of sedimentation of the whole Rocky Mountain region were all more or less similar, and in this particular epoch these several means of time correlation afford more or less harmonious evidence. Unlilce sediments. — Such similar sediments, however, become increasingly rare in the continental deposits of Eocene and Oligocene time. A single life zone, such as the Titanotherium zone, may be contempo- raneous with violent fluviatile action and heavy sedi- than the surface of the earth. For this reason they form superior data for time correlation. This is one of the chief generalizations that have grown out of the long series of observations and studies of the correla- tion of Tertiary geologic events in America and Europe that were specially made in the preparation of this monograph. Life zones of the titanothere epoch. — By the method of determining geologic time by discriminating life zones the whole epoch of the evolution of the titano- theres has been subdivided into titanothere zones, distinguished not only by successive genera and species of titanotheres but by corresponding changes in all the environmental forms of life. Each of these life zones probably represents a very long period of time, for in each there was a very considerable evolution of the titanotheres as well as of other forms. These zones (17-9; see table, p. 9), named in descending order, are as follows: INTRODUCTION TO MAMMALIAN PALEONTOLOGY 17. Titanotherium-Mesohippus zone (Brontops robustus zone, fauna; Chadron C fauna; Brontops dispar zone, Chadron B and Brontops brachycephalus zone, Chadron A fauna) . 16. Theoretic zone (Uinta C 2). 15. Diplacodon-Protitanotherium-Epihippus zone (Uinta C 1 fauna) . 14. Eobasileus-DoKchorhinus zone (Uinta B 2 and Washakie B 2 faunas) . 1.3. Metarhinus zone (Uinta B 1 and Washakie B 1 faunas). 12. Uintatherium-Manteoceras-Mesatirhinus zone (Bridger C and D and Washakie A faunas). 11. Paleosyops paludosus-Orohippus zone (Bridger B fauna) . 10. Eornetarhinus-Trogosus-Palaeosyops fontinalis zone (Bridger A and Huerfano B faunas). 9. Lambdotherium-E otitanops-C oryphodon zone (Wind River B and Huerfano A faunas) . Estimated duration of tJie titanothere epoch. — The duration of the titanothere epoch, from the time of the earhest known member of the family {Larnbdotherium) to that of the last product of titanothere evolution {Bron- totherium) is estimated as 600,000 years. This estimate is based on the calculation of Walcott, made from measurements of the rate of geologic sedimentation, that the entire Tertiary period covered not more than 3,000,000 years. If estimates made by Bar- rell (1917.1, p. 892), based on radioactivity, can be verified the duration of Tertiary time should be extended to 54,000,000 years. If this estimate is accepted the duration of the titanothere epoch alone would extend to 11,000,000 years. Though the geologic esti- mate of 600,000 years for titanothere evolu- tion seems to be too small, the physical esti- mate seems to be too great, and for the present we may regard the estimate based on geologic data as ranging between 600,000 and 1,000,000 years. HAYDEN'S SUBDIVISIONS OF THE EOCENE AND THE OLIGOCENE The geologic formations in which titano- there remains occur and the life zones into which these formations are subdivided have been discovered and described during the last 56 years, the first report on them being that of Meek and Hayden (1862.1), in which the entire Tertiary geologic column is represented in a "General section of the Tertiary rocks of Nebraska," reproduced here in facsimile. There is little doubt that when Hayden described the White River group as "1,000 feet or more" in thickness, as including the "Bad Lands of White River; under the Loup River beds, on the Niobrara, and across the country to the Platte," and as com- posed of "white and light-drab clays, with some beds sandstone, and local layers limestone," he had in mind the area extending from Cheyenne River of South Dakota to the region south of North Platte River, displayed in the accompanying map and panoramic section. This section includes at its base the Titano- therium and Oreodon zones (Chadron and Brule for- mations), from which Hayden listed certain char- acteristic forms of animal life, such as TitanotJierium {=Menodus), Choeropotamus {=Ancodus, Hyopota- mus), "Rhinoceros" {=Caenopus), AncJdtherium {= Mesohippus)' , Hyaenonodon {= Hyaenodon) , Ma- chair odus { = Dinictis). Gemral Section of the Tertiary rocks of Nebraska. Names. SUBDIVISIONS. Thick- LOCALITIES. Foreign Equiva- lents. .a > 2 3 Fine loose sand, with some layers of limestope, — contains bones of Canis, Felis, Castor, Eguiis, Mastodon, Tesiudo, &c., some of which are scarcely dis- tinguishable from living spe- cies. Also Helix, Pliysa succinea, probably of recent species. All fresh water and land types. o o o On Loup fork of Platte River ; extend- ing north to Niobrara River, and south to an unknown distance beyond the Platte. a u > 5 White and light drab clays, with some beds sandstone, and local laj-ers limestone. Fossils, Oreodon, Titanotherium, Ch(?ro- potamus, Rhinoceras, Anchitke- rium,Bycenonodon,A/achairodus, Trionyx, Testudo, Helix, Plan- orbis, Limnma, Petrified wood, &c. &c. All extinct. No brackish water or marine re- mains. i a o o o r-t Bad Lands of White River ; under the Loup River beds, on Niobrara, and across the country to the Platte. o 13 P. Light gray and ash colored sandstones, with moro or less argillaceous layers. Fossils, — fragments of Trionyx, Testudo, with large Helix, Vivipara, Petrified wood, &c. No marine or brackish water types. O O O IM . Wind River valley. Also west of Wind River Mountains. - '3 'a 1 Beds of clay and sand, with round ferruginous concretions, and numerous beds, seams and local deposits of Lignite ; great numbers of dicotyledonous leaves, stems, &c.of the genera Platanus, Acer, Ulmus, Populus, &e., with very large leaves of true fan Palms. Also, Helix, Melania, Vivipara, Corbicula, Unio, Ostrea, Potamomya, and scales Lepidotus, with bones of Trionyx, Emys, Compsemys, Orocodilus, he. U o a u o Occupies the whole country around Fort Union, — extending north into the British possessions, to un- known distances ; also southward to Fort Clark. Seen un- der the White River Group on North Plat- te River above Fort Laramie. Also on west side Wind River Mountains. 1 Figure 6. — The Meek and Hayden Tertiary section of 1862 The deposits named are now known to include the following: "Loup River beds." The lower Pleistocene fauna listed is found in an area that includes deposits of the Pliocene and upper Miocene (Ogalalla formation of Darton) . "White River group," including lower Miocene (Arikaree formation of Darton) and Oligo- cene (Brule and Chadron formations of Darton). The " Choeropotamus" is Ancodus ameTicanus, the ancodont of the Chadron formation (Titanotlierium zone). "Wind River deposits" (summit of the lower Eocene). "Fort Union or Great Lignite group" (basal Eocene). These Titanotherium and Oreodon zones are now regarded as lower and middle Oligocene, respectively, and above them have been discovered the Protoceras and Leptauchenia zones, which embrace the highest sediments assigned to the Oligocene. The combined thickness of the Oligocene at this point is 600 to 650 feet. Above it, to the east, are "light-drab clays," having a total thickness of 500 feet, and these, when combined (1,150 feet), correspond to the "1,000 feet 6 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA or more" of Hayden's section. It therefore appears that Hayden's description of the White River group conforms with the accompanying panoramic section of the Oligocene and lower Miocene exposed on the south side of White River, South Dakota, shown in of his White River group apparently came from beds now classified as Oligocene. The name White River group has therefore for years been restricted to the beds of Oligocene age (Brule and Chadron formations). DISCOVERY OF THE TITANOTHEEES OF THE PLAINS At the base of this great section lies the Titanotherium zone, or " Titano- therium beds" of the Hayden-Leidy memoirs, fully described in Chapter II, composed in part of clays, in part of river-channel sandstones, in which titanothere remains are extraordi- narily abundant. The northern borders of this wonder- ful region appear to have been first explored around Bear Creek, a dry tributary on the south side of Cheyenne River, from which Thaddeus A. Cul- bertson brought back the first collection of fossils in 1850. From these expo- sures of the Titanotherium. and Oreodon life zones were obtained the greater part of Leidy's types, which are de-c scribed in Chapter III. The Brule and Arikaree formations, which overlie the Chadron, belong to a period succeed- ing the titanothere epoch, with which this monograph closes. The physiography of this ancient flood-plain region — its broad level stretches, its meandering rivers, its fringing river-border forests, its distant mountains and active volcanic peaks — as restored from our present knowledge of its fauna and flora, is described in Chapter II. It forms a wide contrast to the mountain-basin region, in the heart of which lie the Wind River de- posits, described by Hayden in 1862. DISCOVERY OF THE MOUNTAIN - BASIN ENVIRONMENT OF THE TITANOTHEEES As the entire lower Oligocene history Figure 7 —Panoramic section of the Big Badlands of South Dakota, looking of the titanotheres is recorded chiefly southeastward across Cheyenne and White Rivers to Porcupine Butte This section of the ancient flood-plain sediments now expoi^ed cuts through five great life zones — the Titanotherium, Oreodon, Leptauchenia, Promenjcochoerus, and Merycochoerus zones. It includes also four ancient river-channel sandstones and conglomerates— the •'Titanotherium sandstones," " Metamynodon sandstones," "Protoceras sandstones," and " Fromcrycochoerus sandstones" — each of which includes a more or less distinct river-border and forest fauna. (See map, fig. 69, vicinity of section B.) Figure 7, as sketched under the direction of Osborn for the United States Geological Survey in 1909. (Osborn and Matthew, 1909.321.) Hayden did not, however, specifically define the upper limit of his group, and all the fossils listed by him as characteristic in the Great Plains region east of the Front Range of the Rocky Mountains, so their entne Eocene history is recorded almost exclusively in the mountain -basin region west of the Front Range, in western Wyoming, northwestern Colorado, and northeastern Utah. The interpreta- tion of these remnants of the great Eocene sediments (given in Chapter II) involves far more difficult problems and has required more prolonged and in- INTRODUCTION TO MAMM.tLIAN PALEONTOLOGY D A K O T A Chadron Formation (Tit f ^ [ unique forms. * A single jaw attributed to one of the aberrant Southi American ungulates has been found in the Eohippus- Corypho Basin, Wyo. I lite zone, "Sand Coulee beds" of Clark Fork Only three of the eleven ungulate orders shown in the table were living in the Rocky Mountain region when the titanotheres arrived — (1) the archaic Amblypoda, represented, as we have seen, by Cory- phodon, extremely smaU-brained, of very clumsy build, heavy-footed, in general proportions somewhat like the African rhinoceroses, RTiinoceros (Cerato- therium) simus and R. (Opsiceros) iicornis; (2) the Condylarthra, represented by a diminutive Phena- codus, also extremely small-brained, contrasting with Coryphodon in its small size and cursorial build, formerly but no longer believed to be ancestral to the higher ungulates; (3) the modernized Perissodactyla, including the ancestors of the horses {Eohippus), tapirs (Systemodon) , and lophiodonts (Heptodon). The newly arriving perissodactyl titanotheres equaled in size and resembled in their general cursorial limb structure the condylarths as well as the horses, tapirs, and lophiodonts. They were greatly surpassed in size by members of the Coryphodon family, some species of which were quadruple the size of the earliest known titanotheres. However, certain of the titano- theres of this stage (Eofitanops) exceeded the condy- larths (Phenacodus) in size. It is noteworthy that the archaic Condylarthra (Phenacodus) were numerically preponderant in the Phenacodus zone, just prior to the arrival of the earliest perissodactyl horses. There was doubtless an incessant competition between all these modernized, alert, large-brained perissodactyl ungulates and the archaic, small-brained ungulates {Coryphodon and Phenacodus), which were especially inferior in the mechanics of their foot structure. When, in the upper Eocene, the clumsily built Amblypoda reached the final phase of their evolution in the gigantic Uintatherium and Eohasileus, they apparently became suddenly extinct, and at the same time the titanotheres suddenly began to develop into more formidable animals. At no time in the Tertiary period was the earth populated in the same region with more than one type of very large quadruped. In the Northern Hemisphere the dominance of the amblypods (in the Eohasileus- Coryphodon epoch) was succeeded by the dominance of the titanotheres (in the closing titano- there epoch), and the titanotheres in turn, when they had reached their largest development, suddenly became extinct with no trace of a preliminary stage of decline. OID AND NEW SYSTEMS OF CLASSIFICATION OLD TERMINOLOGY RETAINED The studies for this monograph were begun by Professor Marsh under the old ideas of classification in mammalogy, derived from Linnaeus and his suc- cessors. These studies were continued by Osborn on the same old lines, as shown in his first paper on the titanotheres. (Osborn, 1896.107.) The discovery of adaptive radiation and of polyphyletic evolution, which was one result of the researches made for this monograph, has developed a new phyletic system of classification. Yet even in this new system it is necessary to adhere to the old Linnaean terminology, for the reason that Linnaean methods have been used during the long period of systematic description in 14 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA which the greater number of genera and species of titanotheres have been described; and the Linnaean generic and specific names can not be replaced unless two systematic names have been given to the same animal. Rather than introduce a new terminology we attempt to place each Linnaean species in its proper phyletic position — that is, in its true phylum — and to connect it with other species by intermediate or transi- tion stages, which are termed mutations, the "ascend- ing mutations" of Waagen as distinguished from the contemporaneous "mutations" of De Vries. LINNAEAN METHODS OF DEFINING SPECIES, GENERA, AND PHYLA OF TITANOTHERES Between 1847 and 1902 as many as 29 genera and 67 species of Eocene and Oligocene titanotheres were defined, but of all the definitions given hardly a single Proceeding along these lines Marsh and Cope defined a number of genera of titanotheres, certain of which have since proved to be closely successive members of the same phylum and consequently members of the same genus. Osborn went to the opposite extreme in attempt- ing to reduce all the titanotheres to a single genus. In his paper of 1896, entitled "The cranial evolution of Ti- tanotJierium" (Osborn, 1896.110),hereached the wholly erroneous conclusion that there had been only a single distinct and definable genus of titanotheres — the origiaal Titanotherium of Leidy — and that all the variations among the titanotheres were of the rank of species, rep- resenting different stages of development. This has proved to be a greater error than that of Marsh, because it was based on the hypothesis that the titanotheres belonged to a single — monophyletic — line of descent. APPEARANCE AND EXTINCTION OF MAMMAL ORDERS IN NORTH AMERICA Archaic J\fa7n7na2s - soled, hlacTo. Mode?-nLze^j \ MIDDLE EOCEP _ \ \ \ ^ jA V-.^\\ V:. V" ^ \\ \\ i i ,, I \\ I y'l EARLY EOCENE \ / Figure 14. — Contrast between the Linnaean and phylogenetic systems of classification of sub- families, genera, and species cover a continuous phylum (namely, of ammonites) — that is, successive hereditary stages, which he named "mutations." Many direct phyla of invertebrate animals have since been made known. In this monograph we first learn the full meaning of a mammalian phylum — namely, a phylum is a con- tinuous geologic line of descent diverging from other phyla (1) in the gradual transformation of every character in size and proportion and (2) in the rela- of teeth, vertebrae, or claws. The "species" of Lin- naeus are now known to be actually superspecies and to include one or more modern species, subspecies, and geographic races and varieties, distinguished by differ- ences in coloring, habit, proportion, or otherwise. These differences are due in part to environment and in part to habit. They represent the different bodily effects produced on animals of similar ancestral stock under different environments, in which somatic changes INTRODUCTION TO MAMMALIAN PALEONTOLOGY 17 are rapid and conspicuous. They are in part hereditary (germinal) diflferences, which pass down for generations unmodified by habit or environment. For example, the American genus Peromyscus (the white-footed mouse), as studied by Osgood (1909.1), (dolichocephalic). Peromyscus may have been widely distributed from some common center during the last 40,000 years, and during this long period there may have been both geographic or space evolution and geologic or time evolution, the evolution in time being Figure 15.- -The family tree of the titanotheres, showing the relation between the branches (phyla), sub- families, and genera, as known to science in 1919 The shaded areas show connections that f 3 well established; the dotted lines show gaps that remain to be filled by future discovery, especially in the Uinta formation of Utah. presents a continuous series of transition changes in color and form in species having a geographic range from Tehuantepec to Alaska. In the northern re- gions Peromyscus is larger and has relatively longer teeth and a skull that may be somewhat elongate comparable to that which we observe in the geologic phyla of the titanotheres. The existing genus Cervus affords another example, significant because its geographic range is similar to that of certain Oligocene titanotheres. 18 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Species and subspecies of the genus Cervus [Table prepared by Qeirit S. Miller, 1918] Name Habitat Nature of habitat Climate Cervus canadensis (American elk = New York and New Jersey southward to Open plains, badlands, Humid to extremely wapiti) . the Carolinas; central western States; sand hills; forests and arid. Nebraska, the Dakotas, and the coun- meadows. try farther west, across the Rockies. C. c. merriami (Merriam's elk) .- New Mexico and Arizona _. _ . Mountains and plateaus; Generally arid; for- forests and meadows. ests wet. C. nannodes (dwarf elk) _ . San Joaquin Valley, Calif., and adjoining Plains and tule swamps Generally arid. foothills. C. occidentalis (Olympic elk) Washington, Oregon, California; formerly Chiefly forested regions; Humid. south to San Francisco Bay. Van- some meadows. couver Island? C. xanthopygus (Bedford's deer; Manchuria and adjoining parts of Siberia Forests .. ... Do. Manchurian stag). C. sibiricus (Altai maral) Baikal, Saiansk, and Altai Mountains; Formerly forests and open Extremely humid to southern Siberia and northern Mon- timberless country; extremely arid. golia. even open high desert mountains. Now re- stricted to forests and meadows. C. songaricus (Tien-Shan stag) Tien-Shan Mountains ^ ._ Mostly arid(?) C. yarkandensis (Yarkand stag) Eastern Turkestan _ _ _ Do. C. macneilli (Kansu stag) Kansu and Szechwan border of Tibet Tibet.. - _ ... .-...- do C. wardi (Ward's stag) do C. hanglu (Kashmir deer; hangul, Vale of Kashmir and adjacent mountains. Chiefly forest; some open Humid. hanglu) . parks. C. bactrianus Russian Turkestan . . Chiefly arid. C. maral (maral) . Persia, Crimea, Caucasus C. e. atlanticus (Norwegian red West coast of Norwav _ Do. deer) . C. e. germanica (red deer). Middle Europe Do. C. e. bolivari (red deer of central Central Spain.. Spain). C. e. hispanicus (red deer of south- Southern Spain. Humid and semiarid. ern Spain) . , C. corsicanus (Corsican stag) Corsica and Sardinia. _ . Semiarid. C. barbarus (Barbary deer) _ _ _ _ _ Morocco, Algiers, palearctic north Africa Chieflv arid. COMPARISON BETWEEN ZOOLOGIC AND PALEONTOLOGIC SPECIES The difference between zoologic and paleontologic species is represented in tlie accompanying diagram (fig. 16), showing the descent and relationship of cer- tain members of the dog family (Canidae). A theo- retic stem or central form is shown from which geo- graphic races have been given off horizontally, as it were, and the ascending mutations and species of the evolutionary line of development from the ancestral form have arisen geologically. It follows that in making an anatomic comparison between the existing geographic species and sub- species of such genera as Peromyscus or Cervus and a geologic phylum of species such as that of Menodus or Brontotherium the same comparative anatomical methods of measurement and observation should be employed. Direct measurements of the length and breadth of the skull should be recorded, by which indices (proportions of single structures like the skull) and ratios (proportions between different parts like the upper and lower segments of the limbs) should be established. The proportional changes technically known as dolichocephaly (elongation of the head), brachy- cephaly (broadening of the head), dolichopy (elonga- tion of the face), brachyopy (abbreviation of the face), dolichopody (elongation of the feet), brachy- pody (abbreviation of the feet), dolichomely (elonga- tion of the limbs), brachymely (abbreviation of the limbs) occur in geographic species and subspecies in their corresponding stages exactly as they occur in geologic phyletic time series. The chief difference is that in the geologic time phyla these differences of INTEODXJCTION TO MAMMALIAN PALEONTOLOGY 19 proportion may be followed through long periods of time from their incipient to their final stages, in which various climaxes of change of proportion are reached, such as extreme length or breadth of head or extreme length or shortening of the feet. PROPORTIONS OF THE SKULL IN BEARS AND IN TITANOTHERES In comparing the Eocene and Oligocene titanotheres with the modern bears {Ursus), for example, as studied by C. Hart Merriam (1918.1), we may note certain parallelisms. The members of each of the eleven subfamilies of titanotheres are distinguished by certain proportions of the skull — that is, they are broad-headed, round-headed, or long-headed — by the shape of the horns and the acceleration or retarda- tion in their development, by the presence or absence of cutting (incisor) teeth, by certain proportions of limb, according as they are swift-footed (cursorial), slow-footed (medi- portal), or heavy-footed (graviportal), and by other minor features. The methods ap- plied to the study of the existing bears may be applied to the study of the skull or other hard parts of the titanotheres. In the titano- theres, however, we may observe all these changes of proportion actually in progress from stage to stage as revealed by paleontol- ogy, whereas in the bears we can observe only certain structural forms, which, so far as our observation goes, appear to be fixed or com- pleted, although they undoubtedly represent stages in a state of actual progression. B. Bridger and succeeding titanotheres — Continued. 6. Manteoceratinae; mesatioephalic to brachycephalic; accelerated development of the horns; mediportal {Manteoceras, Prolitanotherium) . 5. Diplacodontinae; dolichocephalic; accelerated molar- ization of the premolars; imperfectly known (Diplacodon) . 4. Telmatheriinae; mesaticephalic to dolichocephalic {Telmatherium, Sthenodecles) . 3. Palaeosyopinae; brachycephalic; short-limbed {Palae- osyops, Limnohyops) . A. Wind River titanotheres ; face longer than cranium : 2. Eotitanopinae; medium-limbed, mediportal (Eoti- 1. Lambdotheriinae; light-limbed, cursorial {Lambdo- thenum) . The above scheme presents the eleven subfamilies of titanotheres as they were distinguished in 1914. GEOGRAPHIC DISTRIBUTION AT PRESENT TIME True R ZcoUfiical WlPES -GEOGRAPHIC DISTRIBUTION IN PAST TIME FEATURES DISTINGUISHING TITANOTHERES PHYLA OF The first application of changes of propor- tion to the arrangement of the subfamilies of titanotheres is the following synopsis, pre- pared in 1914: Proportions of skull and limbs; presence and absence of characters distinguishing the subfamilies {main phyla) of titanotheres lOsborn, 1914.409] B. Bridger and succeeding titanotheres; cranium longer than face: 11. Brontotheriinae; mesaticephalic to brachycephalic; horns long, transversely flattened, and divergent (Brontolherium) . 10. Megaceropinae; mesaticephalic to extreme brachy- cephalic; horns long, vertically placed; no incisor teeth (Megacerops (—Symborodon)). 9. Brontopinae; brachycephalic; horns short, rounded, or oval; incisors persistent (Brontops {=Mega- ceratops), Diploclonus) . 8. Menodontinae; mesaticephalic to dolichocephaUc; short triangular horns; incisor teeth reduced or wanting; feet and limbs long {Menodus {=Titano- therium), Allops). 7. Dolichorhininae; mesaticephaUc to doUchocephalic; limbs, so far as known, short (Dolichorhinus, Mesatirhinus, Sphenocoelus, Metarhinus, Rhadi- norhinus) . Figure 16. — Theoretic descent of existing members of the dog family (Canidae) from a common ancestor A represents the ancestral type. Dots represent intergradations indicated by paleontologic observations (vertical lines) covering five periods of geologic time. A', B, B', C, and C rep- resent existing forms, and dots represent a few existing intergradations demonstrated by zoo- logic observations (horizontal lines). Heavy lines and the adjacent dots represent the phyla; also the past and present distribution of geographic (ontogenetic and environmental) sub- species, races, and intergrades. Since that time certain phyla have been condensed by the discovery of titanotheres that link together some of these subfamilies, and others have been expanded by the discovery of new subfamilies, such as the Rhadinorhininae. MUTATIONS OF WAAGEN Where the fossil material is abundant the genera and species are found to be connected by a series of intergradations. These intergradations, though con- tinuous, are measurable, and therefore a species is subdivisible into a series of intergrading forms. The monophyletic, systematic, or taxonomic unit division of these species is the mutation of Waagen, which is a subspecific stage in the development of one or more characters. Such an actual sequence of mutations of Waagen may be illustrated in the genus BrontotJierium, as indicated on the following page. 20 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBEASKA Oligocene stages of titanotheres of the Brontotherium 'phylum in the Titanotherium zone Division of zone Stage Species Tiieoretic ascending mutations Brontotherium platyceras Brontotherium ramosum Brontotherium ourtum Species. Subspecies. Mutation. Do. Species. Subspecies. Mutation. Do. Species. Subspecies. Mutation. Do. Species. Subspecies. Mutation. Do. Upper. Do Subspecies. Mutation. Do. Species. Subspecies. Mutation. Do. Middle. Subspecies. Mutation. Do. Brontotherium hypoceras Lower. Subspecies. Mutation. Do. Subspecies. Mutation. Do. " Genus Titanops IVIarsli. ZOOLOGIC AND PALEONTOLOGIC NOMENCLATURE Significance of the table. — The sequence shown in the accompanying table, which presents what is believed to be a generic, monophyletic, or nearly single phyletic series of changes of form, evolving in a single geographic region of South Dakota, illustrates the manner in which the Linnaean binomial system and the muta- tion substages of Waagen may be adapted to express a phyletic sequence. The newer trinomial names of modern mammalogy and the subspecific names may be employed to connect the intergrading mutations. The most primitive species, Brontotherium leidyi, is so notably distinct in size and skull structure from the most advanced species, Brontotherium platyceras, that, if named by zoological standards, it might well be ' Type of genus Brontotherium (Marsli). placed in a separate genus — in fact, several generic names have been suggested for members of this phylum, namely, Brontotherium, Titanops, Bronto- theridion (MS.) — but the subdivision of such a phylum into a number of genera would obscure the all-im- portant monophyletic unity, for such a phyletic genus is defined by its peculiar and distinct evolutionary tendencies. For example, the genus Brontotherium tends toward the evolution of flattened horns, a charac- teristic which begins in a very slight flattening of the posterior side of the horn, as observed in B. leidyi, and develops into the extraordinarily broad, flattened horns of B. platyceras. New phyletic meaning of species. — The species repre- sented by large collections of mammals like those of some of the phyla of the titanotheres, especially the INTBODUCTION TO MAMMALIAN PALEONTOLOGY 21 Brontops phylum, are so closely intergraded and con- nected by "ascending mutations" that the dividing lines between them can be drawn only arbitrarily, according to individual judgment. In the Brontops phylum, for example, the species Brontops hrachyce- pJialus grades imperceptibly into the species Brontops dispar through gradual transitions in a great number of characters, as may be seen in the Hatcher collection in the United States National Museum. There is no evidence of brusque transitions, saltations, or jumps in any structure, such as are presupposed in the mutation theory of De Vries. By contrast, the mutations of Waagen are intergradations between arbitrarily defined species, and through these muta- tions species and genera pass imperceptibly one into another. Evolutionary characters of each phylum. — Thus we reach a clear conception of a phylum of the titanotheres in its osteologic and dental characters. A phylum may be further defined as a succession of interbreeding (syngamic, Poulton) individuals of similar (synepi- gamic, Poulton) ancestry, which may or may not occupy a similar range of country (synpatric, Poul- ton), which follow in every structural character a sim- ilar line of evolution (synphyletic, Osborn) and adap- tation (syntelic, Osborn). In each horn, in each tooth, in every bone of the skull and skeleton, and by inference in all the hard parts as well as in all the soft parts, each phylum has its distinctive mode and rate of transformation in each character, as follows: (1) Distinctive hereditary pro- portion; (2) distinctive tendencies to change of propor- tion; (3) distinctive progressive changes of proportion; (4) distinctive retrogressive changes of proportion; (5) distinctive accelerations and retardations in ontogeny (individual development) ; (6) distinctive rates (veloc- ities) of progression and retrogression in phylogeny in each character. In each phylum are consequently developed distinctive and ever changing proportions and ratios between different single characters and groups of characters, measurable by indices and ratios. Such indices express the degrees of broad-headed, long- headed, broad-footed, short-footed structure and pro- portion, and so on. Each phylum has also its distinc- tive but constantly changing indices and ratios of teeth to skull, of skull to body, of body to limbs, etc., which also are constantly changing as we pass from the lower to the higher geologic levels. Old and new meanings of taxonomic terms. — In the following table a comparison is made between the old and the new meanings of the taxonomic terms used by mammalogists. The definitions given in the sec- ond column are those of the old "special creation" system — followed by Linnaeus — which is based on geographic distribution alone; the definitions given in the third column are those of the new phyletic sys- tem — that of Osborn — which is based on both geologic and geographic distribution. The new system was first used for the rhinoceroses (Osborn, 1900.192) and for the titanotheres (Osborn, 1902.208). Comparison oj ike Linnaean and the phyletic systems of taxonomic terms Term Definitions Old system New system Family A contemporaneous group of similar subfamilies A smaller contemporaneous group of similar genera__ A still smaller contemporaneous group of similar or related species. A group of related subspecies and geographic va- rieties. Nothing corresponding to the geologic mutation of Waagen. Contemporaneous and ancestral phyla that exhibit similar family tendencies of evolution. A branch composed of one or more phyla which exhibit similar generic tendencies of evolution. Part of a single phylum of successive species and muta- tions exhibiting similar tendencies. A series of ascending mutations. Geologic mutation (of Waagen); ascending substages within a specific phylum. Subfamily Genus __ Species Mutation Desired harmony of mammalian paleontology and zoology. — The methods employed by all zoologists, paleontologists, and anthropologists in their observa- tion and measurement of the hard parts of mammals should be the same. The methods pointed out above, first presented by Osborn (1914.412), are founded on the comparison in time of geologic ascending evolutionary phyla of mammals — such as the rhi- noceroses and the titanotheres — with contemporaneous geographic series of species, subspecies, and varieties that may be grouped within a single genus. What applies to the systematic terms used in the classifica- tion and description of animals applies with equal force to those used for single characters, for it is the cumulative sum of evolutionary change in a very large number of single characters which makes up the mutation of Waagen, the species, or the genus, as the case may be. 22 TITANOTHEEKS OF ANCIENT AVYOMING, DAKOTA, AND NEBEASKA SUMMARY OF DIFFERENCES BETWEEN OLD AND NEW SYSTEMS To sum up: (1) The Linnaean genus or species is defined (statically) by the presence of certain propor- tions and by the presence or the absence of certain characters, whereas the phyletic genus or species is defined (dynamically) by the progressive evolution of certain proportions and by the gradual gain or loss of certain characters; (2) the Linnaean genus or species was clearly distinguished from a related genus or species, whereas the phyletic genus or species may gradually fade into its ancestor or successor, and the point where we make the dividing line is largely arbi- trary; (3) consequently the phyletic genus actually has a new meaning, but to avoid innovation in nomen- clature we apply the phyletic term genus to a number of species having a wide range in time and space, in the same manner that Linnaeus applied the term genus to a number of species having a wide range in space only. STUDY OF THE EVOLUTION OF SINGLE CHARACTERS In the hard parts of living as of extinct animals only three kinds of changes are observable — (1) changes of proportion, which the author terms "allo- metrons"; (2) the appearance of absolutely new char- acters, which the author terms "rectigradations"; (3) the disappearance or retrogression of characters. Changes of proportion. — Changes of proportion (allometrons) make up by far the larger part of the evolution of the titanotheres, as of that of all other mammals. At least 95 per cent of the differences between the skeletons of Eotitanops horealis and Brontotherium plafyceras are due to changes of pro- portion, and not more than 5 per cent to additions of absolutely new characters, such as horns. Conse- quently a very careful study has been made of allo- metry — that is, of the methods of calculating, measur- ing, recording, and describing changes of proportion — and the result has been the discovery of a number of general principles that apply to all mammals, extinct and living, including man. Probably also the un- discovered causes of changes in proportions are the same in all mammals, but their discovery constitutes a very difficult problem. (See Chap. XL) In this difficult work the paleontologists may be greatly aided by the zoologists, especially by very precise field observers, such as Allen, Merriam, Miller, Osgood, and Sumner. Although the mammalogists have demonstrated that there is an apparently causal relation, direct or in- direct, between certain types of coloration and of size (harmonic increase or decrease) and the geographic environment, the relation between change of environ- ment and changes in proportion (disharmonic) is very obscure. It is known that a harmonic increase or decrease in size of the entire mammal is correlated with certain differences in habitat, often for the obvious reason that a favorable environment favors development of larger races, whereas an unfavorable environment dwarfs growth. It remains to be determined, however, whether certain environments induce uniformly similar disharmonic changes of pro- portion. Anthropologists, for example, have failed to establish a definite causal relation between environ- ment and the broad-headed (brachycephalic) or the long-headed (dolichocephalic) form of the human head. The chief contribution that the paleontologist has made to this obscure matter is to show that when a proportionate change of head form is once established in a certain direction there is a tendency to go to extremes, so that, for example, extremely long heads or extremely broad heads tend to evolve longer or broader heads. These evolutionary tendencies are illustrated in the titanotheres. Adaptive new characters. — The second mode of mam- malian evolution — by the appearance of absolutely new characters — lies in a field where the paleontologist has a great advantage over the zoologist, because in a series of fossils a new character (rectigradation) can be traced back to its incipient, rudimentary stage, in which it is so inconspicuous that it would not attract the attention of the zoologist. Many characters that eventually may exert a most profound influence on the evolution of a race — that may, in fact, dominate aU other characters — arise, so far as observed, from excessively minute beginnings. These origins of new characters are pointed out with great precision in Chapters V and VI, in which the evolution of the skuU and teeth is described in detail as observed in the Eocene and lower Oligocene titanotheres. This very precise study of the origin and evolution of similar characters in many different lines of descent has led to the important discovery that phyla differ less through the possession of this or that new character than through the different rates of evolution at which the same character arises and evolves. In one phylum a new character like the horns will arise in an early geologic stage and evolve very rapidly, whereas in a related phylum it will arise relatively late in geologic time and will evolve very slowly. Thus a phyletic genus is defined not only by the characters which it exhibits but by the rate of the evolution of these characters. This principle, again, is observable only thi-ough paleontology. The origin of new characters, as manifested in dif- ferent ways in the members of twelve subfamilies of the titanotheres and as indicated by comparison with the INTRODUCTION TO MAMMALIAN PALEONTOLOGY 23 origin of similar characters in other families of Peris- sodactyla, has accordingly been studied with great care. Retrogressive characters. — The retrogression or disap- pearance of characters is illustrated in the history of the titanotheres by the features enumerated below. 1. Reduction of the canine teeth in many later titanotheres. 2. Reduction and occasional loss of incisors. 3. Reduction and frequent loss of first lower premolar. appearance in North America and western Europe of members of nine different families of Perissodactyla, the odd-toed ungulates, which were probably all de- scended from a common ancestral or stem form which lived in Upper Cretaceous time. The probable charac- ters of this stem form are fully described in Chapter X, where it is shown that the ancestral perissodactyl was a comparatively small and simple quadruped not ex- Perissodxictyls CRETACEOUS Peris sodaxtyL Stem, Figure 17.- -Successive invasion of nine perissodactyl families in Nortli America and western Europe between latitudes 40° and 50° Th2 chalicotheres (aberrant clawed perissodactyls with affinities to the titanotheres) are regarded as members of a separate superfamily, the Chalicotheroidea. Diagonal shading indicates the extent to which each phylum is represented by fossil remains. 4. Reduction and loss of protoconule and metaconule in upper molars. 5. Reduction of nasals and their coalescence with frontals. 6. Reduction of the trapezium in later titanotheres. PHYLOGENY OF THE NINE TYPICAL FAMILIES OF THE PERISSODACTYLA The competition of the titanotheres through natural selection was naturally closest with other members of the order Perissodactyla. As shown in the ordinal phylogenetic tree (fig. 17), we observe the successive ceeding half a meter in height, and that it was origi- nally confined to a definite geographic area, feeding ground, and range, very possibly in northern Asia. The eight families that appear in North America and the paleotheres, which appear only in western Europe, were by no means equally distinct from one another. They were originally separated from the stem form not into nine branches but into five great main branches, termed superfamilies, as shown in Figure 17 and in the accompanying table. 24 TITANOTHEEES OF ANCIENT WTfOMING, DAKOTA, AND NEBRASKA Phyla of the odd-toed ungulates Superfamilies Families 1. Titanotheroidea 1. Brontotheriidae : The titanotheres, known chiefly in North America and in eastern Europe. 2. Chalicotheroidea 2. Chalicotheriidae: The chalicotheres, first known in Europe and North America; then in Asia. 3. Hippoidea: Horselike forms _ ._ 3. Palaeotheriidae : The paleotheres, known in western Europe only. 4. Equidae: The horses, first known in Europe; then simultaneously in North America and Europe; subsequently in Asia, Africa, and South America. 4. Tapiroidea: Tapir-like forms 5. Tapiridae: The tapirs, first known in North America; then in Europe and Asia. 5. Rhinocerotoidea: Rhinoceros-like forms _- 6. Lophiodontidae: The lophiodonts, known in North America and Europe. 7. Amynodontidae: The amynodonts, aquatic rhinoceroses; first known in North America; then in Europe. 8. Hyracodontidae : The hyracodonts, cursorial rhinoceroses; upper Eocene and OUgocene of North America only, so far as known. 9. Rhinocerotidae : The rhinoceroses, the typical rhinoceroses; first known in North America and Europe; then in Asia and Africa. In North America the horses (Eohippus) were the first perissodactyls to arrive. They were followed by the tapirs (Systemodon) , which in turn were succeeded by the lophiodonts {Heptodon). It is possible that ancestral titanotheres were living iu northern parts of the American continent, but apparently thej^ did not reach the region near the fortieth parallel until it had become well populated with horses, tapirs, and lophiodonts. By middle Eocene time three more families had appeared — the paleotheres, in Europe only; the rhinoceros-like amynodonts (semiaquatic forms), which first appear in North America and subsequently in Europe; and the cursorial rhinocer- oses known as hyracodonts (Hyrachyus), which appear in North America only and preceded the amynodonts. Toward the beginning of upper Eocene time there first appear in North America, as well as in Europe, ancestors (Eomoropus) of the chalicotheres, animals closely related in tooth structure to the titanotheres, which were separated into a distinct order (Ancylo- poda) by Cope and are here regarded as forms some- what parallel to the Titanotheroidea. WIDE GEOGRAPHIC DISTRIBUTION OF THE PERISSODACTYLA We are first struck with the remarkably wide holarctic distribution of the perissodactyls in Eocene and lower Oligocene time, a fact which points to facihty of migration over the whole Northern Hemis- phere. Only one family, the paleotheres, is exclu- sively European, and one other, the hyracodonts, is, so far as known, exclusively North American. The titanotheres were formerly beheved to be exclusively North American, but two forms have been found in eastern Europe, which correspond very closely with the titanotheres of upper Eocene age from the Uinta Basin in northern Utah. Members of all the other perissodactyl families — the chalicotheres, tapirs, lophiodonts, amynodonts, and rhinoceroses — probably ranged freely to and fro over the great northern continent of Em-asia and North America combined, the geographic region known as Holarctica. The second important fact regarding the Peris- sodactyla is that, although the environment dining middle and upper Eocene time, after the extinction of the archaic imgulates — the Condylarthra and Amblypoda — was especially favorable to the existence of the Perissodactyla, this order reached its maxi- mum expansion in the lower Ohgocene epoch, when all the nine families were existing and apparently flourishing at the same time. It would appear that in upper Eocene and lower Oligocene time Holarctica was dominated by perissodactyls. This period was immediately followed by a period when either the environment was adverse to the existence of the peris- sodactyls or competition with other types of imgu- lates was disastrous to them, because at or before the end of the lower Oligocene epoch five perissodactyl families suddenly disappeared — the titanotheres, paleo- theres, lophiodonts, amynodonts, and hyracodonts. The aberrant chalicotheres, apparently through retreat to forested regions, survived in Europe and probably also in North America until the Pliocene epoch. INTRODUCTION TO MAMMALIAN PALEONTOLOGY 25 .^"^- Figure 18. — Outlines of the body form of the perissodactyls, drawn to the same scale The largest known member o( each family is selected for comparison. The smallest known stem forms of each family are illustrated in Chapter X. The animals are grouped according to their natural relationships, as indicated especially by the pattern of the molar teeth, as follows: Bhinocerotoid group: A, Mclamynodon; family Amynodontidae; graviportal; aquatic; lower Oligocene. B, Hyracodon, family Hyracodontidae; cursorial; middle Oligocene. C, Ceratotherium simum; living white rhinoceros; family Rhinocerotidae; graviportal. Tapiroid group: D, Tapirus terresiris; existing tapir; family Tapiridae; mediportal. Hippoid group: E, Palaeotherium; family Palaeotheriidae; lower Eocene; mediportal. F, Equus pTzewalskii; existing horse; family Equidae; cursorial. Chalicotheroid group: (?, MoTopus; family Chalicotheriidae; clawed perissodactyl; lower Miocene. Titanotheroid group: B, Brontotherium platyceras; family Brontotheriidae; graviportal; lower Oligocene. 101959— 29— VOL 1 4 TITAXOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA INTRODUCTION TO MAMMALIAN PALEONTOLOGY 27 Thus out of the nine original famiUes of the great order of Perissodactyla only three — the horses, tapirs, and rhinoceroses — have survived to the present time, and these during the glacial epoch were greatly reduced both in numbers and in geographic dis- tribution. The consideration of these facts raises the whole problem of the origin and adaptive radiation of the perissodactyls (see Chap. X) and the general problem of the causes of the extinction of the perissodactyls and of other quadrupeds (see Chap. XI). adaptive origin of new characters. The moment of origin of each new character is a very important moment in the history of that character. Does each new character arise fortuitously at this point or that, in an adaptive or inadaptive condition, or does each new character arise in a mechanically adaptive condition, although this condition may be merely incipient? The biologic purpose of the long and dry descrip- tions and tables of measurements given in Chapters V, VI, and VII of this monograph is to direct obser- vation continuously to this problem of the origin of Figure 20. — Periods of expansion and extinction of the perissodactyls and contemporary forms Showing that the expansion of the perissodactyls was coincident with the extinction of the archaic Condylarthra and Amblypoda and that the extinction of many perissodactyls was coincident with the expansion and adaptive radiation of the artiodactyls. CAUSES OF EVOLUTION There can be no doubt as to the survival value of certain finished types of tooth structure and Hmb structure (see pp. 880-881), a principle first formulated by the distinguished Russian paleontologist Kova- levsky (1873.1). Two important questions that the reader must keep in mind in considering the origin of innumerable new characters are (1) whether there is evidence of chance origins and chance rudi- ments of certain types of structure possessing suffi- cient survival value to establish themselves through the principle of the survival of the fittest, or natural selection; or (2) whether there is some other ortho- genetic principle at work causing the definite and new characters. Our general conclusions concerning these two questions are presented in Chapter XI. ADAPTIVE EVOLUTION AND OVEEEVOLUTION OF THE FORM OF SKULL, TOOTH, AND FOOT Whatever may be the causes of evolution its re- sults are definite. The visible evolution of all the hard parts of the body in herbivorous animals is originally mechanical and manifests general adapta- tion to two broad groups of purposes: 1. Prehension of food (lips, teeth, and jaws); com- minution of food (teeth and jaws); conservation and transportation of stored food energy (body and limbs). These purposes involve all the mechanical changes of structure of skull and tooth. 28 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 2. Motion and locomotion; migration in search of food and to escape enemies; adaptation to perform the act of reproduction and to protect the young. These purposes involve all the mechanical changes of the structure of limb and body. The operation of the principle that, under the domi- nance of these modes of mechanical adaptation each organ, structure, and character is adaptively evolved for some special service to the organism is not invariably evident in respect to all changes in the proportion of characters. Certain characters of proportion, such as extreme broad-headedness or extreme long-headedness, seem to interfere with adaptation; they appear to be carried so far in one direction as to render the animal less adapted to survive than its less specialized ances- tral forms. In other words, certain tendencies of evolution may carry a phylum beyond its require- ments in adaptation. Aside from this question of the different degrees of survival or actual elimination value of certain tend- encies of evolution, there can be little doubt that in its origin and development each character, sooner or later, responds and reacts independently to the con- ditions of the environment, quite apart from the question as to the causes of such response. The teeth react to the kinds of food; the feet and limbs to the kinds of soil. The principles of the divergence of quadrupeds from each other in their independent adaptations in the skull, teeth, limbs, and feet are fully discussed elsewhere (see p. 123) in the treatment of the principle of adaptive radiation. Though they may have lived apparently in the same region and have been fossilized side by side in the same sediments, all distinct species of quadrupeds have locally different habits and habi- tats. The structure of the skull, jaws, and teeth re- sponds to their habits and tastes ; the structure of the feet and limbs responds to their habitats — the nature of the ground, etc. PHYLETIC DIVERGENCE IN THE EVOIUTION OF NEW PROPORTIONS IN HORSES AND IN TITANOTHERES All the families of an order of Perissodactyla start their career from a similarly proportioned ancestral stem form such as that described in Chapter X (p. 760) as the stem perissodactyl. Starting with the same complement of characters, divergence in proportions separates the families of perissodactyls more and more widely from one another. In the Equidae (horses), for example, the head form of the earliest known ancestor (EoMppus) is very similar to that of the earliest known ancestor (Eotitanops) of the family Brontotheridae. In both these primitive skulls the orbit is near the center of the head, and in the later forms it apparently moves backward or forward, but what really happens is that the skull is elongated in front of the orbit in the horse and is elongated behind the orbit in the titanothere. (See fig. 21.) A comparison of the forms shown in Figure 21 with those shown in the following figures will demon- strate the marked similarity of the lower Eocene forms and the very wide divergence of the modern forms. The skulls of the ancestral tapir, horse, and titanothere {Systemodon, Eohippus, and Eotitanops) are in many ways much alike, the chief differences consisting in (1) the details of the characteristics of the dentition, (2) the relative position of the orbits, (3) the depth of the head through the back part of the lower jaw, and (4) the size of the muzzle. The primitive titanothere prophetically suggests the titanothere characters in the relatively heavy muzzle and stout lower jaw. The primitive horse Eohippus prophetically suggests the modern horse in the taper- ing form of the slender lower jaw and in the general contour of the skull, except that the eye is placed near the middle of the head, as in other primitive perisso- dactyls. The primitive perissodactyl Systemodon, regarded by Osborn as an ancestral tapiroid, had a somewhat longer, more pointed muzzle but was otherwise very similar to the contemporary horse Eohippus. These differences of proportion between the facial region in front of the orbit and the cranial region behind the orbit are partly correlated in adaptation to the elongation (hypsodonty) of the crowns of the grinding teeth. In the horse and in most of the rumi- nant artiodactyls the face is elongated to accommodate the vertically elongated (hypsodont) grinding teeth. In the titanotheres, which are browsing animals, and in the browsing rhinoceroses of India and of Africa the orbit is directly above the grinding teeth and the cranium is slightly elongated, as shown in Figure 22. Thus it may be stated as a general prin- ciple of skull evolution that in browsing ungulates the cranium tends to be elongated and the face tends to be abbreviated, whereas in grazing ungulates, like the white rhinoceros of Africa, in which the grind- ing teeth are elongated, the face is elongated, and the cranium is abbreviated. It follows that these respective proportions of the region in front and back of the eyes are adaptive; they are part of the general correlation of skull proportions with the functions of the grinding teeth employed in the prehension of food, as provided for chiefly in the shape of the upper and lower lips, which are obtrusible and flexible both in the browsing rhinoceroses and in the grazing horse, which occasion- ally browses. When the horse is browsing it extends its lips very much in the manner of the browsing rhinoceros, except that in the rhinoceros the independ- ent motion and the pointing of the upper lip are more extreme. In the grazing white rhinoceros the upper lip is extremely broad and square. The animal subsists largely on grasses, which it crops with its square lips, exactly in the manner that the horse INTEODUCTION^ TO MAMMALIAN PALEONTOLOGY 29 crops grass with its lips and front teeth. In all the rhinoceroses cropping front teeth are atrophied, the four pairs of incisors and the canines being reduced to a single large pair on either side and being thus analogous to those of certain titanotheres. From these comparisons we deduce the structure of the mouth parts in the titanotheres as restored by Gregory. (See p. 704.) We also deduce the various adaptations to the browsing and grazing habit re- spectively in the different genera of titanotheres, for undoubtedly some were purely browsers and others of the face, with a relatively short skull, and with a very powerful neck, a feature that is also especially characteristic of the titanotheres. Thus there is a general resemblance between the side profile of Brontotherium platyceras and that of the Indian rhinoceros, which is due to analogous mechan- ical evolution, through the principles known as homo- plasy, parallelism, or convergence. The titanotheres pass through a long lower and middle Eocene phase of tapir-like analogies, but when, in middle Eocene time, horns begin to appear the head region develops Figure 21. — Phyletio divergence in the evolution of new proportions in horses and in titanotheres Lower Eocene ancestral horse Eohippus (A) and lower Eocene ancestral titanothere Eotilaijops (C) (both with the orbit in the same relative position on the skull) compared with a modern horse (B) with face extended in front of the orbit and a titanothere of the latest stage (D) with slvuU extended behind the orbit. Thus two very similar heads (A, C) become increasingly dissimilar (B, D). Scales various. tended toward grazing. Thus the orbits, the face, the grinding teeth, the front teeth, the lips, and the bones supporting these structures are respectively transformed in adaptation to the function of prehen- sion and to browsing or grazing habits. The front part of the skull of the rhinoceros, with its terminal dermal horn, is comparable to that of the large-horned titanotheres, with their terminal bony horns. It will be observed that the entire front part of the head of the rhinoceros, in adaptation to the great strain of the horn used as a weapon of offense and defense, is correlated with a flat or a concave line along the top rhinoceros-like analogies. Similar analogous phases also occur to a greater or less extent in the feet of the rhinoceros and the titanothere. On comparing the heads of the types of perisso- dactyls, ancient and modern, we observe that different modes of feeding and of offense and defense guide the dominant adaptations in evolution. The evolution operates under the principles of anatomical correla- tion and compensation, gain or loss in one part being mechanically balanced by gains and losses in every other part. This process includes the principle of physiologic compensation, whereby loss of function in 30 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 22. — Contours of the head and of parts of the mouth in browsing and grazing perissodactyls A, Asiatic rhinoceros {Rhinoceros ' inikusi, chiefly a browser; B, black rhinoceros ot Africa (R. (.Opsiceros) bicornis), chiefly a browser; C, white rhinoceros of Africa (R. (Ccraioiherium) simum), chiefly a grazer; D, domestic horse {Equns caballus), chiefly a grazer; E, American tapir ( Tapirus tenestTis), a browser. ' The generic terminology of the rhinoceroses is not yet fully agreed upon by zoologists. The family tree, like that of the titanotheres, is polyphyletic. INTBODUCTION TO MAMMALIAN" PALEONTOLOGY 31 Figure 23. — Heads of lower Eocene and modern jjerissodactyls, showing changes of proportion and of the lip structure Based on materials in the American Museum of Natural History. Scales various. A, Head of the lower Eocene tapiroid Sijstemodon, very sim- ilar to that of Eohippus and of Lambdotherium; B, head of middle Eocene tapir Hdaleies, in which a prehensile upper lip first appears; C, head of the modern tapir Tapirus, whose prehensile upper lip forms a short proboscis; D, head of middle Eocene cursorial rhinoceros Hyrachyus, still of primitive proportions; E, head of existing white rhinoceros {EhiTicccrcs ( Caalothcriuw) simum) with extremely broad, grazing type of lip structure. 32 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA one part is taken up by some other part. For exam- ple, the loss of the function of the incisors in the pre- hension of food is compensated for by changes in the form and function of the lips. upper lip like that of the tapir necessitates space for the superior retractor muscles, which curl the lip upward and backward. An example of the results of the evoUition of the lower jaw may be seen by compar- FiGtTRE 24. — Restorations to the same scale of the heads of some of the principal t3'pes of titanotheres Drawn by Charles E. Knight, after models made by him under the author's direction. About one-seyenteenth natural size. A, Brontops roiustus Marsh, oblique yiew, middle Titanotherium zone; B, Menodus giganleus, upper Titanotherium zone; C,Megacerops copei Cope, partly oblique side yiew, summit of the Titanotherium zone of Colorado; D, Broniotherium platyceras Scott and Osborn, the final stage in the eyolution of the horns of the titanotheres, summit of the Titanotherium zone of South Dakota; E, Protitano- iherium sp., summit of the Eocene. With the evolution of the lips the structure of the anterior parts of both the upper and lower jaws, of the anterior teeth, and the anterior nasal openings is closely correlated. The development of a prehensile ing Eotitanops gregoryi and Brontotherium {medium) gigas, the whole jaw of the former hardly exceeding in length a single posterior grinding tooth of the latter. (See fig. 25.) INTRODUCTION TO MAMMALIAN PALEONTOLOGY 33 EVOIUTION OF THE LIMBS AND FEET OF THE TITANOTHERES The feet of the titanotheres, like their skulls, pass through a lower Eocene tapir-like phase, which is followed by a middle and upper Eocene rhinoceros- like phase and finally they attain a structure similar to that of the rhinoceroses, as shown in Figure 26, except that all the titanotheres, like the existing tapirs, retained four distinct and functional digits in the fore foot. The fore foot of the tapir resembles the fore foot of the lower Eocene titanothere except that in the latter D. II, III, IV, V were all of nearly equal size, as shown in the diagram (B). This is known as the mediportal stage, for it is adapted to carrying a moderate amount of weight. The foot of the rhinoceros (C, C, C) is like that of the upper Eocene and lower Oligocene titanotheres except that in these there were four weight-bearing digits instead of three. This is known as the graviportal type of foot, in which a large cushion pad is developed at the back to relieve the shock of impact, and the end phalanges of the digits are incased in the horny sheath in front. In the tapir and rhinoceros the main weight passes directly through the center of the median phalanx (D. Ill), but in the tetradactyl titanotheres the main weight passes between D. Ill and D. IV. The concentration of the weight on the central digit of the horse and its resultant monodactylism, correlated with the expansion of the horny hoof and the contraction of the pad, is part of the evolution of a cursorial type of foot, which presents the widest contrast to the graviportal type. In addition to comparing the head structure it was found necessary to compare the foot and limb struc- ture of the titanotheres with that of all the other perissodactyls — not only the bony parts but the musculature. The work done on the musculature led to an exhaustive study of all that is known of the muscular anatomy of the members of the three exist- ing families of perissodactyls. This study, which was directed by William K. Gregory, formed the basis of the restoration of the muscular anatomy of the giant Brontops rolustus presented in Chapter VIII (pp. 722, 723). This restoration of an extinct animal is the first that has been based upon exact comparative study. It presents the titanothere as a superb example of the graviportal type of musculature and skeleton, sur- passed only by the existing elephants. The study of the structure of the foot led to a special investigation of the proportions of the limb bones in the ungulates. This investigation, directed by Osborn and cooperated in by Gregory, resulted in the striking discovery that the proportions of the upper and lower segments of the limbs and of the feet are invariably adjusted, first, to the weight that the limb must carry, and second, to speed of locomotion. These propor- tions are evolved, quite irrespective of ancestry, in adaptation to different modes of progression. Thus similar proportions of limb segments are observed not only in all mammals but in reptiles as well. A study, '^^-^Ga^;^^' I Figure 25. — Lower jaws of the first and the last of the titanotheres One-sixth natural size. A, EotUanops gregoryi, a small-jawed species from the Wind River formation (lower Eocene); B, BrontotTierium medium, from Chadron C level of Chadron formation (lower Oligocene). therefore, which was designed to disclose the habits of the titanotheres led to a thorough investigation of the principles of limb evolution in all the hoofed mam- mals in adaptation to various modes of locomotion and to various loads. This special study forms the subject of Chapter IX, in which acknowledgment is made to previous investigators. Not only the proportions of the upper and lower segments of the limbs but all the bones of the shoulder and pelvic girdles are gradually transformed from the subcursorial stages of Lambdoiherium and Eotitanops through the mediportal tapir-like stages to the gravi- portal stages of the ponderous Oligocene titanotheres. This transformation is continuous, not sudden; it is brought about gradually by the simultaneous and correlated modification of all the bones and muscles involved in locomotion. Function (habit) is evi- 34 TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA dently far more potent than ancestry (heredity) in the determination of general form, yet in comparing the limbs of all the members of the different perisso- dactyl families with one another we can generally, by some family characteristic inherited from the ancestral stem form, distinguish the tapir type, the rhinoceros type, the titanothere type, etc. In the limbs, as in the skull and teeth, the titanothere, rhinoceros, or tapu' ancestry respectively seems to keep the evolution of proportion and form within certain limits, so that, for example, the resemblance between the graviportal scapula of the titanothere and that of the rhinoceros, though it may be very close and deceptive, is never quite complete. The stages of muscular and skeletal the origin of new characters (rectigradations). In this problem of the origin of new characters in the titanotheres we have two principal subjects of study, namely, the origin of horns on the skull and the origin of cusps on the grinding teeth. In the evolution of the grinding teeth the titano- theres are very conservative; in them few new cusp elements originate, though several of the old cusp elements disappear. These animals thus present a striking contrast to the horses in the evolution of the grinding teeth, for in the horses a large number of new cusp elements are successively added. Yet the grinding tooth of the earliest titanotheres {Lambdo- therium and Eotitanops) is in general similar to that Figure 26. — Structure of the feet in extinct and living odd-toed ungulates (perissodactj-ls) A, Sole of the left fore foot of a tapir (Taphus ierresiris), showing the tripod-like arrangement of digits II, III, and IV, and the reduced condition of V; B, sole of the left fore foot of an Eocene titanothere (Mesatirkinus petersoni), restoration based on Princeton Museum specimen No. 10013; C, sole of the fore foot of a rhinoceros, showing the enlarged hoofs of the three digits (II, III, IV) ; C^, side view of same; C^ longitudinal section of same; D', sole of the fore foot of a horse, show- ing the expanded nail; D^, longitudinal section of same. The central pad (/} in A, B, and C is homologous with the relatively reduced pad or frog (/) in the foot of the horse (DO- All but B after Eber. evolution, arranged from latest to earliest, are as follows : 4. Graviportal; ponderous, relatively slow-moving types, such as Brontolherium, Rhinoceros {C eratotherium) simum. .3. Mediportal; of moderate weight and speed, such as Limnohyops, Tapirus. 2. Subcursorial; of light weight and relatively swift move- ments, such as Eolilanops of the lower Eocene. 1. Cursorial; swift moving, Ught frame, such as Lanibdo- therium of the lower Eocene. ORIGIN OF NEW CHARACTERS AS DISTINGUISHED FROM CHANGES IN PROPORTION The continuous gradual changes of proportion in the head, trunk, and limbs (allometrons), as already outlined, present a problem distinct from that of of the earliest horses {Eohippus). In these lower Eocene contemporary mammals the grinding teeth are the same, cusp for cusp. In the horse all these cusp elements are preserved and utilized, and the highest degree of mechanical adaptation to the graz- ing habit is gradually evolved; in the titanotheres the browsing habit is generally conserved, and there is little marked increase of mechanical adaptation; in fact, mechanical inadaptation or imperfection of the grinders may have been one of the probable causes of the extinction of the titanotheres at a time when the conditions favorable to grazing gradually replaced those favorable to browsing. The adaptive radiation of the grinding teeth in the several families of the Perissodactyla from somewhat INTRODUCTION TO MAMMALIAN PALEONTOLOGY 35 similar ancestral forms is shown in Figure 29. The earliest members of every family had low-crowned (brachyodont) molar teeth, of relatively simple pattern, composed of six principal cusps ranged in three pairs — an external pair, the paracone and meta- cone; an intermediate pair, the protoconule and metaconule; and an internal pair, the protocone and hypocone. In the titanotheres, chalicotheres, paleotheres, and horses the internal pair of cusps assume the conical, rounded shape (bunoid), whereas the two external cusps assume the double crescentic shape (selenoid), together forming a W, hence this type of tooth is termed bunoselenodont. These bunoselenodonts apparently formed origi- nally a natural group from which the horses (Eohippus), the titanotheres (Eotitanops), and the chalicotheres (Eomoropus) gradually diverged very early in Eocene time. This is shown in Figure 30. Another group of perissodactyls is the bunolo- phodonts, which includes the tapirs and lophio- donts, in which the internal and external pairs of cusps alike assume an elongate, crested, or lophoid pattern. This group has two main branches, the tapirs and the lophiodonts. The tapirs as forest-seeking animals escaped fossiliza- tion and are rarely found; only isolated remains of them have been found in Europe and America; yet they constituted one of the most persistent of all the perissodactyl phyla. The lophiodonts were tapir-like animals, in which the posterior outer molar cusps were flattened and thus are intermediate in shape between the tapir tooth and the rhinoceros tooth. These animals doubt- less had a wide expansion in the luxuriant forests of Eocene France, and they attained very great size just before their extinction, which occurred contemporaneously with the extinction of the titanotheres in America — that is, in lower Oligocene time. Only one branch of the lophiodonts, the swift-footed Helaletinae, reached North America in lower Eocene time, soon after the arrival of the tapirs (Systemodon) and the horses (EoTiippus). The grinding tooth of the rhinoceroses is lopho- dont — that is, all the cusps are turned into elon- gate crests, of lophoid type, and the posterior outer cusps of the upper grinding teeth are elongated as well as flattened, producing an asym- metry of the cusps of the outer wall (ectoloph) of the crown. A grinding tooth of this kind is far more effective than that of the bunoselenodont titano- theres or of the bunolophodont tapirs. Such a tooth is a very efficient cutting instrument for an animal of either the browsing or the grazing habit. It is also capable of elongation (hypsodonty), and in two subfamilies of the rhinoceroses, the white rhinoc- eroses and the elasmotheres, the grinding teeth become hypsodont, greatly increasing the longevity and consequent reproductive power of each indi- vidual. Figure 27. — Restorations of nine species of titanotheres from the lower, middle, and upper Eocene and the lower Oligocene Drawn by Mrs. E. M. Fulda. About one-fiftietb natural size. The rhinoceroses gave off at least twelve distinct branches (phyla) and were thus more plastic in adapta- tion than the titanotheres. These branches became adapted to every habitat, aquatic as well as terrestrial, to every mode of locomotion — cursorial, mediportal, and graviportal — and to every kind of feeding — brows- ing and grazing. Like the titanotheres some of the rhinoceroses passed from the mediportal to the gravi- 36 TITANOTHBEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA portal stage of locomotion. In doing so they acquired an entirely new set of proportions, which are shown in detail in Chapter IX. The teeth form the readiest means of distinguishing different branches and subbranches of the Perisso- dactyla from one another. The ancestral pattern, whether bunoselenodont or lophodont, is so marked and persistent that it is only partly modified through their evolution, and these give off one mediportal, forest-living branch, HypoMppus. The horses are paralleled by cursorial or subcursorial titanotheres, such as LamhdotJierium, by cursorial paleotheres {Palaeotherium and Paloplotherium) , mistakenly sup- posed by Huxley to be the ancestors of the horses, by two cursorial branches of the lophiodonts (the helale- tids and the chasmotheres), and by two cursorial Figure 28. — Evolution of the skeleton of the titanotheres A, First stage (subcursorial), lower Eocene, Lambdotlierium popoagicum; B, second stage (subcursorial), lower Eocene, Eotitanops horealis; O, intermediate stage (mediportal), middle Eocene, Palaeosyops leidyi; D, final stage (graviportal), lower Oligocene, Brontops Tobustus. From one twenty-eighth to one-thirtieth natural size. branches of the rhinoceroses (the triplopodines and the hyracodonts) . It is shown elsewhere (see Chap. IX) how the cursorial habit, independently assumed in each of these subfamilies, modified not only the limbs but the skull and the entire skeleton into analogous forms that simulate real affinity. In Figure 32 all these cursorial branches, independently evolving in analogous adaptation. The manner in which the skeleton and limbs similarly became adapted inde- pendently to various modes of locomotion and thus assumed analogous forms and proportions is no less remarkable than the independent adaptation of the teeth to similar kinds of food. Of the nine typical perissodactyl families the horses alone are cursorial through the entire period of INTRODUCTION TO MAMMALIAN PALEONTOLOGY 37 ProtitartotfieriuTrh emargrinaticm. Upper Eocene ( upper Uinta) Manteocems manteoceras Middle Eocene (upperBridger) Zimnohyops priscus M/ddle Eocene (lowerBridger) Eotitanops borecdzs Lower Eocene (Wind River) Figure 29. — Evolution of the skull and molar teeth in the titanotheres In EoUtanops the facial part of the skull is longer than the brain case (cranium). In Brontotherium the face is very short and the brain ease is very long. The horn swellings (H) first appear in Manteoceras and become very prominent in the succeeding stages. The top of the skull becomes deeply concave. The outer wall and the V-shaped cusps of the upper molar teeth (paracone, metacone) become very deep, while the inner cusps (protocone, hypocone) retain their low, conical form. The lower molars retain the W-shaped crown throughout, which increases considerably in depth. 38 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA different perissodactyl families, are indicated by dif- ferent kinds of shading. Forest-living habits among perissodactyls are some- what more rare, especially the extreme adaptation to forest living, consisting of relatively slow locomotion and marked special adaptation to browsing on the leaves of trees. Types that are more or less fully Aquatic branches of the perissodactyls are also more or less readily distinguishable. Among the titano- theres we have a group of swamp or river living forms, with short limbs and spreading feet, whose remains are preserved in many river-channel sandstones, namely, the genera DolichorMnus and Metarhinus, which are clearly distinguished from all other titano- FiGURE 30. — Adaptive radiation in tlie evolution of the upper molar teeth in the odd-toed hoofed mammals (perissodactyls) After W. D. Matthew. The earliest members of each family had low-crowned (brachyodont) teeth, of relatively simple pattern. In the titanotheres and paleotheres the internal cusps remain low and the two outer main cusps form a W. In the horses (hypsodont) the whole sur- face of the crown is thrown into complex crests and ridges and the crown becomes very long. In the tapirs (brachyodont) the molar crown takes the form of two sharp cross crests. A somewhat similar pattern is seen in the lophiodonts, e.Kcept that in this family (brachyodont) the outer cusps form an irregular outer wall. In the rhinoceroses (brachyodont to hypsodont) the outer wall (ectoloph) becomes very much flattened, elongate, and oblique, and the cross crests also become oblique. adapted to forest living are represented, we believe, among the chalicotheres, among certain forest-living horses {HypoJiippus), and among certain forest-living tapirs (Tapirus terrestris), all relatively slow in move- ment and all without conspicuous weapons of offense or defense, except that the chalicotheres, such as Moropus, are provided with heavy claws. theres by their apparent adaptations to river-border or aquatic life. Certain tapirs frequent river borders and swim freely for long distances, but they do not acquire distinctive aquatic adaptations. Among the rhinoceroses the pronounced aquatic division is the amynodonts, which have marked aquatic features about the head, simulating those of the hippopotami. INTRODUCTION TO MAMMALIAN PALEONTOLOGY The great family tree of the perissodactyls may be interpreted as shown below. Family tree of the perissodactyls 39 Primitive ancestors Ancient branches Families, extinct and living A. Bunoselenodont branch of basal Eocene time: inner cusps bunoid, conical; outer cusps selenoid, crescentic. 1. Titanotheres. 2. Chalicotheres. 3. Paleotheres. 4. Horses. Perissodactyls of Upper Cretaceous and basal Eocene time: four digits on the fore foot, three on the hind foot; six rounded cusps on the upper grind- B. Bunolophodont branch of basal Eocene time; inner cusps crested, outer cusps symmetrically crested and more or less flattened. 5. Tapirs. 6. Lophiodonts, mediportal and graviportal; confined to Europe. Helaletids, cursorial lophodonts; reaching America. ing teeth. C. Lophodont branch of upper Eocene time; inner cusps crested, outer cusps asymmetrical, greatly flattened. 7. Amynodonts (aquatic). 8. Hyraoodontidae (cursorial and medi- portal) . 9. True rhinoceroses (mediportal and gravi- portal), variously adapted to browsing and grazing; distinguished by variations in the evolution of the horns. The mediportal structure, in which the skeleton and limbs are adapted to moderate speed and weight, embraces those intermediate stages in several different families in which there was moderate body weight and moderate speed, as in the tapirs. In the tapirs this is the last term of evolution, but in the titanotheres and in many rhinoceroses the mediportal stage is simply a gateway to the graviportal stage, in which the proportions of the limbs and trunk are adapted to weight bearing, more or less rapid progression, and active offense and defense. The interpretation of these phenomena of analogous, parallel, and convergent evolution under the princi- ple of adaptive radiation, presented on pages 121-127, simplifies the problem of the anatomy of the group as a whole as well as of the several adaptations seen in the skull, skeleton, limbs, and teeth. Each perissodactyl family appears to exhibit an innate potentiality to evolve in many different directions and thus to meet new conditions of life. In this sense each family is plastic. Here we are not wit- nessing the direct action of the environment: we are witnessing the direct response of the organism, through largely unknown causes, to develop its poten- tial heredity characters along certain new lines. If the supply of new potential characters is exhausted, if a mechanical stage is reached out of which no addi- tional stages can be developed, the animal will tend to become extinct unless it can retire to the recesses of the forests, as did the chalicotheres, and thus escape a struggle for existence in competition with more plastic forms, better adapted to the grazing life. The interpretation of these processes, however, has been the most difficult and baffling of all the problems that have arisen in the research made for this monograph. The interpretation of the modes and causes of the origin and evolution of new characters and of new proportions in response to new conditions of life (see pp. 834-849) is extremely difficult. Explanations that at first seem obvious appear on close analysis not to be explanations at all. As this monograph is the most exhaustive and most detailed study thus far made of any group of mammals it seems important to show the bearing of all the observations on each of Figure 31. — Three types of teeth of members of nine typical families of perissodactyls Bunoselenodont (A), bunolophodont (B), and lophodont (C) types of teeth displayed in the short-crowned Cbrachyodont) stage. the current theories of evolution. It appears that, as is fully set forth in Chapter XI, we are still very far from even a preliminary understanding of the causes of many of the processes of mammalian evolution. VELOCITY IN THE DEVELOPMENT OF CHARACTERS AND IN PHYLOGENY The earliest explanations of evolution were purely mechanical; we are now passing through a phase of 40 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA chemical explanations; but it appears that we may be led to the adoption of certain physical conceptions and the use of certain physical terms (Osborn, 1917. 462) for what has been described above as the rate of evolution of certain characters as distinguishing genera. For the term "rate" we will substitute the term "velocity." Ontogenetic velocity. — The velocity of the evolution of certain characters in embryonic development — in fact, throughout the whole course of individual development — has long been a very familiar feature of adaptation. From the embryo onward a char- isms, and we shall see that the most plausible explana- tion of it thus far offered is the theory of natural selec- tion proposed by Darwin. Phylogenetic velocity. — Another kind of velocity, however, may be noted in the perissodactyls and may be measured and calculated with great precision in the numerous phyla of titanotheres here considered. This velocity may be called phylogenetic velocity. Its postulation rests upon the fact that a given character may evolve much more rapidly in the members of one phylum than in the members of a related phylum, al- though the environment of both phyla may be the FiGiTRB 32. — The family tree of the perissodactyls, showing adaptive radiation of the nine families and thirty- five subfamilies Exhibiting their divergence In limb and foot structure into cursorial, forest-hving, mediportal, and graviportal types and in tooth structure into browsing and grazing types. acter may be either hurried along or slowed down in its rate of development, and in consequence it will appear in earlier or later stages of individual life. For example, certain adult proportions of the limbs are needed at birth in all cursorial animals; these adult proportions are consequently hurried forward during the foetal life, so that the animal is at birth able to run immediately with almost the same speed as the parent. This kind of velocity of development is called ontogenetic; it is appurtenant to every char- acter in every stage of its development, it is closely connected with the survival of certain young organ- same. For example, in twelve subfamilies of titano- theres we observe homogeneous characters evolving independently — the same cusps on the teeth, the same horns on the skull. How, then, do the subfamilies differ from one another? They differ because the evolution of each character in each phylum proceeds with its distinctive velocity. In a phylum that is evolving rapidly a certain character appears early in geologic time; in a phylum that is evolving slowly the same character appears late in geologic time. The titanotheres of one phylum may at a particular geologic period be completely hornless, whereas those of a con- INTBODUGTION TO MAMMALIAN PALEONTOLOGY 41 temporaneous phylum may have well-developed horns. In the former the horns may appear much later and may never acquire very great momentum in develop- ment. We can thus note the incipiency of the differ- ences between the short-horned titanotheres and the long-horned titanotheres. This principle of unequal phylogenetic velocity in the development of the same characters enables us to distinguish different genera and species. In one genus the development of the internal cusps of the premolar grinding teeth shows high velocity; in another genus it shows low velocity. Apparently these internal cusps are useful adjuncts of the tooth; they make the tooth more effective for grinding up food. Similarly the horns are useful adjuncts of the head in warding off enemies. Yet these characters evolve so slowly in certain phyla that it is unreasonable to believe that utility and natural selection are the prime causes of their evolution. There would seem to be physiological and physical (or chemico-physical) causes of these different velocities. It is the data on the different velocities of the developmen-t of the same characters in related phyla which give the principal biologic value to the long series of detailed measurements and justify the large number of figures that are presented in Chap- ters V and VI. This suggests a summary of the bio- logic aspects of the problems of this monograph and of the features that distinguish this particular field of biologic research. SUMMARY OF THE EVOLUTION OF THE TITANOTHEREG The known titanotheres were confined to a relatively small area near the fortieth parallel in western North America and to Europe and Asia. The direct lines of descent and the continuous changes in many branches in different or successive life zones were complicated by the occasional incursion of new families from out- side larger regions, probably from northern America and perhaps from northern Asia. (See appendix.) Nevertheless the localities in western North America where the remains of titanotheres have been found were apparently near the main geographic center of the evolution of the family, for the series of known fossils enables us to follow almost every step in the slow transformation of forms that were small and defenseless to forms that were huge and well armed. The remains of the titanotheres now collected repre- sent the most complete evolutionary series of mammals thus far discovered except those of the horses. The horses, however, are much less highly differentiated. In the titanotheres we see the growth of a great and vigorous family tree, giving off numerous branches (phyla), which diverge in characters and habits while retaining hereditary resemblances and certain heredi- tary trends and tendencies of transformation. Each of these branches is made up of slowly transforming successive stages (mutations of Waagen), which appear 101959^29— VOL 1 5 to be the more continuous and unbroken by sudden change the more thoroughly we explore the geologic levels where they successively occur. The evolution of the soft parts can only be inferred. The hard parts evolve in a variety of ways, chiefly through increase of size, through changes in proportion, through addi- tion of new parts, and in less measure through loss of parts. Actual addition or loss of parts in the titano- theres is rare; general increase in size is almost uni- versal, though in a few branches the size is diminished or arrested. Changes in the proportions (allometrons) of struc- ture were brought about by different velocities of phylogenetic evolution (acceleration and retardation) in the skeletal framework as a whole and in each of its parts. No less important is the definite and successive addition of new characters (rectigra- dations), each developing from infinitesimal begin- nings until it reaches a stage of usefulness and each apparently having its individuality (biocharacter) and its separate history. Throughout this wonderful transformation, which is in general adaptive, there were certain manifest germinal (hereditaiy) tendencies and certain unkno\vn interactions between these germinal changes and' the external, habitudrnal, and environmental influences. The more carefully we study the detailed characters in each branch the more evident it becomes that the causes of evolutionary development are neither exclu- sively external nor exclusively internal but are to be sought hypothetically in the interactions between germinal, habitudinal, and environmental forces. The changes in the proportions of the skeletal characters and the new elements added to the teeth and skull, which are the outward expressions of these hypothetic germinal and environmental reactions, become visible more or less contemporaneously but not simultane- ously in all members of the branches and sub- branches of the great family tree — that is, the same characters appear, but at different periods and with different velocities of development. The whole process is an orderly one, which is, however, not predetermined in the germinal constitution of the titanotheres but results from certain innate or germinal potentialities of evolution, which are evoked in response to certain environmental and habitudinal conditions. The struggle for existence, or natural selection, is operating continuously and more or less strongly on every single character according as its survival value is greater or less. In each successive geologic level we witness alterations of the family tree — its impover- ishment through the extinction of certain branches or its augmentation through the survival of other branches and the immigration of branches which evolved in other regions. The individual members of all the branches (with two exceptions) become more imposing and more diverse as time goes on. Finally, 42 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA at the climax of the general trend of transformation and at the very height of the grandeur of development, we observe the apparently simultaneous extinction of the whole titano there family, seemingly through failure to cope with changed environmental conditions or to compete successfully with other herbivorous types. This contribution to biology is therefore important chiefly as a study of the actual modes of evolution as observed in the skeleton and teeth of many different members of a great family of extinct animals which existed throughout a long period of geologic time — from the early Eocene through the early Oligocene — a time reckoned as hundreds of thousands of years. It is merely suggestive as to the causes of evolution. SECTION 3. BIBLIOGRAPHY OF LITERATURE CITED OR CONSULTED IN THE PREPARATION OF CHAPTER I Barrell, Joseph. 1917.1. Rhythms and the measurement of geologic time: Geol. Soc. America Bull., vol. 28, pp. 745-904, Dec. 4, 1917. Darwin, Charles. 1859-2. The origin of species by means of natural selection, or the preservation of favored races in the struggle for life, 502 pp. London, John Murray, 1859. FOBSTER-COOPER, C. 1913.1. Thaumastotherium osborni, a new genus of perisso- dactyls from the upper Oligocene deposits of the Bugti Hills of Baluchistan (preUminary notice): Annals and Mag. Nat. Hist., 8th ser., vol. 12, pp. 376-381, October, 1913. 1913.2. Correction of generic name [to Baluchitherium]: Annals and Mag. Nat. Hist., 8th ser., vol. 12, p. 504, November, 1913. Hayden, Ferdinand Vandiveer. 1873.1. United States Geological Survey of the Territories; First, Second, and Third Annual Reports, reprinted in one 8vo volume, 1873. (First and second first issued in Rept. Commissioner of General Land Office for year 1867, Wash- ington, 1867; Third issued independently as Preliminary Field Report of U. S. Geol. Survey of Colorado and New Mexico, 1869.) Kovalevsky, Dr. Woldemar. 1873.1. Monographic der Gattung Anthracotherium Cuv. und Versuch einer nattlrlichen Classification der fossilen Hufthiere: Palaeontographica, Band 22, Heft 3, pp. 131-210, Taf. 7-9, 1873; pp. 211-346, Taf. 10-17, 1874. Leidy, Joseph. 1869.1. The extinct mammalian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America: Acad. Nat. Sci. Philadelphia Jour., 2d ser., vol. 7, pp. 1-472, 30 pis. Linnaeus, Caroltjs. 1758.1. Systema naturae . . . Editio decima, reformata, Holmiae, 1758. Matthew, William Diller. 1901.1. The Carnivora and Insectivora of the Bridger Basin, middle Eocene: Am. Mus. Nat. Hist. Mem., vol. 9, pt. 6, pp. 291-567, pis. 43-52, 1901. Meek, F. B., 1862.1 (and Hayden, F. V.). Descriptions of new lower Silurian (Primordial), Jurassic, Cretaceous, and Tertiary fossils collected in Nebraska Territory by the exploring expedition under the command of Capt. Wm. F. Raynolds, U. S. Top. Engrs., with some remarks on the rocks from which they were obtained: Acad. Nat. Sci. Philadel- phia Proc, vol. 13, pp. 415-447, 1862. Merriam, C. Hart. 1918.1. Review of the grizzly and big brown bears of North America (genus Ursus), with description of a new genus, Vetularctos: U. S. Dept. Agr. Bur. Biol. Survey North Am. Fauna, No. 41, 136 pp., 16 pis., Feb. 9, 1918. OsBORN, Henry Fairfield. 1896.107. Titanotheres of the American Museum of Natural History: Am. Naturalist, vol. 30, No. 350, pp. 162-163, February, 1896. 1896.110. The cranial evolution of Titanoiherium: Am. Mus. Nat. Hist. Bull., vol. 8, pp. 157-197, July 31, 1896. 1900.192. Phylogeny of the rhinoceroses of Europe, Rhinoceros Contributions No. 5; Am. Mus, Nat. Hist. BuU., vol. 13, pp. 229-267, Dec. 11, 1900. 1909.321. Cenozoic mammal horizons of western North America, with appendix, Faunal lists of the Tertiary Mammalia of the West by William Diller Matthew: U. S. Geol. Survey BuU. 361, 138 pp., 1909. 1914.409. Recent results in the phylogeny of the titano- theres: Geol. Soc. America BuU., vol. 25, No. 3, pp. 403-405, Sept. 15, 1914. 1914.412. Rectigradations and allometrons in relation to the conception of the "mutations of Waagen" of species, genera, and phyla: Geol. Soc. America BuU., vol. 25, No. 3, pp. 411-416, Sept. 15, 1914. 1917.462. The origin and evolution of life on the theory of action, reaction, and interaction. New York, Charles Scribner's Sons, 1917. Osgood, Wilfred H. 1909.1. Revision of the mice of the American genus Peromyscus: U. S. Dept. Agr. Bur. Biol. Survey North Am. Fauna, No. 28, 285 pp., 7 pis., map, Apr. 17, 1909. Waagen, W. 1869.1. Die Formenreihe des Ammonites subradiatus, Versuch einer palaontologischen Monographic: Geognostisch-palaontologische Beitrage heraus- gegeben * * * von Dr. E. W. Benecke, Band 2, pp. 179-257, 1869. White, C. A. 1868.1. First and Second annual reports of progress by the State geologist and the assistant and chemist on the Geological Survey of the State of Iowa, etc., 284 pp., Des Moines, 1868. CHAPTER II ENVIRONMENT OF THE TITANOTHERES AND EFFECT OF ADAPTIVE RADIATION ON THEIR VARIATION SECTION 1. GEOLOGY AND GEOGRAPHY CORRELATION OF EARLY TERTIARY EVENTS IN THE ROCKY MOUNTAIN REGION WITH THOSE IN WESTERN EUROPE The recorded history of the titanotheres extends from the upper horizons of the lower Eocene series (upper Ypresian or upper Wind River horizon) through the middle and upper Eocene to the top of the lower Oligocene (Sannoisian or Chadron horizon), covering a period estimated at 450,000 to 600,000 years. This estimate is based on the assumption that 9,000 to 12,000 feet of sediment was deposited during the period from basal Eocene to lower Oligocene time and that the average rate of deposition was 1 foot in every 100 years. The Eocene type formations (Wasatch, Bridger, etc.) of the Rocky Mountain region in North America have gradually acquired a time significance, similar to the stages (etages) into which the Eocene and lower Oligocene of Europe are divided, as shown in the following table. The correlation in time between France and America is close for some periods, as, for example, between the Sparnacian and lower Wasatch and between the Sannoisian and Chadron. For other periods the correlation is provisional, because the faunal relations are interrupted. Provisional correlation of European and American geologic stages and life zones of the tifanothere epoch Epochs Stages (Stages) ot Europe Type formations ot America Major type life zones Maximum thiclvuess of sediments in feet, deducting overlaps Lower Oligocene. Sannoisian. Chadron (Nebraska and South Dakota). (Extinction of titanotheres.) Tilanotherium-Mesohippiis. 500 Upper Eocene. Ludian. Uinta (northeastern Utah). Diplacodon-Protitanotherium-Epi- hippus. 600 Bartonian. Lutetian. Ypresian (upper). Bridger (southwestern Wyoming). Uintatherium-Manteoceras-Mesa- tirhinus. Palaeosyops paludosus-Orohippus. Eometarhinus - Trogosus - Palaeo- syops fontinalis. Middle Eocene. 1,875 Lower Eocene. Ypresian (lower). Sparnacian. Wasatch (western W3'oming). Coryphodon. (First titanotheres.) 2,025 Transition. Cernaysian. Thanetian. O 03 Torrejon (northwestern New Mexico). Puerco (northwestern New Mexico) . Pantolambda. Polymastodon. 6,000 Basal Eocene. Total 11,000 Cretaceous. ° Montian. Danian. Lance ( = in part Laramie and Denver)." Triceratops. " The United States Geological Survey classifles the Lance formation as Tertiary (?), the Laramie formation as Upper Cretaceous, and the Denver as Eo author of this monograph believes that the Lance formation is equivalent in part to the Laramie and Denver formations and that it is of Cretaceous age. 43 44 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA All estimates orgeologic time are highly provisional, because they involve two unknown quantities — the amount of overlap and the relative rate of deposition. The rate of the deposition of sediments varies enor- mously. For example, certain Fort Union sediments of Montana, aggregating 6,000 feet in thickness, are at present considered contemporaneous with Torre- Lambdotheruum popoaoicum £otUanoDS orinceps Figure 33.- -Outlincs of the bodies of titanotheres at different stages of evolution jon sediments of New Mexico, which aggregate only 385 feet. It would therefore appear that sedimenta- tion in Montana was more than thirteen times as rapid as in New Mexico. The only sedimentary stage which appears fairly uniform in several geo- graphic localities is the Wasatch, which exhibits beds of approximately the same thickness in many different regions. If an average rate of deposition of a foot in a century is assumed, the period from basal Eocene to lower Oligocene time, inclusive, is estimated as not exceeding 1,100,000 years, a moderate estimate considering the great biologic changes that took place in the titano- theres and other groups during this period. The epoch of the titanotheres is roughly estimated at 500,000 years or more, during which they steadily increased in size, from the geologically earliest animals, which are no larger than a sheep, to some of the latest members of the race, which exceeded in size the largest rhinoceroses, standing over SJ^ feet at the shoulders. The recorded history of the titanotheres is nearly unbroken, but there have been two evolu- tionary gaps, one between the lower and the mid- dle Eocene, which was filled in 1918 by explora- tions of the Huerfano (Osborn, 1919.494), and one between the upper Eocene and the lower Oligo- cene, which will be filled by the exploration of the upper part of the Uinta formation (theoretic faunal zone 16, still unknown). The record also shows sudden transitions caused by invasions of animals from other regions. The geographic range of the titanotheres was probably continent wide in America and also ex- tended across Asia into the Balkan region of south- eastern Europe. In the relatively small Rocky Mountain and western plains region, where most of the fossil remains have been discovered, we observe the successive invasion of new kinds of titanotheres, which had apparently evolved pre- viously in other regions, probably in areas to the north and east. The geologic age of the little-known European titanotheres is somewhat uncertain. The type and only known specimen of Brachydiastemaihe- rium, an animal about the size of Diplacodon, is recorded fi'om a formation in eastern Hungary that was originally assigned to the lower Eocene, but this animal is in a stage of evolution corre- sponding to that of the uppermost Eocene titano- theres of America, and the same European forma- tion has yielded remains of a primitive rhinoce- ros (ProTiyracodon) of upper Eocene or even lower Oligocene type. Brachydiastematlierium is there- fore probably not of lower Eocene age. The animals described as Menodus rumelicus and Titanotherium hohemicum are in all respects sim- ilar to American titanotheres of lower Oligocene age, but as the localities and horizons from which these fragmentary specimens were obtained are in doubt they may be imported American fossils to which a European origin has been erroneously imputed. The correlation of the chief geographic, geologic, climatic, and faunistic events during the Tertiary period in the Eocky Mountain region with those in western Europe has been studied by the author con- tinuously during the last 20 years, with the coopera- ENVIRONMENT OF THE TITANOTHERES 45 tion of Depgret in France and of Matthew, Merriam, Granger, Brown, Peterson, Douglass, Riggs, Darton, Stanton, Berry, Knowlton, and others in this country. The theoretic correlations reached are shown in the accompanying tables (pp. 43, 48). The comparison of similar stages in the evolution and migration of floras and faunas is partly independent of changes in the surface of the earth and in climate and is partly related to them. The general succession (Osborn and Matthew, 1909.321; Osborn, 1910.346) of the four Eocene and Oligocene life phases of North America is as follows: Phase IV (lower Oligocene) , approximation. — A similar mam- mal fauna in western America and western Europe. Extinction of archaic fauna and invasion of modern fauna. Phase III {upper and middle Eocene), estrangement. — Inde- pendent mammal fauna of western America and western Europe; gradual diminution of archaic fauna. Phase II (lower Eocene), approximation. — Closely allied and similar fauna of western America and western Europe; first invasion of modernized fauna. place this after the first Rocky Mountain (Laramide) revolution in post-Laramie time — that is, after the end of typical Laramie deposition in Colorado. Others, among them the author of this monograph, place it at the time of the extinction of the great land and marine reptiles of Europe and America — that is, after Lance time.'' The Fox Hills formation, which underlies the Lance, represents the end of uniform widespread marine sedimentation. At some places the Fox Hills is continuous with overlying fresh- water deposits laiown as Laramie; at others it is con- tinuous with overlying deposits known as the Lance. Thus Laramie time and Lance time, in our opinion, are in part the same — that is, they overlap at some places. Lance and Fort Union flora. — New physiographic and climatic conditions arose during the initial period of the Rocky Mountain uplift, when uplands and plateaus were formed. Knowlton and Berry have shown that the Fort Union flora extends back into land areas Forme Known fossil areas iigration areas Figure 34. — Map showing the known areas (black) and the hypothetical areas (oblique lines) of titanothere migration and habitat Phase I (basal Eocene) , approximation. — Partly similar archaic mammal fauna of western America and western Europe. Final Mesozoic phase. — Gradual extinction of the upper Cretaceous dinosaur fauna and appearance of ancestors of the archaic Eocene fauna. This alternate approximation and estrangement of the mammal life of western America and western Europe points to periods during which conditions favored intermigration and intervening periods when geographic, climatic, or forest barriers may have stood between these widely separated regions. The basal Eocene American forests — those of the Fort Union epoch, for example — were very luxuriant and were unfavorable to migration. lATE CRETACEOUS AND EARLY TERTIARY CLIMATES End of the Cretaceous period. — The initial point in the correlation of geologic time in both the Eastern and the Western Hemisphere is-the end of Cretaceous deposition. (See table on p. 48.) Some geologists Lance dinosaur time, regarded by the author as late Cretaceous. The Lance flora is prevailingly a rela- tively warm temperate flora as compared with the antecedent Laramie and other Upper Cretaceous floras in the same region, and the climate in Lance time was about like that of the present Atlantic Coast States from North Carolina southward. In the Rocky Mountain province (Berry, 1914.1, pp. 153-154), in the zone of transition from the Cretaceous to the Eocene, a large number of local floras appear, such as those in the Arapahoe and Denver formations of Colorado, the Livingston formation and the Lance formation ("Hell Creek beds") of Montana, and the typical Lance formation of Wyoming. The forma- tions in which they occur consist of lacustrine, fluviatfle, and terrestrial deposits eroded from the rising land area of the Rocky Mountain province. These early so-called post-Laramie floras are said to 8 The United States Geological Survey classifies the Lance formation as Terti- I ary (?). The author of this monograph regards it as Cretaceous. 46 TITANOTHEEES OF ANCIENT "VA^yOMING, DAKOTA, AND NEBRASKA 1. Sweet Grass County, Mont. Fort Union formation. 2. P. T., San Juan Basin, N. Mex. and Colo. Puerco and Torrejon forma- tions and "Tiffany beds." 3. W., near Evanston, Wyo. Typical Wasatch group. 4 Big Horn Basin, Wyo. Wasatch for- mation. 5. W. R.. Wind Eirer Basin, Wyo. Typical Wind River formation. 6. Beaver Divide, Wyo. Eocene and Oligocene section. 7. H., Huerfano Basin, Colo. Typical Huerfano formation. 8. B., Bridger Basin, Wyo. Typical Bridger formation. 9. W. K., Washakie Basin, Wyo. Typi- cal "Washakie formation" of Hay- den. 10. U., Uinta Basin, Utah. Typical Uinta and older Eocene deposits. 11. Wh. R., White River, S. Dak. Typi- cal White River group. 12. Powder River and Pumpkin Buttes, Wyo. Fort Union and Wasatch formations. 13. F. U., Fort Union, N. Dak. Typi- cal Fort Union formation. 14. P., Red Deer River, Alberta. Paska- poo formation. Oligo Oligocene flood plain Figure 35.— General geologic sketch map of the Rocky Mountain region, showing existing topography and drainage areas and their relation to areas of Eocene and lower Oligocene sedimentation Each of the numbered areas e.'cccpt 13 and 14 is also represented in geologic section in this chapter. Topography after the United States Geological Survey, 1911 (See tables on pp. 48, 57, .=.8.) ENVrROlSTMENT OF THE TITANOTHERES 47 be distinct from those of the true Laramie and to be more closely allied to those of the true Fort Union above. The true Fort Union floras of basal Eocene (Thane- tian) age include between 500 and 600 species of trees, which were apparently derived from areas farther north, certainly not from areas farther south. These forests, which were contemporaneous with the Puerco and Torrejon mammals, indicate a climate in the Rocky Mountain region between the fortieth and fiftieth parallels that was far from tropical, yet moderately warm and humid, with mild winters, favor- able to the growth of palm, fig, and camphor trees, as well as other warm-temperate trees, including gink- gos and sequoias. This flora, which is characteristic of the early uplift period of the Rocky and Uinta Mountains in Colorado and Wyoming, indicates a somewhat cooler climate than that of the subsequent lower Eocene (Green River) epoch in the same region and a much cooler climate than the subtropical climate of the South Atlantic States in early Eocene time. In fact, both in the Rocky Mountain region and farther south the American climate became milder and more tropical as the Eocene epoch advanced. EOCENE GEOGRAPHY OF WESTERN NORTH AMERICA AND ITS RELATION TO FAUNAI MIGRATIONS GEOGRAPHIC DIVISIONS AND THEIR BEARING ON MIGRATION The main topographic features of western North America were established between late Cretaceous and middle Eocene time. In late Cretaceous and early Eocene time certain routes of migration connected the animal life of the central Rocky Mountain region with that of Eurasia and probably with that of South America. The key to these routes of migration and to the geographic distribution of these animals is afforded by the results of researches made since 1853 by the geological surveys of the United States and Canada. The foundation of the descriptive geologic history of the Rocky Mountain region is laid in the report of F. B. Meek and F. V. Hayden (Meek and Hayden, 1862.1). The entire Cordillera region extends from Bering Strait to the Isthmus of Tehuantepec, a distance of 4,500 miles, and has an average width of 500 to 600 miles. The main geographic divisions of the Cor- dilleran region, named in order from east to west, are the following : Rocky Mountain Range, Bering Sea to Colorado, including — Front or eastern range, facing the Great Plains. Rocky Mountain basins between the eastern and western ranges, forming the central north and south migration routes of mammals. Westerly ranges, facing the interior plateaus. Central interior plateaus, intermontane belt region (main migration routes of herbivorous mammals) : Northern interior plateaus, Alaska to Washington. Columbia Plateau. Nevada-Sonora Plateau (Great Basin). Colorado Plateaus. Mexican Plateau. Pacific mountain system, British Columbia: Sierra Nevada. Pacific mountain basins between the Sierra Nevada and the Coast Ranges. Coastal migration routes of mammals. Pacific Coast Range. A transverse section of the Cordillera on the 41st parallel exhibits clearly the main confines of these mountain ranges, basins, and plateaus. The great plateaus and the mountain basins may have pre- sented bordering forests and central grassy plains and jungles, interspersed with swamps, marsh lands, rivers, and lakes similar to those in the plateau and mountain (Kenya, Kilimanjaro) region of equa- torial Africa to-day. Migration from north to south or from south to north was possible along three routes. Our only knowledge of the late Cretaceous and Eocene mammal life of North America is afforded by the remains of mammals of the Rocky Mountain basins and foothills from Alberta to northern New Mexico. During the Oligocene epoch the life of the Columbia Plateau is revealed in the John Day forma- tion of Oregon. The life of the Great Plains first appears in the lower Oligocene formations in South Dakota, Wyoming, Nebraska, and Colorado, which border the Rocky Mountains on the east. The Eocene mammalian life of the country that stretches east- ward from the Rocky Mountain Front Range to the Mississippi and the Atlantic coast is entirely unknown. 48 TITAKOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBEASKA Correlation of late Cretaceous and early Tertiary stages in Europe and in North America Epochs European stages Rocky Mountain and Plains formations Changes in flora and climate Chief forms of reptile and mammal Upper Eocene. Ludian. Bartonian. Uinta formation {Diplacodon zone), upper part of "Wa- shakie" formation (Washakie B), and (?) upper part of Bridger formation (Bridger E). . Ancestors of horned titan- otheres. Middle Eocene. Lutetian. Upper Ypresian. Lower Eocene. Lower Ypresian. Sparnacian. Transition. Cernaysian. Basal Eocene. Uppermost Creta- ceous." Upper Cretaceous. Danian. Maestrichtian. Lower part of Bridger forma- tion (Bridger A, B, C, and D), lower part of "Washakie" formation (Washakie A), and upper part of Huerfano for- mation (Huerfano B). Rapid evolution of titano- theres (upper Bridger). Post- Wasatch and post-Green River uplift, Uinta Mountains, Utah.- Wind River, Green River, and Wasatch formations and low- er part of Huerfano forma- tion. Green River flora, show- ing affinity to tropical flora of the south; climate warmer than Fort Union. Post-Fort Union mountain uplift, Montana and Colorado. Fort Union, Torrejon, and Pu- erco formations. Swamp, la- goon, forested flood-plain sediments; lignitic and coal ' deposits. Lance (upper part) , Denver and Arapahoe formations. Ris- ing land area of Rocky Moun- tain region; brackish-water estuarine, fluviatile, and chan- nel sediments. Fort Union flora of mod- ernized types. Appearance of titanothe- res (Wind River time). Appearance of modernized families (lower Wasatch time) . Archaic mammals of Pu- erco, Torrejon, and Fort Union time. Extinction of the dino- saurs and large marine reptiles. Fort Union flora. Warm and humid climate similar to that of south- eastern coastal States; mild winters, flora not tropical. Low-lying forested swamps in the plateau region. Open flood plains surround- ing the mountain slopes. Triceratops-Tyrannosaurus fauna. Mammals of Lance time. Ancestors of Puerco and Torrejon placentals, mar- supials, multitubercu- lates. Paskapoo mam- mal fauna of Alberta (more recent). Beginnings of Laramide revolution; Rocky Mountains (Colo.), Uinta Mountains (Utah), Wasatch Mountains (Utah). Uppermost of the conformable series sediments of Rocky Mountain and Plains region: Laramie formation ( = low- er part of Lance). Fox Hills sandstone. Pierre shale. Edmonton flora of Al- berta (similar to Fort Union) . Laramie flora transitional to modern. Upper Cretaceous flora. Climate warmer than Fort Union. Edmonton dinosaur {Lep- toceratopa) fauna (suc- ceeding Belly River), of Fox Hills (?) age; Ojo Alamo (N. Hex.) dino- saur fauna similar to Judith River fauna; Judith River (Mont.) and Belly River (Al- berta) dinosaur fauna; Monoclonius of Pierre age. ■■ The Lance formation is classified by the United States Geological Survey as Tertiary (?) and the Denver and Arapahoe formations as Eocene. Note.— Near the end of Cretaceous time the chief uplift of the Laramide revolution in the Rocky Mountains began in the Front Range of the Colorado Rockies after the Laramie and before the Arapahoe. In the northern (the Montana) Rooky Mountains the chief uplift occurred at the end of the Fort Union. In southern Colo- rado and in northern New Mexico uplifts occurred both before the Puerco and after the Torrejon. (Ransome, 1915.1, pp. 360-3C2.) ENVIKONMENT OF THE TITANOTHERES 49 Figure 36. — Map of western North America showing supposed routes of migration of animals This map shows the general early Tertiary topography of the Great Plains, mountain ranges, northern and southern plateaus, and coast basins and illustrates the supposed lines of Asiatic migration from the north and South American migration from the south. Modified after F. L. Hansome (1915.1). 50 TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBEASKA Figure 37. — Map showing the orogeny of the western mountain and plateau region After F. L. Ransome (1915.1). Key to the numerals is given in Figure 36. ENVIRONMENT OF THE TITANOTHEKES 51 CHARACTER OF THE MOUNTAIN-BASIN, PLATEAU, AND PLAINS REGIONS The geographic history of the mountain-basin region and of the Plains region presents some resem- blances and some contrasts. Both regions were subject to slowly progressive elevation during this period. Nearly all the Eocene deposits of the moun- tain basins were laid down in broad, fiat valleys and on mountain plateaus, which were drained largely by the same great river systems that drain them to-day, whereas those of the Plains region were widely scattered over broad flood-plain areas in which the rivers frequently changed their courses, the present river courses being cornparatively modern. In the mountain basins, from the basal Eocene of the Fort Union, Puerco, and Torrejon formations to the summit of the upper Oligocene as represented in the John Day formation of the Columbia Plateau, the older Tertiary rocks were at very few places worked over into newer deposits, but at many places deposition was continuous. Despite continuous ero- sion since Oligocene time large areas of the historic Eocene sediments of the mountain-basin region have been preserved in their original purity and con- tinuity for the geologist and paleontologist. By contrast, in the Plains region large areas of the original Oligocene strata were in part worked over to form Miocene strata, and part of these in turn were eroded to form Pliocene strata; again all three contributed to the Pleistocene strata; and finally all four are now contributing to the alluvium of the Great Plains. EOCENE TOPOGRAPHY IN THE ROCKY MOUNTAIN REGION, MONTANA TO NORTHERN NEW MEXICO By middle Eocene time the topography of the Rocky Mountain region from Montana to northern New Mexico had become broadly similar to that of to-day. The existing sharply sculptured ranges of the Big Horn, Wasatch, Uinta, and San Juan Mountains are remnants of much loftier ranges, which had their birth in late Cretaceous and early Eocene time. The two great drainage systems of the region — (1) Big Horn, Yellowstone, and Missouri Rivers on the north and (2) Green, White, San Juan, and Colorado Rivers on the south — were probably well established at the end of Eocene time. According to Ransome (1915.1) and Lindgren (1915.1) the general uplift of the land in the Rocky Mountain region near the end of Cretaceous time was not uniform at different points either in its incep- tion or in its intensity. Apparently the earliest move- ment occurred after the deposition in the Denver Basin of the conformable series of Cretaceous beds that is now called the Laramie formation, which over- hes the Fox Hills sandstone. The Front Range of central Colorado arose at this time, before the deposi- tion of the Arapahoe formation of Colorado (Ran- some, 1915.1, p. 361). Andesitic tuffs and flows occur in the Denver formation, which immediately overlies the Arapahoe. At the south end of the Rocky Moun- tains, in northern New Mexico, great uphfts occurred both before and after the deposition of the basal Eocene Puerco and Torrejon formations. In con- trast, in the typical Rocky Mountains of Montana the principal uplift appears to have taken place at the end of Fort Union time — that is, subsequent to basal Eocene time. In the Park Range province of Colorado there was uplift and vigorous erosion at the end of the Cretaceous period and renewed uplift after the deposition of the lower Eocene Wasatch and Green River sediments. The separate history of the great mountain ranges in the basin region also shows that the upward move- ments began early in Eocene time. The Big Horn Range of northern Wyoming (Darton, 1906.1) arose as an anticline from the nearly horizontal strata of the Plains to a height of 9,000 feet in early Eocene time. Its uplifted peaks were truncated, and the larger features of the present topography were outlined. The major uplift of the Wind River Mountains, which produced a broad, low, somewhat broken anticline, also took place in early Eocene time (Fisher, 1906.1). In the Wasatch Range of western Wyoming, an east- ward-dipping monocline cut off along its western side by a great fault, there was only a slight uplift at the end of the Jurassic, the main uplift taking place at the end of the Cretaceous (Boutwell, 1907.1). Subse- quent movement took place in post-Eocene time. East of the Wasatch Range is the exceptional east and west anticline of the Uinta Mountains, which extends eastward and westward as a broad central plateau, 150 miles long and 30 miles wide, forming a dividing line between the Bridger and Uinta Basins. The for- mation of the Uinta arch began at the end of the Cretaceous period (Emmons, 1907.1, p. 302), as is shown by the fact that the flanking Tertiary beds lie unconformably over the upturned edges of the older strata, which stand at angles of 30° or more. The Eo- cene formations — the Wasatch, Green River, Bridger, and Uinta — are upturned against the flanks of the Uinta Mountains, in a position which means that the continued rise of the mountain mass has dragged up the edges of the adjoining beds. Powell estimated that the summit of the Uinta anticline rose 25,000 feet above the level of the ad- jacent country — the Bridger and Uinta Basins. This altitude is equivalent to that of the Himalaya Moun- tains. Certainly in Eocene time the Uinta was a lofty, majestic range. The Colorado Front Range arose between the time of the deposition of the Laramie and Arapahoe formations, to the south, and the San Juan Mountains arose at the end of Cretaceous time and again after the deposition of the basal Eocene Puerco and Torrejon formations. 52 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA ENVIRONMENT OF THE TITANOTHEKES 53 The entire topography of the mountain-basin region was thus broadly defined at the end of the Cretaceous period and was accented by uplifts during and after Fort Union (Puerco and Torrejon) time; also after Wasatch and Green River time, following which, from the present Canadian border to northern New Mexico, there was a continuous very gradual uplift. In gen- eral this uplift was earlier and more rapid in Colorado and New Mexico — that is, it occurred before the Fort Union epoch — and more retarded in Montana, where it occurred after the Fort Union epoch. In the Huer- fano Basin the upturn of the western edge of the Huerfano beds amounts to 84°, and although this uplift is local it indicates a considerable movement in the Sangre de Cristo Range after Wind River time (W. Granger, letter, 1919). Ransome (1915.1, p. 362) believes that a large part of the Rocky Mountain uplift followed the deposition of the Fort Union formation. CONTRAST IN PHYSIOGRAPHIC CONDITIONS EAST AND WEST OF THE ROCKY MOUNTAIN FRONT RANGE During and after the deposition of the conformable Cretaceous formations (such as the Fox Hills and the Laramie) the country bordering the Rocky Mountain range on the east presented a marked physiographic contrast to that lying within the Rocky Mountain basins. Sedimentation east and west of the Rockies was not contemporaneous. East of the Rockies. — On the east flanks of the Front Range great river flood-plain systems began in the north in Pierre time and extended toward the south after Fox Hills time. Thus on the western borders of the present Great Plains region rivers had long been spreading out sand over their flood plains in Alberta, forming such deposits as the Belly River sandstone in Pierre time and the Edmonton sandstone in Fox Hills time, and extending southward through Montana to deposit the Judith River sandstone in Pierre time, the Laramie formation of Colorado, the "Hell Creek beds" of Montana, the great Lance sandstones of Converse County, Wyoming, and the Denver and Arapahoe formations of Colorado after Fox Hills time. The fact that the Lance sandstones were laid down at the end of Cretaceous time ^ is shown by the remains of the horned and carnivorous dinosaurs found in them, especially Triceratops and Tyrannosaurus. At about the same time Triceratops alticornis flour- ished east of the Front Range of Colorado, during the deposition of the Denver formation, wliich overlies unconformably (by erosion and uplift) the Laramie, the topmost formation of the "conformable Cretaceous series." These great flood-plain deposits, correlated both by their dinosaurs and by flora of the older Fort ' The United States Geological Survey classifies the Lance formation as Ter- tiaryC?), but the author regards it as of Upper Cretaceous age. Union type, mark the beginning of the Rocky Moun- tain revolution as it affected the country to the east. At certain localities, notably along Hell Creek, Mont., south of the Missouri, these fans of much disturbed channel sand and gravel are contemporaneous with undisturbed beds that appear to be lithologically exactly like those of the Fort Union; consequently Fort Union sedimentation began in some regions early in post-Laramie time. This long period of mountain erosion and sedi- mentation east of the Rockies came to an end either through heavy forestation or high-gradient river ero- sion, which deposited materials farther east. It is a very significant fact that in the region east of the Rocky Mountains, between South Dakota and northern New Mexico, only sparse lower Eocene sediments (Huer- fano A and Cuchara) are known between Fort Union (basal Eocene) and Chadron (lower Oligocene) time, whereas in the region west of the Front Range sedi- mentation continued through the entire Eocene epoch. West of the RocTcies. — In the mountain-basin region from southern Montana to New Mexico the condi- tions during Lance time were very different from those that prevailed east of the Rockies. There was ap- parently erosion and rapid transportation rather than deposition. Within the mountain basins — except around Medicine Bow, near Laramie, and around the Agathaumas sylvestris locality, near Black Buttes, Wyo. — relatively few deposits of Lance age {Tricera- tops zone) have thus far been identified by means of fossils. The Evanston formation, above the Adaville formation, in the typical Wasatch section of south- western Wyoming, according to Berry, contains plants of Fort Union and of Wasatch rather than of Denver age. Similarly the oldest Eocene deposits of the San Juan Basin (the Puerco and Torrejon) are comparable with the Fort Union and not with the older Lance formation; they overlie unconformably beds of prob- able Montana age. In brief, few deposits of Lance time (Triceratops zone) have thus far been identified within the mountain-basin region, although they may be found hereafter. At many places the oldest sedi- ments of the mountain basins lie upon the eroded sur- faces of unquestioned Cretaceous and older formations with pronounced unconformity. Physiographic conditions again changed, apparently, for after Lance time sedimentation began vigorously in the mountain-basin region and continued through- out the Eocene until it formed deposits having a com- bined thickness of 9,000 to 11,000 feet. (See table on p. 43.) Not until Oligocene time, when the deposi- tion of these mountain-basin beds probably ceased, was great fluviatile and flood-plain sedimentation re- sumed east of the Front Range, forming the lower Oligocene Chadron beds. 54 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA LATERAL AND MAIN RIVER SYSTEMS IN THE MOUNTAIN- BASIN REGION The great mounfain-hasin valley. — The contour lines of the basal Eocene and lower Eocene sediments of the mountain-basin region in northern New Mexico and Montana are very illuminating. They show the presence of a series of broad, relatively level basins — ■ a chain of flat uplands or valleys — in which the prod- UINTA BAS Period of volcanic ^ dust eruption in ^ soutlnern Wyoming, Q> Utah. and Colorado, sandstones washakie basinQ, mingled with erosion '^ products. Chiefly dacite > tuffs and sandstones Figure 39. — Chronologic relations of formations in the mountain-basin region This diagram exhibits the overlapping of sediments and the falls of volcanic ash in eight widely separated areas (Nos, 2-5 and 7-10, flg. 35), which, when combined, cover the entire Eocene epoch. ucts of erosion and the volcanic dust that were gath- ered by streams from the surrounding mountains were spread wide, indicating that although the mountain streams had high gradients and great erosive power the larger rivers had low gradients and little trans- porting power. The uniform elevation of the moun- tain-basin region at the north and the south and the low river gradients were favorable to sedimentation. We observe, moreover, that in basal Eocene time the conditions of climate and of sedimentation were some- what uniform in the Puerco and Torrejon deposits of the San Juan Basin in New Mexico, laid down by tributaries of Colorado River, and in the typical Fort Union deposits of Montana, laid down by tributaries of Missouri River. The rates of sedimentation were different. Similar basal Eocene sediments probably underlie some of the Wasatch (lower Eocene) deposits in the intermediate basins of Wyoming and Utah, for they have been exposed in the San Juan Basin only by the removal of the overlying Wasatch. A new sedimentary phase was begun in Wasatch time, and a third phase in Bridger time. The contrast in the physio- graphic conditions east and west of the Front Range has a very important bearing upon the paleontologic records. The mountain-basin sediments af- ford a marvelous and almost unbroken record of mammalian evolution in the Eocene, but little or nothing in the Oligo- cene, doubtless because large areas of Oligocene sediments have been eroded away. Only two spots remain — Bates Hole and Beaver Divide, in Wyo- ming. Piedmont, flood-plain, and la- custrine deposits. — King led the earlier geologists in presenting the theory that the mountain basins were once filled with a chain of lakes. This theory was adopted by Marsh, Cope, Scott, and Osborn. Leidy, as early as 1869, cast doubt upon the lake theory as applied to the White River group east of the moun- tains. The lake theory has grad- ually been replaced by the flood- plain theory through the studies of Haworth (1897. 1), Gilbert (1896.1), Matthew (1899.2), Davis (1900.1), Johnson (1901.1), and Hatcher (1902.3). For the highly diversified mountain-basin region throughout the very long period of the Eocene, with its considerable climatic vicissitudes, no single theory of deposition is adequate. We have seen that in the basal Eocene, during Fort Union, Puerco, and Torrejon (Thanetian) time, there were doubtless great level areas, heavily forested, with dense undergrowth, favor- able to the formation of peat and lignitic deposits Period of mountain erosion of granitic, calcareous, rhyolitic early volcanic and sedimentary areas ENVIRONMENT OP THE TITANOTHEEES 55 and subject to heavy silting of fine sediments from annual floods. These were like the flooded areas of the forest belt in the Amazon delta. Such still-water areas were contemporaneous with areas in the pied- mont regions close to the mountains, where stream erosion was active. The conditions that prevailed in general during Wasatch (Sparnacian) time are nearly- paralleled by those now found in the flood plains of Parana, Paraguay, and Uruguay Eivers, which are carrying down vast masses of gravel, sand, and clay from the mountain chains of Brazil, as reported by John Ball in his " Notes of a naturalist in South America "(1887.1). The annual rain- fall in these mountains ranges from 100 to 136 inches, and it rapidly disinte- grates the yieldingrocks and discharges a vast quantity of detrital matter over the broad plains of Argentina and Uruguay. The mountain streams have thus built up wide, level areas in these countries, and the lower rivers, ploughing their channels through the vast deposits over which they must make their way, extend their banks with every increment and thus continually make additions to the outskirts of the formation they are depositing. In this way deposits covering an area of 200,000 square miles have been formed from the mountains of Brazil. The period of flood-plain and piedmont deposition in the Rockies was followed by the great lacustrine period of Green River time and of Wind River (Ypresian) time, in which the climate was much warmer. In the same region there ensued the flood-plain period of the Bridger. Eocene basin deposition of another kind and climatic change are indi- cated in the widespread horizontal ' ' white layers "that divide the Bridger into several geologic and faunistic levels. These white layers indicate periods of lagoon leveling by annual uniform flooding and evaporation, similar to that of the existing playa lakes of the Great Basin in Nevada. In middle Eocene time new conditions of foresta- tion and erosion and the presence of volcanic atmo- spheric dust in the Bridger and Washakie Basins are indicated. Sinclair showed (1906-1909) that the Bridger formation was composed chiefly of volcanic material that has been more or less rearranged by stream action, and that clouds of volcanic dust doubtless filled the atmosphere during the Bridger epoch (middle and upper (?) Eocene). This interest- ing discovery was confirmed by thorough analyses made by Johannsen in 1914. The rocks of the upper and middle Eocene formations consist chiefly of volcanic tuff. Although the minerals of this tufl' are those of a dacite (quartz andesite), the quartz grains may be of sedimentary origin and the volcanic rock may be andesite (Johannsen, 1914.1, p. 210). The presence of dacite tuffs in the lower Bridger levels (B and C) indicates that the atmosphere was charged with volcanic dust, which also settled upon the con- temporaneous deposits of the Washakie Basin, 100 miles to the east, as well as on the Uinta Range, 60 .25, 26a, 26 b STACK MT^ CO > < / U LOWER BROV FiGURK 40. — Section of deposits near Barrel Springs, Washakie Basin, southern Wyoming (No. 9, fig. 35) Showing alternating beds of tuff, siliceous and calcareous deposits, and sandstone. Johannsen (1914.1), after Granger, with modifications. The numbers refer to lithologic specimens examined by Johannsen. miles to the south. Thus during middle and upper Eocene time the atmosphere over the present Bridger, Washakie, and Uinta region was at times charged with volcanic dust. Specimens of lower and basal Eocene rocks indicate sediments of more normal type, and whatever volcanic material they contain is so much altered by re-sorting and mixing with normal sedi- ments that it is not clearly recognizable. The manner in which the layers of dacite and glass tuffs alternate -with the heavy river-channel sand- stones is clearly displayed in the analysis of sediments from the Washakie Basin by Johannsen. Tuffs are 56 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA also scattered, but more sparingly, through the earlier Torrejon, Wasatch, and Wind River formations, along with river-borne material derived from the decay and erosion of older rocks. SECTION 2. EOCENE AND LOWER OLIGOCENE FORMATIONS AND FAUNAL ZONES FIRST FAUUAI PHASE (BASAL EOCENE) SEVENTEEN LIFE ZONES Largely as the result of explorations and researches made for this monograph, the major Eocene and Oligo- cene type life zones that were recognized by Leidy, Cope, and Marsh up to the year 1900, such as the " Coryphodon beds," " JJintaiherium beds, "Diplacodon beds," and " TitanotJierium beds," have gradually been differentiated, through the work of Osborn, Granger, and Matthew, into 16 known life zones, each distinguished by the presence of a highly varied mammal fauna and by the appearance or disappearance of certain groups of mammals and reptiles. There is also one theoretic life zone, between known upper Eocene and known lower Oligocene time, making 17 in all. Each of these life zones corresponds with a series of sediments rang- ing in thickness from 300 to 600 feet. Many of them correspond with changes in climate, temperature, and forestation, and some of them are clearly defined and sharply demarcated from others. A single generic name, such as Coryphodon, rarely suffices to distin- guish them, because many genera and even certain species may survive for long periods of time. Each of these faunal zones is defined paleontologic- ally by one or more of the life forms it contains, geo- logically by the locality v/here it is best preserved, to some extent botanically by the flora it contains, and lithologically by the character of its rocks as shown by microscopic analyses. Thus, for example, we have the typical upper Wind River zone — the "Lost Cabin " or LamhdotJierium-Eotitanops-CorypJiodon zone, a rather cumbersome designation, which indicates that only during this period did these three types of mammals exist together. In this zone Lambdotherium is the most distinctive genus. Sediments in different geo- graphic basins are correlated in such a manner as to present the whole life story of the Eocene epoch, as shown in the accompanying diagram. Of the two chief faunistic gaps that have been recognized, that between the Wind River and the Bridger has now been filled by explorations of the Huerfano, so that there remains only that between the Uinta and the White River. These 16 known life zones will doubtless be multiplied to 20 or more by future discovery. They are shown in the following table: ENVIfiONMENT OF THE TITANOTHEEES Synopsis of life zones 57 Epoch Life zones Horizon Characteristic species and genera 17. Titanotherium-Mesohippus. Brontops robustus, Menodus gigan- Lower Oligocene. Chadron B teus, Brontotherium platyceras, Allops crassicornis. Brontops dispar, Menodus trigono- ceras, Allops marshi, Brontothe- rium hatcheri. Brontops braohycephalus, Menodus heloceras, Brontotherium leidyi. 16. Theoretic zone (no fauna) 15. Diplacodon-Protitanotherium- Epihippus. 14. Eobasileus-Dolichorhinus 13. Metarhinus Uinta C 2. Uinta CI Diplacodon, Protitanotherium, Epi- Upper Eocene. Uinta B 2 and Washakie B 2. Uinta B 1 and Washakie B 1. hippus, Protoreodon. Eobasileus, Dolichorhinus. Metarhinus, Amynodon. 12. Uintatherium-M a n t e c eras - Mesatirhinus. 11. Palaeosyops paludosus-Orohip- pus. 10. Eometarhinus-Trogosus-Palaeo- syops fontinalis. Washakie A and Bridger C and D. Uintatherium, Manteoceras, Mesati- rhinus. Palaeosyops paludosus, Orohippus. Bridger A and Huerfano B. Palaeosyops fontinalis, Eometarhinus. Lower Eocene. 9. Lambdotheri um-Eotitanops- Coryphodon. 8. Heptodon-Coryphodon-Eohippus. 7. Systemodon-Coryphodon-Eohip- pus. 6. Eohippus-Coryphodon. Huerfano A, Wind River B, and Big Horn E. Big Horn (Wasatch) D and Wind River A. Big Horn (Wasatch) C__. Big Horn (Wasatch) B-._ Lambdotheri um, Eotitanops, Cory- phodon, Meniscotherium. Heptodon, Eohippus, Coryphodon. Systemodon, Eohippus, Coryphodon. Eohippus, Pelycodus, Coryphodon. Transition basal Eo- cene to lower Eocene. 5. Phenacodus-Nothodectes-Cory- phodon. Big Horn (Wasatch) A___ Phenacodus, Nothodectes, Corypho- don, Champsosaurus. Pantolambda, Tetraclaenodon, Claen- odon. Torrejon A._ . . .- -- Deltatherium, Mioclaenus, Haplo- Basal Eocene. conus. Polymastodon, Oxyclaenus. Puerco A._ _ Ectoconus, Champsosaurus. Cretaceous." Triceratops-Tyrannosaurus Lance and Denver forma- tions. Triceratops, Tyrannosaurus, Champ- sosaurus, Meniscoessus. Judith River and Belly River formations. Monoclonius, Deinodon, Eodelphis. » The United States Geological Survey classifies the Denver formation as Eocene and the Lance formation as Tertiary (?) . 101959— 29— VOL 1 6 58 TITA^TOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Lower Tertiary geologic horizons and life zones and their hoofed mammals Epoch Geologic horizon Chiet life zones. (Titanotheres, horses, and other mammals.) Chief titanotheres and other perissodactyls Oligocene. Leptauchenia, Miohippus, and Oreodon. Extinction of titanotheres Chadron A, B, and C. 17. Titanotherium-Mesohippus. Giant titanotheres — Menodus, Brontops, Brontotherium, etc. Upper Eocene. Uinta C. 16. Theoretic zone (fauna un- . known). 15. Diplacodon-Protitan othe- rium-Epihippus. Protitanotherium, early horned ti- tanotheres. Washakie B and Uinta B. 14. Eobasileus-Dolichorhinus. 13. Metarhinus. Dolichorhinus cornutus, Mesati- rhinus. Bridger C and D and Washakie A. 12. Uintatherium-Manteoceras- Mesatirhinus. Manteoceras, ancestors of Ohgo- cene titanotheres. Middle Eocene. Bridger B. 11. Palaeosyops paludosus-Oro- hippus. Palaeosyops and numerous other titanotheres. Bridger A and Huerfano B. 10. Eometarhinus-Trog osus- Palaeosyops fontinalis. Palaeosyops fontinalis (primitive). Big Horn E ("Lost Cabin"), Wind River B (typical Wind River of Cope), and Huerfano A. 9. Lambdotherium-Eotitanops- Coryphodon. Appearance of titanotheres Lower Eocene. Big Horn D ("Lysite") and Wind River A. 8. Heptodon-Coryphodon-Eo- hippus. Big Horn C ("Gray BuU"). 7. S3'stemodon-Coryphodon- Eohippus. Earliest tapiroids, Tapiridae. Big Horn B ("Sand Coulee"). 6. Eohippus-Coryphodon. Earliest Equidae (horses). Transition. Big Horn A ("Clark Fork") of Wyoming and " Tiffany beds " of southwestern Colorado. 5. Phenacodus-Nothodectes- Coryphodon. Earliest Phenacodus (condylarths) . Earliest Coryphodon. Notho- dectes, similar to Pleisiadapis of Cernay. Upper horizon of Torrejon forma- tion. 4. Pantolambda. Ancestors of the Amblypoda. Basal Eocene. Lower horizon of Torrejon forma- tion. 3. Deltatlierium. ^ Upper horizon of Puerco forma- tion. 2. Polymastodon. M ultitub erculata. Lower horizon of Puerco forma- tion. 1. Ectoconus. Earliest known Taligrada. ENVIEONMENT OP THE TITANOTHEEES 59 Figure 41. — Eocene and lower Oligocene mammalian life zones in eleven typical correlated areas in New Mexico, Colorado, Utah, Wyoming, South Dakota, and Montana, located as shown on the general geologic map (fig. 35) Arranged by Osborn (1919) after original studies made in the field, chiefly by Granger, but also by Hatcher (Oligocene), Hills, Peterson, and Gidley (Eocene) . The 16 Icnown life zones numbered 1 to 15 and 17 are indicated in the diagram by darlc horizontal lines. The nonfossiliferous areas are indicated by light oblique lines. These life zones and sections also correspond with the detailed geologic sections in this chapter. The United States Geolojical Survey classifies the Lance formation as Tertiary (?), Eocene (?). The author of this monograph regards it as Cretaceous. 60 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA BASAL EOCENE TIME IN MONTANA AND NEW MEXICO Fori Union formation of Montana. — The typical Fort Union formation of Hay den (Meek and Hayden, 1862.1, p. 433), at the junction of Yellowstone and Missouri Rivers, lies east of the center of an ancient forested swamp in which was laid down the upper part of Hayden's "Great Lignite Group." One of the most interesting results of discoveries made in 1901 (Doug- lass, 1902.1) is revealed in an exposure of the Fort Union in Sweet Grass County, Mont., near the head- waters of the Musselshell, containing a rich fauna of the archaic species of basal Eocene animals, some of which are identical with those found on the head- waters of San Juan River, in northern New Mexico, a thousand miles to the south. Both lie near the one hundred and seventh meridian. The presence in large numbers of animals belonging to similar species shows that uniform climatic and physiographic con- ditions existed in this great mountain-basin region for a very long time, because similar generic fonns of life (Olaenodon, Pantolambda) persist through 3,000 feet of Fort Union sediments. The remains of the oldest of these mammals are found immediately above the dinosaur-bearing beds at a level which is here identical with that of the Lance formation; and the present opinion is that sedimentation may have been continuous throughout Upper Cretaceous and basal Eocene time in this region in Montana. The mode in which these Fort Union beds were formed has not yet been positively determined, but the masses of fresh-water shells which they contain in certain localities indicate that they were in part laid down in shallow lagoons and swamps, which were in some places crossed by river channels. At some places the beds contain multitudes of leaves, which give us a complete record of the forest life of the time. Vast areas of warm-temperate and more hardy trees were interspersed with areas where swamp vegetation accumulated rapidly enough to form great beds of lignite. Amid the glades of these forests there wan- dered swamp turtles, alligators, and especially the choristoderan reptiles of the characteristic genus Champsosaurus. Puerco and Torrejon formations of New Mexico. — A southern center of this archaic mammal life is the type locality of the Puerco formation, on the divide between the Rio Grande and the San Juan, in north- western New Mexico, a formation described by Cope (1875.1) as the "Puerco marls." Cope listed the first mammalian fauna from those beds in 1881, opening a new epoch in mammalian paleontology. In 1885 he assigned to the formation a thickness of 850 feet and distinguished it from the underlying beds, which he supposed to be Laramie but which have since been divided into the Qjo Alamo sandstone, the Kirtland shale, and the Fruitland formation, all probably of Montana age, older than Laramie. The Puerco of Cope appears to be a single formation geo- logically, deposited with apparent conformity between the upper and lower divisions, but it is sharply divided faunistically into two main life zones, a lower, which retains the name Puerco, and an upper, to which the name Torrejon was given by Wortman in 1895 (Osborn and Earle, 1895.95, pp. 1-3A). In 1910 Gardner (1910.1) applied the name Nacimiento group to both divisions. In 1897 Matthew (1897.2) separated the mammal fauna of the two levels, and in 1912 and 1913 Sinclair and Granger (1914.1) estab- lished in this group no less than four faunistic levels, which are shown in the accompanying section (fig. 43). Two faunistic levels were observed by Wortman in the Puerco, and two distinct faunistic levels are dis- tinguished by Granger, Sinclair, and Matthew in the Torrejon. These four successive changes in the archaic fauna occurred during a period of continuous sedimentation, for no unconformity has been observed between the Puerco and Torrejon. The rate of deposition of the 800 feet of Puerco and Torrejon sediments was rela- tively slow as compared with that of the deposition of the 6,000 feet of the corresponding Fort Union sedi- ments to the north. As the mammals distributed through 4,000 feet of the northern part of the Fort Union deposits correspond chiefly with those of the Torrejon, it appears possible that the underlying Puerco fauna may belong in part in upper Lance time. We observe that the Fort Union was deposited upon the Lance continuously, without recognized notable unconformity, whereas the Puerco lies upon the eroded surface of the Ojo Alamo, which, because of its dinosaur fauna, is considered of probable Judith River and Belly River age. The close resemblance of the crestless trachodont dinosaur, Kritosaurus navajovius, from the Ojo Alamo, to a corresponding form from the Belly River forma- tion of Alberta also suggests a close correlation in time.* In 1912 and 1913 Sinclair and Granger thoroughly explored the basal Eocene deposits of the San Juan Basin, with the results enumerated above. SUMMARY OF FAUNAE EVENTS OF BASAL EOCENE TIME In addition to the four fossiliferous zones observed tn the Puerco and Torrejon formations, all distinc- tively basal Eocene, there is an overlying zone in the "Tiffany beds," beyond the border of Colorado, deter- mined by Gidley (1909) and Granger (1916). These beds contain a fifth fauna, which is strictly interme- diate between basal Eocene and lower Eocene. This transitional basal-lower Eocene zone is described on pages 64-65. The basal Eocene mammalian life > See Parks, W. A., The osteology of the trachodont dinosaur Kritosaurus incur- limanus: Univ. Toronto Studies, Qeol. series, 1920. ENVIRONMENT OF THE TITANOTHERES 61 Figure 42. — Section of Upper Cretaceous and basal Eocene (Fort Union) deposits in Sweet Grass County, Mont. After Stanton (1909.1), Stone and Calvert (1910.1), and Gidley (1919). This very significant exposure (No. I, flg.35) is in an outlying area of the Fort Union formation and its mammal fauna corresponds broadly with that of the Torrejon formation of northwestern New Me-xico, although the section has not yet been divided into separate life zones. It affords the most satisfactory means of correlating the Fort Union and Puerco and Torrejon formations. The United States Qeological Survey classifies the Lance formation as Tertiary(?), but the author of this monograph re- gards It as of UpperTCretaceous age. 62 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 43. — Section of Eocene deposits in the San Juan Basin, northwestern New Mexico (No. 2, fig. 35), showing the base of the Puerco formation resting upon the eroded surface of the Ojo Alamo sandstone, as observed by Sinclair and Granger (1914.1) This section displays the close geologic continuity of the Puerco and Torrejon beds, which are subdivided faunistically into four distinct life zones, named, in ascending order, (1) Ecloconus and (2) Poly mastodon zones, Puerco formation; (3) Deltatherium and (4) Fantolambda zones, Torrejon formation. The Ojo Alamo sandstone is perhaps of Judith River age. ENVIRONMENT OF THE TITANOTHEEES 63 of the Puerco and Torrejon formations in northwestern New Mexico and southwestern Colorado is accord- ingly related as follows: Transition epoch: 5. Phenacodus-Nothodectes-Conjphodon zone. Represented in the "Tiffany beds" of southwestern Colorado and in the Wasatch formation (horizon Big Horn A, "Clark Fork") of Big Horn Basin, Wyo. Basal Eocene epoch: 4. Pantolamhda zone. Represented in the upper levels of the Torrejon formation of northwestern New Mexico and in the upper part of the Fort Union formation of Montana. 3. Deliatherium zone. Represented in the basal part of the Torrejon formation and in part of the Fort Union formation of Montana. 2. Polymastodon zone. Represented in the upper part of the Puerco formation of northwestern New Mexico. Not yet recorded in the Fort Union formation. 1. Ectoconus zone. Represented in the lower part of the Puerco formation. Not yet recorded in the Fort Union formation. BASAL EOCENE FAUNAL ZONES ZONES 1 AND 2: ECTOCONUS AND POLYMASTODON ZONES [Puerco fauna; part of Thanetian of Europe] No equivalent of the most ancient Puerco fauna has thus far been discovered in the Fort Union beds of the North or in Europe; it is at present unique. Puerco mammals and reptiles. — The Puerco mammals are extremely archaic, mostly Meseutheria (Osborn) or paleoplacentals (Matthew), representing groups of placentals that became extinct during the Eocene. The Puerco contains no remains of modern orders or families of mammals except three, one (Miacidae) which is related to the doglike Carnivora, a second which is related to the primitive Insectivora, and a third which is related to the primitive Edentata. No rodents or lemuroid primates have been discovered, and certainly no perissodactyl or artiodactyl ungu- lates were in this region at this time. Matthew (1914.1, p. 383) is of the opinion that most of these archaic placentals have "no known predecessors in the Lance formation. About 10 per cent of the fauna consists of rodent- like multituberculates, an extremely ancient order re- lated to the existing monotremes or to the marsupials. These animals are nearly related to ancestral forms in' the Lance. Didelphiid marsupials are also present. Similarly the reptiles all belong to families that originated in Belly River or Pierre time (Upper Cretaceous) or earlier. The Choristodera (Champso- saurus) became extinct in basal Eocene time. Note- worthy is the absence of the prevailing Tertiary families of chelonians (Emydidae, Testudinidae), which, with the modernized mammals, first appear in the lower Eocene. On comparing the life of the Puerco with that of the Lance we find a mammalian fauna that indicates no very wide gap in time — a fauna that is somewhat more ancient than the Torrejon and known Fort Union, also more ancient than the Cernaysian and upper Thanetian of France. It is therefore probable that the Puerco corresponds with the lower Thanetian of France, but its life has no known equivalent either in Europe or in this country. The opinion of Cope that the ancestry of the modernized mammals should be sought among these Puerco forms lacks adequate confirmation. The op- posite opinion — that the Puerco mammals are not ancestral to the modern mammals — was developed by Osborn (1893.82, 1894.89) when he applied to them the name Mesoplacentalia (Meseutheria), indicative of their archaic or Mesozoic characteristics. They repre- sent the first known adaptive radiation of the placen- tals into archaic flesh eaters and herbivores. We note the presence of three families of archaic Carnivora (Creodonta) and remote relatives {Psittacotherium) of the Edentata. Among the archaic ungulates we find one varied family (Periptychidae) of the Amblypoda (Taligrada) and two families (Phenacodontidae, Mio- claenidae) of the Condylarthra. Puerco sedimentation and physiography . — The Puerco formation is not separated from the overlying Torrejon formation by any lithologic or stratigraphic break. (Sinclair and Granger, 1914.1, p. 308.) The absence of erosional unconformity between the Puerco and Torrejon was also observed by Gardner (1910.1, pp. 722-723) and by Bauer (1916.1, p. 277). That the Puerco and Torrejon formations represent a very long period of geologic time is demonstrated by the recorded 6,000 feet of Fort Union sediments, which have yielded the Torrejon fauna alone; and, like the Fort Union, they represent a very long period of uniform conditions of climate and sedimentation. The pres- ence of fish, crocodiles, turtles (Trionyx), and other genera in the same strata with the bones of mammals and at the same level shows conclusively that these deposits were formed by water. That the streams were of low gradient is shown by the complete absence of pebbles in the Puerco and by the wide horizontal extent of some of the clay bands. Bogs, apparently formed in back waters in the channels, were filled with accumulations that preserved impressions of the leaves of figs (Kc-Ms), plane trees (Platanus), poplars {Populus), relatives of the bread fruit (Artocarpus) , and numerous shrubs (Paliurus, Viburnum). The quantity and vari- ety of these plant remains, together with the abundant large drift logs in the clays, indicate a heavy growth of vegetation along the streams. The species of Ficus. Paliurus, Viburnum, and Artocarpus are also found in the Denver and Raton formations of eastern Colorado ; and other species indicate Fort Union age (Knowlton, cited by Sinclair and Granger, 1914.1, p. 306). The mode of occurrence of the fossils in the still-water clays and occasionally in the river-channel sandstones shows that some of the skeletons may have been 64 TITANOTHEEES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA washed into the streams during heavy rains and scattered by the action of crocodiles, carnivores, tur- tles, and fish. Other skeletons show traces of gnaw- ing, probably by small Ptilodontidae, which proves that many of the bones lay for some time on the surface of the ground before reaching the streams or being covered in flood time by water-borne sediments. ZONES 3 AND 4: DEITATHERIUM AND PANTOLAMBDA ZONES [Torrejon and Fort Union faunas; in part Thanetian of Europe] The mammals of the Torrejon formation of north- western New Mexico, whose remains are found in a stratum about 250 feet above the base of the Puerco mammal-bearing level, are somewhat larger, con- siderably more diversified (perhaps because more fully known), and of slightly more progressive type. They show very close affinity to the Fort Union mammals of Montana and some affinity to the Cernaysian forms discovered in the conglomerat de Cernay, near Rheims, France. The multituberculates, which occur in the Holarctic region in upper Triassic (Rhaetic) time, now make their last appearance abundantly; of the Ptilodonti- dae, Ptilodus (or Neoplagiaulax) is found in New Mexico, Montana, and Cernay; the large Polymasto- don that distinguishes the upper Puerco zone does not recur. Here also are five families of archaic carnivores (Creodonta), among which, in the Miacidae, there is a genus (Didymidis) which appears to lead through the civet and doglike forms of the lower and middle Eocene into forms related to the modern Carnivora. Among the three Torrejon families of Insectivora the existing Centetidae (tenrecs) are possibly related to the genus Palaeorydes , a very primitive form resem- bling the modern Cape golden moles {(JhrysocMoris of South Africa, Necrolestes of South America). The ancestors of the modern edentates are highly diversi- fied (Edentata, Ganodonta) and include slothlike animals, indicative of present or former migrations into South America. Of the families of archaic ungu- lates two (Phenacodontidae and Mioclaenidae) repre- sent the Condylarthra, and two (Periptychidae and Pantolambdidae) represent the Amblypoda. Of the Amblypoda Pantolambda cavirictus, which is also found in the Fort Union, is very characteristic. Of the bearlike Creodonta (Arctocyonidae) Claenodon ferox, which is closely related to the Arctocyon of the Thanetian of France, occurs also in the Fort Union of Montana. Most of these mammals of the Torrejon, like those of the Puerco, were ancient adaptive radiations of the Mammalia. They were small-brained, had de- fective foot structure, and were unfitted to compete with the ancestors of the modernized mammals, which begin to appear immediately above the Noiho- dedes zone. Six families approached extinction at the end of the Torrejon — the Plagiaulacidae of the Multi- tuberculata; the Oxyclaenidae of the Carnivora; the Conoryctidae of the Edentata; the Periptychidae and Pantolambdidae of the Ambyploda (which, however, are related to the succeeding coryphodons) ; and the Mioclaenidae of the Condylarthra. The Plagiaula- cidae and Oxyclaenidae, however, survive into the early Wasatch, the Periptychidae into the "Tiffany beds." Torrejon time thus ends with the extinction of a large number of families of archaic mammals, though several families survived, passing into the succeeding lower Eocene. Unconformities of the Torrejon with the underlying Puerco have not been found. (Sinclair and Granger, 1914.1, p. 312; also Gardner, 1910.1, p. 722, and Bauer, 1916.1, pp. 273-277.) There is no doubt about the aqueous origin of either the Puerco or the Torrejon deposits. The Torrejon carries less petrified wood than the Puerco, but it contains Z7mo-bearing beds, which occur repeatedly in the gray clays, and abundant shells of land moUusks {Pupa), which are found in the clays that contain bones of mammals. Lithologically, the Torrejon closely resembles the Puerco, except that gravels of quartzite, jasper, red shale, etc., occur in some of the channel sandstones. Mammals appear principally in the zones filled with small rusty calca- reous concretions, which occur in clays that range in color from red mottled with green to gray. The upper boundary of the Torrejon is everywhere marked by the presence of Tetradaenodon (ancestor of PJiena- codus) and of the two amblypods PeriptycTius rhabdo- don and Pantolambda. The total thickness of the Torrejon differs at different places, ranging from 240 to 660 feet, whereas the approximately contempora- neous Fort Union of Montana, which possibly also represents the Puerco, attains a thickness recorded as nearly 6,000 feet. The top of the Tori'ejon is in unconformable contact with sandstone that indicates a cycle of deposition of coarse sediments and alluvial fans, attributed to Wasatch time. SECOND FAUNAI PHASE (LOWER EOCENE) TRANSITIONAL BASAL EOCENE FAUNAS ZONE 5; PHENACODUS-NOTHODECTES-COETPHODON ZONE [Base of Wasatch formation of Big Horn Basin, first Wasatch life zone, Big Horn A; Cernaysian of Europe] The first Wasatch life zone is represented in the "Tiffany beds" of southwestern Colorado, in the basal part of the Wasatch formation (horizon Big Horn A= "Clark Fork") of the Big Horn Basin, Wyo., and probably in the summit of the Fort Union formation of Montana. In southwestern Colorado, near the headwaters of the San Juan, are the "Tiffany beds" of Granger, which contain a fauna characterized by the last appearance of PeriptycTius and by the first appearance of Phenacodus and of Coryphodon, a genus ENVIKONMENT OF THE TITANOTHEEES 65 characteristic of the Sparnacian of France. Notho- dectes of the "Tiffany beds" is particularly interest- ing because of its structural affinity to Plesiadapis of the Cernaysian of France. The multituberculates are represented in Wyoming by Ptilodus (" Sand Coulee" and "Clark Fork"?). Of the four specimens of ptilodontids from Wyoming, one found by Granger was from the Big Horn B horizon ("Sand Coulee beds"). Three found by Stein were probably from the same horizon but may have been from the under- lying Big Horn A horizon (the "Clark Fork beds"). Undoubtedly ptilodontids occur in the "Clark Fork," but we can not furnish any positive evidence (W. Granger, 1919). This mammal fauna as a whole actually resembles that of the Torrejon more closely than that of the lowest overljang horizon (Big Horn B, "Sand Coulee") of the Wasatch. A significant discovery in the No- thodectes zone is Zanyderis, a bat showing affinities with the vampires (Phyllostomatidae) of South America. The NotTiodedes zone ("Tiffany" and "Clark Fork") is basal Eocene, as indicated by the absence of the four orders Primates, Perissodactyla, Artiodactyla, Rodentia; it is lower Eocene, as indicated by the presence of Phenacodus and Coryphodon. The mammalian life of the "Clark Fork" beds in the Big Horn Basin of Wyoming, to the north, is very similar (Granger, 1914.1, p. 204) to that of the "Tif- fany beds" in Colorado. These "Clark Fork beds," 500 feet in thickness, are characterized by the pre- dominance of the Condylarthra (PJienacodus and Ec- tocion), remains of which constitute three-fourths of the fossils collected from them. The amblypod un- gulates are represented by CorypJiodon and by the first appearance of an animal (Eohathyopsis) ancestral to Baihyopsis, of the Wind River formation, which in turn is ancestral to the horned UintatJierium of the Bridger formation. Among the Reptilia is the last surviving Champsosaurus from the Fort Union and the Cretaceous, a distinctively basal Eocene type. EARLY EOCENE TIME General correlation. — Lower Eocene (Wasatch) time began, it may be said, with the first appearance of Coryphodon and Phenacodus in the "Clark Fork" and "Tiffany beds" described above as the Phenacodus- Nothodectes- Coryphodon zone, in which is found the first phase of the Coryphodon fauna. The modernization occurred later, in the "Sand Coulee beds" (Eohippus zone), which overlie the "Clark Fork." The Sparnacian of Europe is broadly parallel with part of the Wasatch formation {Coryphodon zone) of America. It is typified in France by the deposits of Soissons, Meudon, and Vaugirard; in England by the Woolwich beds, which contain a rich flora. In these fluviomarine, lagoon, and lacustrine deposits of Europe mammals are rare, and homotaxis with America is afforded through the large coryphodons, the perisso- dactyl Lophiodon, and the creodonts Palaeonictis and Pachyaena. This sparse European fauna, which in its early stages lacks Equidae (Hyracotherium) , has almost a counterpart in that of the Nothodectes zone of the Rocky Mountain region. The two upper zones of the lower Eocene (Wasatch) are correlated with the Ypresian of Europe. Wasatch and Sparnacian floras. — According to Berry (1914.1, p. 148) the earliest Eocene beds of Europe (Sparnacian and Ypresian stages) contain the flora found in the Oldhaven, Woolwich, and Red beds of England, largely unstudied, and the small flora from the Paris Basin recently described. The Woolwich beds have yielded the fig (Ficus), the locust (JRohinia), the tulip tree (Liriodendron) , and Grevillea, a pro- teaceous plant now confined to Australia. Berry believes (letter to the author, April 1, 1918) that in lower Wasatch time the Fort Union flora persisted over the Rocky Mountain basin region. This belief implies that the climate was then prevailingly warm-temperate but that there were occasional incursions of trees of subtropical type. Sedimentation during Wasatch time. — As the Sparna- cian stage of Europe, which is equivalent to part of the Wasatch, derives its name from Epernay (Latin Spar- nacum), so the Wasatch stage of mammalian life derives its name from the typical Wasatch group of Hayden in western Wyoming, a single mammal-bearing member of which is the Knight formation (Veatch, 1907.1), 1,750 feet in thickness, containing Cope's types of Eohippus index, E. vasacciensis, Phenacodus primaevus, Coryphodon radians, C. semicinctus, C. latipes. These species of mammals do not represent the oldest Wasatch fauna; they are of the same age as the species found at the "Lysite" horizon (life zone No. 8) of the Big Horn Basin. Among the chief sources of Wasatch mammals are the following: Feet 1. Knight formation (Veatch), top of typical Wasatch group (Hayden), southwestern Wyoming; red and yellow sandy clays 1,750 2. Wasatch formation, Big Horn Basin, Wye; red, brown, and gray sandstones and clays 2, 025 3. Wasatch formation ("Bitter Creek" of Powell and "Vermilion Creek" of King), Washakie Basin, Wyo.; red and gray clays and sandstones 4, 000-5, 500 4. Wasatch formation of the San Juan Basin, N. Mex 1,500 5. Wasatch formation of the Uinta Basin, Utah (White, 1878) 2,000 6. Wasatch formation of the Powder River Basin, Pump- kin Buttes, Wyo 2,400 The estimate made by King (1878.1) of the thick- ness of the sediments in the Washakie Basin (4,000- 5,500 feet) is considered high (Granger). It is inter- esting to note that the mean thickness (about 2,300 feet) of the Wasatch sediments in the six areas listed above exceeds somewhat that of the Bridger formation (1,875 feet). 66 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The earliest Wasatch sediments are those in the Big Horn and Clark Fork Basins of northern Wyoming, from which we obtain the whole range of lower Eocene fossil mammals, beginning with the end of basal Eocene time. fossils. Douglass found a considerable faima in the Wasatch of the Uinta Basin. Systemodon occurs there. Wortman has reported (letters) a Coryphodon from the Wasatch of the Washakie Basin. To the south, in the San Juan Basin, there were laid down, over the Torre j on, thick beds of sand and fluviatile sediments, which form the New Mexico Wasatch. These beds, which are divided by Granger (1914.1) into an upper ("Largo') and a lower ("Almagre") divi- sion, have a combined thickness of 1,500 feet, throughout the greater part of which mam- malian fossils are found. These Wasatch beds in New Mexico have much the same general appearance as the Wasatch in other localities, consisting of red, gray, and ocherous bands of shale and sandstone, without evidence of unconformity throughout the series. The most recently identified Wasatch sediments are those of Pumpkin Buttes, in the Powder River Basin, Wyo. The correlation of the faunal horizons in these sedimentary areas by the species of mammals which they contain was determined with remarkable precision by the American Museum expedition under Granger, as shown in the accompanying table (p. 67). Wasatch pJiysiograpliic and climatic condi- tions favorable to modernized fauna. — All the Wasatch sediments indicate a profound and somewhat abrupt change in the physiographic and climatic conditions of the mountain-basin region from those that prevailed during Fort Union, Puerco, and Torrejon time. In general, still-water sedimentation in level forests and lagoons ceased. Fluviatile, flood-plain, fluvial- fan, and channel deposits containing a larger percentage of coarser materials were wide- spread. There is evidence of open stretches of country exposing sand, gravel, and clay, subject to occasional desiccation and aridity. The Wasatch of the Big Horn Basin repre- sents the filling of an intermontane trough of downwarp. (Sinclair and Granger, 1912.1, p. 66.) Materials were transported by streams Figure 44.— Columnar section of Cretaceous and Eocene sediments ex- f ^.^^ ^^le surrounding mountains, as shown by posed -along Bear River near Evanston, in extreme southwestern ^^e lithology of the gravel, sand, and clay. The Wyoming (No. 3, fig. 35) , sliowing tlie typical Wasatcli group of Hayden (1869). Chiefly after A. C. Veatch (1907.1) Mammals similar to those in zone 8 (Eohippus, Phe-nncodus, lieplodon, and Coryphodon) occur in a narrow fossiliferous stratum which may be referred to the Heptodon- Coryphodon- Eohippus zone. Above are Oreen River and Bridget beds; below are 4,600 feet of beds (without mammals) belonging to the Wasatch group (Fowkes and Almy formations), which are imderlain by the Evanston formation, containing Fort Union plants, and the AdaviUe formation, containing Montana plants and invertebrates. The author of this monograph regards the Evanston formation as uppermost Cretaceous. Similar heavy and continuous sedimentation also occurred during Wasatch time, in both the northern and the southern Uinta region, in the Bridger and Washakie Basins on the north, and in the great Uinta Basin south of the mountains. Few of these vast masses of sediment have thus far yielded mammalian underlying Fort Union was uplifted before sedimentation began, and the synclinal basin was inclosed more or less completely to the east, south, and west by anticlinal mountains. Erosion from the mountain rocks represents all the members of the typical section from the Archean to the Fort Union, usually by stream transportation and deposition in river channels and over broad flood plains. No beds of volcanic ash have been found, nor is there evidence of transportation by wind. The deposits of clay show a more or less regular alternation of red and bluish- gray layers, which may be due to climatic changes. ENVIRONMENT OF THE TITANOTHEKES 67 The excess of iron salts in the red clays may have accumulated and oxidized to hematite during dry climatic cycles; the blue clays were probably deposited in a moister climate, which is less favorable to the concentration and oxidation of the iron. Similar alternations of red and blue clays in the desert basins of Lop and of Sewistan have been described by Hunt- ington, who also associates the colors with the recur- rence of moist and arid climatic cycles. Sinclair and Granger (1914.1) ascribe the color banding of the Wasatch and Wind River clays to a similar cause — the alternation of moist and dry climatic conditions — but they have not found any other evidence of excessive aridity, the fauna of the red and blue bands being the same. The fact that the blue clays of the Wasatch are here and there lignitic and are at some places associated with skeletal remains suggests that they may have been formed during cycles of rather abun- dant rainfall, when the surface of the intermontane basin was prevented from drying out rapidly. That these climatic and physiographic conditions were not local is shown by similar color banding in the Wasatch of all the mountain-basin regions. The name "Ver- milion Creek" was applied by King to the Wasatch because of the red color of the rocks through which that creek flows in southern Wyoming and north- western Colorado. Microscopic examination of the feldspars in the Wasatch deposits of the Big Horn Basin does not favor the idea of luxuriant subtropical forests and a warm, humid climate, with the formation of a deeply decayed humus, but rather suggests a dry, not necessarily arid climate, with rapid changes of temperature, favorable to splintering of the ledges of hard rock; rapid trans- portation of the fragments for short distances; and burial of these beyond reach of carbonated waters. A cursorial ungulate fauna. — This conception of a drier lower Eocene climate in the basins during Wasatch time accords with the successive appearance in this region of four families of the modernized types of perissodactyl mammals — horses, tapirs, lophio- donts, and titanotheres — with light, cursorial limb and foot structure adapted to rapid locomotion and wide seasonal migration. Correlation of lower Eocene life zones of Wyoming and New Mexico {after Granger, with modifications) New Mexico (Wasatch — "Largo" and "Almagre") 'Largo" (typical). Eohippus, Menis- cotherium, Am- Vjloctonus. 'Almagro" (typical). Eohippus, Anaco- don. Unconformit3' be- tween Wasatch and Torrejon. In south- ern Colorado "Tif- fany" (typical) . No perissodactyls. Torrejon. No peris- sodactyls. Fauna more primitive than in "Clark Fork." Evanston (typical Wasatch) Green River. Knight (typical) . Heptodon, Eohip- Wind River Basin (typical Wind River) Lambdotherium zone ("Lost Cabin"; typical) . H y- rachyus, Eotita- nops, Lambdothe- rium, Heptodon, Eohippus, Menis- cotherium. Heptodon zone ("Lj'- site"; typical) . Heptodon, Eohip- pus. Big Horn and Clark Fork Basins Lambdotherium zone ("Lost Cabin"). Lambdothe- rium, Heptodon, Eo- hippus, Ambloctonus. Heptodon zone ("Lj'site"). Heptodon, Eohippus, Anacodon. Systemodon zone ("Gray Bull," typical). Syste- modon, Eohippus. Eohippus zone ("Sand Cou- lee," typical). Eohippus (abundant), etc., first ar- tiodactyls, rodents, and primates. Phenacodus zone ("Clark Fork"; typical). No perissodactyls, artiodac- tyls, rodents, or primates. Fauna more advanced than in Torrejon. End of lower Eocene. First titanotheres ap- pear. First lophiodonts ap- pear. First tapirs appear. First horses appear. Arrival of modern- ized mammals. End of basal Eocene. Archaic mammals onlv. 68 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA A very significant fact, clearly presented in tlie table on page 67, is that these small, light-limbed, cursorial ungulates appear not simultaneously but at successive horizons. At the lowest level are the horses (Eohip- pus) ; at a higher level the pseudo tapirs (Systemodon) ; at a still higher level the lophiodonts (Heptodon) ; and then, toward the end of the lower Eocene, the titano- theres {Lambdoiherium) . Figure 45. — Generalized section through Upper Cretaceous and basal and lower Eocene deposits near Pumpkin Buttes, Powder River Valley, AVyo. (No. 12, fig. 3.5) Adapted from C. H. Wegemann (1917.1). Though the results of our observations may be modi- fied by further discoveries the successive rather than simultaneous appearance of these advancing waves of perissodactyl migration is what a study of modern migrations should lead us to expect. All these animals, as shown elsewhere in this monograph, have similar cursorial foot structure, which indicates extensive areas of dry land and open meadow, in which the small, defenseless Herbivora could easily escape the attacks of the Carnivora. Habitat of Wasatch mammals. — The conditions that prevailed in Wasatch time have been determined very interestingly by Loomis in his "Origin of the Wasatch deposits" (1907. 1, pp. 356-364). In adaptation to various habitats the known species of vertebrates are divided as follows: Aerial, 3 per cent; cursorial, terrestrial, and arboreal, 75 per cent; amphibious, 12 per cent; aquatic, 10 per cent. The light-limbed horse, Eohippus, typical of a plains or partly open country, alone makes up 32 per cent of the total collections from the Systemodon zone ("Gray Bull" horizon). All the other odd- toed ungulates are light-limbed, in- cluding the tapiroids {Systemodon), lophiodonts (Heptodon), and primitive titanotheres {LamhdotJierium) , as well as the surviving archaic condylarths (PJienacodus and Edocion). The feet of all these animals indicate dry rather than swampy or forested land, because they are more slender than those of the modern tapir. On the other hand, the coryphodons were certainly marsh dwellers and perhaps in part stream dwellers. The small percentage of species of truly aquatic animals, such as crocodiles, fishes, and turtles, whose remains are mingled with those of the prevailing land animals, probably be- came stranded in lagoons far from the rivers. The presence in the rivers of rather large fishes is shown by the re- mains of the large Clastes. Remains of river-living turtles (Trionyx) have also been found in the Wasatch. LOWER EOCENE FAUNAL ZONES ZONE 6: EOHIPPUS-CORYPHODON ZONE [Second Wasatch life zone. Big Horn B; lower Sparnacian of Europe] Below the EoMppus-CorypTiodon zone in the Clark Fork Basin of Wyo- ming lies the first Wasatch life zone {PJienacodus - Nothodectes - CorypJiodon zone) described on pages 64-66. Near the head of the Big Sand Coulee, on the Clark Fork of the Yellowstone, which adjoins the Big Horn River basin on the west, is a series of about 200 feet of red-banded shales, which overlie the Phenacodus zone ("Clark Fork beds," transition basal Eocene) and contain a mam- malian fauna that is radically different from that of the underlying "Clark Fork." These beds (the "Sand Coulee beds " of Granger) mark the first appear- ance in the Rocky Mountain basin region of four modernized orders of mammals — the lemuroids. ENVIEONMENT OF THE TITANOTHEEES 69 rodents, artiodactyls, and perissodactyls. Of the Perissodactyla only one family occurs, the Equidae, represented by a prhnitive specific form of EoJiippus (E. horealis). There are two or possibly three species of Eohippus in these "Sand Coulee beds," which are not yet separable from the species found in the "Gray Bull" horizon above. Here also occurs Palaeanodon, an ancestral armadillo, which left descendants in lower and middle Eocene time. This lower Eocene horizon, described by Granger (1914.1, p. 205), appears to constitute the beginning of Sparnacian time in the Rocky Mountain region. It contains the oldest known modernized fauna (perisso- dactyls, artiodactyls, rodents, etc.) found in America. The antiquity of these beds is indicated by the last recorded appearance of the primitive order Multi- tuberculata, as represented by remains of Ptilodusf sp. The horizon is also distinguished by the absence of tapirs (Systemodon) . Here occur the first known species of the primitive lemuroid Notharctidae (Pely- codus) and the peculiar ungulate Hyopsodus, now re- garded as a condylarth. No other exposures con- taining this very primitive Wasatch fauna have thus far been discovered. ZONE 7: SYSTEMODON-COEYPHODON-EOHIPPUS ZONE [Third Wasatch life zone, Big Horn C; upper Sparnacian of Europe] The "Gray Bull beds" of Granger (1914.1, pp. 203, 204), in the Big Horn Wasatch, lie at a horizon that is distinguished by the presence of the earliest tapirs — the pseudotapirs (Systemodon). These beds were at first called the Ralston,^ a name that had been pre- occupied. They are exposed principally in the Clark Fork and Big Horn Basins south of the Yellowstone (PI. V, B) and are at least 600 feet thick. They may be correlated with part of the "Almagre" of the Wasatch of New Mexico. As this is the first appearance of the tapirs, and as their remains are mingled with those of horses, this horizon may be known as the Syste- modon-Corypliodon-EoMppus zone. These beds are exposed chiefly along the south side of GreybuU River, where they extend over many miles. From this horizon was made the larger part of Cope's col- lection from the lower Eocene of the Big Horn Basin, including the classic skeleton of Phenacodus primaevus, as well as the skeleton of P. copei '° and that of Eohippus, besides many species of CorypTiodon. One of the most common forms is the pseudotapir >"Ealston" was the name given by Sinclair and Granger (1912) to the Clark Fork beds. "Clark Fork" was substituted by Granger (1914) because "Ralston" had been previously used otherwise. Sinclair and Granger (1912) referred the beds between the "Lysite" and the "Ralston" to the "Knight" formation. Granger (1914) separated the " Knight beds " into two horizons, which he called " Gray Bull " and "Sand Coulee." The "Gray Bull" and the overlying "Lysite" of Buffalo Basin constitute the "Big Horn Wasatch" of Cope and Wortman. The "Gray Bull" is exposed almost entirely in the Big Horn Basin, although a small area of it overlies the " Sand Coulee" beds at the head of Big Sand Coulee in the Clark Fork Basin (Granger, 1919). 1 'The type of Phenacodus wortmani is from Wind River. Cope's reference of the small Big Horn skeleton to this species is not correct. Granger (1915) renamed the skeleton P. copei. Systemodon, which includes the species S. tapirinum, and it is noteworthy that this genus, which is in- directly related to true tapirs, does not appear in the overlying beds. ZONE 8: HEPTODON-COEYPHODON-EOHIPPUS ZONE [?ourth Wasatch life zone. Big Horn D and Wind River A ; lower Ypresian of Europe] To zone 8 belong the "Lysite beds" (PI. V, A) of the Big Horn Basin Wasatch, Wyoming (Big Horn D); the lower level of the Wind River formation (Wind River A) ; a part of the Knight formation of the typical Wasatch group; and parts of the "Almagre" and "Largo" of the New Mexico Wasatch. In this life zone Heptodon takes the place of Systemodon, which disappears or is not thus far recorded. The grace- ful lophiodont Heptodon appears at the very summit of the underlying "Gray Bull beds," is abundant in the "Lysite," and continues into the "Lost Cabin," its presence being one of the means of correlating the fauna of these beds with that of the typical Wasatch group in the Knight formation. This Knight fauna occurs in the CorypJiodon-henrmg layer, which Cope describes as 500 feet above the base of this division of the typical Wasatch of the Evanston region, or about the middle third of the formation according to Granger. The typical Heptodon zone (= "Lysite") of the Wind River beds, 350 feet in thickness, is distinguished by the absence of titanotheres (LamidotJierium, Eotitanops), which are very abundant in the super- imposed "Lost Cabin beds." The "Lysite" or Heptodon zone in the Big Horn Basin is 400 feet thick. Anacodon, one of the arctocyonid creodonts, which has flattened or pavement-like teeth, is characteristic of the Heptodon zone. This zone is faunistically but not lithologically separated from the overlying Lamh- dotJierium zone. ZONE 9: 1AMBD0THEEHTM-E0TITAN0PS-C0RYPH0D0N ZONE [Fifth Wasatch life zone. Big Horn E, Wind River B, and Huerfano A; upper Ypresian of Europe] Geology and fauna. — To zone 9 belong the typical Wind River of Hayden and of Cope in the Wind River Basin, Wyo. ( = the "Lost Cabin" of Granger and Sinclair); the "Lost Cabin" (Granger) of the Big Horn Basin Wasatch; part of the "Largo beds" (Granger) of the San Juan Wasatch of New Mexico; part of the Green River lacustrine formation of Wyoming; and the lower level of the Huerfano for- mation (Hills) of Colorado or Huerfano A. This is the typical Wind River life of all the literature of Cope. (See PI. VI.) The Lamldotherium life zone is distinguished by the arrival in the Rocky Mountain basin region of the first titanotheres, which are abundantly represented in remains of the smaller, cursorial Lambdotherium and the larger, mediportal Eotitanops. It includes the 70 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA FiGUHE 46. — Composite section of the Eocene deposits of tlie Big Horn and Clark Fork Basins, Wyo. This section contains ttie entire Big Horn Basin Wasatch of Cope's descriptions, which is now divided into very clearly defined ascending life zones, as follows: 5, Phenacodus-Nothoiectes-Coryphodon zone; 6, EoMppus- Corijphodm zone; 7. Systemodon-Corypho- don- Eohippus zone; 8, Heplodon- Coryphadon-Eohippus zone; 9, Lambdotliermm-Eotitanops-Coryphodon zone. A few characteristic species of mammals from each horizon are indicated in the right-hand column. Chiefly after Granger (1918). ENVIEONMENT OF THE TITANOTHERES 71 last surviving species of Coryphodon and of the con- dylarth Phenacodus among the archaic ungulates. The presence of the condylarth Meniscotherium serves to correlate the Wind River with the upper levels ("Largo beds") of the Wasatch of New Mexico. While the Wind River life on the whole represents a continuation of that of the preceding stages of the Wasatch, with which it possesses several genera and eleven species in common, it also includes nine new genera that survive in the Bridger formation of middle Eocene time. The Wind River marks the end of the lower Eocene, the last period of certain highly distinctive lower Eocene forms like Cory- phodon, but it is also prophetic of the middle Eocene in the presence of lemuroids like Notharctus, Anapto- somewhat like a slender, diminutive tapir in body proportions. In skull structure and dentition Eoti- tanops foreshadows the true titanotheres of the middle Eocene; its feet are more slender than those of its successors, and it was doubtless a more agile animal. The special life conditions surrounding these early titanotheres are more fully set forth in the descriptions of the Wind River titanotheres in Chapter V, section 3. Olimate and physiography during the deposition of the Wind River and Green River sediments. — For Wind River life in general the reader is referred to section 3 of this chapter. Here we may speak of the whole basin region. While fluviatile and flood-plain sediments were being deposited in the Wind River Basin of northern f!t» - ""."-i**^*, Figure 47. — A typical "Lost Cabin" locality, on the north side of AlkaU Creek about 8 miles east of Lost Cabin, Wind River Basin, Wyo. Lambdotherium-Eotitanops- Coryphodon zone (Wind River B). A characteristic view of tlie red-banded beds that have yielded the greater part of the fauna of the Lambdotherittm zone. (Compare PI. VI, B.) After Granger (1910.1), Am. Mus. negative 17792. morphus, and Shoshonius; of true doglike or civet-like carnivores like Viverravus and Vulpavus; or of rodents like Sciuravus and Par amy s. Remains of Equidae are rather rare and are represented by several species of Eohippus, of which E. venticolus is the most pro- gressive, and those of titanotheres, especially Lamb- dotherium, are very abundant. Lambdotherium, one of the earliest titanotheres, was a small, light-limbed form, about the size of a coyote {Canis latrans). It represents a distinct cursorial side branch of the titanothere family, re- sembling the contemporary horses and lophiodonts in its light limb and foot structure. Eotitanops ("the dawn titanothere") was a true and very primi- tive titanothere about the size of a sheep {Ovis aries), Wyoming there lay to the south a large, shallow lake, covering about 5,000 square miles, in which were deposited 800 feet of impure limestone at the base, followed by about 1,200 feet of thin, fissile calcareous shale. (King, 1878.1, p. 381.) The deposition of these lake sediments (Green River) began near the end of Wasatch time. They contain abundant and well-preserved remains of insects and fishes. The presence of sting-rays and other fishes of marine or coastal type indicates that these originally marine forms had become landlocked, as did the existing marine survivors in the Caspian Sea and Lake Titicaca. Many of the fishes of the Green River shales are related to forms now found chiefly in the southern continents, especially South America. 72 TITA.NOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA Green River forests. — Our present knowledge of the Green River flora, which, according to Berry (1914.1, p. 164) was mid-Eocene, indicates a considerably warmer climate than that of the basal Eocene Fort forests differed from the tropical forests of the Georgia coast in the presence of genera Hke Bex, Juglans, Myrica, Planera, Quercus, Rhus, Salix, and Zizyphus, most of which are temperate types. Thus the Green MESOZOIC and PALEOZOIC Pe/ycoc/us Sh osh on fas /^bsaroh/L/S ' Didyrnic^iS \//verr3\/us Miacis Vu/pavus Pafripfe/is Pro/i'mnocyon S/nopa Trif-emnoc/on ^Hapa/odecfes CynodonfomyS Diacodon Pariofops D/de/pnodaS ,Pa/seosinopa {Esfhonyx ' Paramys Mysops ? Palaeanodon 5/y/inodon Phenacodus Pcfoconus Menisco^her/um Pyopsodus Coryphodon Baihyops/s Eoffranops Lambdoiher/um Hyrachyus Hep^odon fiobippus [O'Scodex/s Pe/ycodus Omomys Te fortius Abs3ro/ Caenopus Ischyromys Poebro therium Menodus he/oceres ?I)ipioicodo7v zoTze^ Amynodon ? anficjuus Protoreodon Camelodon ^ Pro tifan other/ um Zamhdotherhwv zone Lambdofherium Coryphodon, Phenacoo/us_, Hepfodon, Eohippus Figure 52.— Section of exposures from lower Eocene to lower Oligocene at Green Cove, on Beaver Divide, at the southwestern border of the Wind River Basin, Fremont County, Wyo. Includes deposits in Wind River, Bridger (?), Uinta (?) and White River time. Chiefly after Granger (1910.1). This is a most significant section, for the base ot the TiimolUnum zone (Chadron A) uncontormably overlies beds originally referred to Uinta C 1 (Diplacodon zone). ENVIEONMENT OF THE TITANOTHEEES 77 Wasatch Heptodon-Coryphodon-EoMppus zone through the Wind River Lamhdotherium-Eotitanops-CorypTiodon zone upward into the Oreodon zone of Ohgocene time. This is the only undoubted Eocene-Oligocene sedi- ment thus far determined in the Rocky Mountain basin region. Its total thiclmess is 1,080 feet, and it represents relatively slow sedimentation. There is a single period of erosional unconformity at the end of the upper Eocene. The life of the Wind River beds of this section is distinctly of upper Wind River ("Lost Cabin") time, corresponding with Wind River B and Huerfano A, for it includes the titanothere Lambdotherium popo- agicum, a CorypTiodon, two species of Equidae {Eohippus craspedotus and E. venticolus), and two species of Heptodon (H. calciculus and H. ventorum), which are characteristic of closing Wasatch time. The presence of remains of garpikes and crocodiles in this fauna shows that these deposits were fiuviatile and indicates that Wind River shales were of flood-plain origin, though they include many channel fillings of coarse arkose. We thus have glimpses of a faunistic period broadly corresponding with the lower Ypresian of France, cer- tainly extending from Wyoming to Colorado, and probably spreading much more widely in the Rocky Mountain and the adjacent Plains region. Though it includes surviving members of the older Wasatch life and incoming members of the succeeding Bridger life, the Wind River and Huerfano life stands directly intermediate between these; in fact, the representa- tives of archaic families destined to become extinct and those of modernized families destined to populate the earth are very nearly balanced, including 21 genera (30 species) of archaic mammals and 22 genera (36 species) of modernized mammals. Simultaneously with the decline of the coryphodons the uintatheres reappeared in the genus Bathyopsis, ancestral to the giant Uintatherium, which character- izes Bridger C and D. THIRD FAUNAI PHASE (MIDDLE AND UPPER EOCENE) CORRELATION OF AMERICAN ZONES WITH EUROPEAN STAGES There is strong evidence of uniform and favorable en- vironment and persistent evolution throughout middle and upper Eocene time in the Rocky Mountain basin region. The changes show progressive modification and adaptation rather than breaks by migration or extinction. Both the archaic and the modernized families increased in size and variety. The surviving archaic mammals appear to have flourished and in- creased, especially in size and muscular power. Near the very end of Eocene time only two new famihes of quadrupeds appear, the ancestral camels (Camelidae) and the oreodonts (Oreodontidae), whereas in western Europe new families repeatedly appear from the south. east, and north. The general correlation of the Euro- pean stages and the American zones is given on page 78. 78 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBEASKA. Correlation of middle Eocene and upper Eocene American life zones and European stages Epoch American life zone Approximate European stage 15. Diplacodon-Protitanotherium-Epihippus zone (Uinta C). Ludian. Upper Eocene. 14. Eobasileus-Dolichorhinus zone (Washakie B 2, Uinta B 2). 13. Metarhinus zone (Washakie B 1, Uinta B 1). Bartonian. 12. Uintatherium-Manteoceras-Mesatirhinus zone (Washakie A, Bridger C and D). Lutetian. Middle Eocene. I 11. Palaeosyops paludosus-Orohippus zone (Bridger B). i 10. Eometarhinus-Trogosus-Palaeosyops fontinalis zone (Bridger A, Huerfano B) . TYPICAL BBIDGER FORMATION; MIDDLE EOCENE (LUTETIAN AND BARTONIAN OF EUROPE) Character of sediments. — The Bridger formation, the most important and the most thoroughly explored of the Eocene Tertiary, occupies a great area north of ditions. The Bridger formation attains its maximum thickness of 1,875 feet near the Uinta Mountains and thins out northward. Beyond the margins of the Green River lacustrine deposits the Bridger overlies upper members of the Wasatch group. Figure 54. — Map showing the Eocene sediments encircling the Uinta Mountains of southwestern Wyoming and northern Utah Modified after King (1876.1). U, Uinta Basin, typical Uinta formation of King and Marsh. (The area mapped includes older and possibly younger rocks than the true Uinta formation— Z)/piacoion zone.) B, Bridger Basin, typical Bridger formation of Hayden. WK, Washakie Basin, typical " Washakie" formation of Hayden. G, Green River formation. W, Typical Wasatch group of Hayden. X, Type locality of Coryphodon and associated Wasatch fossils. C, Cretaceous. the Uinta Mountains and east of the Wasatch. Unlike the Wasatch, the lower Bridger (horizon B) is unique; no contemporaneous fossiliferous deposition is known. At the base its sediments pass gently into the Green River shales, and the lower levels of Bridger A show gradual transition from lacustrine to flood-plain con- Unlike the lower Eocene Wasatch and Wind River sediments the Bridger is not composed chiefly of material derived by erosion from the adjacent moim- tains (Sinclair, 1906.1, p. 278) but consists of great series of deposits of volcanic ash and dust, solidified into tuffs, which weather into picturesque and in ENVIRONMENT OF THE TITANOTHEHES 79 places highly colored "badlands." Apparently the greater part if not all of these tuffs were distributed from unlinown eruptive volcanic centers by wind; but at four periods they were deposited in great shallow playa lakes and partly worked over by stream, delta, and flood-plain deposition. From the general absence of coarse materials such as would be transported by streams of high gradient, it is inferred that the Bridger formation accumulated in a relatively level area. (Sinclair, 1906.1, p. 279.) Exploration of the Bridger formation. — The Bridger formation has been explored almost continuously by geologists and paleontologists, first by Hayden (1869- 1871), next by King (1878), who regarded the Bridger as an ancient lake basin deposit, then by Osborn and Scott (1877-1878), and again by Endlich (1879). In 1902 the American Museum parties, guided by Matthew and Granger, under the direction of Osborn, undertook to determine whether the Bridger can be divided into a series of life zones. After four years of careful geologic field work by Granger and Matthew (1902-1905), who had at hand the level record of every specimen, the Bridger was subdivided lithologically and faunistically into five levels, A to E. Bridger A is relatively barren. Of these levels A and B were grouped into the lower Bridger (Palaeosyops paludosus- OroMppus zone), characterized by the absence of Uintatherimn, and C and D, the upper Bridger {Uintatherium- Manteoceras- Mesatirhinus zone), distin- guished by the appearance and great abundance of TJintaiherium. Similar faunistic surveys in the Washakie Basin, east of the Bridger Basin, and in the Uinta Basin, south of the Uinta Mountains, have given very complete correlation of the local subdivisions of the section as follows: Correlation of middle and upper (?) Eocene sections of the Uinta, WashaTcie, and Bridger Basins Uinta Basin Washakie Basin Bridger Basin Life zones Uinta C. Absent. Absent. Theoretic zone (No. 16); fauna unknown. 15. Diplacodon-Protitanotherium-Epihippus. Uinta B. Washakie B. Bridger E (barren beds). 14. Eobasileus-Dolichorhinus. 13. Mctarhinus. Uinta A (barren). Washakie A. Bridger D. Bridger C. 12. Uintatherium-Manteoceras-Mesatirhinus. Barren beds. Barren beds. Bridger B. 11. Palaeosyops paludosus-Orohippus. Bridger A. 10. Eometarhinus-Trogosus-Palaeosyops fontinalis. Volcanic ash deposits. — The petrographic analysis of the rocks of the Bridger formation serves to support their correlation with the deposits of the Washakie Basin, to the east, and of the Uinta Basin, to the south. The recognition by Sinclair (1906.1, pp. 273-280) of the fact that the entire Bridger series was in large part originally volcanic dust and the later careful petro- graphic analysis by Johannsen (1914.1) led to the conclusion that the Bridger rocks are largely tuffs perhaps modified in part by sufficient transportation to add the numerous grains of quartz they contain, and that these grains may be of sedimentary origin although the material of the tuffs is mostly andesitic. Johannsen's analysis of the Bridger rocks is essentially as follows: Bridger D. Irregular grains of quartzite, feldspar, hornblende, etc. : dacite tuff. Bridger C. Fragments of quartz and hornblende; groundmass of glass tuff. Bridger B. Smith's Fork; fragments of quartz, feldspar, horn- blende: ?dacite tuff. Bridger B. Church Buttes; fragments of quartz, feldspar, etc.: altered tuff, probably dacite tuff. Bridger A. North of Church Buttes, fragments of quartz, feld- spar, hornblende. No glass tuff seen. Thus the Bridger is composed chiefly of dacite tuff, of altered dacite, and of glass tuff containing irregular grains of quartz, feldspar, and hornblende, which are at some places contained in a groundmass made up of entirely coarse angular particles of stringy glass full of bubbles. The Huerfano formation of Colorado, which is in large part older than the Bridger, is com- posed largely of glass tuff. The deposits in the Washakie Basin, east of the Bridger Basin, are com- posed chiefly of dacite and glass tuffs. Playa lalce deposits. — Conspicuous features of the Bridger formation are four hard "white layers,'' which were laid down at intervals in the series of beds. 80 TITANOTHEBES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Some of these "white layers" have been traced over many square miles. They are composed of tuffaceous shale and marl or of calcareous shale and are in places filled with fresh-water shells. They mark periods during which the deposition of volcanic dust was less rapid, when the Bridger Basin was temporarily base- leveled and the waters rose into wide, shallow playa lakes, in which sedimentation was slow. That these four relatively thin "white layers," which are vari- FlGURE -Geologic section of the entire Bridger formation in the Bridger Basin, Wj'o. Shows the division by the four chief "white layers" and the main divisions by three principal zones — A, Palaeosyops fontinalis zone; B, Palaeosyops paludosus-Orohippus zone; C and Utniatherium zone. ously known geographically as the "Cottonwood white layer," the "Burnt Fork white layer," the "Lone Tree white layer," and the "upper white layer," correspond with long periods of geologic time is shown by the marked faunal differences that separate them, which indicate that extensive migration occurred before and after the deposition of each of these layers, but especially the first, which separates the lower from the upper Bridger life zone. Life environment in Bridger time. — From observa- tions made by Hay (1905.1, pp. 327-329) while he was collecting fossil turtles in the Bridger in 1903, he con- cluded that the Bridger deposits were almost solely the result of fluviatile and flood-plain action, that this basin was a nearly level country, which was probably covered with vegetation and well forested. The dis- tribution of fossil remains in all parts of the Bridger area indicates that the animals lived near the places where they became buried and that they were chiefly such as may inhabit well-wooded regions. The river- channel beds, which are composed of coarse ma- terials, show that streams with rapid currents traversed the basin. These streams were bor- dered by swamps in which were formed beds of impure lignite, or by fresh-water bays in which the shells of fresh-water mussels accumulated. The finer deposits indicate shallow, muddy bays, in which the remains of the larger quad- rupeds are occasionally found in positions indicating that they had been mired in a standing posture. The old stream channels have yielded remains of several species of bowfins (Amiidae), garpikes (Lepidosteus) , and siluroids. Crocodiles were numerous and diversified. The reptiles suggest that the climate was Floridian, or south temperate, and we may picture a partly open, partly forested country, somewhat similar to the existing bayou region of the Mississippi Delta of Louisiana. Analysis of the Testudinata by Hay (1908.1) has also afforded a clear idea of the physiographic conditions in Bridger time. The soft-shelled river turtles (Trionychoidea) were represented by at least 25 species, and there are now in the world only 26; the Bridger rivers and brooks fairly swarmed with these creatures, some of them equal in size to the largest existing Asiatic species. There are indications of 4 species of the family Emydidae (order Cryptodira), as compared with the 12 species now living in the Missis- sippi Valley. The genus Baptemys, of the same order, has its nearest relatives at present in Central America, and a third genus (Anosteira) is reported by Lydekker in the upper Eocene of England. The presence of : and D, extcusive stretches of land is indicated by the true land tortoises (Testudinidae) of the genus Hadrianus, including giant tortoises nearly 3 feet long, which probably lived on dry lands bordering the sluggish Bridger streams. The ancient Lower Cretaceous order Amphichelydia is also represented here by four species belonging to two genera. The environmental adaptations of the animals of the Bridger Basin were classified by Matthew (1901.1, pp. 309, 310) as follow?: Land animals: 1. Aerial: Remains of birds rare and fragmentary, as in nearly all geologic formations. ENVIHONMENT OF THE TITANOTHEEES 81 2. Arboreal: Primates, many Carnivora, and some Insec- tivora and Rodentia. Out of 1,007 specimens, belong- ing to 46 genera, 13 genera (184 specimens) were certainly arboreal and 11 genera (485 specimens) were probably arboreal. 3. Terrestrial (cursorial and ambulatory) : Some of the carnivores and all the ungulates (17 genera, 314 specimens). Also some lizards and chelonians. 4. Fossorial: Certainly fossorial, 3 genera (S specimens). Some of the insectivores may also have been fossorial. 5. Amphibious: One insectivore (Pantolestes) (1 genus). Probably certain carnivores. Water animals: 6. Fresh- water: Numerous crocodiles, aquatic turtles, fish, and fresh-water mollusks. 7. Marine: No marine animals. Contrast this lack of types with the types of fish in the preceding Green River formation. The Bridger life thus included many arboreal, terrestrial, and aquatic forms, the last mostly reptiles, fishes, and invertebrates. The slow-moving, ambula- tory quadrupeds form a relatively large proportion of the mammals, but the cursorial types, such as the Equidae (Orohippus), are relatively rare; also the fos- sorial types. The Bridger life seems to be that of a partly forested flood plain. The remains of large mammals are so numerous as to indicate abundant open, gladed areas, comparable to the partly forested and partly open delta regions along certain rivers of modern time. The foot structure of the Bridger quadrupeds gives less certain evidence of an open plains country, favorable to cursorial types, than that of the Wasatch (lower Eocene) quadrupeds of the same region. No impressions of leaves from the Bridger forests have been discovered. It is probable that the forests of Green River type, described on pages 72-73, per- sisted into Bridger time and that the climate then was warm-temperate, almost subtropical. The faunal history of the Bridger as a whole shows a gradual reduction in the number of archaic mammals of Mesozoic stock and a steady increase in the number of their competitors among the modernized mammals, the numerical relations between these two groups in upper Bridger time being as follows: Genera Species Archaic mammals 15 35 Modernized mammals 57 146 Duration oj the Bridger epoch. — Matthew (1909.1), following the earlier geologists, believes that the lacustrine conditions in Green River time arose from the uplift of the Uinta Mountain range, which blocked the basin and caused the formation of the great lake in which the material that formed Green River shale was laid down. As the river gradually cut its way through the east end of the Uinta Range the lake gave way to the broad Bridger flood plain, in which was deposited the volcanic ash washed down from the slopes of the Uinta Mountains to the south, and the deposit was worked over and sorted by the streams that flowed across the plain. The Bridger Basin was subject to intermittent overflow, which gave rise to large but shallow lakes of clear water. If we should assume that the Bridger formation occupied one- tenth of total estimated Eocene time — 90,000 to 100,000 years — the fossiliferous beds, which are 1,100 feet thick, have accumulated at an average rate of 12 inches per century. This estimate would allow 110,000 years for the deposition of the Bridger forma- tion exclusive of the "white layers " formed at intervals when deposition was arrested. The titanothere re- mains of the Bridger indicate a long period of evolu- tion, but not so long as that of the Chadron (lower Oligocene). Chief localities and exposures of the Bridger formation in ike Bridger Basin Bridger E: Uppermost exposures: Sage Creek Mountain. Henrys Fork Table. Twin Buttes. Bridger D: Upper exposures: Level Twin Buttes D 1-5 Spanish John's Meadow D 1-5 Cat Tail Spring D 1-5 Henrys Fork, Burnt Fork post office D 1-5 Henrys Fork, Lone Tree post office D 1-5 Summers Dry Creek D 1-5 Henrys Fork Hill D 1-5 Sage Creek Spring D 1-5 Lane Meadow D 1-5 Bridger C: Lower exposures: Henrys Fork, Lone Tree post office C 4-5 Lane Meadow C 3-5 Spanish John's Meadow C 3-5 Henrys Fork Hill C 3-5 Twin Buttes C 1-5 Dry Creek C 1-5 Henrys Fork, Burnt Fork post office C 1-5 Church Buttes, third bench C 1-3 Bridger B: Upper exposures : Cottonwood Creek Typical B 4^5 Millers ville, 6 miles southeast of B 4-5 Cottonwood Creek, middle of B3 Grizzly Buttes B 3 Church Buttes B 2-3 Lower exposures: Cottonwood Creek B2 Grizzly Buttes Typical B 2 Exposure B, 5 miles south of Granger B 2 Millersville B 1-2 Cottonwood Corral, Blacks Fork ,. B 1-3 Exposure A, 5 miles south of Granger B 1 Church Buttes B 1 Bridger A: Hams Fork Bluff; Granger to Opal, 25 miles. Mouth of Big Sandy Creek {IPalaeosyops -fontinalis zone). Big Muddy exposures between Carter and Granger. Blacks Fork Bluffs, east of Granger. 82 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA ZONE 10: EOMETAEHINUS-TEOGOS0S-PALAEOSYOPS FONTINAIIS ZONE [Bridger A and Huerfano B ; lower Lutetian of Europe] The lower Bridger (Bridger A and B) corresponds with the "calcaire grossier superieur" of the Paris Basin. The correlation of Bridger A with Huerfano of some 200 feet of calcareous shale alternating with tuff (Matthew, 1909.1), which are exposed principally around the eastern, northern, and western margins of the Bridger Basin. It is therefore supposed that Bridger A, which passes down into the lacustrine Green River shales, is partly of lacustrine, partly of Figure 56. — Map of the Bridger Basin, Wyo. (No. 8, fig. 35) Showing the principal topographic features, Twin Buttes and Henrys Fork Table, and a diagrammatic section of the Bridger formation (A, B, C, D, and E) capped by the Bishop {"Wyoming") conglomerate (W). After Matthew and Granger, 1902, 1909. B has recently been established through the discovery in each of the mammalian species Palaeosyops (Lim- nohyops) fontinalis Cope. (Osborn, 1919.494.) In these beds vertebrate fossils are rare and include, besides the titanothere above mentioned, remains of crocodiles, turtles, and fishes only. Bridger A consists fluviatile origin and is transitional both geologically and in its fauna between Green River (upper Wind River) and Bridger B time. Sinclair describes this horizon as consisting of "buff and pale-green tuffaceous shales and sandstones, often containing in enormous numbers shells of Paludina and Unio." ENVIRONMENT OF THE TITANOTHEEES 83 z LJ O O liJ U _J Q Q no q: DQ B3 Pa/aeosyops majo/; neofype {?)Limnohyops laevidens, type Palaeosypps palicdosits- Pa/aeosyops major, ref. Limnohyops monoconus, type L/mnohyops matfhewi, type L/mnohyops priscus, type Limnohyops monoconus OroTiippus typicus GRIZZLY BUTTES FAUNA Notharctus Harpagolestes Hyradiyus agrarizis MetadieiroTnys dasypus Orohippus atavus TUlothjeriLawfodiens A5 ,'' Pa/aeosyops fonf/na/is cr Uo q: CD NO MAMMALS oo cc ^ LL t_> u bj F UJ > < a ^ 5 o '^ e Figure 57. — Section of the lower part of the Bridger formation in the Bridger Basin, Wyo. (No. 8, fig. 35), showing the succession of the species of titanotheres and other mammals The section is 650 feet thick. The principal geologic features are represented in the center. After the studies of Osborn, Granger, and Matthew. 84 TITANOTHEBES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA ZONE 11: PAIEOSYOPS PALUDOSUS-OEOHIPPUS ZONE [Bridger B; upper Intetian of Europe] The richly fossiliferous deposits belonging to the Paleosyops paludosus-OroMppus zone (Bridger B) are exposed chiefly in the northern area of the Bridger formation, near Fort Bridger, along Blacks Fork and its tributaries. They represent a very long period and consist of 450 feet of tuffs and sandstones (fig. 4) divided into two principal escarpments, which are separated by Cottonwood Creek valley. In this zone the titanotheres and other mammals undergo notable progressive evolution, and there is a marked succession of species. (See fig. 57.) The succession of the species of titanotheres in Bridger B, in descending geologic order, is as follows: Limnohyops monoconus Os- born, type. Limnohyops matthewi Osborn, type. Palaeosyops paludosus Leidy. Palaeosyops paludosus, re- ferred. Palaeosyops paludosus, type. ?Mesatirhmus Junius Leidy. Palaeosyops major Leidy, hy- potype. Limnohyops laevidens Cope, type. Palaeosyops major Leidy, re- ferred. Limnohyops monoconus Os- born, referred. The species of titanotheres found in Bridger B belong exclusively to the subfamily Palaeosyopinae and represent the two generic branches Palaeosyops and LimnoTiyops, closely related animals with broad spreading feet and heavy limbs, slow in gait. The reference to Mesatirhinus of the species P. Junius Leidy is somewhat doubtful. The lower half of Bridger B at Grizzly Buttes (PI. VII, B) , an escarp- ment along Smiths Fork, is by far the richest collect- ing ground in the Bridger Basin; thousands of speci- mens have been taken from it, including many more or less complete skulls and skeletons, all recorded from Bridger B 2. Beds at a slightly higher level, in Bridger B 2 and in Bridger B 3, on the escarpment along Cottonwood Creek, have yielded a number of complete skeletons, including those of several species of Equidae (OroTiippus), a variety of catlike and dog- like creodonts {Limnocyon, Harpagolestes), abundant small civet-like creodonts (Viverravus, Sinopa), an- cestral canids (Miacis, Uintacyon), a surviving (?) condylarth (Hyopsodus) ; also ancestral Edentata {Metacheiromys, armadillo-like) and the rodent-like tillodonts {Tilloiherium fodiens, Trogosus). They have also yielded many rodents (Paramys, Sciuravus), as well as a rich primate fauna of lemuroids (Noth- arctus). The entire fauna has been very carefully reviewed and analyzed by Matthew (1909.1, pp. 298-302). Rich as is the fossil life of the lower Bridger, many mammalian subfamilies and many genera and species are lacking which occur abundantly in the upper Bridger. Noticeable is the absence of uintatheres (Uintatherium) and of three important genera of ti- tanotheres {Manteoceras, Telmatherium, Mesatirhinus), which appear abundantly in the upper Bridger. The "Cottonwood Creek white layer," marking the summit of Bridger B, indicates a long period of shallow lake flooding of the Bridger Basin during which the large amblypod uintatheres and the more advanced titanotheres entered the basin. Vintaihe- rium is not found in Bridger B, but it occurs at the very base of Bridger C, the lowest level of the upper Bridger. ZONE 12: UINTATHERIUM-MANTEOCERAS-MESATniHINUS ZONE [Bridger C and D, Wasliakie A, and Uinta A; part of Bartonian of Europe] The fauna of zone 12 in the Bridger Basin, which includes deposits 725 feet thick (Bridger D, 375 feet; Bridger C, 350 feet; see fig. 58), may be clearly dis- tinguished from that of zone 1 1 (lower Bridger = Bridger B and A) by its content of the remains of the animals listed below: Titanotheres : Palaeosyops robustus Leidy. Palaeosyops copei, type. ?Telmatherium validum, type. Manteoceras manteoceras. Mesatirhinus petersoni, type. Palaeosyops leidyi, type. Limnohyops laticeps, type. Mesatirhinus megarhinus, type. ?Telmatherium cultridens. Other mammals : Hyrachyus princeps (cursorial rhinoceros). Patriofelis ferox (catlike creodont). Isectolophus latidens (tapir). Uintatherium robustum (four-horned amblj-pod) . Notharctus crassus (large lemuroid). Pantolestes natans (aquatic insectivore) . Homacodon vagans (primitive artiodactyl) . LTintatherium mirabile (amblj'pod uintathere). Orohippus sylvaticus (primitive equine). Bridger C. — The lowest beds of the horizon Icnown as Bridger C are exposed at the foot of Sage Creek Mountain, along the southern slope of Henrys Fork Table; also at the foot of Twin Buttes and along the slopes north of Twin Buttes. They consist of 350 feet of gray and greenish-gray tuffs, divided into a lower and an upper half by the "Burnt Fork white layer" and bounded above by the "Lone Tree white layer." After careful analysis of the fauna of Bridger C, Mat- thew concluded (1909.1, p. 304) that its marked dis- tinction from the fauna of Bridger B was due to the immigration of several new genera into the Bridger Basin. Among these especially are the titanothere genera Manteoceras, TelmatJieriurn, and MesatirJiinus, which appear to be really newcomers and not in any sense descendants of the lower Bridger genera Palaeo- syops and Limnohyops. The two genera last named, however, are represented in Bridger C by distinctly new specific forms, much more progressive than those in Bridger B. Thus Bridger C is characterized both by marked evolutionary changes in mammals that pass over from the lower levels and by the introduction of a fauna that is more or less new. Of this new fauna BNVIKONMENT OP THE TITANOTHERES 85 TJintaiherium is closely related to the ancestral BatJiyop- sis, which is found in the Wind River Lambdotherium zone and in the long antecedent first Wasatch zone. The pseudotapir Isedoloiyhus is related in tooth struc- ture to Systemodon, which is characteristic of the third Wasatch zone. We are therefore disposed to regard the life of the upper Bridger ( Uintatherium) zone as the result of a local immigration from the adjacent Rocky Mountain or Plains region into the Bridger Basin, and not as the result of a continental immigra- tion such as is made manifest in the lower Eocene. Bridger D. — Upon the "Lone Tree white layer" lie the 375 feet of strata that form Bridger D, in which are found five faunistic levels, D 1 to D 5. The fos- siliferous sediments of this closing period of the Bridger consist of 350 feet of "gray and greenish-gray sandy and clayey tuffs, with one or more ash beds," including the upper "white layer," which lies about 75 feet below the top of the formation. Among the tita- notheres of this zone are descendants of species of Palaeosyops, Limnohyops, Manteoceras, and Telma- therium, which continue to increase in size and which represent advancing mutations that are exhibited in the comparative measurements shown in the tables on pages 304, 313, 341, 364. It is noteworthy that there is no very marked faunistic change in the species of titanotheres that persisted from Bridger C to Bridger D. For example, Manteoceras manteoceras persists from the lower to the higher levels, and Mesatirhinus peter soni is recorded in both C 2 and D 3. Exceptions to this slow evolution are seen in two species — Palaeosyops copei, which represents in certain characters an ad- vanced stage of evolution allied to a stage found in the lower sediments of the Washakie Basin, and Telma- therium validum, assigned to Bridger D, which shows a distinct advance upon Telmaiherium cultridens, as- signed to Bridger C 5. Bridger E. — Bridger E is theoretically correlated with Washakie B and Uinta B (upper Eocene). The topmost beds of the Bridger formation, 500 feet thick, include sediments that are almost barren of fossils, but the few fragments of mammals they have yielded are of undoubted Bridger age. The 500 feet of soft banded tuff containing at intervals thick layers of volcanic ash indicate increasingly active volcanism. The layers of gypsum found at this horizon were probably deposited in playa lakes (Sinclair, 1906.1), like those in the Humboldt Basin of the present time. The dark-red bands in Bridger E may indicate an arid climate. The correlation of Bridger E with Washakie B, to the east, is purely conjectural, for neither con- tains determinable remains of mammals. Matthew (1909.1, p. 306) attributes the paucity of life in this zone to violent volcanic eruptions, observing that the thick and generally unsorted beds of ash indicate great volcanic activity and that the large amount of gyp- sum and the absence of fossils might be due to the consequent destruction of vegetal and animal life, which converted the region into a barren plain that was alternately submerged and desiccated. The UintatJierium zone in the Washakie Basin (Washakie A) is described on pages 85, 87, in the description of the deposits of that basin. The barren deposits in the Uinta Basin (Uinta A) that correspond to the Uintatherium zone are described on pages 91-92, in the description of the Uinta Basin. WASHAKIE BASIN, WYO. STRATIGRAPHY OF THE BASIW Deposits and faunal zones. — The Washakie Basin lies about 50 miles east of the Bridger Basin, and the two contain similar volcanic sediments. The basin was described by Hayden in 1869-70 (1871.2, p. 73), and more fully by Cope in 1873 (1873.4). Its fau- nistic levels were studied by the Princeton expedition (Osborn and McMaster, 1881.8) and by expeditions of the American Museum of Natural History, under Wortman (1893, 1895) and Granger (1906). Granger (1909.1, pp. 13-32) gave the first complete and accu- rate description of the geology of the Washakie Basin in his "Faujial horizons of the Washakie formation of southern Wyoming" (1909.1, pp. 13-32). King treated the deposits of the Washakie Basin as of Bridger age and of lacustrine origin. Osborn (Osborn and McMaster, 1881.8) favored the theory of separate deposition, and Scott (1899.1) showed that where the fauna of the Washakie Basin departs from that of the Bridger it approaches that of the Uinta. The dis- covery of the true upper Bridger fauna in horizon A of the Washakie Basin was due to the American Museum expeditions of 1893, 1895, under Wortman. The Washakie Basin, with its vivid coloring and its alternation of hard and soft layers of tttft' and sand- stone, affords the most picturesque geologic views to be found in the Rocky Mountain Eocene basins. Haystack Mountain ("Mammoth Buttes" of Cope), a long ridge of badlands near the north end of the basin, which in places rises 400 feet above the plain, forms the northern border of an extensive semicircular "central basin" that has the appearance of a gigantic crater. The floor of this basin is rather level and regular, being broken only by a few low tables and buttes, which have long been preserved from erosion by their capping of hard sandstone, though their sides are trenched by innumerable deep, vertical- walled canyons, which present a great variety of architectural forms that are illuminated by brilliant coloring. Washalcie A {TJintaiherium zone, middle Eocene). — ' The "lower brown sandstone" of the Washakie Basin, known as Washakie A (fig. 60), contains the fauna of the Uintatherium- Manteoceras- Mesatirhinus zone. It was deposited contemporaneously with the upper Bridger (Bridger D), to the west, and probably with the non- 86 TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 3 lIj u z UJ u O Ld U -J Q Q D3 TITANOTHERES GEOLOGIC SECTION OTHER MAMMALS Pa/aeosyops robustus Mesai/rh/nusjan/us Pa/aeosyops robusius Pa/aeosyops cope/, type " robustus Mesat/r/j/nuspetersor?/, type Pa/aeosyops /e/dy/ Manteoceras manteoceras Limnohyops /at/ceps Pa/aeosyops /e/'c/yJ No t/i a/ 'c/./JS CI -assi fs Pairiofclis fcro.x- Z//'/ilcU/ior/u/n leic(ya/iurrh (?JJsec/o/op/ias latldens ~~ Uintat/ieriL/yn. HENRYS FORK H/LL L ONE TREE WH/TE Z A YER UirifafJteri'iim att/rrps '^'Elac/i oce? -as "par vu/ri C5 fPj Pa/aeosyops /e/oy/ 'Je/mather/umcu/ir/dens,type /^esat/rh/nus megarb 's xoTie -- Pa/aeosyops /e/dyi, type CD C3 C2 BURNT FORK WH /TE LAYER ? Limnohyops /at/ceps,type ^L,^,j^j_ i— j_j.^ ' Te/matber/um cu/tr/c/ens /^esatirb/nus peterson/ Pa/aeosyops granger/.tjpe ?/i^anteoceras manteoceras Homacodon vaf/ans Jfyraehyus inipe/ia/L-i Oro/i/ppiis sy/vat/C7/s COTTONWOOD W/i/TE LAYER BENCH UintaiJi eriurn ^^ B5 _SANO^.O.fii£ Palaeosyops paZudosits- Ir^roTiippiis :zoTie \ ^ ' / COTTONWOOD CREEK BENCH Figure 58.— Section of the upper part of the Bridger formation in the Bridger Basin, Wyo. Shows the vertical distribution of the titanothere species on the left, the principal geologic features in the center, and the distribution of the other species of mammals on the right. Principally after Osborn, Granger, and Matthew. ENVIRONMENT OP THE TITANOTHERES 87 fossiliferous Uinta A, to the south. Its contempo- raneity with Bridger D is established through the common presence of the following species: Uintatherium robustum. Uintatherium mirabile. Manteoceras manteoceras. Notharctus tenebrosus Leidy. Hyrachyus princeps Leidy. Sinopa. Palaeosyops copei Osborn. Mesatirhinus megarhinus. Mesatirhinus petersoni. Hyopsodus. Paramys. I bench, which constitutes the lower rim of the basin I on its northern border. This "lower brown sand- I stone" passes at a low angle southward beneath the floor of the basin. Below it, and apparently conform- able with it, lie gray sandy shales, which are pro- visionally referred to the Green River but which were probably laid down in lower Bridger time (Bridger A and perhaps Bridger B). As these deposits I show no marked evidence of erosion it seems probable fShale? Nos. 35,36,37 [Sandstone Nos. 25, 26a, 26 b Nos. 20,21, 22 STACK MT^ LOWER BROV Figure 59. — Diagrammatic vertical section of deposits near Barrel Springs, Washakie Basin, southern Wyoming Shows the alternation of tuffs, siliceous, calcareous, and sandstone materials. Johannsen (1914.1), after Granger, with modifications. The numbers refer to lithologic specimens examined by Johannsen. This fauna of the Uintatherium zone occurs in 260 feet of Washalde A, which is composed largely of altered eruptive rocks, probably dacite tuffs, of cal- careous and siliceous shales, and of glass tuffs mingled with grains of quartz, hornblende, feldspar, according to the analysis of Johannsen (1914.1, p. 214). The "lower brown sandstone" layer yields a rich fauna of uintatheres. This layer forms a persistent that the Washakie Basin was filled wi^h a lake in Green River time whUe Bridger A was being deposited to the west. Washakie B {Metarhinus and Eohasileus-DolicJio- rhinus zones, upper Eocene). — The upper Eocene Washakie B horizon is described on pages 89-90, in the description of upper Eocene faunal zones 13 and 14, to which it belongs. TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 60. — Diagrammatic horizontal section of the Washaliie Basin, southern Wyoming, from north to After Granger (1909.1). This section stiows tlie Uintatheriiim- Manteoceras zone (Washakie A, lower brown sandstones), discovered by the American Museum in 1893; Metarhinus zone (Washalcie B 1), base of the original "Washakie" formation of Hayden and Cope; DolichoThinus-Eobasileus zone (Washakie B 2), sumrnit of the original "Washakie" formation of Hayden and Cope; "Adobe Town," rougtJy eroded area in which Amymion antiquus, Achaenodon, etc., were dis- covered by the Princeton espedition of 1878. The numbers show locations of lithologic specimens studied by Johannsen. FiGTJEE 61. — Sketch map of the Washakie Basin region, in southern Wyoming After Granger (1909.1) from Clarence King (1876.1). The shaded area is the "Washakie" formation of Hayden, mapped by King and the United States Geological Survey as the Bridger formation. ENVIRONMENT OF THE TITANOTHERES Mammalian life of the WashaMe Basin 89 Washakie A (Uintatherium-Manteooeras-Mesatirhinus zone). Washakie B 1 and B 2 (Eobasileus-Dolichorhinus zone and Meta- These forms are found also in Bridger C and D, to the west rhinus zone). These forms are found also in Uinta A and B, south of the Uinta Range. Primates : Lemnroids Notharctus sp. Hemiacodon sp. Rodents Paramys cf. P. delicatus. Paramys leptodus, type. Paramys grangeri. Carnivores : Creodonts Thinocyon cledentis, type. Harpagolestes immanis. (Giant creodont of the Patriofelis ferox. family Mesonychidae.) Sinopa rapax var. lania, type. Synoplotherium lanius, type. Miacids (doglike Miacis washakius, type. Limnocyon potens. (An oxyaenid creodont.) carnivores) . Miacis medius. Oodectes? pugnax, type. Condy larths Hyopsodus cf. H. despiciens. Uintatherium grande, type. Ambly pods Eobasileus cornutus, type. (Giant amblypod Uintatherium speirianum, type. with the front horn directly above the eyes; first appearance.) Eobasileus galeatus, type. Eobasileus furcatus, type. Eobasileus pressicornis, type. Artiodactyls Homacodon sp. Achaenodon insolens, type. (First of the elo- theres.) Achaenodon robustus, type. ?Protylopus sp. (A cameloid form.) Perissodactyls : Titanotheres Palaeosyops copei? (Last of Palaeosyops. Palaeosyops sp. Manteoceras manteoceras, type. ("Prophet- horn" titanotheres.) Manteoceras washakiensis, type. Mesatirhinus megarhinus, type. (Ancestor of Metarhinus earlei, type. (Fluviatile type.) DoHchorhinus.) DoUchorhinus hyognathus, type. (Dohchorhi- Mesatirhinus petersoni. nus cornutus stage.) Dolichorhinus vallidens, type. Rhinoceroses and rhi- Hyrachyus sp. (Cursorial rhinoceros.) Hyrachyus sp. (Cursorial rhinoceros of Bridge noceratoids. Triplopus cubitalis. type.) Triplopus sp. Amynodon antiquus, type. (First of the amyno- donts (aquatic rhinoceroses).) Chalicotheroids Eomoropus amarorum, type. (Forest-living ancestral chalicothere; ancestor of Moropus.) Lophiodonts Helaletes sp. Desmatotherium guyoti, type. Dilophodon minusculus, type. Dilophodon minusculus? ZONES 13 AND 14: METARHINUS ZONE AND EOBASHEUS-DOIICHORHmUS ZONE [Uinta B 1 and Washakie B 1; Uinta B 8] The great life division known as Washalde B, 380 feet in thickness, contains a new and dominant fauna, which is not represented at all in Bridger D or Wash- akie A. It is significant that this unit is divided into two zones by its fauna, exactly as Uinta B is divided into two zones, the Eoiasileus-DoIichorMnus zone (Washakie B 2 = Uinta B 2), and the Metarhinus zone (Washakie B 1 = Uinta B 1 = (in part) Bartonian of Europe). Certain of the older mammalian families and genera (as Uintatherium) begin to disappear and 101959— 29— VOL 1 S new generic and specific forms replace them. Con- spicuous among these is the amblypod Eobasileus, first described from this region by Cope, which re- places Uintatherium. A full list of this fauna is given above. Among the distinctive forms are the fol- lowing : Eobasileus cornutus Cope, type; DoHchorhinus hyognathus Scott and Osborn, type. Uintatherium speirianum Osborn, type. Triplopus cubitalis Cope, type. Eomoropus amarorum Cope, type. Amynodon antiquus Scott and Osborn. Achaenodon insolens Cope. Metarhinus earlei Osborn, type. 90 TITANOTHEKES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA Washakie A is characterized by "rusty brown nodu- lar sandstones," and Washakie B by "coarse white, pink, and sabnon-colored sandstones" and by "the extremely coarse green sandstones or feldspar con- glomerates. The rocks first recognized as sandstones GEOLOGIC LEVELS OFSPECIES SUMMIT OF HAYSTACK M Zp^y\Eobas/7eus comufus, type "°?S-<5-v>.\ EobcLsileiLS- ict)_i-s^E=^ Dolichorhimis zone ZONAL LEVEL APPROXinATE Eomoropus amarorum, type Lepforeodon marsh/, type \Do/ichorhinus hyoqnathus, type level (A.M. No. 13164, Co/U906) '■ a.^i^?l%'^'^^^i^hMelarl>/nus earlei, type L£V£L OF S^r^ A my no don _=ti -"^ —^''^pnt/quas type ADOBE TOW/V Achaenodon insolens, type -■-'■'■ -~ '^-^M anleoceras washak/ensis, type level Winlalherium speirlanum, type level Mesatirhinus megarhinus type Palaeosyops cope/ Manteoceras manteoceras type Uintafherium sp. div. Figure 62. — Columnar section of the Washakie Basin, Wyo. life zones Shows the principal genera of the lower and upper life zones and the actual level of certain characteristic species. Chiefly after Granger (1909.1). This section includes the Uintaiherium zone (Washakie A), lower brown sand- stones; Meiarhinus zone (Washakie B 1); and Eobasileus-DolichorMnus zone (Washakie B 2) , upper gray-green beds. ' Numbers in column show position of lithologic specimens examined by Johannsen. prove to be interspersed with dacite and glass tuffs." (Johannsen, 1914.1, p. 215.) The sandstones, which were derived from granite by erosion, consist of grains of quartz, hornblende, and feldspar embedded in a shghtly devitrified groundmass. (See PI. IX.) The composition of these sediments indicates the presence in this region of active volcanoes, which were discharging great clouds of dust. Unlike the sediments of the Bridger Basin the sediments of the Washakie Basin were deposited in rather turbulent water and contain none of the "white layers" that indicate the still water that prevailed in the upper Bridger. Turbu- lent water is not favorable to the preservation of the remains of small mammals. Only one of the smaller perissodactyl ungulates (Triplopus) has been found, and no remains of Equidae. The first aquatic rhinoceroses {Amynodon) belong to a river- frequenting type; the first of the entelodonts {Achaenodon) is also a river-frequenting form; the first of the forest- dwelling chalicotheres {Eomo- ropus) also occurs. Thus the Washakie Basin has preserved for us mainly the larger swamp and river-border fauna but has yielded little record of either the arboreal or plains -living cursorial fauna of the time. In the fauna of the Wa- shakie Basin (a list of which is given in the table on p. 89) the large hoofed animals pre- dominate, especially those adapted to stream borders, swampy land, rivers, and streams. A small fauna of in- sectivores, lemuroids, carni- vores, and ancestral artiodac- tyls doubtless abounded, but the environment was unfavor- able to the preservation of such remains, and the micro- fauna has been found only rarely. The small titanothere MetarUnus is highly distinc- tive of this Washakie B 1 life zone. (Kiggs, 1912.1.) Uinta .B.— Exactly the same physiographic condi- tions prevailed at the same time in the great basin south of the Uinta Mountams while the sediments known as Uinta B were being deposited. These sedi- ments, which have a combined thickness of 800 feet, contain exactly the same riparian fauna, including a 35), showing ENVIRONMENT OF THE TITANOTHERES 91 AMVNODON SANDSrONE large number of identical species, and therefore con- stitute an extension of the Eobasileus-DolichorMnus and Metarhinus life zones to the south. The fauna and deposits of Uinta B are more fully described on pages 91-99, in the description of the Uinta Basin. UINTA BASIN, UTAH PHYSIOGRAPHIC, CUMATIC, AND VOLCANIC CONDITIONS IN THE UINTA BASIN DURING MIDDLE (?) AND LATER EOCENE TIME It is a striking fact that the later Eocene sediments in the Uinta Basin are composed mainly of altered eruptives, probably dacite tuffs, as indicated by analyses of nine samples by Johannsen (1914.1, pp. 212-214). The rocks of the lower levels ^ described as "brown sand- stones" comparable in litho- logic appearance to Washakie A, contain a large element of tuff and consist microscop- ically of irregularly broken and rounded fragments of quartz, lime-soda feldspar, hornblende, biotite, and frag- ments of andesite or basalt in a brown groundmass, which is chiefly chlorite but contains some calcite. On the lower levels (in Uinta A) brown is the prevailing color, as in Washakie A. In Uinta B sediments of this color pass into pinkish-brown and red- dish-brown sediments, and in Uinta C into pale-green and gray fine-grained rocks con- taining considerable glass. Many rocks that look like sandstones prove under the microscope to resemble flow breccias. Uinta A as now defined is entirely unfossiliferous but is here correlated with the middle Eocene fossil- iferous horizon A of the Washakie Basin {Uintathe- rium zone). Uinta B 1 (in some previous reports included in Uinta A) contains a rich river-border fauna, like that of Washakie B 1. Uinta B 2 (formerly constituting all of Uinta B) contains a larger land and river-border fauna, like that of Washakie B 2. BARREN {DiplacodoTV- EpUtippus zone EobcLsiZeus - DolichorTuruLS zoTte TYPICAL U/NTA MEADOW FAUNA TRANSITION FAUNA ^:metarhinus sandstone" 'XfluviatiTe' ^=r £^?^ ^_^1 MetarhiniMS FLU VI ATI LE FAUNA Figure 63. — Diagrammatic section of the Uinta formation exposed in tiie nortii wall of White River Canyon 3 miles below mouth of Evacuation Creek, Utah GEOLOGIC HORIZONS IN THE UINTA BASIN Uinta fauna of Marsh 100 feet above "Amynodon sandstone." The deposits of horizons A and B of the Uinta Basin are not those of the typical Uinta formation of Marsh (1871.3), of King (1878), or of Scott and Osborn (1891.1), all of which belong to Uinta C, the Diplaco- don zone; they form the lower part of the section (Uinta A and Uinta B), determined by the American Museum expedition of 1894 under Peterson (Osborn, 1895.98) and successively explored with remarkable results by Peterson, Douglass, and Riggs, whose obser- vations and exact records of the vertical distribution of genera and species have firmly established the stratigraphy of the Uinta Basin section as presented in Figure 65. (See PL IX.) After observations of Peterson, Douglass, and Riggs. Uinta A, columnar sandstones, unfossiliferous; Uinta B 1, MetarUnus zone capped by ** Metarhinus sandstone," containing a fiuviatile fauna; Uinta B 2, Eobasileus-DaUchorhinus zone, capped by "Amynodon sandstone," containing a transition fauna; Uinta C, Diplacodon-EpiMppus zone, containing the typical Uinta C contains the typical Uinta (Diplacodon) fauna. The sediments in the Uinta Basin between the Diplacodon zone and the Green River formation were classified by White (1878.1, p. 37) as Bridger, although no fossils were found in it, and wore treated as con- temporaneous with the Bridger deposits. We now know that the sediments that form Uinta B were cer- tainly laid down after Bridger C and D had been deposited, but they may have been contemporaneous with the unfossiliferous Bridger E. During the American Museum explorations of 1893-94 Peterson 92 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA discovered 150 fossil mammals in the lower series, I by the subsequent explorations and publications of which were first correlated by Osborn (1895.98, p. 72) 1 Douglass (1909.1) and Eiggs (1912.1). The strati- N. Mesatirhirvus superior, type Metarhinus ripartus , •■ r Sthenodectes Anvynodon.CF>inte^-medzzts A Figure 64. — Section of the Uinta formation (No. 10, fig. 35) from Kennedy's Basin to White River Canyon, Utah This section includes Uinta A, the barren sandstones; Uinta B 1, the Metarhinus zone capped by prominent bluffs of "Metarhinus sand- stone"; above this Uinta B 2, the Eobasileus-Dolichorhinus zone, capped by the "Amynodon sandstone." After E. S. Eiggs (1912.1); see also F- B. Weeks (1907.1). with the typical "upper Washakie," now known as Washakie B. The determination of the stratigraphy as well as the faunistic succession has been modified graphic order of the later Eocene deposits of the Uinta Basin and the correlated fauna may be presented as follows : Later Eocene deposits and fauna in the Uinta Basin, Utah Formation and nature of deposits Geographic conditions and mammalian fauna Uinta of King, Marsh, and White: Diplacodon elatus beds of Marsh; horizon C of Peterson, Douglass, and Riggs. Dacite tuffs and sandstones, grayish and greenish. Ferruginous. Thickness, about 600 feet. Uinta B 2 of Peterson and Osborn: Doliehorhinus cornutus zone of Osborn (1895.98). Amynodon beds of Riggs (1912. 1, p. 22). Coarse brownish dacite tuffs and sandstones, capped at the summit by the "Amynodon sandstone," immediately underlying Uinta C. Thickness, 285 feet. Uinta A of Peterson and Osborn, in part [Telmatotherium megarhinum beds of Osborn = Metarhinus fluviatilis zone, Osborn, upper Metarhinus zone of Riggs]: Capped by the "Metarhinus sandstones" of Riggs, with underlying coarse- grained brownish dacite tuffs and sandstone ledges; channel beds, varying in thickness from 5 to 30 feet, containing abundant remains of Metarhinus. Thickness, 266 feet. Uinta A of Peterson and Osborn, lower levels (lower Metarhi- nus zone of Riggs) : Capping of columnar sandstones, under- lain by friable sandy shales, interspersed with ledges. Thickness, 585 feet (Douglass, 1913). Unfossiliferous. Underlain by Green Ri\'or formation. Meadow, forest, and river fauna. Large titanotheres: Diplacodon elatus, Protitanotherium emarginatum, etc. Artiodactyla: Protoreodon, Leptotragulus, primitive camels. Small equines (Epihippus uintensis). Other large and small members of the true Uinta fauna. No traces of Amblypoda. Fauna chiefly stream border and fluviatile and some small forms. Last uintathere (Eobasileus). Aquatic rhinoceros (Amynodon intermedius) abundant. Entelodonts (Protelothe- rium uintense). Rare cameloids (Protylopus) . Rare eden- tates (Stylinodon). Numerous long-headed titanotheres (Doh- chorhinus cornutus, D. fluminalis, Sthenodectes). In the upper levels, first long-horned titanothere (Eotitanotherium of Peterson) ; ancestral Symborodon-like titanotheres (Rhadinorhi- nus). Titanotheres e.xtinct at this level or not recorded from it are Mesatirhinus and Metarhinus. Abundant fluviatile and forest fauna, of small variety. Tita- notheres: SmaU lowland varieties of Metarhinus very abund- ant, including several distinct specific forms; also the long- headed Doliehorhinus superior, the short-headed Sphenocoelus, Metarhinus earlei, M. riparius, M. fluviatihs, Rhadinorhinus, Doliehorhinus longiceps, an ancestral form of Dohchorhinus cornutus. The amblypods Eobasileus or Uintatherium. The large creodont Mesonyx obtusidens. No fossil mammals certainly recorded by Peterson, Douglass, or Riggs from this level. ENVIEONMENT OF THE TITANOTHEEES 93 U DQ CD < h Z 100 300' 400' Dolichorhinus fluminalis Dolichorhinus y- cornufus, type ' Sfhienodectes incisivus, iype Dolichorhinus heterodon Dolichorhinus hyognafhus (cornulus) Sfhenodectes incisivus R had in orhinus diploconus Dolichorhinus longiceps, type Dolichorhinus I =-,^^-==2:=". T Diplcuxtdort Amynodon skel-.ATn.Mus. N9J933 26"ATrvynocLoTh sandstoTve^j^ArrvyTzodon, irvtermediics ProtelotheriuTTh idntense ; Eobasileus- 400 super/or, type Melarhinus n'parius, lype Melarhinus earlei Telmalolherium D olich orhinus longiceps Rhadinorhinus abbotti Melarhinus fluvialilis, lype Sphenocoelus uinlensis, lype Melarhinus cristalus, lype (?Dolichorhinus longiceps) Melarhinus ripar/us 1: I EobasileiLS < StyLLnodorv I ProtylopzLS sWidstone^^ Harpccg'olestes Eobasileits iiznte.nsis, type FieldMus. 12170 iEobcLsiLeiis Triplopics MesoTvyx obtusidens Crocodilus ? TriplopiLS i< 500 NO MAMMALS RECORDED Figure 65. — Section of the Eobasileus-Dolichorhinus and Melarhinus zones in tlie Uinta Basin, Utah, show- ing stratigraphic distribution of species of titanotheres The species of titanotheres are shown in the left-hand column, the geologic strata in the middle column, other characteristic mammals in the right-hand column. After observations made by Peterson, Douglass, Eiggs, and Osborn. 94 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBEASKA The researches of Peterson, Douglass, Riggs, and Osborn prove that Washakie B and Uinta B comprise two distinct faunal divisions — a lower, Uinta B 1 {MetarTiinus fluviatilis, M. earlei zone), probably cor- responding with the lower levels (B 1) of Washakie B, and an upper, Uinta B 2 {Eohasileus-DolichorJiinus (cornutus) Tiyognathus zone), probably corresponding with the upper levels (B 2) of Washakie B. UINTA B 1 (METARHINUS ZONE = ZONE 13) Riparian fauna. — The fauna of the MetarTiinus zone was evidently that of a riparian lowland and was in part fluviatile or aquatic, as is indicated by its adapta- tions to aquatic and lowland life, which are inde- pendently developed in members of several different families. These adaptations are indicated by some of the specific names, such as MetarTiinus riparius, M. fluviatilis, two diminutive titanotheres, and DolicTio- rTiinus fluminalis. The animal last named is a short- limbed swamp-dwelling form, a fit companion of the river-seeking rhinoceros Amynodon intermedius , which begins to show aquatic adaptations in the structure of the orbit. The generic aspect of this fauna is almost identical with that of Washakie B, with the single exception that near the summit of Uinta B the ente- lodont ProtelotTierium replaces AcTiaenodon of Washa- kie B. The fauna contains a single new titanothere, RTiadinorTiinus , which is closely related to MetarTiinus. Some of the specific forms are identical with those of Washakie B and some exhibit more recent phases of evolution, which may be represented in the unfossilif- erous upper levels of Washakie B. We consequently reach the broad generalization that Washakie B 1 and Uinta B 1 were not only contemporaneous sediments but that they indicate the prevalence of similar physiographic and climatic conditions at this time on the north and the south sides of the Uinta Range. River-cTiannel fauna. — Remains of the small titano- there MetarTiinus have been found in ancient river channels, as determined by Riggs. This genus is by far the most distinctive fossil of this life zone and is apparently confined to it, although at certain levels primitive species of DolicTiorTiinus {D. longiceps) are found in equal abundance (Riggs, 1912.1, p. 20). This life zone, which is 400 feet thick, is composed chiefly of massive ledges of sandstone alternating with layers of sandy shales or indurated clays. In all the ledges there are traces of cross-bedding, and at many places there are beds of coarser river sand containing pebbles of quartzose material, sandstone, and clay shale. In these beds are found disarticulated bones of mammals, as well as the branches and at some places the trunks of trees, all pointing to the action of swiftly flowing streams that swept through a flood plain. Many skulls are found embedded in gravel, with their narial or orbital cavities filled with pebbles such as could be carried only by rapidly flowing water. Another evidence of stream action lies in the dissocia- tion of the parts of single skeletons. Whole skeletons have exceptionally been found but little disturbed, lying in a fine-grained homogeneous sandstone, ap- parently deposited in quieter water, such as deep pools or eddies. Remains of the long-headed titano- there DolicTiorTiinus are found only in the heavy sand- stones, so that this animal was apparently confined to the vicinity of streams. Supposed river-frequenting species of MetarTiinus, always found in sandstone, in- clude M. fluviatilis, M. riparius, and M. earlei. The species last named is found also in the lower levels of Washakie B, north of the Uinta Mountains. As we ascend in this MetarTiinus zone we find, according to Riggs (1912, p. 24), increasing numbers of upland forms. The "MetarTiinus sandstone" ledge that caps this zone has yielded the type specimens of Doli- cTiorTiinus superior, MetarTiinus riparius, and M. earlei, the last-recorded appearance of these animals in the Uinta Basin. UINTA B 2 (EOBASILEUS-DOLICHORHINUS ZONE = ZONE U) In the beds of the Eohasileus-DolicTiorTiinus zone the river sandstones and channel deposits gradually give place to shales and clays, indicating physiographic changes in this part of the Uinta Basin. In the lower 100 feet of bluish or grayish shales, which are overlain by 40 feet or more of fine red clays, little evidence of mammal life is found, but certain thin beds contain frag- ments of Eohasileus. As we rise in the formation the gray "clays" begin to yield a mixed fauna of lowland and plains forms, including Protylopus and Stylinodon, together with remains of DolicTiorTiinus (cornutus) TiyognatTius and Amynodon intermedius. The massive "Amynodon sandstone," which forms the summit of this life zone, yields the type specimen of the long- headed titanotheres DolicTiorTiinus (cornutus) Tiyog- natTius, D. fluminalis; also of StJienodectes incisivus. This is the last appearance of the genus DolicTiorTiinus in the Uinta Basin. Doubtless the massive "Amyno- don sandstone" terminated the active period of fluviatile and flood-plain deposition in this locality. The D. (cornutus) TiyognatTius zone yields the large enteledont RrotelotTierium uintense, which is inter- mediate between AcTiaenodon insolens of Washakie B and ElotTierium of the White River group. This sandstone contains also the lophiodont DesmatotTie- rium guyoti, which is a forerunner of Colodon of the White River (Oligocene) group. ZONE 15: DIPLACODON-PEOTITANOTHERIUM-EPIHIPPnS ZONE [Uinta C I; Xudian of Europe] To zone 15 belong the Uinta of King and Marsh, the "Brown's Park beds" of Powell, and the Uinta(?) of the Beaver Divide, Wind River Basin. This zone (Uinta C 1) is correlated with the European stage that was named Ludian, after the "marnes de Ludes" in the Paris Basin, a stage typified by the "gypse de Montmartre," made famous by the classic researches ENVIRONMENT OF THE TITANOTHEEES 95 of Cuvier. The lower Ludian yielded the type speci- men of the equine LophiotJierium, a horse in the same stage of evolution as the diminutive American Epihip- pus of the Uinta. The American beds contain a rich titanothere fauna. They include the "Diplacodon beds" of Marsh (1877.1, p. 354) and contain the robust titanothere Protitanotherium, which is inter- mediate between the "prophet-horn" Manteoceras and the horned titanotheres of the lower Oligocene; also the type of Protitanotherium superbum, an animal greatly exceeding in size the earlier Oligocene titanotheres. Of great interest is the survival of the ancestral genus Manteoceras in the species M. uintensis, a genus first occurring in the upper Bridger, and the Bridger genus TelmatJierium in the species T. ultimum. plains fauna (Hypertragulidae and Camelidae) rep- resented respectively by genera believed to be ancestral to the tragulids (Leptotragulus) and to the camels (Protylopus, Camelomeryx) ; also members of the oreodonts (Protoreodon), and the agriochoerids {Agrio- choerus). The diminutive tylopod Protylopus has been selected as the possible ancestor of the great family of American camels. The fauna found near the base of the true Uinta thus includes a considerable light-limbed meadow and plains element, transitional to the plains fauna of the lowest Oligocene of the White River group. The occurrence of this fauna near the base of Uinta C indicates that the Uinta formation probably passes up into lower Oligocene time. The beds represent Figure 66. — Badlands near the mouth of White River, Uinta Basin, Utah (No. 10, fig. 35) Wortman and Peterson collecting. This view shows the typical Uinta formation (Uinta C 1) and the Diplacodon zone in the fore- ground, with Uinta C 2 (unfossiliferous) in the distance. After Osborn (1910.346). Am. Mus. negative 17663. Compare Plate XII, B. The amphibious rhinoceros Amynodon occurs in the species A. antiquus. It should be noted that the remains of all these large mammals were found not far above the base of Uinta C, and that all the speci- mens in the chief collections of small Artiodactyla (Protoreodon, Leptotragulus) and of Perissodactyla (Triplopus, LopModon, Isectolophus , a tapiroid, Epi- Jiippus) (Peterson) were obtained from the lower levels of Uinta C. With Epihippus were found the only primate that has been discovered in the Uinta Basin, NotJiarctus? uintensis, a lemuroid, and the supposed condylarth or insectivore Hyopsodus. The few surviving ancient creodonts are represented by Oxyaenodon and by the giant Harpagolestes uintensis. Especially important is our first knowledge of the a considerable change in local physiographic condi- tions from those of Uinta B. The fine-grained soft material, composed of altered eruptives, probably dacite tuffs, is of much the same texture as the char- acteristic " Titanotherium beds" (Chadron formation) of South Dakota, except as to its color, which is brick- red ; in fact, a reddish tinge prevails throughout the sediments of Uinta C During this latest part of the Eocene epoch the titanotheres of the Rocky Mountain basin south of the Uinta Mountains distinctly approach in character the titanotheres of the Great Plains. The appearance in this layer and near the summit of Uinta B of two or three entirely new forms of titanotheres (Eotitano- tJierium, Diplacodon, Protitanotherium) is less indica- 96 TITANOTHBEES OF ANCIENT AVYOMING, DAKOTA, AND NEBEASK.V tive of new migrations into the Rocky Mountain region than of new physiographic conditions favor- able to the fossilization of some of the upland and meadow Herbivora that had been evolving in the adjacent Plains region but had not mingled with the fluviatile, swamp, and forest-border fauna that inhabited the Uinta Basin in Uinta B time. The following summary of the later Eocene faunas of the Uinta Basin should be examined in connection with Figures 63-66. Composite section of mammalian faunas of tlie late?' Eocene sediments of the Uinta Basin [After Peterson, Osborn, Riggs, and Douglass] Uinta C (true Uinta formation = Diplacodon zone); 600 feet. Badlands like those of South Dakota, but of brick-red color. Brownish and reddish ferruginous sand- stones and clays (Peterson). Uinta B 2 (Eobasileus-Dolichorhiniis zone) ; 300-400 feet. Section along gilsonite vein No. 2 (Riggs). Includes "Amynodon sand- stone," gray and greenish clays, ferruginous sandstones, bluish and greenish shales. Two red layers with fossiliferous sandstone between (Douglass). Supposed base of horizon B 2. Uinta B 1 (Metarhinus zone = upper Meta- rhinus zone of Riggs) ; 400 feet. Section on divide between White River Canyon and Coyote Basin (Riggs) . Also section 3 miles below mouth of Evacuation Creek (Riggs) : "Metarhinus sandstone." "Eobasileus sandstone" = massive ledges of reddish sandstone, alternating with layers of sandy shales. Indurated clays. Uinta A (lower A of Peterson, lower Meta- rhinus zone of Riggs) ; 500 feet (Riggs) ; 585 feet (Douglass). Section in north wall of White River Canyon (Riggs) : "Columnar sandstones, about 300 feet thick, weathering as bold cliffs, or but- tresses along the canyon of White River. Color slightly more grayish than the underlying shales, but brown predominates (Riggs). "Two hundred feet friable sandy shales, weathering in steep slopes, with hori- zontal outcroppings of nodular or sandy layers, or by massive ledges of limited extent" (Riggs). Green River (?) formation. Shaly gray sand- stone of lacustrine origin. Titanotheres Diplacodon elatus, type. Protitanotherium emarginatum. Protitanotherium superbum, type. Telmatherium ultimum, type. Manteoceras uintensis, type. Eotitanotherium osljorni, type. Dolichorhinus cornutus ( = hyogna- thus), type. Dolichorhinus fluminalis, type. Dolichorhinus heterodon, type. Sthenodectes incisivus, type. ?Rhadinorhinus diploconus, type. Dolichorhinus longiceps, type (near base) . Dolichorhinus superior, type. Metarhinus riparius, type. Metarhinus earlei. "Telmatherium," large jaw. Dolichorhinus longiceps. Rhadinorhinus abbotti, tj-pe. Metarhinus fluviatilis, tyjje. Dolichorhinus longiceps, skeleton. Metarhinus riparius (from base). Sphenocoelus. Heterotitanops parvus. (=?Meta- rhinus), from base of B 1. None. Other forms of life First oreodonts. Cameloids. Dichobunids. Aquatic rhinoceros (Amynodon). Small perissodactyls (tapiroids, lophio- donts, Epihippus). Last creodonts, Meson\-chidae and Oxyaenidae. No amblypods found. Last ambly pod (Eobasileus). First cameloid (Protylopus). Last taeniodonts (Stylinodon). Aquatic rhinoceros (Amynodon). Primitive entelodont (Protelotherium uintense) . Remains of plants and fishes; oc- casionally plentiful in sandstone ledges (Peterson) . Last primates ("Notharctus"). Giant creodonts, Mesonychidae (Har- pagolestes) . Giant creodont (Harpagolestes). Crocodiles. Turtles. Giant amblypod (Eobasileus). Light-limbed perissodactyl (Triplopus). Creodonts, Mesonychidae (Mesonyx). None. No mammals (Peterson, Riggs). Fragments of turtles. Unios. Remains of plants and occasionally large tree trunks in sandstone ledges (Peterson). Remains of plants, fishes, and insects in the shales (Peterson). ENVIEONMENT OF THE TITANOTHERES 97 SUMMARY OF FAUNAS OF UINTA B AND C Though the whole later Eocene section of the Uinta is 1,900 feet thick it includes 500 feet of un- fossiliferous beds both at its base and at its summit, so that the fossiliferous beds cover only about 900 feet. The stages of evolution are best measured in the suc- cessive species of DolichorJiinus, which are found both at low and at high levels in the fossiliferous part of the section. The archaic mammals that play so large a part through lower and middle Eocene time diminish in number and approach extinction at the end of Eocene time. The numerical inferiority of the waning archaic mammals and the rapid increase in the numbers of modernized mammals are indicated in the following table, prepared in 1910: Transition in mammalian life at end of Eocene time Genera Species Archaic mammals: .2 1 5 6 Condylarthra ( H vopsodontidae) 2 5 8 13 Modernized mammals: Primates _ 2 1 3 3 9 ?3 Rodentia __ 3 Carnivora (Miacidae) 4 4 Perissodactyla 16 18 30 The Amblypoda culminate in the gigantic Eobasileus, which disappears at the end of Uinta B, when the gigantic creodont Mesonychidae and the catlike Oxyaenidae appear for the last time. It is note- worthy that these animals attain their largest size in this, their waning period. The lemuroid primates are found in greatly diminished numbers as compared with those in the Bridger, possibly because the con- ditions were unfavorable to the fossilization of re- mains of arboreal animals; in fact, we know nothing of the forest or the arboreal fauna during the entire period of Washakie B and Uinta B because of pre- vailing fluviatile conditions of deposition. ADAPXrVE RADIATION OF THE TITANOTHERES IN THE UINTA BASIN GENERA AND SPECIES HEPRESENTED Through these 650 feet of fossiliferous sediments the titanothere fauna of the Uinta Basin is revealed as extraordinarily large and varied, no less than 11 genera and 22 species having been described. The animals range in size from the small Metarhinus flu- viatilis, some of which were not so large as a tapir, to the huge ProtitanotJierium superbum. The titanothere Metarhinus is abundant and char- acteristic in Uinta B 1, ranging from the base to the summit but not extending into Uinta B 2 as here de- fined. (In previous reports horizon B 2 has been included in Uinta A.) According to Riggs (1912.1, p. 27) the genus includes two phyla — the first comprising the small MetarJiinus fluviatilis Osborn and M. riparius Riggs, with long, narrow skull; the second including the broad-skulled forms M. earlei Osborn (which is also found in Washakie B) and M. cristatus Riggs. Metarhinus was a companion of its long-skulled rela- tive Dolichorhinus in and near the rapidly flowing streams, its remains being usually found in coarse and semigravelly sandstones. (Riggs, op. cit., p. 24.) In Uinta B 2 rapid streams, apparently the favorite haunt of Metarhinus, were less abundant than in Uinta B 1 (Riggs, op. cit., p. 25), which partly ac- counts for the apparently sudden disappearance of these animals from the sediments. Sphenocoelus uintensis, which is also probably from the Metarhinus zone (Uinta B 1), is known only from the hinder half of a skull. This strange animal is clearly a member of the Metarhinus-Dolichorhinus group and may be closely related to the long-skulled Metarhinus riparius. The Metarhinus series as a whole is clearly related to the older and more primitive Mesatirhinus megarhinus of Washakie A and Bridger C and D, which is also structurally ancestral to Dolichorhinus . The name Heterotitanops parvus Peterson has been applied to the skeleton of a very young animal from Uinta B 1. It was found, articulated, in a hard sand- stone concretion and lower down in Uinta B 1 than any mammalian remains heretofore described from that horizon. (Peterson, 1914.2.) In the opinion of Gregory the characters of the deciduous dentition and of the facial region of the skull of this animal indicate that it probably represents the newly born young of some of the Metarhinus-Rhadinorhinus group. Rhadinorhinus is distinguished from Metarhinus by its tapering nasals and by the reduced infraorbital process of the malar bones. One species, R. abhotti Riggs, is found in Uinta B 1 , and another, R. diploconus Osborn, is recorded from Uinta B 2. Riggs suggests that Rhadinorhinus was an upland rather than semi- aquatic form. Gregory noted in 1902 that it fore- shadows the long-horned titanothere Megacerops (Symiorodon) of the lower Oligocene in the abbrevia- tion of the face and in the characters of the dentition. The long-skulled Dolichorhinus is represented by two species in Uinta B 1 (one of which, D. longiceps Douglass, extends into the base of Uinta B 2) and by four species in Uinta B 2. The most primitive species, D. superior, is in general intermediate in structure between the ancestral Mesatirhinus and the later species of Dolichorhinus. The most advanced species, D.fluminalis Riggs, is from the upper levels of Uinta 98 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA B 2. The allied D. cornutus is believed to be specifi- cally identical with D. hyognathus of Washakie B. In Uinta B 1 remains of Dolicliorliinus, as noted by Riggs, are frequently found associated with those of Metarhinus in coarse pebbly sandstone laid down in rapid streams; but in Uinta B 2 they are frequently found in lenticular sandstones, which were apparently deposited in quiet water, for they show little evidence of water currents, and which yield skulls associated with mandibles and parts of skeletons. Riggs accord- ingly infers that Dolichorliinus inhabited the low grounds near quiet waters rather than the swift currents preferred by Metarhinus. Contrasting with the elongate and straight-sided skull of Bolichorhinus is the broad, robust skull of StJienodedes incisivus (Douglass), a titano there with exceptionally massive incisor and canine teeth and broad, low-crowned upper molars. The type skull was found by Douglass in a thick deposit of sandstone and small gravel, evidently of stream origin, near the middle of Uinta B 2, whereas Riggs's specimen was found in lenticular sandstones at about the same level. Sthenodectes shares many characters in common with the Bridger genera Manteoceras and Telmaiherium and appears to be an advanced member of that macrodont group. With the possible exception of Rhadinorhinus all the titanotheres so far noted from Uinta B 1 and B 2 belonged to aberrantly specialized side lines, which are not found in later formations and apparently became extinct. One titanothere recorded from near the summit of Uinta B 2, namely, Eotitanoiherium osborni Peterson (1914.1 ; 1914.4), is highly progressive toward the giant plains-living types of the uppermost Eocene (Uinta C) and of the lower Oligocene. This animal, represented by parts of two skeletons in the Carnegie Museum, surpasses even ProtitanotTierium of Uinta C in the development of a pair of large oval bony protuber- ances above the eyes. One of the most remarkable features of Eotitanotherium is the progressive sub- molariform character of its third and fourth upper molars, which are, indeed, slightly more advanced in type than those of certain lower Oligocene titano- theres. The animal was about as large as a rhinoceros, and throughout the skeleton are mingled the earlier characters of its Eocene predecessors with progressive, plains-living adaptations prophetic of some of the Oligocene titanotheres, especially those of the meno- dontine group. The titanothere fauna of the true Uinta (Uinta C), though less extensive in genera and species than that of Uinta B 1 and B 2, is none the less of prime impor- tance to the historian of the family. Thus the titano- theres of the true Uinta include, first, certain conserva- tive phyla {Manteoceras, Telmatherium) , which repre- sent the little-altered descendants of genera either of Uinta B 1 and B 2 or of Washakie and Bridger types; second, two very progressive and different phyla, Diplacodon and ProtitanotTierium, of uncertain relation- ships, which appear to be immigrants from other localities. ADAPTIVE RADIATION OF PHYLA There seem to be at least four contemporary phyla, representing wide local adaptive radiation : First, the robust, short-limbed forms, one of which, Manteoceras uintensis Douglass, found in gray sand- stone in the red beds of the lower portion of Uinta C, is considerably larger than the more primitive species of Manteoceras in the upper Bridger and Washakie A but is little modified otherwise. Its horn swellings, if developed at all, were not large, and it is strongly macrodont in type, like Telmatherium and Sthenodectes. Second, the long-limbed, long-headed, relatively hornless Telmatherium, which is distinguished espe- cially by its deep malar bones and the high sagittal crest and is represented in Uinta C by the great Telmatherium ultimum and the gigantic T. altidens. T. ultimum is practically hornless, having only the slightest rugosity at the naso-frontal junction in the type skull. Accordingly the species Manteoceras uintensis and Telmatherium ultimum and the genus Sthenodectes, while advancing in the direction of the Oligocene type in various characters, are apparently excluded from direct ancestry to the later types by certain specializations, such as marked enlargement of the incisors and canines, and by the lack of develop- ment of effective horn swellings. Third, Diplacodon elatus Marsh of Uinta C, a progressive titanothere, which is known chiefly from the upper dentition and takes its generic name from its submolariform third and fourth premolars. The precise relations of this animal are still in doubt. The premolars and molars may have been derived from the type represented by Rhadinorhinus diploconus of Uinta B 2, which is the only one of the older titano- theres that has the dentition and skull at all like those of Diplacodon. In other respects the Diplacodon dentition suggests that of the lower Oligocene titano- there Menodus trigonoceras , and in still another respect it resembles that of Eotitanotherium of Uinta B 2, though it differs from that genus in the more molariform con- dition of the third upper molar. Fourth, Protitanotherium emarginatum Hatcher, which is known from the facial part of the skull and the lower jaw of the type specimen. It is a large animal, which, so far as Imown, approaches the Oligocene type of Brontops. It has oval horn swellings which are less protruding than those of the type of Eotitanotherium; its nasals are wide distally, in con- trast to the tapering nasals of Eotitanotherium; its canines are very stout and acutely conical; its stout upper incisors form a flattened arch. Altogether it seems to represent a phylum distinct from Eotitano- ENVIRONMENT OF THE TITANOTHEEES 99 therium and of doubtful relationship both to earlier or to later titanotheres, although it was possibly derived from Manteoceras. Of the same phylum is Protitanotherium swperhum, a gigantic animal, with a jaw 24 inches long and premolars and molars of very progressive type. It is much larger than the smaller titanotheres of the lower Oligocene. Another titanothere of uncertain relationship is BrachydiasUmatherium from Transylvania, eastern Hungary (now Rumania). The geologic age of this animal is not certain, but it is in an upper Eocene stage of evolution as compared with the titanotheres of America. FAUNA UNREPRESENTED The sequence of titanothere species in the Uinta Basin illustrates the vagaries of the fossil records of the Rocky Mountain basin region caused by local physiographic changes; each kind of sedimentation exhibits only a part of the fauna. For the entire period covered by the lower sediments of the Uinta Basin little or no knowledge of the small terrestrial fauna has come to light, none of the arboreal fauna, and none of the plains and upland fauna, in contrast with the surprisingly extensive knowledge of the fluviatile and the swamp-dwelling fauna. Gradually conditions changed, and Uinta B 2, as we ascend, affords an increasing knowledge of the cursorial meadow fauna; but it is not until Uinta C (true Uinta) that local conditions became favorable to the pres- ervation and fossilization of the small cursorial mam- mals of the artiodactyl and perissodactyl divisions of the ungulates. The sudden appearance of these animals might be attributed to immigration, but it is equally probable that they were all evolving in the same region or in the adjacent Plains region. Thus the data do not necessarily suggest immigration or migration; these animals may have been brought into the field of observation by changing conditions of fossilization. The manner in which these numerous phyla of titanotheres enter this field is shown in the following table: Geologic and geographic range of phyla {here "subfamilies" and certain genera) of titanotheres [Showing their successive immigration from the north and their evolution in their i column; the later immigrants are named in order from bottom to top. ew habitat. The earliest immigrants are those named at the bottom ot the first The difference in the length of the blaclc bars has no significance] Phylum Wind River B « (" Lost Cabin ") (middle Eocene) Bridger (upper middle Eo- cene) Washakie (upper Eocene) Uinta (upper- most Eocene) Chadron (Oligo- cene) A' B C = D » E A ' B A B" C A B C 1 1 ^^^ ___ 1 — — ■^ ---- — ^ ! » Wind River B = Huerfano A. I Bridger A = Huerfano B. ■ Bridger and D = Washakie A. ' Washakie A = Bridger C and D. • Uinta B = Washakie B. 'Diplacodon, Eotitanotherium, Protitanotherium. ZONE 16: THEORETIC UINTA C 2 Titanotheres have thus far been determined from only the lower 100 feet of Uinta C. They are at present only partly known. When fully known we shall probably find close generic if not specific corre- lation between the upper fauna (now unknown) of Uinta C and the fauna of the lower levels (Chadron A) of the White River group. The passage from Eocene to Oligocene time probably occurs within the period of Uinta C (true Uinta) deposition. Scott is disposed to put all of Uinta C in the Oligocene. COMPOSITE EOCENE AND LOWER OIIGOCENE SECTION AT BEAVER DIVIDE, WIND RIVER BASIN, WYO. Most of the Oligocene sediments in the Rocky Mountain basin region have been eroded away. The only locality where fossil-bearing lower Oligocene sedi- ments still overlie those of the upper Eocene is on the southern border of the Wind River Basin, Wyo., where the true Titanotherium zone overlies sediments containing a fauna similar to that of the Diplacodon zone (Uinta C). The geologic section observed at this point by the American Museum expedition of 1909 100 TITANOTHEHBS OF ANCIENT WYOMING, DAKOTA, AND NEBEASKA under Granger and N. H. Brown, who discovered this fauna in 1908, is as follows: Oreodon zone = Brule for Summit of lower Oligocene mation Base of lower Oligocene, Titanotherium zone=Chadron formation Upper Eocene, Diplacodon zone = Uinta(?) formation — Middle Eocene (?), unfossiliferous = Bridger (?) forma- tion Lower Eocene, Lamhdotherium zone = upper part of Wind River formation Feet 540 O/'eodoTL zone A single tooth of either Diplacodon or Protitano- therium has been found at Beaver Divide, Wyo. The correlation with Uinta C rests upon Camelodon arapahovius Granger, a species somewhat more pro- gressive than Protylopus of Uinta C and somewhat more simple than Leptotragulus , characteristics that combine to place it among ancestral camels, in the Camelidae. In certain characters it agrees with Lep- totragulus profedus of the Titanotlierium zone of Pipe- stone Springs, Mont. The Amy- nodon found here agrees with the species A. antiquus, originally determined in Washakie B ( = Uinta B). Two specimens of Protoreodon are referable to P. parvus, from the base of Uin- ta C or the summit of Uinta B. Above this Diplacodon (?) level is a very marked erosional unconformity between the up- per Eocene and the lower Oli- gocene; broad, shallow valleys (Sinclair and Granger, 1911.1, p. 99), indicating fairly mature topography, were excavated in the sediments of the Diplaco- don(1) zone. After these val- leys were cut the first deposits laid down were fine-grained buff-colored tuffaceous shales. In this tuff the American Mu- seum exploring party of 1909 found a skull of Menodus heJoce- ras, which belongs to the lower level of the Titanotherium zone, corresponding with Chadron A. The volcanic ash comprising the sediments of the Oreodon titanothere zone, a few feet thick, is covered with a mud flow of volcanic material 46 feet thick, above which lies 540 feet of fine, wind-blown buff ash and dust. No clays have been found at this middle Oligocene horizon , which corresponds in age with the Brule formation of the White River group — only wind- laid ash and coarse gravel, perhaps deposited by torrents during occasional heavy rains. None of these sediments ap- pear to have been much dis- turbed by water, and Sinclair Diagrammatic section of deposits at Green Cove, Beaver Divide, Wyo. ^^^ Grander (1911.1 p. 114) Oreodon Cyllndrodon Caenopus Ischyromys Poebro therlum Menodus he/oceras ? DiplcLcodoTz zone Amynodon ? anfiquus Protoreodon Camelodon Pro titan o ttierium Lamhdotherium zone Lambdotherium Coryphodon, PlienacoduSj hieptodon , Eohippus Figure 67 ^^^ ^^„„ „ ^„ , (No. 6, fig. 35), from the Lambdotherntm zone (Wind River) to the Oreodon zone """ .""'"""" t"*h lie e that they accumulated under a drier (White River) at the summit Chiefly after Granger (1910.1). ENVIEONMENT OF THE TITANOTHEEES 101 climate than that which prevailed in Eocene time. These upper sediments contain a true Oreodon zone fauna. FOURTH FAUNAI PHASE (LOWER OLIGOCENE) LOWER OLIGOCENE MAMMALS COERELATION OF EUROPEAN AND AMEEICAN FORMS The lower Oligocene mammals represented by the fossils thus far discovered are listed below. Peculiar to Europe: Paleotheres. Anoplotheres. Oenotheras. Gelooids. Amphicyonids. Viverrids. Cricetines (hamsters). Theridomyids. Sirenians (Hahtherium). (Horses not recorded.) Common to Europe and North America: Titanotheres (central Europe). Chalicotheres. Rhinoceroses (aceratheres and diceratheres) . Amynodonts. Anthracotheres. Suillines. Entelodonts. Opossums. Hyaenodonts. Canids (dogs). Mustelids (martens). Machaerodonts (saber-tooth cats). Peculiar to North America: Horses. Hyracodonts (rhinoceroses) . Oreodonts. Camelids. Hypertragulids. Leptiotids. Chrysochlorids? (inseotivores) . Ischyromyids (rodents). Leporids (hares). ZONE 17: TITANOTHERIUM-MESOHIPPUS ZONE [Chadron A, B, and C; Sannoisian of Europe] The forms that constituted this rich world of lower Oligocene mammalian life were distributed through the Rocky Mountain basin region, but the sediments that contained the fossils have been eroded away except in a few isolated areas, such as those along Pipestone Creek, Mont.; at Beaver Divide, Wyo., south of the Wind River Basin; and at Bates Hole, Wyo. The areas in which these sediments were deposited lie east of the Rocky Mountains, in Sas- katchewan, North Dakota, South Dakota, and Colo- rado. The chief fossil-bearing sediments exposed are in the localities shown below. Recorded thickness of the Titanotherium zone in thirteen exposures of lower Oligocene deposits Feet 1. Cypress Hills, Saskatchewan (Lambe, 1908) 50-500 2. Pipestone Creek, Jefferson County, Mont. (Douglass, 1903) 300 + 3. White Butte, N. Dak. (Douglass, 1903) 120 4. Big Badlands, S. Dak. (Hatcher, Darton) (typical area of Titanotherium zone) 180 5. Goshen Hole (Scotts Bluff), southeastern Wyoming (Darton), maximum thickness 200 6. Hat Creek, South Fork, Cheyenne River, Dawes County, Nebr 100 ± 102 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA ENVIRONMENT OF THE TITANOTHERES 103 7. Near Dickinson, S. Dak. (Douglass) 40-50 8. Pine Ridge, S. Dak. (Darton) 30-60 9. Beaver Divide, Wyo. (Granger) 10. Bates Hole, Natrona County, Wyo 11. Adelia, Sioux County, Nebr. (Darton), about Between Platte River and Arkansas River drainage (Darton). ("Monument Creek group" of Hayden; Castle Rock conglomerate of Richardson, 1912.1) 300 Horsetail Creek, northeastern Colorado (Matthew,1901.1), not over 46 (?) 12. 13 between the upper fauna (now unknown) of Uinta C and the fauna of the lower levels (Chadron A) of the White River group. The passage from Eocene to Oligocene time probably occurs within the period of deposition of Uinta C. Scott is disposed to put all of Uinta C in the Oligocene. General Section of the Tertiary rocks of Nebraska. 100 The deposits at these localities, some of them indicated on the accompanying map, represent only the exposed parts of the lower Oligocene deposits of the great flood-plain sys- tem now known as the Chadron and corre- lated formations, the larger part of which is covered by the Brule and Arikaree formations. This flood plain extends 325 miles north and south and 300 miles east and west. We do not know whether it was wholly continuous. Such an area would embrace 97,500 square miles, which would not exceed the present Andean flood plains. At the base of these sediments in South Da- kota and northern Colorado there are abundant remains of titanotheres, certain of which are in stages of evolution no more advanced than those found at the base of Uinta C, Diplacodon zone. Consequently the faunistic relation be- tween the titanotheres living in the mountain basins and those living on the Plains remains to be solved by future discovery. This rela- tion may be revealed in the "missing" faunal zone. At present we may divide the life zones, in descending order, as follows: 17. Titanotherium zone: Chadron C, levels 3, 2, 1: Brontops robustus. Menodus giganteus. Brontotherium platyceras. Chadron B: Brontops dispar. Menodus trigonoceras. Brontotherium hatched. Chadron A, levels 1, 2, 3: Brontops brachycephalus. Menodus heloceras. Brontotherium leidyi. 16. Theoretic zone of Uinta C (upper levels, or Uinta C 2): Unknown or "missing." 15. Diplacodon zone of Uinta C (lower levels, or Uinta C 1) : Protitanotherium emarginatum. P. superbum. Diplacodon elatus. It is very important to recall the fact that titanotheres have thus far been determined from only the lower 100 feet of Uinta C, that they are only partly known, and that when fully known we shall probably find a close generic if not specific correlation NameB. SUBDIVISIONS. Thick- LOCALITIES. Foreig:n Equiva- lentB. S 3 Fine loose sand, with some layers of limestope, — contains bones of Canis, Felis, Caxtor, Equus, Mastodon, Testudo, &c.., some of which are scarcely dis- tinguishable from living spe- cies. Also Helix, Physasucclnea, probably of recent species. All fresh water and land types. o On Loup fork of Platte River ; extend- ing north lo Niobrara River, and south to an unknown distance beyowd the Platte. a § > s s White and light drab clays, with some beds sandstone, and local layers limestone. Fossils, Oreodon, Titanotherium, Cliaro- potamus, Rhinoceros, Anchithe- rium, Hycenonodon, Afachairodus, Trionyx, Testudo, Helix, Plan- orbis, Limncea, Petrified wood, &c. &c. All extinct. No brackish water or marine re- mains. o a o 8 O Bad Lands of White River ; under the Loup River beds, on Niobrara, and across the country to the Platte. a o o .£■2 Pig Ti p. Light gray and ash colored sandstones, with more or less argillaceous layers. Fossils, — fragments of Trionyx, Testudo, with large Helix, Vivipara, Petrified wood, &c. No marine or brackish water types. o o § . -21 r-t Wind River valley. Also west of Wind River Mountains. »- 5 'S 13 "§ 3 D Beds of clay and sand, with round ferruginous concretions, and numerous beds, seams and local deposits of Lignite ; great numbers of dicotyledonous leaves, stems, &c. of the genera Platanus, Acer, Ulmus, Populus, &o., with very large leaves of true fan Palms. Also, Helix, Mclania, Vivipara, Corbicula, Unio, Ostrea, Potamomya, and scales Lepidotus, with bones of Trionyx, Emys, Compsemys, Crocodilus, &c. a u o o o o Occupies the whole country around Fort Union, — extending north into the Britisli possessions, to un- known distances ; also southward to Fort Clark. Seen un- der the White River Group on North Plat- te River above Fort Laramie. Also on west side Wind River Mountains. § Figure 70. — Facsimile of the Meek and Hayden Tertiary section of 1862, showing original definitions of White River group and Wind River formation The deposits are now known to include the following: "Loup River beds" (lower Pleistocene fauna listed). The area includes deposits of the Plio- cene and Miocene (Ogalalla formation of Darton). "White River group," including lower Miocene (Arilcaree formation of Darton) and Oli- gocene (Brule and Chadron formations of Darton). The " Clioeropotamus" is Ancodus amcricoKKs.the ancodont of the Chadron formation (Titanotherium zone). "Wind River deposits" (summit of the lower Eocene). "Fort Union or Great Lignite group" (basal Eocene). OLIGOCENE FLOOD-PLAIN SEDIMENTATION IN THE WESTERN GREAT PLAINS REGION CONDITIONS OF DEPOSITION A very long period of extremely slow sedimentation, east of the Rocky Mountains of Wyoming and Colo- rado, began in lower Oligocene time and extended without interruption to lower Miocene time, laying down the great deposits originally described as the White River group by Meek and Hayden (1862.1, p. 433) in the following language: 104 TITANOTHEBES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA White River group * * * Wliite and light drab clays, with some beds sandstone, and local layers limestone. Fossils: Oreodon, Titanotherium, Choeropotamus, Rhinoceros, Anchithe- rium, Hyaenodon, Machairodus, Trionyx, Testudo, Helix, Planorbis, Limnaea, petrified wood, &c., &o. All extinct. No brackish- water or marine remains * * * I^OOO feet or more * * * Badlands of White River; under the Loup River beds, on Niobrara, and across the country to the Platte. * * * Miocene. This original definition appears to include all that has been determined subsequently and mapped by the United States Geological Survey (Darton, 1905) under three formations, namely, Chadron, Brule, and Arikaree, as shown in the accompanying illustration east. This fact is significant. It would appear, as stated at the beginning of this chapter, that sedimen- tation in this region was suspended after Denver, Lance, and Fort Union time. The Eocene gradients were so high that there were long periods of erosion, during which large areas of Upper Cretaceous beds were laid bare in the region that now includes North and South Dakota, western Nebraska, and Colorado, so that the lowest Oligocene sediments of the White River group, composing the Titanotherium zone (Chadron A), lie in gentle valleys of ancient formation that range in age from the Algonkian to the Denver formation and Dawson arkose. In Hayden's typical Figure 71. — Map showing tributaries of Cheyenne River, S. Dak., from the southeast and tlie type locality (X) of the " Titanotherium beds" of Hayden (Chadron formation), on Bear Creek; also principal collecting ground of Hatcher (dotted area) , the chief fossiliferous area in the Big Badlands (fig. 69). Meek and Hayden did not, however, specif- ically define the upper limit of theii' White River group, and all the fossils listed by them as characteristic of the White River group apparently came from beds now classified as Oligocene. The name White River group has therefore for years been restricted to the beds of Oligocene age (Brule and Chadron formations). This great flood-plain deposition was preceded by a long period of erosion in Eocene time. No sedi- ments of Wasatch, Bridger, or Uinta age have been found on the Plains east of the Front Range of the Rocky Mountains, except in a small area of Huerfano sediment which lies within a mountain basin farther locality of the White River group — the Mauvaises Terres of early explorers — the Big Badlands between the Cheyenne and the White River of South Dakota — the underlying beds are composed entirely of the Pierre (Upper Cretaceous). At some places (Loomis, 1904.1, p. 432) the rivers depositing the Titanotherium- bearing beds washed out along theu" banks masses of the Pierre shale that contained characteristic Pierre fossils — Baculites and the bones of Cretaceous rep- tiles — and redeposited them in Oligocene sediments. On this level, the gently undulating surface of the Pierre, east of the Rocky Mountains and the Black Hills, meandered broad, sluggish streams, whose chan- ENVIEONMENT OF THE TITANOTHEEES 105 nels ranged in width from a few hundred feet to half a mile. Beside these stream channels there were lagoons and areas of back water, some of them spread- ing into shallow lakes but none into vast sheets of fresh water. Savannas were interspersed with grass- covered pampas traversed by wide, meandering rivers that frequently changed their course. In these chan- nels were deposited conglomerates and river sandstones, marked by cross-bedding, as well as calcareous grits In the shallow lagoons and back waters were deposited the fine clays and layers of fuller's Qarth. The de- posits of gypsum represent periods of evaporation. In the lower part of the Titanotherium zone the de- it spread over the great area on which it has left its traces by the deposition of its peculiar sediments. * * * The basin-like character of this formation is most admirably shown." In the same memoir, Leidy (1869.1, p. 25) expressed some doubt as to the lacustrine theory, observing: "It is a remarkable cir- cumstance that among the large quantity of fossil bones brought from the Mauvaises Terres and sub- mitted to the examination of the author, certainly amounting to several tons in weight, there was de- tected no trace of remains of birds or fishes; and the same may be said of reptiles, except one species of turtle." Juan Ot/l^^ Ul 'lo he.iLi' ol UjJ.yiU-J Figure 72. — Type locality of the " Tiianoiheriuvi beds ol llaj deu, oii Bear Creek, S. Dak. Panoramic view, connecting at X. Upper section, looking southeastward, up Bear Creek; lower section, looking northwestward, down Bear Creek. Am. Mas. negatives 104722-104726. posits consist chiefly of fine flood-plain or overflow sediments interspersed with river sandstones and conglomerates, perhaps locally lacustrine, and occa- sional layers of volcanic ash. This theory that the deposits of the western Great Plains region are of flood-plain and fluviatile origin has gradually replaced the older lacustrine theory that they were laid down in great fresh-water lakes. The lacustrine theory originated with Hayden, who, in his geologic introduction to Leidy's memoir of 1869 (1869.1, p. 18), observes: "One of the most interesting features in regard to this great fresh-water lake is the evidence of its growth from a germ, as it were, until 101959^29— VOL 1 9 The lake-basin theory was generally adopted by geologists and paleontologists, reaching its apex in King's development of the lake theory both for the Plains and the mountain region. Johnson (1901.1), Gilbert (1896.1), Haworth (1897.1), and especially Davis (1900.1) reviewed the whole subject broadly in a critical way, developing the theory of fluviatile and flood-plain origin. Fraas (1901.1), Hatcher (1902.3), and more recently Darton (1905.2) set forth strong evidence for the theory of deposition in river channels, flood plains, back waters, lagoons, and shallow lakes. Among paleontologists Matthew (1899.2; 1901.1) was the first to attack the lacustrine theory of the 106 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA origin of the Brule clay of the White River group and to advance reasons for believing that the sandstones were formed by river and flood-plain sedimentation and the clays in part by back water and lagoon and chiefly by eolian sedimentation. His paleontologic Porcupine Butte CANIC ASH LAYER t-: H.). Id s Brontops dispar. Skull (erroneously m 1 lettered P). Nat. Mus. 4245. Skull J, Nat; Mus. 4738. Hat Creek, Lower B, probably correct (J. B. H.). Brontops brachycephalus?. Skull M, Nat. Mus. 4259. -(-.55.6feet(J.B.H.). Menodus trigonoceras. Skull G', Nat. Mus. 4291. Skulls N, A, B No.? (a large skull). Level B, probably correct (J. B. H.). Brontops brachycephalus. Skull i", Nat. Mus. 4258. -f71.4 feet (J. B. H.); -f48.5feet(N.H. Darton). ENVIKONMENT OF THE TITANOTHEKES Geologic succession of Oligocene titanoiheres in the Chadron formation — Continued 115 Level Brontops A Hops Menodus Megacerops Brontotherimn Classification uncertain 3 Brontops brachycephalus. Skull X', Nat. Mus. 1214. Level probably correct (J. B. H.). Brontops brachycephalus?. Skull m, Nat. Mus. 4940. Level probably correct (J. B. H.). Brontotherium ?hypoceras. Skull K', Nat. Mus. 4702. Level very doubt- ful (J. B. H.). Skulls V, I'. B s 2 Diploclonus tyleri. Type skull. 35 feet above Pierre shale and 165 feet below top of formation (R. S. Lull). Allops marshi?. Skull E, Nat. Mus. 1213. Level proba- blycorrect (J.B.H.). Brontotherium leidyi (type). Skull R, Nat. Mus. 4249. Level cor- rect (J. B. H.). 1 1 Brontops dispar??. Skull P, Nat. Mus.? (not 4245). "This skull in pieces found July 4, 1887, right at base of beds" (J. B. H.). Brontops brachycephalus. Skull c, Nat. Mus. 4261 (type) . Lower levels (J. B. H.). Skull b, Nat. Mus. 4947 (marked 1991). +14.4 feet; 130.6 feet below top (J. B. H.). Very young skull (new born?), Univ. Wyoming. "From extremely low level" (W. H. Heed). Bates Hole, Wyo. Allops walcotti (type) . Skull Q, Nat. Mus. 4260. "Probably lower beds, level A, fine-grained sand- stones" (J. B. 11.). Menodus heloceras. Skull, Am. Mus. 14576. At base of beds near Hailey, Wyo. (W. Granger). Brontotherium leidyi. Skull, Carnegie Mus. 93. Hat Creek, Nebr., 15 or 20 feet from bottom of lower beds (J. B.H.). HATCHER'S COHECTIONS, 1886-1888 According to a report delivered orally by Hatcher to the author in 1901, the collections made by him in 1886 included 24 skulls, some from Hat Creek, Nebr., and some from the Big Badlands of South Dakota, which were designated in his records and field notebooks by the letters A, B, C, etc., but which now bear United States National Museum numbers. In 1887 Hatcher collected from Sioux County, Nebr., mostly from Big Cottonwood Creek (adjoining Hat Creek), a second series of skulls, which he similarly designated by the letters A to K. Later in the same season he moved camp to the South Dakota Badlands and collected the skulls L to Z and a to w. Thus during the season of 1887 he collected 45 skulls. During the season of 1888 he collected another series of 24 skulls in the South Dakota Badlands, which he designated by the letters a', b', c', to z' but which Professor Marsh later relettered A' to Z' . Subsequently the catalogu ers of the United States National Museum assigned numbers to all these skulls. These revisions of the records have caused confusion, so that it is now doubt- ful whether certain skulls that bear capital letters and United States National Museum numbers belong to the series of 1886 from Hat Creek, Nebr., or to the series of 1887 from Big Cottonwood Creek and the South Dakota Badlands. Such uncertainty, of course, involves equal uncertainty as to the localities and geologic levels at which the specimens were obtained, but nearly all uncertainties have been settled by Dr. W. K. Gregory through careful examination of all the available evidence, with the assistance of Mr. J. W. Gidley, of the United States National Museum. The above table is based on these original and revised records. This remarkable collection, now preserved in the United States National Museum, constitutes the reference standard as specifically determined by the author with the assistance of Messrs. Gidley and Gilmore and includes the skulls and jaws indicated below, which are enumerated in detail under the respective genera in Chapter VI : Allops phylum: 24 skulls and lower jaws in four specific stages. Diploclonus phylum: 1 skull in one specific stage. Brontops phylum: 58 skulls and jaws in three specific stages. Brontotherium phylum: 42 skulls and jaws in nine specific Megacerops phylum: 7 skulls and jaws in three specific stages. Menodus phylum: 26 skulls and jaws in four specific stages. Figure 76. — Section showing the results of stratigraphic leveling in the Chadron formation (Titanotherium zone) in the badlands of White River, S. Dak., in June, 1901, by N. H. Darton The results are affected by dip, by unconformity, and by variation in the thickness of the beds. In determining the dip the beds showing the nearest reliable con- tacts of the Chadron with the Pierre formation were selected for all the levelings, and as most of the distances determined were short and were measured along the strike of the low-dipping beds the angle of the dip is unimportant . The Chadron formation lies on a smooth plane of unconformity, and its basal member is gen- erally continuous but was doubtless laid down against a sloping shore, and the layers are not synchronous throughout its extent. Nearly all the bones listed in the text, however, were found in an area so small that this unconformity is unim- portant. The variation in the thickness of beds is the most important factor aflecting the determination of the stratigraphic levels and one that could not be accurately determined, for the beds present so much variation in character that they can not be followed for a distance long enough to afford a basis for strati- graphic subdivision of the formation. A horizon 30 feet above the base of the formation at one point may represent a horizon 45 feet above it at another point; thus a bone found at A may have been deposited at the same time as a bone found at B. 116 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA SOURCES OF ERROR IM DETERMINING STRATIGEAPHIC LEVELS It should be borne in mind that owing to the great difference in the thickness of the " Titanotherium beds" in different locaUties and to the irregular topog- raphy of the Pierre shale upon which the beds rest it often happens that the base of these beds at one point may correspond to the middle Titanotherium zone at others, so that an exact stratigraphic subdi- vision of the Chadron formation over wide areas is some 165 feet below their summit. Hence this skull is assigned to Hatcher's level A 3, although its large size and progressive structure would lead one to infer that it came from the upper Titanotherium zone (Chadron C). Notwithstanding these discrepancies we are able to follow the evolution of five separate phyla of titanotheres, from the small animals of the lower Titanotherium zone (Chadron A), which have small Adaptive radiatiorL of the subfaTnilies of PerLssodcLctyls Haiiits OTui habitats AQUATIC Kv-3 MEDIPORTAL WpM GRAVI PORTAL ESS Figure 77. — The family tree of the Perissodactyla Adaptive radiation of the 9 families and 35 subfamilies. Their divergence in limb and foot structure into cursorial, forest-living, mediportal, and graviportal types, and in tooth structure into browsing and grazing types, is indicated by respective symbols. not at all possible. In spite of such opportunity for error only a few well-authenticated records (such as that of the type of Brontops dispar) appear to con- tradict Hatcher's statement that the titanotheres of advanced structural development are confined to the upper levels of the beds. One such striking exception is recorded by Prof. R. S. Lull (1905.1), who states that he found the type of Diploclonus tyleri at a point only 35 feet above the Pierre shalei at the base of the Titanotherimn beds, which was there horns, through intermediate types to the latest forms, which have highly specialized skulls, from the top of the " Titanotherium beds." This evolution was rather even and regular in the phyla Brontotherium and Menodus {= Titanotherium) , but in the Brontops phylum it appears that some of the primitive types of the lower zone survived with little change into the middle zone (Chadron B), and that other primitive types evolved gradually into the more specialized species of the middle and upper zones. ENVIEONMBNT OP THE TITAN0THEEE5 117 Height in feet above Pierre shale at which remains of titanotheres were found as determined in 1901 hy J. B. Hatcher and N. H. Barton'^ Skull Y' (?). "Big flat-horned skull in National Museum"; Nat. Mus. 1211 (?) Skull Q. Indian Draw. Probably skull "small q" was meant (Nat. Mus. 4946, Brontotherium curtum), ' ' wrongly lettered Q" Large-horned red skull. Nat. Mus. 4256, Brontotherium medium (type) Skeleton. Am. Mus. 518, Brontops robustus? Skull v. Indian Draw. Nat. Mus. 4711, Megacerops copei (type) Long-horned skull. West branch of Indian Draw. Brontotherium ramosum Skull M. Near Middle Corral Draw. Nat. Mus. 4259, Brontops brachycephalus Skull "F." Quinn Draw, South Dakota. ?Nat. Mus. 4258, Brontops brachycephalus Skull O'. South side of west fork of Corral Draw. Nat. Mus. 4705, Megacerops "bucco," female Skull "I." Quinn Draw, South Dakota. Nat. Mus. 2151, "AUops serotinus," female Skull "H." Quinn Draw, South Dakota. Nat. Mus. 4251, Allops serotinus (type) Skull "little F." Quinn Draw, South Dakota. Nat. Mus. 4703, Brontops dispar Little skull "B." On fork of west fork of Corral (?Quinn, J. B. H.) Draw. Probably skull b, Nat. Mus. 4947, Brontops brachycephalus, female, aged 81 C) 65.4 "93. 3 81. n 55. 6 71.4 46. 7 80 77-34 62 65. 3 62. 3 55. 6 48. 5 46. 7 43.5 40.7 39.0 14.4 • In a letter to the author, dated July 31, 1901, Hatcher expressed grave doubts as to the accuracy of these levels on account of practical difficulties encountered in the field. » From horizon ot skull O', Nat. Mus. 4705, to horizon of this skull there is a vertical upgrade of 46.6 feet. ' 27 leet above skull V. •I Very high, 8 to 10 leet from top of titanothere zone (Hatcherl. • 32 leet below the 3-foot sihceous limestone layer at top of Titamtherium zone. The Pierre shale contact was far away, and although it was on a line of levels the dip in interval could not be ascertained precisely (Darton). MAMMALIAN LIFE OF THE LOWER OLIGOCENE TITANOTHEEIUM ZONE The most highly characteristic feature of the Ohgo- cene mammals as a whole, compared with the Eocene mammals, is their decided modernization, which is shown in the following table giving the percentages of the modern and the archaic families of the Oligo- cene Plains fauna as compared with those of the Eocene mountain-basin fauna. Percentages of modern and archaic families in Eocene, Oligocene, and Miocene time Basal Eocene Lower and middle Eocene Upper Eocene Lower Oligocene Miocene Modern fami- lies or those closely related or ancestral to modern families Archaic fami- lies supposed to be wholly extinct and not closely related to modern types This modernization of mammalian life is in part real and in part apparent, because the Plains fauna presents for the first time the full aspect of the upland, plains, and meadow life, especially the smaller and larger herbivorous ungulates. This life is, however, only partly revealed in the Titanotherium zone, in which conditions for the fossilization and preserva- tion of the land fauna were less favorable than in the overlying Oreodon zone (Brule clay). In fact, re- mains of the small ungulates, such as the horses of the period (Mesohippus) , are very rarely preserved in either the coarser or the finer sediments of the Chad- ron of South Dakota but are found more abundantly in the sediments of Pipestone Creek, Thompson Creek, and other areas in Montana and in the Swift Current Creek area of Saskatchewan. The entire Titanotherium zone fauna as listed by Osborn and Matthew (1909.321, pp. 103, 104) contains representa- tives of 6 orders and 24 families of mammals, which are of interest and value as showing the principal types of mamnxals that were in competition with the titanotheres in the struggle for existence. 118 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA Fish, reptile, and mammal Jauna contemporary with the titanotheres Common name or comparable form, habits or habitat, nature of deposits, etc. Classific name Region inhabited PISCES Actinopterygii: Amiidae — Do. Do. Gar pikes (Lepidosteus) ; rivers and streams. - Lepidosteidae — Do. Catfislies (siluroids) ; rivers and streams Siluridae — Do. Do. Do. Crocodiles and alligators; rivers and streams __ REPTILIA Crocodilia: Crocodilidae — Do. South Dakota. Squamata: Anguidae — Helodermoides tuberculatus Douglass Montana. Do. Palaeophidae — Subterrestrial ; wet and forested places Chelonia: Dermatemydidae — Xenochelys formosa Hay South Dakota. Emydidae — Land tortoises; characteristic of uplands, open country. Testudinidae — Do. Do. Soft-shelled turtles; aquatic; remains found in fluviatile sandstones. Trionychidae— PIat3'peltis leucopotamica Cope Saskatchewan, South Da- MAMMALIA Marsupiaha: Didelphidae — kota. Cursorial, predacious, like the Thylacinus of Tasmania; resembling modern wolves in Ferae : Hyaenodontidae — "Pseudopterodon" minutus" (Douglass) Montana. Comparable with hyena of Africa; powerful Saskatchewan, South jaws. Hyaenodon cf. H. crucians Leidy Dakota. Do. Canidae — Daphaenus cf. D. hartshornianus Cope Daphaenus cf . D. felinus Scott ** Saskatchewan. Do. South Dakota. Analogous to the marten and mongoose Nothocyon "^ paterculus (Matthew) Montana. South Dakota. Mustelidae — Bunaelurus inf elix Analogous to the marten and polecat Montana. " This is, in fact, an undescribed genus, more primitive than Hyaenodon and Pterodon, allied apparently to Sinopa and Tritemnodon. >> If Mr. Lambe's figure is correct this can hardly be Z>. felinus; it agrees much better with D. dodgei Scott. ' CynodicHs is not applicable to the American OlJgocene species that have been called by that name. Nothocyon is next in priority among available names. The type, however (N. geismarianus) , is a large and rather specialized species from the John Day formation. It maybe necessary to separate the small species from the middle and lower Oligocene under the name Pseudocynodictis (Schlosser). ENVIKONMENT OP THE TITAN OTHERES Fish, reptile, and mammal fauna contemporary with the titanotheres — Continued 119 Common name or comparable form, habits or habitat, nature of deposits, etc. Classific name Region inhabited Analogous to the leopard and cheetah MAMMALIA — Continued Ferae — Continued. Felidae— South Dakota. Possibly like the modern Gymnura of the East Insectivora: Leptictidae — Montana. Do. Do. Do. Do. Do. Allied to the Solenodon of the West Indies(?) _ ?Solenodontidae — Montana. Fossorial, like the Cape golden moles Burrowing; analogous to marmots, prairie dogs, and the like. Squirrels and ground squirrels (spermophiles). ? Chry sochloridae — Montana, Wyoming. Montana. Saskatchewan. Montana, Wyoming. Assiniboia. Montana. "Xenotherium" "^ unicum Douglass Nothocyon "lippincottianus" (Cope) Glires (Rodentia) : Ischyromyidae — Titanotheriomys veterior (Matthew) Titanotheriomys ' ' Ischyromys typus Leidy ' ' _ Do. Prosciurus ?saskatohewensis (Lambe) Saskatchewan. Castoridae — katchewan. Do. Found in Chadron clays; like pocket mice; Perognathus. Rabbits; remains found in the Chadron clays-. Heteromyidae — Adjidaumo(Gymnoptychus) minor Douglass- Adjidaumo (Gymnoptychus) minimus Mat- thew. Leporidae — Montana. Do. Do. Do. Saskatchewan. Grazing, upland rhinoceroses; cursorial; found in the Chadron clays. Perissodactyla: Hyracodontidae — Amphibious rhinoceroses; found in the channel Amynodontidae — South Dakota. Do. Small rhinoceroses of rather slender propor- tions, probably of browsing habit; remains found chiefly in clays. Rhinocerotidae — Do. Do. Leptaceratherium trigonodum Osborn Do. Assiniboia, South Dakota. Do. Caenopus cf. C. platycephalus Osborn South Dakota. Very small, slender-limbed horses, cursorial; grazers and browsers. Abundant in clays; Lophiodontidae — Colodon ( = Mesotapirus) occidentalis Leidy. Equidae — Dakota. South Dakota. Colorado, South Dakota. Mesohippus westoni Cope Saskatchewan. Do. Do. Mesohippus precocidens Lambe Saskatchewan. I Name preoccupied by Xenotherium Ameghino, 1904, a genus of edentates. 120 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA Fish, reptile, and mammal Jauna contemporary with the titanotheres — Continued Common name or comparable form, habits or habitat, nature of deposits, etc. Classiflc name Region inhabited Suillines abundant and characteristic. Diffi- cult to place. Small, compact, dldactyl feet and fairly long limbs, cursorial. Ribs and abdomen small. Common in clays and sandstones. MAMMALIA — Continued Artiodactyla: Entelodontidae — South Dakota. Do. South Dakota. Dicotylidae ( = Tagassuidae) — South Dakota.- Leptochoeridae — Stibarus montanus Matthew -- Montana. Analogous to pigs. Occur chiefly in the clays-_ Anthracotheriidae — South Dakota. Do. Peccary-like, but of grazing habits. Rather Oreodontidae ( = Agriochoeridae) — Montana. Do. Browsing. Agriochoerus partly arboreal proportions like the larger cats. Do. Oreodon (= Merycoidodon) hybridus Leidy. Oreodon (= Merycoidodon) buUatus Leidy_- Oreodon (= Merycoidodon) affinis Leidy Oreodon ( = Merycoidodon) " culbertsonil Leidy." Agriochoerus maximus Douglass Agriochoerus minimus Douglass South Dakota. Do. Do. Saskatchewan. Montana. Do. South Dakota. Saskatchewan. Analogous to existing chevrotains of Africa — Hypertragulidae — Trigenicus socialis Douglass ?Trigenicus mammifer Cope Montana. Saskatchewan. Do. Do. South Dakota. Do. Grazing, upland, cursorial, like the smaller antelopes of Africa and the guanacos of South America. ?Heteromeryx transversus Cope " Anthracotherium pygmaeum" Lambe " Camelidae — Saskatchewan. Do. ?Leptotragulus profectus Matthew Montana. « Based upon a part of a "right upper molar," which from Mr. Lambe's figure appears to be a left lower molar, probably of a hypertragulid comparable to Heieromeryx. NOTES ON THE HABITAT OF THE FAUNA OF THE CLAY AND SANDSTONE AS A WHOLE Matthew was the first to distinguish between the upland forms, found chiefly in the clays (flooded plains), and the lowland and aquatic forms, found in the sandstones (river channels). The following dis- criminations have been made: 1 . Typical grazing group oj open plains. — Hyracodon, Oreodon, Mesohippus, Eotylopus, Poehrotherium. Note the cropping front teeth, associated with delicately cut and progressively long-crowned grinders, small, compact feet, and, except in Oreodon, long, slender limbs. Colodon may belong here. 2. Browsing group of hush country and forest. — Titanotheres, Metamynodon, Caenopus, lEntelodon, 1 Anthracotherium, lAncodon, ''.Agriochoerus. All large- sized fighting beasts, with coarse, heavy enamel on cheek teeth; front teeth adapted to lip browsing. Metamynodon may very likely have been amphibious ; the others probably were not. Entelodon is somewhat of an enigma; Sus is the nearest analogue but not a close one. 3. Small hush or forest-dwelling browsers. — Hetero- meryx, Leptomeryx, Trigenicus. Analogous to the modern tragulines and probably of similar habits. 4. Carnivora. — The hyaenodonts are analogues of the wolves. The ancestral canids are analogues of the mustelines and viverrines. True mustelines are scarce. Dinictis is the only cat. 5. Rodentia. — Rabbits much like modern "cotton- tails" of the Great Plains. Heteromyids have ap- peared, but no true mice (Muridae) imtil the middle ENVIHONMENT OF THE TITANOTHEEES 121 Oligocene. Ischyromyids are abundant and include terrestrial (?) and arboreal (?) forms; whether fos- sorial forms existed or not is not proved. Eutypomys, though referred to the Castoridae, is not at all analo- gous to the modern beaver but rather to a large squirrel or spermophile. 6. Insedivora. — The leptictids have rather sharp- cusped teeth and are intermediate in type between opossums and tree-living erinaceids. The moderate wear of the teeth is evidence against the theory that their food was worms or other terrigenous forms. There are no obvious arboreal adaptations in the limbs and feet; perhaps they may have been semi- arboreal. Their survival, unaltered as to cheek teeth, from the basal Eocene is suggestive of some special protection, such as spines. As for the zalamb- dodonts, they may have been fossorial, Xenotherium being molelike, but the evidence is insufficient. 7. Marsupialia. — Rare. Precisely like small opos- sums in the structure of the teeth. 8. Aves. — No birds have been recorded in this fauna, although they probably existed and may have been even numerous and varied. 9. Reptilia. — Crocodiles and trionychids occur in the sandstone lenses; probably they were aquatic forms analogous to modern crocodiles and soft-shell turtles. In the clays Testudo occurs; also Xenochelys, probably similar in habits to modern land tortoises and marsh turtles. The lizards are apparently analo- gous to the Gila monster and to some of the swift- footed anguid lizards. Burrowing amphisbaenids occur in the Oreodon zone but have not yet been dis- covered in the lower Oligocene; no doubt they formed part of the fauna; also other lizards and many snakes. 10. Batrachia. — No batrachians have been recorded, but there is no reason to suppose that they were absent or rare. 11. Pisces. — A few fragments of fresh-water fishes, similar to those characteristic of muddy rivers of to-day, are recorded from the Swift Current beds in Canada. They will doubtless be found in the sand- stones and other stream deposits of the Titanotherium- bearing beds of the United States. SECTION 3. ADAPTIVE RADIATION, PRIMARY AND SECONDARY, THROUGH CHANGE OF ENVIRONMENT A CAUSE OF THE DIVERSIFICATION OF THE TITANO- THERES HABITAT OF THE UNGULATES The present geographic features of modern equato- rial Africa, consisting of a high central plateau, river borders, savannas, and forests, exhibit a close parallel to what we believe were those of the known titanothere region of North America in Eocene and lower Oligocene time. These conditions may also be compared with those found in the existing flood plains at the head- waters of the great rivers of South America east of the Andes in the warm temperate and subtropical but not in the tropical belt. 101959— 29— VOL 1 10 Adaptive radiation: Favorite habitats of existing perissodactyls and elephants [See fig. 78] RHINOCEROSES Rhinoceros sondaicus. Java. Typically a forest dweller, occasionally found in alluvial swamps. A browser. Rhinoceros (Dicerorhinus) sumatrensis. Hilly forest districts of Sumatra. A browser. Rhinoceros {Opsiceros) bicornis. Bush-covered country and open plains; forested foothills in the dry season. Fairly abun- dant on the top of the Aberdare, British East Africa (elevation 9,000 feet). A browser, feeding on shrubs, roots, leaves, etc. Rhinoceros unicornis. Grassy jungles of India. A grazer. Rhinoceros (Ceratotherium) simus. Savannas and grassy plains, with swamps or water holes for wallowing. A grazer. TAPIRS AND ELEPHANTS Tapirus roulini. Pinchaque tapir of the high region of the Andes and Cordilleras. A browser. Elasmognalhus bairdi. A hill dweller, seeking lowlands during rainy seasons. A browser. Tapirus terrestris. A forest dweller. Lowlands of Brazil and Paraguay. A browser, feeding on palm leaves, fruits, water plants. Tapirus indicus. Lowlands and forests of India. A browser. Elephas (Loxodonta) africanus. Less typically a forest animal than E. indicus. Savannas, dry country, and forests. Ranges from the seacoast to points beyond the alpine heath zone of Mount Kenia and the bamboo belt of other African mountains. Ascends and descends steep places with wonderful facility. A browser and grazer. HORSES AND ZEBRAS Equus burchelli. Essentially a plains dweller; often found in sparse savannas. E. grevyi. Grevy's zebra. Low plateaus, thorn bush and feather grass country that has gravelly soil. Essentially a dweller in open plains and savannas. E. quagga (extinct). The quagga. A karoo dweller. Fre- quents open, arid plains. E. zebra. Mountain zebra. Hilly and mountainous country. E. przewalskii. Przewalski horse. Gobi Desert. A steppe dweller. ASSES Equus asinus. Abyssinian ass. Wiry hedge and upland country. E. hemionus kiang. The kiang. Desolate plains in the vicinity of lakes and rivers. High table-lands of Tibet (15,000 feet) . Coarse wiry pasture and rough hard yellow grass. E. asinus somalicus. Striped African ass. Borders of the Nubian Desert. E. hemionus onager. Persian wild ass. Migrates from the plains to the hills in summer. The onager of Persia. POLYPHYLY AMONG HOOFED MAMMALS THE TITANOTHEEES AND OTHER EXTINCT FORMS It is astonishing to find within relatively small geographic areas both Eocene and Oligocene remains of many kinds of titanotheres, which lived close together under very similar climatic conditions, the more so because the known geographic distribution of the titanotheres in Eocene time is confined to the cen- tral Rocky Mountain region and extends only from the Wind River Basin of Wyoming on the north to the White River Basin of Utah on the south, a distance of 122 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA kl ^ ^ ■5 1^ ■<5 ^ ~ci ^ \ ro .fo 1 ~^ S ^ f:; ^ /2.O00 to /O.OOO 6.000 fo 2,000 /oo' ^ ^ ^ « ^ I S >0 Vi I I /-/yr3p opo fa/77/ Z?C/^0/7^ N £ as/'nus {/Africa) £.przew3/sA/ (y^sia s/eppesj £. ze.&r^ fj^fr/ca p/3:^e.aus) NypoAippic/S— ttypc h/pp/c/s £. zebrs £. iurc/ie/// £.^revy/ £. ^cjsgga ■5 Tap/rus C/t/g/7 (Soc/M /Indi'^s) A/7?e r/ca) Tap/rus ba/rc// ^oL//h /imer/'ca) Tap/ras /nd/cus 77 smer/canus (/.Oiv/anc/.r S. Amer/ca) Te/ma'fheres (H/^h p/3ins) Menodonfs— Symboroc/onfs (Hi'//y regions) Man/eoceraf/nes Bronfofheres ^f/oocf p/aJ/7s) -Menodonts Bron fop/n es DoZ/chorh/nes Mefarhines Pa/aeosyop/nes Figure 78. — Geographic cross section showing the nature of the habitats of the larger existing ungulates and of the titanotheres as illustrating adaptive radiation The upper row shows the present geographic distribution ot the ungulates in continental Africa and the theoretic geographic features of the Rocky Mountain region in Eocene and Oligocene time— namely, high valleys, plateaus, foothills, plains, river valleys, flood plains, bot- tom lands, and river and lake borders. The second row shows the corresponding present distribution of the plant foods of different types of browsing and grazing, cursorial, graviportal, and semiaquatic quadrupeds. The four next lower rows show, in descending order, the corresponding adaptive radiation of the rhinoceroses, extinct and living; of the elephants and mastodons and the typical aquatic hippo- potami and sirenians; of the plateau, plains, and forest types of horses; of the mountain, foothill, and lowland types of tapirs. The bot- tom row shows the theoretic adaptive radiation of the principal types of titanotheres— telmathcres and menodonts of the higher levels; symborodonts in the foothills; manteoceratines, brontothercs, brontopines, and menodonts on the flood plains; dolichorhines, metarhines, and palaeosyopines on the lowlands and river borders. ENVIRONMENT OF THE TITAN OTHEEES 123 480 kilometers (298 miles). The continental extent of the distribution of the titanotheres, which is still unknown, was undoubtedly far greater, including, perhaps, the larger part of the North American continent and certainly extending into Asia. In Oligocene time the known geographic distribution was somewhat larger, including an area extending from Colorado to southern Alberta and measui'ing from north to south about 1,200 kilometers (746 miles). Titanotheres lived also in eastern Europe, both in Transylvania' and Rumelia, also in Mongolia. Our present Icnowledge of the geologic horizons of the titanotheres is still extremely meager regarding certain strata. The extent of our knowledge is sum- marized below. Geologic horizons of the known genera and subgenera of the titanotheres Lower Oligocene; upper, middle, and lower levels: Brontops, Diplocloniis, Alloys, Menodus, Brontotherium, Megacerops. Upper Eocene; Uinta C (true Uinta) : Telmatherium, Man- teoceras, Diplacodon, Prolitanoiherium, Eotilanolherium. Upper Eocene; Washakie B and Uinta B: Metarhinus, Rhadi- norhinus, Mesatirhinus, DoUchorhinus, Manteoceras, Tel- matherium, Diploceras. Middle Eocene; upper part of Bridger formation : Mesatirhinus, Manteoceras, Palaeosyops, Telmatherium. Middle Eocene; lower part of Bridger formation: Limno- hyops, Palaeosyops, Eometarhinus. Lower Eocene; Wind River formation: Lamhdotherium, Eoti- tanops. As compared with what we observe among the hoofed animals living to-day these titanotheres certainly dwelt near one another under very similar conditions of climate but in different feeding ranges and local habitats; they sought the same watercourses, and their remains were entombed in similar deposits. As the whole tendency of discovery up to the present time has been to multiply the phyla, to separate and diversify the titanotheres, the probability is that many other kinds of titanotheres lived in other parts of North America and Asia. The evolutionary principle underlying these diversi- ties Osborn (1902. 214, p. 353) has called adaptive radiation, which is the application to paleontology of the idea of divergence as conceived and developed successively by the studies of Lamarck, Darwin, Huxley, and Cope." Radiation is a broader principle than divergence, because it implies evolution in every direction possible to the organism. The idea of radiating branches from central forms assists the imagination, because the known radiations of extinct animals must be supplemented by the unknown radia- tions, and it is most remarkable how these missing radii have been discovered in group after group of animals. Such adaptive "radiation" is either "con- tinental" — that is, it occurs where diversities in food, " See also Osborn, H. F., 1902.214; 1905.267; 1910. 345; 1910. 346; Stevenson- Hamilton, J., 1912. 1; Sclater, P. L., 1894. 1; Lydekker, K., 1893. 1; Gregory, J. W.^ 1896.1; Blanford, W. T., 1888.1; Kobelt, W., 1902.1; Schimper, A. F. W., 1903.1; Lonnberg, E,, 1912. 1; Roosevelt and Heller, 1914. 1. soil, or climate prevail over large areas — or "local" — that is, it occurs where marked diversities prevail in relatively small areas. The radiation among the titanotheres in southern Wyoming and northern Utah seems to have been largely "local," indicating that the physiography of the mountain basin was highly diversified. One of the results of adaptive radiation is poly- phyletic evolution, the existence within families of a large number of independent minor branches that may pursue more or less divergent evolution in local or continental regions but that may come together in river and flood-plain basins, so that their fossil re- LIMBS AND FEET Short-limbed, plantigrade,] AMBULATORY pentadactyl, ung:uicu-> or late stem ^ ^^JTORRESTRIAL CURSORIAL Digitigrade OMNIVOROUS (Grass Herb Shrub Fruit ^. „.„„o|..„.. ™<" "~-^^[Carr' MYRMECOPHAGOUS Dentition reduced Stem INSECTIVOROUS Figure 79. — Original radiation of the unguligrade Herbivora, Carnivora, and Inseotivora, showing the adaptations of teeth, limbs, and feet to various habits and environments mains are found in the same localities and deposits. Polyphyletic evolution has been discovered so fre- quently, among both the mammals and the lower forms of life, that it may be considered the rule and mono- phyletic evolution along single lines the exception. Some of the examples of polyphyletic evolution among extinct mammals that ha^'e been determined in com- paratively recent years are the following: Contemporaneous branches, or phyla Oreodonts (Cope, Wortman, Peterson, Matthew, Doug- lass) 7-9 Lophiodonts (Osborn, Deperet) 5-7 Anthracotheres (Stehlin, Deperet, Andrews) 6-8 Rhinoceroses (Osborn) 8-9 Horses (Osborn, Gidley, Matthew) 8-9 Titanotheres (Osborn) 10-12 Elephants and mastodons (Osborn) 7-10 124 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA THE EXISTING AFRICAN ANTELOPES The polyphyly among the titanotheres and other extinct Perissodactyla presents a marked contrast to the impoverished conditions among tlie existing mem- bers of the same order wlien we consider that in all parts of Asia and Africa only five kinds of existing rhinoceroses can be distinguished by the characters of the skeleton and teeth alone, that only six or eight kinds of horses, asses, and zebras in the same great region can be distinguished by their hard parts, and that, similarly, among the tapirs of Asia and South America only three kinds can be distinguished. This contrast between present monophyly and former poly- phyly is due to the fact that the order Perissodactyla, though formerly a dominant group, is now a declining group. In the existing Bovidae, especially those in the groat continent of Africa, we have a parallel to the ancient polyphyly of the titanotheres and other Perissodactyla. The Bovidae is a family that includes the cattle and antelopes and that is now in the highest stage of ra- diation and adapted to a great variety of physiographic and biotic conditions, as shown in the primary and secondary adaptations in the seven subfamilies of the African antelopes. The African antelopes: Subfamilies, habits, and environment Subfamilies and habits Antilopinae (browsers and grazers) Gazelles Pallahs (impalas) Springbucks Gerenuks Saigas Bubalidinae (mostly grazers) : Gnus Hartebeests Blesboks Sassabies Tragelaphinae (browsers and graz- ers): Elands Koodoos Bush bucks Bongos Situtungas Hippotraginae (grazers) : Roan antelopes Sable antelopes Gemsboks Addaxes Neotraginae (browsers and graz- ers): KUpspringers Oribis Dik-diks Cephalophinae (mostly browsers) ; Duikers Environment Plains and deserts. Thorny bush and glades. High veldts. Deserts and bush. Steppes. Open plains. Open forests or plains. Open rolling country. Open forests and flats. Stony hills. Forests. Dense forests. Swamps and lagoons. Thin forests. Rolling uplands. Open deserts. Waterless deserts. Hills, mountains. Thin forests. Dense forests and bush. The African anteloyes: Subfamilies, habits, and environment — Continued Subfamilies and habits Environment Cervicaprinae (grazers on suc- culent plants near water) : Open forests and stony hills. Reed swamps, river bor- ders. Open swampy plains. Slopes of hills. An incipient or attempted adaptation to a grazing life is seen in the teeth of certain titanotheres. Most titanotheres are browsers. Broadly speaking, her- bivorous animals that live on open plains are grass eaters and tend to become gregarious in habit and cursorial in locomotion, whereas those that prefer the shady depths of the forests are browsers, are of soli- tary habit, and are mediportal in locomotion. There are exceptions, such as the black rhinoceros (Rhi- noceros {Opsiceros) Mcornis), which now frequents the treeless plains of East Africa but which is habitually a browser, although it is at times a grazer. The long- necked giraffes are fond of rather dry and fairly open country and are not found in strictly forested regions, yet they are wholly browsers, being especially fond of the leaves of certain thorny acacias, notably Acacia girajfa, and the related short-necked okapi, which is found only in the dense forests of the Congo, is a browser. The principles of adaptation shown in the skull and teeth of Perissodactyla to browsing and grazing habits are described in Chapters V and VI of this monograph. The adaptation of the limbs of the Perissodactyla to speed and weight are described in Chapter VII. In general, the competition and range for food among hoofed animals is accompanied by lengthen- ing of the limbs from medium-paced (mediportal) types to either swift-moving (cursorial) types or heavy- bodied (graviportal) types. Similarly, adaptation of the grinding teeth to browsing habits is seen in the short-crowned (brachyodont) types, and transition to the grazing habit is accompanied by lengthening (hypsodontism) of the crowns of the grinding teeth. Such changes are accompanied by changes in the pro- portions of the head to adapt the action of the teeth to browsing or to grazing. We observe a passage from short-headed (brachycephalic) to long-headed (dolichocephalic) forms of skull. In adaptive radia- tion every possible combination of lengthening and of shortening of skull, tooth, limb, and foot may arise, as well as notable coincidences of structure in different forms, for similar kinds of food may be found and ENVIRONMENT OP THE TITANOTHEEES 125 similar feeding habits may be acquired in widely separated habitats or greatly different environments. Contrasts in structure, such as those shown below, are equally notably. Contrast in structure between browsing and grazing types Browsing types (brachyodont) Grazing types (hypsodont) Short-headed (brachycephalic) Long-headed (dolichocepha- lic). Straight-headed (orthooephalic) _ _ Bent-headed (cyptocepha- hc). Short-limbed (braehymelic, bra- Long-limbed (dohchomelic, ch ypodal) . dolichopodal) . Grazing on the harder siliceous grasses of dry plains and uplands Gazelle Addax Grazing and browzing on the tender leaves and twigs of plains, thin for- est and brush country Sable and roan Grazing and browzing on the tender grasses of moister land and swampy plains "Puku Cob Reedbuck Leaf, bark, and twig eaters in forests Duiker Browsing on tender leaves and shrubs of partly forested countries Bushbuck Waterbuck Figure Browsing on succulent aquatic plants of swampy lands Sitatunga Lechwe ). — Adaptions in the structure of the skull and teeth of Herbivora to diverse habits of feeding Double or even multiple adaptive radiation is continually in operation, first, in the structure of skull and tooth, which is dependent on the nature of the food, and, second, in the structure of foot and body, which is dependent on the nature of the soil. Thus may arise cursorial (long-limbed) grazers (long- toothed), graviportal (heavy-limbed) grazers (long- toothed), or cursorial (long-limbed) browsers. There is no fixed law of correlation of structure of skull and tooth such as was supposed by Cuvier. The law of correlation as restated by Osborn (1902.214) is as follows : Structure of feet (correlated chiefly with structure of limb and body) and structure of teeth (correlated chiefly with structure of skull and neck) diverge inde- pendently in adaptation respectively to obtaining food (by feet) and eating food (by teeth) in different environments. Each structural feature is evolved directly to perform its own mechanical functions or purposes, yet in such a manner that each is consonant with the other. CONTINENTAL ADAPTIVE RADIATION OF THE AFRICAN ANTELOPES The African antelopes are divided into seven sub- families, all mediportal to cursorial in limb structure but widely different in tooth and skull structure, as shown in the table on page 124. The 133 or more species (Sclater, 1894.1) embraced in these seven subfamilies seek food and protection from enemies on the varied surface of the African continent in habitats including no less than 17 differ- ent kinds of country. Each type of habitat has food peculiarly favorable to certain feeding habits to which the structure of the teeth and skull is speciflcally adapted. Each type of CURSORIAL Distance carrying Gazelle Gemsbuck (Oryx) Addax SALTATORIAL Leaping and springing Springbuck MEDIPORTAL ^ Klipspringer Medium-weight bearing Hartebeest Gnu Sable and antelopes SEMI-ARBOREAL Progressively on branches of trees Impala GRAVIPORTAL Heavy-weight bearing Greater kudu E'and \ \ FOSSORIAL Digging and uprooting AMPHIBIOUS Swamp and river-living Waterbuck Puku Cob AQUATIC Reedbuck Partly fluviatile, largely feeding and seeking safety in the water Lechwe Sitatunga Figure 81. — Convergent adaptations in the structure of the limbs and feet of ungulates Certain gazelles are independent of watercourses. The adaptive radiations indicated above occur independently within different subfamilies. habitat demands modiflcations of limb, foot, and hoof structure for movement in the search for food and escape from enemies. The theory of the evolution of the antelope is that in mid-Tertiary time a divergent primary radiation 126 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA divided them into seven subfamilies, each with its distinctive mode of life. During a long period of geologic time the Bovidae have undergone secondary radiations (A-Q, fig. 82), by which certain branches of these subfamilies have become adaptively convergent toward certain branches of other subfamilies through the adoption of similar habits and habitats. Thus, analogous genera and species arise independently in each subfamily. For example, waterless deserts were sought both by the addax, among the Hippotraginae, and by the gazelle, among the Antilopinae; reeds, river borders, and lagoons were sought both by certain Figure 82.- c -Adaptive radiation in the feeding habits of antelopes, as observed by Stevenson-Hamilton in 1912 1-7, Primary radiations; A-Q, secondary radiations Cervicaprinae, such as the lechwes and kobs, and by certain of the Tragelaphinae, such as the situtungas. ADAPTIVE RADIATION IN THE FEEDING HABITS ANTELOPES OF The habits and habitats of the antelopes, as noted by Stevenson-Hamilton (1912), are as follows: 1. Antilopinae. — The impalas {Apyceros melampus) eliiig to neighborhoods of dense thorny bush, to which they fly for refuge. More partial to brovv^sing than to grazing. Food con- sists largely of leaves and shoots, but they eat young and tender grass freely after early rains. Staple diet leaves and fruit of certain acacias, also twisted bean pods of the same. In March fruit of marula is eaten. Toward the end of the dry season they completely strip the bush of everything edible up to the extreme height which they are able to reach. * * * The springbucks (Antidorcas euchore) are typical of the high veldt fauna of South Africa. The only member of the gazelle group in this region. Love high, open tablelands. * * * Xhe typical African races of gazelles include 14 species. Grant's, Thomson's, Speke's, etc. Inhabitants of wide, open plains or sandy deserts. Largely independent of water. 2. Bubalidinae. — Antelopes of large size, large, moi.st rhinari- um; including Buhalis (= hartebeest), Damaliscus (= bastard hartebeest), Connochaetes (= gnu, or wildebeest). Buhalis (= hartebeest), eight species, with everywhere same charac- teristics; frequent open or forest countrj' or treeless plains; essentially grass eaters; like to drink regularly. Young carried about eight months. * * * Damaliscus, bonte- buck {D. pycargus), blesbuck, tsessebe (sassaby) (both D. albifrons), grass-eat- ing antelopes, favoring rather open and fairly flat country, never hills or thick jungle, partiality for shady patches of bush or forest for shelter during the hot hours. * * * Connochaetes (gnu, or wildebeest), white-tailed or black (C ()7iu) and brindle or blue (C. tautrinus). Prefer open, rolling country interspersed with thick thorn or other bush. Some- times remain in the open, bare spaces or plains where they can see for long dis- tances. Essentially grass-eating ani- mals. Pasture cropped closely. Socia- l5le, gregarious. 3. Tragelaphinae. — Elands and bush- bucks, inyalas, kudus, situtungas. Elands {Taurotragus oryx), plains type, graze with horses, donkeys, and cattle but browse by preference, favoring the grass only when fresh and green, sometimes cropping the tops of young river reeds. Gestation period eight and a half months. * * * Bongo {Boocercus eurycoros), fond of the most dense forest, leaves and twigs of a certain kind of undergrowth, which grows from 6 to 8 feet in height. Young shoots all nipped off if bongo have been feeding. Also (?) bark eaters. * * * Bushbucks {Tragelaphus scrip- txis), forest dwellers, solitarj', nocturnal, prefer densely wooded gullies, or kloofs, of South Africa. Browsers on the leaves of various small shrubs and trees; eat grass sparingly when the latter is fresh and green; roots and tubers form fur- * * * Inyalas {Tragelaphus angasi), Exhibit intense localization, probably due to the presence of some peculiar foodstuff, limited in quantity but necessary to the health of the individual animal. Probably browse on various leaves, shrubs, and fruits, bean pods and acacias, fruit of the marula; grass eaten when it is young and of good quality. * * * Sitiitnnga.s (Tragelaphus spekei) , semi- aquatic animals, almost amphibious by nature. Great elonga- tion of hoofs. Strong swimmers. Rapid locomotion upon dry land very difficult. Frequent extensive reed and papyrus swamps bordering lakes and large rivers. * * * Kudus {Strepsiceros strepsiceros) , love stony or rather broken ground, covered with thorn scrub. Gregarious, more than most antelopes, a browser, subsisting chiefly on the leaves of thorn acacias and bush shrubs, together with the fruits of the marula and other trees. ther articles of diet, very local and rare. ENVIRONMENT OF THE TITANOTHERES 127 4. Hippotraginae. — Sable and roan antelopes Hippotragus, oryx and addax, distinguished by the presence of horns in both sexes and small rhinarium or bare space on the muzzle. Sable (H. niger) to a great extent, though not entirely, a grass eater. Prefers thin forest country, interspersed with alternate thickets for shade, and open vleis for grazing. Regular drinker, seldom found more than a few hours from water. Gestation period about 270 days. * * * Roan antelope {H. equinus) favors rather upland, rolling country, not too thickly wooded, such as the middle veldt, but when persecuted takes readily to forest or the same environment as the sable antelope. A grass eater, and drinks regularly * * * [Genus Oryx.] The gemisbuck (0. gazella) of South Africa separated from its nearest generic relative (0. beisa) of German East Africa by an interval of 1,500 miles. Fairly numerous in Kalahari Desert, main- taining its security owing to its independence of water, able to quench thirst from moist tubers and roots. Generally found in small troops. The beisa (0. beisa) inhabits the Kilimanjaro district, British East Africa, Somaliland and the Sudan, east of the Nile. Sometimes found in herds of 50. Period of gestation eight and a half to ten months. White oryx (0. leucoryx) is found west of the Nile. Essentially a desert animal and like the gemsbuck apparenth' associates in small parties. [Genus Addax.] The addax [A. nasomacidatus\ distantly related to both oryxes and roan and sable antelopes, pale sandy color. An inhabitant of waterless sandy deserts of northern Africa. 5. Neotraginae. — Klipspringers {Oreotragus orcotra.gus), like the chamois, prefer small shrubs and grasses growing among the stones. Live on natural moisture of the grass and nightly dews. * * * Oribi (Ourebia) frequents open grass country or plains not too thickly forested. Grass feeders, seldom found any distance from water. Eight species. 6. Cephalophinae. — Lovers of dense bush and forest of central and southern Africa. Thirty-eight species. Duiker (Cephalo- phus grimmi), solitary animal, fond of bush country. Never far from covert. Mainly browsers. Nibbles leaves and young shoots of various acacias, small shrubs. Grass consumed when. young and fresh. Red duiker (C natalensis), dense forests and bush. Blue duiker (C monticola), essentially a browser, favors shelter or dense covert. 7. Ceruicaprinae. — Animals of large or medium size. Water buck (Cobus ellipsiprymiius), open forest country, eastern Africa, favor banks of large rivers, prefer succulent herbage, but are partial to rough and broken country, stony hillsides, and vicinity of fairly thick bush; grass feeders. During dry season frequent banks of streams for succulent herbage. * * * Sing-sing water buck {Cobus defassa), habits similar to above. * * * The lech we (C. lechwe) is smaller than the water buck. Hoofs elongated and pointed. Frequent great reed swamps and river borders, northern Rhodesia. Next to the situtunga, the most aquatic of all antelopes, stand- ing knee or even belly deep in large shallow lagoons. Come ashore to graze, food consisting of grass and young reeds. * * * Gray's water buck (C maria), frequent river bottoms and reedy grass. Stand in shallow water. * * * Puku (C. vardoni), less aquatic than the lechwe, approaching in this respect the water buck — that is, found close to but not in the water. Frequent swampy plains. * * * Uganda cob (C. thomasi), fond of open, rather swampy plains, near rivers or permanent water. Grazes on young shoots of grass. * * * Common reedbuck {Cervicapra arundinum) , lowlands of Natal and Zululand, Transvaal bush country, etc. Favors grassy or reedy valleys near streams or permanent water of some kind. Occasionally met with in thin bush. Food consists entirely of grass. Do not take to water when alarmed. * * * Moun- tain reedbuck {Ceruicapra fulvorufula), lower slopes of hills covered with rocks and loose stones, mingled with scattered bush and long grass. Grass eaters, at night descending from hills to nearest w-ater. Affecting sides rather than tops of hills. * * * Bohor reedbuck {Cervicapra redunca),, favor- ing open vleis and bush or swamp land. Like the neighbor- hood of water. * * * Gray rhebuck {Pelea capreolus) , unlike mountain reedbuck, frequent flat tops of the table mountains; common in South Africa as well as higher levels of the ranges. Grass feeders, and descending at night to drink after the manner of the mountain reedbuck. [Note vertical physiographic distribution of the genus Cervicapra.] CAUSES OF VARIATION AND POLYPHYLY AMONG QUADRUPEDS Change of physical environment. — A series of meteoric and biotic changes — that is, changes of season, of climate, or of rainfall, the appearance of new enemies, the introduction of new plants or the crowding out of old ones — will cause a change of food supply, which will cause a change of habitat, which in turn will cause a change of browsing or grazing habits that will affect locomotion — the use of the limbs in the search for food — and modify the form of the hoofs, because of the change of soil. The browsing mountain moose (Alces) of eastern Idaho, for example, has a hoof of very different form from that of the water- living forest moose of Maine. Among the new enemies that may appear are certain insect pests, such as flies or ticks, which may drive quadrupeds away from feeding ranges that are otherwise favorable into regions, perhaps not far distant, where food is scarcer and the general conditions are more adverse, and where, perhaps, the young are exposed to new dangers. Such changes may bring about (1) a change of habit or (2) a change in habitat or environment, either of which, as a general law, culminates in (3) change of function, followed by (4) change of struc- ture. (5) A change of function or habit certainly brings about a new "incidence" of selection or new set of causes tending to survival or extinction. Change of appetite. — Variations in appetite are un- doubtedly among the chief causes of local divergence. Stevenson-Hamilton (1912.1, pp. 97-158) noted the fastidious choice of food by each of the principal species of African antelopes, and other wild animals are very fastidious and seek an astonishing variety of food in the course of a single season. The predilection for certain kinds of food is very strong, and departures from it lead to adaptive radiation. Similarly Sampson (1905.1) records that the white-tailed deer (Odocoileus virginianus) browse on many kinds of plants in the course of a year. Local polyphyly through reunion of phyla. — Animals that have diverged through migration or through geographic segregation or separation may later be brought together in one region. For example, the mule deer {Odocoileus hemionus) and the white-tailed deer (0. virginianus), which have probably evolved in different regions of the United States, are now found 128 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA together in the same region in the West. In Miocene time the American rhinoceroses were joined in the western plains by certain European rhinoceroses. Thus continental radiations from great countries like Africa, Asia, or America may pour some of their branches into a single small region, mingling many distinct phyla. Hypsodont or grazing types may mingle with brach- yodont or browsing types in the same locality through their choice of grasses or of shrubs as their principal article of diet. Independently in the same region in southern Wyoming two of the branches of the titano- theres {Telmatherium and DolichorTiinus) began to acquire long-crowned teeth, while two others {Palaeo- syops and Limnohyops) retained persistently short- crowned teeth. HABITS OF THE EHINOCEROSES PARALLEL TO THOSE OF THE OLIGOCENE TITANOTHERES Mingling of hrowsing and grazing rJiinoceroses in Africa. — In equatorial Africa the Nile is an insuperable barrier between two species of rhinoceros, the "white rhinoceros," which is confined to the west bank, and the "black rhinoceros," which ranges along the east bank; yet these two species were formerly found together in the same regions of South Africa. The large grazing "white rhinoceros," R. (Ceraiotheriwn) simus, has hypsodont teeth and grazes in the open country, particularly in the wide, grassy valleys, though it was frequently met on the high veldt of Matabele and Mashonaland, feeding at night or in the cooler parts of the morning and evening. Its food consists entirely of grasses. Its sight is bad, but its scent and hearing are acute. On the other hand, the smaller browsing "black rhinoceros," R. (Opsiceros) hicornis, which has brachyodont teeth, was formerly common on the slopes of Table Mountain and on the Cape Flats and closely overlapped R. (Ceratotherium) simus in certain parts of its range; it frequented bush- covered country more than open grass lands and was often found in rocky, stony districts. It is partly nocturnal in its habits. Its food consists entirely of leaves, twigs, and sometimes of the roots of certain bushes and shrubs, but seldom of grass (Roosevelt and Heller, 1914.1). Its adaptations are essentially those of a browser, for it prefers the twigs and small roots of certain shrubs which it finds on the treeless plains of East Africa (Stevenson-Hamilton, 1912.1). It has a considerable vertical geographic range,'- being found also on the high plateau near the glaciers of Mount Kenya. (J. W. Gregory, 1896.1, p. 267.) Habits of Asiatic rhinoceroses. — The existing species of Asiatic rhinoceroses differ in habitat; they do not mingle. Rhinoceros unicornis or indicus, which has relatively hypsodont grinders, frequents the swampy, grassy jungles of the plains of India. The R. sondaicus " Gregory attributes this range to the white rhinoceros, but his observation actually refers to the black rhinoceros, as Heller has pointed out. of Burma and Java has shorter grinders. As observed by Blanford (Lydekker, 1893.1, vol. 2, sec. 4, p. 470), it "is more an inhabitant of the forest than of the grass, and although it is found in the alluvial swamps of the sudarbans, its usual habitat appears to be in hilly countries. It has been observed at considerable elevations both in Burma and Java." Indeed there is much evidence that it probably ascends occasionally to as much as 7,000 feet above sea level. Its food consists largely of twigs and smaller branches. The third species of Asiatic rhinoceros, the Sumatran rhinoceros {R. (Dicerorhinus) sumatrensis) , which has relatively short-crowned teeth, inhabits hilly forest districts and has been observed in Tenasserim at an altitude of 4,000 feet above the sea. According to Lydekker, it is a good swimmer and is said to have been seen swimming in the sea in the Mergui Archi- pelago, possibly traveling in search of new feeding grounds or to avoid certain unfavorable conditions. Thus we find among the rhinoceroses three lines of adaptation to habitat and to food radiation — first, both hypsodont (grass-loving) and brachyodont (browsing) forms; second, a considerable geographic vertical range both in R. (Ceratotherium) simus and R. sondaicus; third, the occasional assumption of semiaquatic habits. All these conditions were partly paralleled among the Oligocene titanotheres, which, however, attained no extreme hypsodontism. HABITS OF THE EXISTING TAPIRS PARALLEL TO THOSE OF THE EOCENE TITANOTHERES The Eocene titanotheres, although inferior in the structure of their grinding teeth, were nearest in form and in body adaptations to the existing tapirs. In the Tapiridae we find these principles of adaptive radiation — great vertical geographic range, including choice between upland and lowland habitat, and assumption of more or less aquatic life. The teeth are short-crowned (brachyodont), are crested (lopho- dont), and are superior in mechanism to the cone and crescent (bunoselenodont) grinders of the titano- theres. These principles are observed as follows: 1. According to J. E. Gray (1872.1, p. 486) Tapirus pincJiaque ascends to very great heights in the Andes. M. Goudot "obtained a young female tapir at an elevation of about 1,400 meters — nearly up to the snow level on the Peak of Tolima in New Granada — about 1843." According to Gray (1872.1, pp. 487, 488) Tschudi, in the "Fauna peruana" (p. 213), says, " This species of tapir [T. roulini] is found in Peru on the eastern slope of the Cordilleras at an elevation of 7,000 or 8,000 feet, which is above the snow line. " 2. On the other hand, the tapirs (T. hairdi) from Mexico and the Isthmus of Panama, which have been referred to the genus Elasmognathus by Gill, are more generally confined to the lower hills or occupy an intermediate habitat. Captain Dow observes (1867.1, p. 214): ENVIRONMENT OF THE TITANOTHEKES 129 Thus far all examples of T. bairdi [Elasmognathus] have been found exclusively on the Atlantic side of the Isthmus [of Panama], and north of the Chagres River. Their favorite haunts appear to be in the hills lying at the back of Sion Hill and the adjoining stations of the Panama Railway. It is only during the rainj' season that they seem to seek the lowlands, for it is only in that season they are captured. Similarly Tapirus {Elasmognathus) dowi was found in the highlands of Guatemala, Nicaragua, and Costa Rica. 3. The opposite extreme from mountain-living habits is furnished by the typical South American tapir (T. terrestris), which inhabits the forest districts of Brazil, Paraguay, and the northern part of Argen- tina. This species is fond of gamboling in the water and rolling in soft mud and swims and dives like a capybara; it is not improbable that it may also walk along the beds of shallow rivers and lakes, as was observed to be the habit of a specimen of the Malayan tapir (Tapirus indicus). In Brazil, in districts remote from cultivation, the food of the tapir is composed largely of palm leaves, but at certain seasons of the year these animals subsist almost exclusively on fallen fruits, and in some districts swampy grasses and water plants form their chief food. VERTICAL GEOGRAPHIC RANGE OF QUADRUPEDS The rhinoceroses as a group have a wide vertical geographic distribution, ranging from sea level to the snow belt. The black rhinoceros, although it prefers the lower grassy plains, is found also on the high plateaus near the glaciers of Mount Kenya. (Gregory, 1898, op. cit., p. 263. '') As above noted, the tapirs as a group range from sea level to the snow belt, 8,000 feet above sea level. Some species are exclu- sively low-level forms {T. terrestris); others range from sea level well up into the mountains {T. iairdi); still others inhabit the higher Andes {T. pinchaque). The elephants also enjoy a wide vertical range; Elephas (Loxodonta) africanus is said to ascend and descend steep places with wonderful facility, and t footprints of the modern Asiatic elephants have been seen among the eternal snows of the highest mountains (Pohlig, 1891.1, p. 328). VERTICAL GEOGRAPHIC RANGE OF THE TITANOTHERES Thus, judging by analogy with the other Perisso- dactyla and from what we know to be true also of the horses, it is probable that the titanotheres enjoyed a considerable vertical geographic range in the Rocky Mountain region in Eocene time and that this may have entered into the causation of their local adaptive radiation. TEN CHIEF HABITAT ZONES OF MAMMALS Wide climatic and physiographic differences, if concentrated in a geographically restricted area, facilitate local adaptive radiation. For example, " Gregory inadvertently attributes this range to the white rhinoceros. grassy meadows favorable to shrubs bring grazers and browsers together. That much more extreme con- trasts are by no means unnatural is shown along the coasts of Mexico, where there is an abrupt transition from an extremely moist, warm lowland to a dry, . cool upland. Similarly abrupt transitions are ob- served in parts of the Andes and the Himalayas. It is consequently not difficult to account for the fact that seven or eight different phyla of titanotheres lived together in southern Wyoming and northern Utah in middle and upper Eocene time, for the entire region was varied and rnountainous. The life zones of mammals have been set forth admirably by Kobelt (1902.1) and should be studied in connection with the vegetation zones of Schimper (1903.1). Some mammals are strictly confined to their typical habitat zones — that is, they are intensely localized. Certain antelopes, such as Tragelaphus angasi, the inyala (Stevenson-Hamilton, 1912.1, p. 135), probably feed upon only a single plant and are limited in range to its distribution. Many Herbivora, such as elephants, rhinoceroses, and horses, are very plastic and have great diversity of habitat in the course of the change in seasons and under varying conditions of competition. Life zones are defined by land and water, by mois- ture and aridity, by depression and elevation, by low and high temperature, by the distribution of insects, and especially by the presence of vegetation adapted to grazing or browsing. Life zones are therefore de- fined sharply in some places and feebly in others. The ten zones discriminated are described below. 1 . Mountain or alpine liaiitat. — High mountains and mountain ranges with the snow and timber lines at altitudes of 6,000 to 12,000 feet or more. Thinly forested or tundra-like lands, adapted both to grazing and browsing ungulates having relatively short limbs and feet adapted to climbing. The Artiodactyla are represented by many forms, some of which range far above timber line, including goats (Capra), rupi- caprines {Rupicapra, NemorJiaedus, Oreamnos), moun- tain sheep (Ovis), vicunas {Lama vicunna) at certain seasons, Pudu deer {Pudua). The Perissodactyla that invade these high forest zones are only certain tapirs of the Andes {Tapirus pinchaque and T. roulini). 2. Mountain forest habitat. — Lower mountain ranges and foothills, dry or well watered, well wooded, with river valleys. This zone includes the dry tropical woodlands (such as those of India), which are favorable to the larger ungulates; also the tropical rain forests (Asia, Africa, North America), generally unfavorable to large ungulates. In Asia the especial habitat of many deer, bovines, antelope, browsing perissodactyls, such as Rhinoceros sondaicus of Java, typically a forest dweller, R. {Dicerorhinus) sumatrensis of Sumatra. In the northern latitudes of North America, the typical home of the deer {Odocoileus), moose {Alces), wapiti 130 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA (Cervus), mountain caribou (Rangij'er) , at elevations of 2,000 to 8,000 feet. On these levels in South America are found among the Tapiridae T. {Elasmo- gnathus) bairdi, a hill dweller seeking the lowlands during the rainy season ; also T. (ElasmognatTius) dowi. In the equatorial belt of Africa both the high forests and lower forested foothills favorable to the growth of shrubs and trees attract also the elephants. Vertical distribution qf lije zones oj ungulates Alluvial bottom lands. Family or phylum Peaks and high- est mountain ranges; 6,000 to 12,000 feet. Browsers High desert and drier uplands and plains, table-lands, plateaus, mesas; 5,000 to 15,000 feet. Grazers Lower mountain ranges, foothills, well wooded and watered; forest lands; tributary river valleys; 2,000 to 8,000 feet. Browsers Great plains and larger river valleys, broad grassy meadows, rolling country; sea level to 6,000 feet. Grazers delta and flood-plain deposits, swamps and jungles, forests and partly forested low- lands; river or sea level. Browsers and grazers Rivers and lakes, river or lake level. Euminants Mountain sheep, goat, deer, and elk (summer). Pronghorn antelope Deer, moose, elk, cari- bou (winter). Buffalo and wapiti, or elk. Rbinocerotidae (re- Hyracodon nebrascensis. Rhinoceros sondaicus. Rhinoceros unicornis- in- Rhinoceros sondaicus. Metamynodon cent and extinct Three long toes. Java; typically a forest habits grass jungles. Occasionally seen in A mph ibious types). Rhinoceros (Geratothe- dweller. R. (Ccratotherlum) simus. alluvial swamps. rhinoceros. rium) simus. Meadows R. (Dicerorhinus) su- Large two-horned rhi- Aceratherium. Four- and sparse forests. matrensis. Inhabit- ant of hilly forest dis- tricts. R. (Opsiceros) bicornls. Two-horned " black " rhinoceros of Africa. Often seen on slopes of table mountains; feeds on roots, leaves, etc. noceros of Africa: inhab- itant of grassy valleys on high veldt. R. (Opsiceros) bicornls. Found on Cape flats, in bush-covered country. Coenopus.and Dlcerathe- rium. Three-toed ani- mals. toed rhinoceros. Equidae (horses, zebras, asses). Equus Demionus, E. ki- Equus zebra, mountain Equus hemionus onager. ang. Kianganddzigge- zebra. Migrates to the hills in tai. Inhabit table- summer. lands of Tibet, 16,000 feet E. asinus somallcus. In- high. Prefer desolate habits Nubian desert. places near lakes and E. quagga. The quagga rivers, and coarse wiry of South Africa; extinct. pasture of rough, hard A karroo dweller. yellow grass. E. burchelll. Burchell's E. onager. The onager of zebra; found north of Persia. Orange River; often seen E. hemippus. Syrian in sparse forests, but wild ass. predominantly a plains E. asinus. Feeds on wiry dweller. desert grasses. E. grevyi. Low plateaus E. zebra. Feeds on plains with gravelly soil. Seen grasses. in thick thorn bush and E. przewalskil. Inhabits tall feathery grass. Es- northern deserts. sentially an inhabitant of the open plains. Tapiridae (tapirs).. Tapirus roulini Tapirus roulini.. Tapirus bairdi. Hill Tapirus americanus. T. pinchaque. T. pinchaque. Tapir of dweller, seeking low- Common tapir of for- Inhabits -slopes the high regions of the lands at rainy season. ests and lowlands of of Cordilleras. Andes. T. dowi. Brazil and Paraguay. A forest dweller, feed- ing on palm leaves, fruits, and water plants. T. indicus. Proboscidea (ele- Elephas africanus. As- Elephas africanus. Less phants). cends and descends steep places with won- derful facility. typically a forest animal than E. indicus; found in comparatively open country; also in forests. E. indicus. Typically a forest animal. Sirenians, chalico- Macrotherium. Moropus. Manatee. theres, hippopo- Dugong. tami. Chalicotherium. Hippopotamus. ENVIRONMENT OF THE TITANOTHEEES 131 3. Boreal forest Tiaiitat. — Characteristic of north temperate zones with cold winters. The "temperate rain forests" of Schimper, partly interspersed with meadowlands. This zone includes the whole of primi- tive northern Europe and North America south of the tundra zone. In Asia it includes the whole of Siberia, grading on the south into the high "steppe" and high "plateau" regions and on the north into the Arctic tundras or barren grounds. It is the great boreal zone of North America, favored both by woodlands and meadows and by sufficient rainfall. The ungulates are very numerous, especially genera of Bovidae, Cervidae, and Suidae. 4. Tundras and barren ground habitat. — In this low- lying, north circumpolar region trees are scarce or absent, except the willows and birches of the river bottoms, and the subsoil is frozen throughout the year. The ungulates are now represented only by the musk ox {Ovibos moschatus) and several species of reindeer (Rangifer); formerly by the mammoth and the horse in Alaska and Siberia during the period of greater forestation. 5. Higher plains and plateaus. — Mesas, table-lands (as in Tibet and the Himalayas), and the desert plateaus of the Rocky Mountains and Andes, altitude 3,000 to 6,000 feet or more; vegetation scattered, sparsely forested, both grasses and shrubs abundant; or rocky and open country with occasional forests. Climate generally severe in winter. This zone grades into the "high steppes" of Asia, the veldt of South Africa, the high plains of North America. It is mostly open country adapted to grazers with hypsodont teeth, long limbs, and slender feet, or to the cursorial and gregarious Herbivora. 6. High steppe and desert habitat. — Treeless and arid wastes, steppes, and deserts of central Asia (such as the Desert of Gobi) or of Persia and Asia Minor, reaching an altitude of 6,000 feet, usually not so rich in flora and fauna as the high plateau. Climate extremely severe in winter. Inhabited chiefly by grazers. In Asia, among the Equidae we find the kiang {Eguus Jciang) of Tibet, the dziggetai {E. hemionus) of Mongolia, the wild horse {E. przewalslcii) of the Desert of Gobi or the Kobdo district of western Mongolia. The kiang of Tibet and Turkestan prefers desert places near lakes and rivers, seeking coarse, wiry pasture and rough, hard grasses. The dziggetai ranges from the lowland steppes of Turkestan to the high plateaus (1,680 meters) of the deserts of Mongolia. In this zone among the Artiodactyla we find the wild Bactrian camel {Camelus bactrianus), the saiga ante- lope {Saiga tartarica), and the Persian gazelle {Gazella gutturosa). 7. Low desert habitats. — Steppes and sandy deserts of northern Africa, Syria, Arabia, Mesopotamia, and the northern borders of the Arabian Sea; rocky countries covered with sparsely vegetated areas and thin forests, scattered shrubs, and thorny bushes. Except in temperature and altitude this zone is like that of the high steppes; its vegetation is sought mostly by cursorial browsers and grazers with colora- tion of the desert; in Africa Gazella dorcas, Addax, Oryx leucoryx, and among the Equidae the north African wild ass {Equus asin'us), the Somaliland ass {Equus somaliensis) , the Assyrian E. hemippus, and the onager {E. onager), which grazes in the low deserts of Kutch and Rajputana. Neither the rhinoceroses nor the tapirs have ever had representatives in these low-lying desert belts. 8. Plains habitat. — Great plains and larger river valleys; broad, grassy meadows bordering glades partly forested or not forested at all, extending from sea level to an altitude of 6,000 feet in northern latitudes. The tropical grasslands or savannas of Africa, the llanos of the Orinoco, the campos of Brazil, the semiarid karoos and veldts of South Africa are partly included in this zone, although they also approach the high steppe habitat. This zone is generally adapted to grazing, hypsodont types, mostly long-headed and cursorial. It is the natural habitat on the Great Plains of North America of the buffalo (Bison bison), of the pronghorn antelope (Antilocapra americana), and formerly of the wapiti (Cervus canadensis). Similarly on the plains of equatorial Africa are found numerous species of antelope (mostly grazers), oxen (grazers), giraffes (tfue browsers), the black rhinoceros, R. (Opsiceros) bicornis (browsers and grazers), and all species of zebra. The ungulates in this open country are either cursorial or graviportal and are well defended by horns. The Tapiridae have never been adapted to a country of this kind. The giraffes frequent the savanna and the thorn- forested country (xerophilous woodland of Schimper). 9. Lower river valleys habitat. — Alluvial bottom lands, delta and flood-plain deposits, swamps and jungles, forested or grassy lowlands near rivers or sea level, typically the home of browsers rather than grazers, with feet and limbs adapted to soft soil, j limbs both of mediportal and graviportal type, with some cursorial types (such as situtungas) having spreading feet. The Artiodactyla include many bovines, some antelopes (such as situtungas), chevro- tains, suillines, the Liberian hippopotamus {Choer- opsis liberiensis) , and the primitive traguline (Dorca- therium) of West Africa. Among the Asiatic rhino- ceroses R. sondaicus, a browsing, brachyodont type, 132 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA is occasionally seen in these alluvial bottoms. Simi- larly, the Sumatran rhinoceros, R. (DicerorJiinus) sumatrensis, also brachyodont, occasionally frequents such a region. Tapirus indicus inhabits this low forest belt in India, and T. terrestris is the common tapir of the forests and lowlands of Brazil and Para- guay. Among extinct forms the chalicotheres {Moropus, etc.) are found here. 10. Aquatic, river and lalce iorder habitat. — Rivers, bayous, and lakes, frequented especially by aquatic browsing types with limbs adapted to swimming or to aquatic life and the teeth adapted to the softer kinds of food. Here we find the ungulates represented by their partly degenerate and specialized offshoots the sirenians, the Artiodactyla by the common hippopot- amus or the water buffalo of the Philippines. Either the lower river valleys or the rivers themselves were undoubtedly the habitat of the extinct rhinoceroses known as amynodonts; also, possibly, of the Miocene Teleoceras, a short-limbed river-frequenting animal. Among the titanotheres there are evidences of aquatic adaptation in species of the genus Mesatirhinus. CONCLUSIONS AS TO HABITATS OF THE TITANOTHERES We have no evidence that titanotheres formerly inhabited alpine, high steppe, or low desert regions. Neither the teeth nor the feet predispose us to specu- late upon such a habitat, nor have we any geologic evi- dence of it. There remain to be considered the "moun- tain," the "forest," the "boreal," or north temperate, the "plains," the "river valleys," the "rivers and la- goons." The earliest known types of titanotheres, which are subcursorial in limb structure, developed in a partly open and partly forested country, frequenting mead- ows, lower river valleys, and plains that were flooded during certain seasons of the year. There is reason to believe that one of their upper Eocene radiations (MetarJiinus) became amphibious or even aquatic. Some authors (Riggs, 1912.1, p. 36) believe that DolichorMnus , as well as the short-footed Palaeosyops, was semiaquatic. The habits of these animals are more 'fully considered in Chapter V. In Oligocene time the titanotheres entered the savanna-like Great Plains region of western North America, which was in part open country, in part country traversed by undulating rivers and by river bottoms bordered with forests. In dentition the titanotheres, both in Eocene and Oligocene phyla, are chiefly a browsing family, though they show incipient indications of adaptation to the grazing habit. SECTION 4. BIBLIOGRAPHY FOR CHAPTER II Ball, John. 1887.1. Notes of a naturalist in South America, .xiii, 416 pp., 1 map, London, 1887. Bauer, Clyde Max. 1916.1. Contributions to the geology and paleontology of San Juan County, N. Mex. — 1, Stratig- raphy of a part of the Chaco River valley: U. S. Gaol. Survey Prof. Paper 98, pp. 271- 278, pis. 6^71, Nov. 24, 1916. Maps western part of Puerco-Torrejon area and gives short suraraary of the two formations. Berby, Edward Wilber. 1914.1. The Upper Cretaceous and Eocene floras of South Carolina and Georgia: U. S. Geol. Survey Prof. Paper 84, 200 pp., 29 pis., 12 figs., 1914. Blanford, W. T. 1888.1. The fauna of British India, including Ceylon and Burma — Mammalia, 1888-1891. BouTWELL, John Mason. 1907.1. Stratigraphy and structure of the Park City min- ing district, Utah: Jour. Geology, vol. 15, pp. 434-458, 1907. Brown, Barnum. 1914.1. Cretaceous-Eocene correlation in New Mexico, Wyoming, Montana, Alberta: Geol. Soc. America Bull., vol. 25, pp. 355-380, Sept. 15, 1914. "Through this eastern exposure I have often found it im- possible to establish any definite line of demarcation between the two beds (Fox Hills and Lance)" (p. 3.5S). "The insensible gradation from marine through brackish- water into fresh-water sandstones is not confined to the eastern exposures of the 'Lance' on Hell Creek. The same transition is found on the border of the Lance formation on Alkali Creek, Sevenmile Creek, and Robber's Koost, all tributaries of the Cheyenne River in Weston County, Wyo." Calvert, William R. 1910.1. See Stone, Ralph Walter, 1910.1. Clark, William Bullock. 1891.1. Correlation papers — Eocene: U. S. Geol. Survey Bull. 83, 173 pp., 1891. Resume of work of various writers. Deposits of Bridger and Washakie Basins considered as one formation. Regards the Puerco as probably of Eocene age (p. 138). Eocene of the Atlantic coast. Gulf States, Pacific coast; historical sketch of the Eocene of the interior. Table showing relative posi- tion of interior Eocene deposits. Extensive bibliography. 1896.1. The Eocene deposits of the middle Atlantic slope in Delaware, Maryland, and Virginia: U. S. Geol. Survey Bull.' 141, 167, pp., 40 pis., 1896. COCKERELL, THEODORE DrU AlISON. 1906.1. The fossil fauna and flora of the Florissant (Colorado) shales: Colorado Univ. Studies, vol. 3, pp. 157-176, 5 figs., June, 1906. Birds, fishes, insects, mollusks, plants. CoMSTOCK, Theodore Bryant. 1873.1. On the geology of western Wyoming: Am. Jour. Sci., 3d ser., vol. 6, pp. 426-432, 1873. Bridger classed as upper Miocene. Cope, Edward Drinker. 1872.3. On Bathmodon, an extinct genus of ungulates: Am. Philos. Soc. Proc, vol. 12, pp. 417-420, 1872. Describes the first mammal from this horizon (.Bathmoion) . 1872.4. On a new genus of Pleurodira from the Eocene of Wyoming: Am. Philos. Soc. Proc, vol. 12, pp. 472-477, 1872. Gives a detailed account of the fossil-bearing beds along Bear River, near Evanston, Wyo. ENVIRONMENT OF THE TITANOTHEEES 133 Cope, Edward Drinker — Continued. 1873.4. The monster of Mammoth Buttes: Penn Monthly, vol. 4, pp. 521-534, 1 pi., August, 1873. A popular account of the finding of tlie sltull of Eobasileus cornutus. 1875.1. Report on the geology of that part of north- western New Mexico examined during the field season of 1874: U. S. Geog. Surveys W. 100th Mer. Ann. Rept. for 1875, pp. 61-97, pis. 2-6, 18 figs., 1875. Tlie original description of " Puerco marls. " Type locality, head of Puerco River. Gives section (p. 96) showing relation of Puerco and other beds in that vicinity. No mammalian fossils, but the marls are referred to the Eocene for stratigraphic reasons. 1877.1. Report upon the extinct Vertebrata obtained in New Mexico by parties of the expedition of 1874: U. S. Geog. Surveys W. 100th Mer. Rept., vol. 4, pt. 2, 370 pp., pis. 22-73, 1877. Extensive account of geology of the Wasatch beds and their fauna. Quotes former article (pp. 17, 18), but says the beds may represent Fort Union or the lignites of upper Missouri. The thickness of the Puerco is given as 500 feet. 1879.1. The relations of the horizons of extinct Vertebrata of Europe and North America: U. S. Geol. and Geog. Survey Terr. Bull., vol. 5, pp. 33-54, 1879. Correlation of Mesozoic and Cenozoic horizons of Europe and North America. 1879.2. Second contribution to a knowledge of the Miocene fauna of Oregon: Am. Philos. Soc. Proc, vol. 18, pp. 370-376, Dec. 30, 1879. John Day formation, Oligoeene. 1880.1. The badlands of Wind River and their fauna: Am. Naturahst, vol. 14, pp. 745-748, October, 1880. Eocene. 1880.2. Observations on the faunae of the Miocene Ter- tiaries of Oregon: U. S. Geol. and Geog. Survey Terr. Bull., vol. 5, pp. 55-69, 1880. See also Paleont. Bull. No. 30, Dec. 3, 1878, and Am. Philos. Soc. Proc, vol. 18, pp. 63-78, Dec. 30, 1878. John Day formation, Oligoeene. 1881.1. Mammalia of the lower Eocene beds : Am. Natural- ist, vol. 15, pp. 337-338, April, 1881. The first mammals are described, but they were not l^nown definitely at that time to be from the Puerco formation. 1885.1. The Vertebrata of the Tertiary formations of the West: U. S. Geol. Survey Terr. Rept., vol. 3, XXXV, 1009 pp., 134 pis. (pis. l-75a), 38 figs., 1885. Contains a general r63Um6 of the Wasatch. The deposits of the Bridger and W'ashaliie Basins and small area on White River in the Uinta Basin considered contemporary. Table of formations in this worli places Puerco as "post-Cretaceous," but in the text the author places it definitely in the Eocene. 1885.2. The relations of the Puerco and Laramie deposits: Am. Naturalist, vol. 19, pp. 985-986, October, 1885. states that the thickness of the beds near the type locality is 850 feet. The author points out the distinctions from Laramie but considers the possibility of "post-Cretaceous" age. 1885.3. The White River beds of Swift Current River, Northwest Territory: Am. Naturalist, vol. 19, p. 163, February, 1885. Oligoeene, White River. Cope, Edward Drinker — Continued. 1886.1. The Vertebrata of the Swift Current Creek region of the Cypress Hills: Canada Geol. and Nat. Hist. Survey Ann. Rept., new ser., vol. 1, for 1885, appendix I to article C, pp. 79-85, 1886. Oligoeene. Cdlbertson, Thaddeus a. 1851.1. Journal of an expedition to the Mauvaises Terres and the upper Missouri in 1850: Smithsonian Inst. Fifth Ann. Rept., appendix 4, pp. 84-145, 1851. Bear River [=Bear Creek] (p. 9.3), a southern tributary of the Cheyenne. First collection [in the Oreodon zone, Brule clays] (p. 94), rhinoceros skull (,A. oiddenlah) , several good heads, excellent teeth and jawbones, etc. Report to Baird (p. 105). Dall, William Healey. 1892.2 (and Harris, G. D.). Correlation papers — The Neocene of North America: U. S. Geol. Survey Bull. 84, 349 pp., 3 pis., 43 figs., 1892. See especially chapter 6, on the supposed Neocene of the in- terior region, considered by States (pp. 280-317); table showing the vertical range of the Neocene of the interior (p. 279); map (p. 178); list of names applied to the Cenozoic beds and formations of the United States (p. 320). 1898.1. A table of the North American Tertiary horizons correlated with one another and with those of western Europe, with annotations: U. S. Geol. Survey Eighteenth Ann. Rept., pt. 2, - pp. 327-348, 1898. Marine Tertiary horizons of the Atlantic coast'and the Gulf States correlated with one another, with those of the western United States, and with those of western Europe. Darton, Nelson Hokatio. 1896.1. Catalogue and index of contributions to North American geology, 1732-1891: U. S. Geol. Sur- vey Bull. 127, 1045 pp., 1896. 1903.1. Preliminary report on the geology and water resources of Nebraska west of the one hundred and third meridian: U. S. Geol. Survey Prof. Paper 17, 69 pp., 43 pis. (inch 9 maps), 23 figs., 1903. Titanotherium zone (Chadron formation) of western Nebraska, along the North Platte, Scott Bluffs, Sioux County, etc. 1905.1. Age of the Monument Creek formation: Am. Jour. Sci., 4th ser., vol. 20, pp. 178-180, 1905. Menodus {TiianoiheriuTTi) remains. Oligoeene. 1905.2. Preliminary report on the geology and underground water resources of the central Great Plains: U. S. Geol. Survey Prof. Paper 32, 433 pp., 72 pis., 18 figs., 1905. 1906.1. Geology and underground waters of the Arkansas Valley in eastern Colorado: U. S. Geol. Survey Prof. Paper 52, 90 pp., 27 pis., 2 figs., 1906. " Monument Creek formation," containing Menodus (^Titano- iherium) of White River age (p. 34) . Nussbaum formation, of late Tertiary age (p. 34) . 1906.2. Geology of the Big Horn Mountains: U. S. Geol. Survey Prof. Paper 51, 129 pp., 47 pis., 14 figs., 1906. Brief reference to the Eocene rocks (p. C7). See especially Bridger [Wind River] formation (p. 70). Davis, William Morris. 1900.1. The fresh-water Tertiary formations of the Rocky Mountain region: Am. Acad. Arts and Sci. Proc, vol. 35, pp. 346-373, 1900. History of opinion on mode of formation; evidence against lake-bed hypothesis and in favor of fluviatile origin. 134 TITANOTHEBES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Dawkins, W. Boyd. 1880.1. The classification of the Tertiary period by means of the Ivlammalia: Geol. Soc. London Quart. Jour., 1880, pp. 379-405. Tertiary and Quaternary horizons and faunas of Great Britain, France, and Italy correlatRd. Deperet, Charles. 1893.1. Note sur la succession stratigraphique des faunes de mammifcres pliocenes d' Europe et du Pla- teau central en particulier: Soc. g volcanic ash deposits of Bridger age in southern Wyoming of animals. (After De ■D. S. GEOLOGICAL SURVEY MONOGRAPH 65 PLATE II A. OJO ALAMO, SAN JUAN COUNTY, N. MEX., LOOKING NORTH Contadl (indicated by arrows) betw^een Ojo Alamo sanditone and clay of Puerco formation is observed dire(5tly back of the trading store EjJqc '"*^*f*^/$0'^ ;i. EXPOSURES OF PUERCO FORMATION EAST OF OJO ALAMO, N. MEX. sfts on eroded surface of Ojo Alamo sandftones; contaiS: indicated by dotted line. The dark Stratum at top to the right is the lower level i^£tocoyiu.s 2:one) of the Puerco formation CRETACEOUS AND BASAL EOCENE CONTACTS IN NEW MEXICO Photographs by W. J. Sinclair,'1913. (After Sinclair>nd Granger, 1914.1) U. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE III / rlij-(jf }!tt^4/> X -y A. UPPER BEDS OF TORREJON FORMATION, WEST FORK OF TORREJON ARROYO, SANDOVAL COUNTY, N. MEX. The Torrejon is overlain unconformably^y the basal sand^ones of the Wasatch(?) formation JPotyrn-fy-stodnTi zo/ie •■' iliiiii if^,. . ^^T u ' . ■ < ^}*^mmi B. EXPOSURES OF PUERCO FORMATION 3 MILES EAST OF OJO ALAMO, N. MEX. BASAL EOCENE AND LOWER EOCENE CONTACTS IN NEW MEXICO Photographs by W. J. Sinclair, 1913. (After Sinclair and Granger, 1914.1) U. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE IV jM-'Of ** * J ^ A. EOHIPPUS-CORYPHODON ZONE (LOWER PART OF WASATCH FORMATION, LEVEL BIG HORN B), LITTLE SAND COULEE Fir^ appearance of Eohipptis. Am. Mus. negative 18565 B. PHENACODUS-NOTHODECTES-CORYPHODON ZONE (BASAL PART OF WASATCH FORMATION, LEVEL BIG HORN A), ABOUT 4 MILES NORTH OF RALSTON Am. Mus. negative 18563 LOWER WASATCH STRATA RESTING ON BASAL WASATCH STRATA, CLARK FORK BASIN, PARK COUNTY, WYO. U. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE V A. TYPICAL "LYSITE" LOCALITY, AT COTTONWOOD DRAW, NORTH OF LOST CABIN, WIND RIVER BASIN, WYO. Shows the Heptodon-Coryphodon-Eohippus zone (level Wind River A), with Paleojoic hills in the background. (After Granger, 1910.1.) Am. Mus. negative 18393 B. TYPICAL "GRAY BULL" LOCALITY, 4 MILES SOUTH OF OTTO, BIG HORN BASIN, WYO. Shows the Syilemodon-Coryphodon-Eohippus sone flevcl Big Horn C), with the excavation of the skeleton of Eohippus osboi in the foreground. Am. Mus. negative 18450 EXPOSURES OF WASATCH FORMATION AND TYPICAL WIND RIVER DEPOSITS IN WYOMING TJ. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE VI ( SiA.'Cr'f'cu-vo JJ } I*, iMtoik^'' ■Msiti-- »m ^^f ^^avjiKKsaitM A. A T^'I'ICAI. HL'ERFANO LOCALITY, 2 MILES WEST OF GARDNER, HUERFANO BASIN, COLO. Palaeosyops fontinalis zone (level Huerfano B). A cedar-covered ridge in midale diftance, and eruptive peaks in the background. The type of Eometarhinus and referred specimens of Palaeosyops fontinalis were found at this site. Am. Mus. negative 104715 J^ cirrti do ih4^riz<^n^ zorve imm B. A TYPICAI- "LOST CABIN" LOCALITY, ON ALKALI CREEK, EAST OF LOST CABIN, WIND RIVER BASIN, WYO. Lamhdotheriunt sone (level Wind River B). The types of Lambdotherium po^oagicum, Eotitanops horealis, and E. gregoryi -were found at this site. Am. Mus. negative 18392 TYPICAL HUERFANO FORMATION OF COLORADO AND WIND RIVER FORMATION OF WYOMING V. S. GEOLOGICAL STJBVET MONOGRAPH 55 PLATE VII A. HENRYS FORK TABLE, LOOKING NORTHWARD ACROSS HENRYS FORK, BRIDGER BASIN, WYO. [UintatheriUTti zone (levels Bridger C and D) and ^etarhinus zone (level Bridger E) -with Bishop ("Wyoming") conglomerate at the top. The Burnt Fork "white layer" (w and arrow) separates level Bridger C 2 from Bridger C 3. Am. Mus. negative 18152 B. GRIZZLY BUTTES, SOUTH OF MOUNTAIN VIEW, UINTA COUNTY, WYO. Palaeosyops paludosuS'Orohippiis zione (level Bridger B). Excavation by Granger of the type skull of Limnohyops priscus (Am. Mus. 11687). Am. Mus. negative 18089 TYPICAL BRIDGER FORMATION (LEVELS UPPER C, D, E, AND LOWER B) OF WYOMING, MIDDLE AND UPPER (?) EOCENE n. s. cKoi.onirAL survey MONOGRAPH 55 PLATE VIII A. NORTHWEST POINT OF HAYSTACK MOUNTAIN, HEAD OF BITTER CREEK, SWEETWATER COLINTY, WYO. Eohasileus-Dolichorhinus and Metarhinus aones (levels Washakie B 2 and B 1). (After Granger.) Am. Mus. negative 18213. (Se figs. 60 and 61) Wash-akyoe £ WasTtaJtie- A B. VIEW SOUTHEASTWARD FROM LACLEDE STATION ON OVERLAND STAGE TRAIL, SWEETWATER COUNTY, WYO. Lower brown sandstones of \Jiy\tat\\,^rium 2;one (level Washakie A) in middle di^ance. Hayrack Mountain and the Eobasileus' 1 the background. (After Granger.) Am. Mus. negative 18223 :i sandilonei of \Jiy\taihe.rium 5c lis and Metarhinus : TYPICAL "WASHAKIE" FORMATION (LEVELS B 2 AND B 1 OVERLYING LEVEL A 1), WYOMING; MIDDLE AND UPPER EOCENE U. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE IX A. COLUMNAR SANDSTONES, TOP OF LEVEL XJINTA A, WHITE RIVER CANYON, UINTA BASIN, UTAH (After Riggs, 1912.1.) Field Mus. negative B. PANORAMIC VIEW, WHITE RIVER CANYON, 4 MILES BELOW WAGONHOUND BEND, UINTA BASIN, UTAH Bluffs on right bank of river belong to the unfossiliferous level Uinta A. Photograph by Riggs. Field Mus. negative MIDDLE EOCENE OF NORTHERN UTAH (LEVEL UINTA A) tr. S. GEOLOGICAL SURVET MONOGRAPH 55 PLATE X A 7 11 J\.myru)do7T^ sandstoTve JSobcLsile-us - DolicJioj'Tzisn A. NORTHERN BOUNDARY OF COYOTE BASIN, UINTA BASIN. UTAH Showing greenish clays of the 'Exibas^Xe^KS'■T>6iich Uirvtco C J Dif^^a?^ B. BADLANDS SOUTH OF WHITE RIVER, UTAH Showing Diplacodon 5one (level Uinta C 1, upper Eocene) in foreground and level Uinta C 2 in diilance- (Compare fig. 66.) Am. Mus. negative 17665 101959— 29— VOL 1 D. S. GEOLOGICAL SURVEY MONOGRAPH 56 PLATE XIV EXPOSURES AT QUINN DRAW, BIG BADLANDS, S. DAK. Showing summit of lower Oligocene Chadron formation (Titanotherium zone) and, at the top, base of younger Brule formation (Oreodon :;one). The sandstone columns in the center indicate a river channel betvveen underlying and overlying claya. Am. Mus. negative 36012 V. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE XV A. SOUTH END OF SHEEP MOUNTAIN, NEAR HEAD OF CORRAL DRAW, BIG BADLANDS, S. DAK. Showing Oreodon zone (Brule formation). Am. Mus- negative 36006 B. CEDAR CREEK, BIG BADLANDS, S. DAK. Showing Oreodon zone (Brule formation) overlying Titanotherium zone (Chadron formation). Am. Mus. negative 36013 BRULE AND CHADRON FORMATIONS OF SOUTH DAKOTA CHAPTER III DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS OF THE TYPES SECTION 1. HISTORY OF DISCOVERY Full descriptions of the geologic and geographic positions of the several types and kinds of titano- theres are given in Chapter II. The present chapter relates the history of the explorations and of the gradual discovery of the character and relations of the titanotheres. THE OLIGOCENE TITANOTHERES THE PIONEER PERIOD: PROUT, OWEN, EVANS, LEIDY (1846-1873) The Big Badlands of South Dakota and north- western Nebraska are even now practically unknown to most Americans. As these lands lie in an arid region far from navigable rivers — a region that was formerly occupied by hostile Indians and that offers little attraction to either the prospector or the set- tler — it is not surprising that their fossil wonders long lay hidden from the world. The fossil remains of the great animals described in this monograph were known to the Indians and referred to in their mythology as "thunder horses." (See Preface, p. xxi.) In 1846 Dr. Hiram A. Prout, of St. Louis, sent to Professors Dana and Silliman of Yale College a cast of a remarkable fossil that he had received from "a friend residing at one of the trading posts of the St. Louis Fur Co. on the Missouri River." Front's brief notes, together with a crude sketch of one of the lower molars, were accordingly published in the American Journal of Science and Arts. (Prout, 1846.1, pp. 288, 289.) In a later communication Prout (1847.1) stated that this fossil (fig. 85) was discovered in the "Mauvais Terre, on the White River, one of the west- ern confluents of the Missouri." This was the famous specimen described by Prout as a "gigantic Palaeo- therium," which Leidy tells us (1852.1, p. 551) was "the first of the many mammalian remains which have been brought to the notice of the scientific world from the vast Eocene cemetery of Nebraska." It thus gave the first hint to scientists that "the region of Nebraska Territory of the United States appears to be as rich in the remains of Mammalia and Chelonia of the Eocene period as the deposits of the same age of the Paris Basin." (Leidy, 1852.1, p. 539.) The fossil jaw described by Prout represented an animal of great size. "The entire jawbone," he says, "must have been at least 30 inches long, which far exceeds in size the PalaeotJierium magnum." The reference to Cuvier's PalaeotJierium was, under the circumstances, very natural, because the lower molars of Front's specimen were surmounted by crescentic cutting surfaces somewhat like those of Palaeotherium. This discovery evidently attracted attention abroad, for in 1849 the French paleontologist Pomel (1849.1, pp. 73-75), after carefully considering Prout's descrip- tion and figures, stated that the fossil represented a new subgenus of paleotheres, for which he proposed the name Menodus giganteus, the generic name re- ferring to the crescents of the lower molars, the specific name to the great size of the animal. Meanwhile (in 1839, 1840-1849) the United States Government geologist. Dr. David Dale Owen, was making his extensive geologic reconnaissance of Wis- consin, Iowa, and adjacent States. In his final report (Owen, 1852.1, p. 194) he tells us that he was "de- sirous, if possible, to connect the geology of the Missis- sippi Valley, through Iowa, with the Cretaceous and Tertiary formations of the upper Missouri, a matter very important to the proper understanding of the formations of the intervening country, which it had been made my particular duty to explore." Finding it impracticable to explore the Missouri region himself he detailed to this work one of his assistants, Mr. John Evans. Late in the field season of 1849 Evans "finally reached that most curious unexplored region, the corner of the 'Badlands' (Mauvaises Terres), lying high up on White River, a locality which seemed likely, above all others, to furnish satisfactory information regard- ing the precise character and age of the Tertiary de- posits of the upper Missouri country." (Owen, 1852.1, p. 195.) From Evans's report (p. 197) Owen gives the fol- lowing description of the Mauvaises Terres of White River: To the surrounding country, however, the Mauvaises Terres present the most striking contrast. From the uniform, monoto- nous open prairie, the traveler suddenly descends, one or two hundred feet, into a valley that looks as if it liad sunk away from the surrounding world, leaving standing, all over it, thousands of abrupt, irregular, prismatic, and columnar masses, frequently capped with irregular pyramids and stretching up to a height of from one to two hundred feet or more. So thickly are these natural towers studded over the surface of this extraordinary region tliat the traveler threads his way through deep, confined, labyrinthine passages, not unlike the narrow, irregular streets and lanes of some quaint old town of the European continent. Viewed in the distance, indeed, these rocky piles, in their endless succession, assume the appearance of massive artificial structures, decked out with all the acces- sories of buttress and turret, arched doorway and clustered shaft, pinnacle and finial, and tapering spire. One might almost imagine oneself approaching some magnifi- cent city of the dead, where the labor and the genius of for- gotten nations had left behind them a multitude of monuments of art and skill. On descending from the heights, however, and proceeding to thread this vast labyrinth and inspect, in detail, its deep, intricate recesses, the realities of the scene soon dissipate the 141 142 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA delusions of the distance. Tlie castellated forms which fancy had conjured up have vanished, and around one, on every side, is bleak and barren desolation. Then, too, if the exploration be made in midsummer, the scorching rays of the sun, pouring down in the hundred defiles that conduct the wayfarer through this pathless waste, are reflected back from ' the white or ash-colored walls that- rise around, unmitigated by a breath of air or the shelter of a soli- tary shrub. The drooping spirits of the scorched geologist are not per- mitted, however, to flag. The fossil treasures of the way well repay its sultriness and fatigue. At every step objects of the highest interest present themselves. Embedded in the debris lie strewn, in the greatest profusion, organic relics of extinct animals. All speak of a vast fresh-water deposit of the early Tertiary period and disclose the former existence of most re- markable races that roamed about in bygone ages high up in characters belonging now to the above three orders; for the molar teeth are constructed after the model of those of the hog, peccary, and babyroussa; the canines as in the bear; while the upper part of the skull, the cheek bones, and the temporal fossa assume the form and dimensions which belong to the cat tribe. Another, the Oreodon of Leidy, has grinding teeth like the elk and deer, with canines resembling the omnivo- rous thick-skinned animals, being, in fact, a race which lived both on flesh and vegetables and yet chewed the cud like our cloven-footed grazers. Associated with these extinct races we behold also, in the Mauvaises Terres, abundant remains of fossil Pachydermata of gigantic dimensions and allied in their anatomy to that sin- gular family of proboscidate animals of which the tapir may be taken as a living type. These form a connecting link between the tapir and the rhinoceros; while, in the structure of their grinders, they are intermediate between the daman and rhinoc- FiGUKE 83 — Mauvaises Terres, Nebraska. After David Dale On en, 1851 the valley of the Missouri, toward the sources of its western tributaries, where now pastures the big-horned Ovis montana, the shaggy buffalo or American bison, and the elegant and slenderly constructed antelope. Owen continues (p. 198) with a popular description of the extinct animals found: Every specimen as yet brought from the Badlands proves to be of species that became exterminated before the mammoth and mastodon lived and differ in their specific character, not alone from all living animals, but also from all fossils obtained even from cotemporaneous geological formations elsewhere. Along with a single existing genus, the Rhinoceros, many new genera never before known to science have been discovered, and some, to us at this day, anomalous families, which com- bine in their anatomy structures now found only in different orders. They form, indeed, connecting links between the pachyderms, plantigrades, and digitigrades. For example, in one of the specimens from this strange locality, described by Dr. Leidy under the name Archiotherium, we find united eros; by their canines and incisors, they connect the tapir with the horse, on the one hand, and with the peccary and hog on the other. They belong to the same genus of which the labors of the great Cuvier first disclosed the history, under the name of Palaeotherium, in publishing his description of the fossil bones exhumed from the gypsum quarries of Montmartre, near Paris, but are of distinct species; and one at least, of this genus, dis- covered in the Badlands (Palaeotherium proutii), must have attained a much larger size than any which the Paris Basin afforded. In a green, argillo-calcareous, indurated stratum, situated within 10 feet of the base of the section, a jaw of this species was found, measuring, as it lay in its matrix, 5 feet along the range of the teeth, but in such a friable condition, that only a portion of it could be dislodged; and this, notwith- standing all the precautions used in packing and transportation, fell to pieces before reaching Indiana. A nearly entire skeleton of the same animal was discovered, in a similar position, which measured, as it lay embedded, 18 feet in length, and 9 feet in height. But here, as in the former case, the crumbling condition of the bones rendered it impos- DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS sible to disinter them whole; and the means of transportation to the Missouri were insufficient, even if these interesting remains could have been extracted in good condition. Figure 84. — "Vertical view of the posterior tooth belonging to the lower jaw of Mr. Prout's Palaeoiherium; natural size." After Prout, 1846 Owen also gives (1852.1, p. 200) a tabular "Section of beds constituting the early Tertiary (Eocene) of the Badlands." This section, reproduced below, was doubtless taken by Evans. 143 The detailed description of the mammalian and chelonian fossils collected by Owen, Evans, and others was intrusted to Dr. Joseph Leidy, of Philadelphia, and was published in Owen's report of 1852 (1852.1, pp. 551, 552), already cited. In this publication Leidy describes Prout's original specimen and gives a poor figure of Evans's principal specimen, an imperfect lower jaw of a titanothere. He adopts provisionally the name Palaeotherium? proutii Owen, Norwood, and Evans but con- cludes his description of these fos- sils with the following significant remark : AU the preceding specimens, except, probablj', the latter two (fragments of upper molars), I suspect belong to a different genus from either Palaeotherium or A nchitherium, and should the suspicion prove correct, Titanotherium would be a good name for the animal, as expressive of its very great size. Section of beds constituting the early Tertiary {Eocene) of the Badlands (Mauvaises Terres) 30 [Numbered in descending order] 1. Ash-colored clay, cracking in the sun; con- taining siliceous concretions 2. Compact white limestone 3. Light-gray marly limestone 4. Light-gray indurated siliceous clay (not effer- vescent) 5. Aggregate of small angular grains of quartz, or conglomerate, cemented by calcareous earth; slightly effervescent 8 6. Layer of quartz and chalcedony (probably only partial) 1 7. Light-gray indurated siliceous clay, similar to No. 4 but more calcareous, passing down- ward into pale flesh-colored indurated siliceous marly limestone (effervescent) ; turtle and bone bed 25 8. White and light-gray calcareous grit; slightly effervescent 15 9. Similar aggregate to No. 5 but coarser 8 10. Light-green indurated argillaceous stratum (slightly effervescent) ; ' ' palaeotherian bed" - 20 Some of the specimens brought back by Evans were referred to in a brief notice pub- lished by Owen, Norwood, and Evans (1850.1), in which the name "Palaeotherium proutii" was proposed "in compliment to Dr. Prout, of St. Louis." The next year (1850) after Evans's journey Mr. Thaddeus A. Culbertson visited, under the auspices of the Smithsonian Institution, the same region (Leidy, 1854.1, p. 12) and "made a good collection of its animal remains," including skulls of Oreodon culbertsoni and the titanothere upper premolars which Leidy afterward described (1852.2, p. 2) under the names Rhinoceros americanus and Eotherium americanus. The locality was Bear Creek, a dry tributary of Cheyenne Kiver. (See Chap. II, p. 104.) Figure 85. — Original figures of Prout's "gigantic Palaeotherium," the first titanothere discovered. After Prout, 1847 A, "Fragment of the inferior maxillary of the left side," one-fourth natural size; B, last lower molar, left side, four-fifths natural size. Thus was proposed the name Titanotherium, which has figured so largely in the literature of American paleontology and was consequently chosen as the basis for the title of this monograph. Two years later Leidy (1854.1) gave a revised and extended description of the available remains of titano- 144 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA theres, which was accompanied by excellent litho- graphs of Prout's and other fragmentary specimens. At this stage of knowledge the only parts figured under the name Titanotherium proutii included the lower molars, a lower premolar, a lower canine, a frag- mentary upper molar, and two upper premolars (Leidy's types of Rhinoceros americanus). Fragments of large upper molars were named Palaeotherium giganteum. The "palaeotherian bed" of Owen and Evans is referred to by Leidy (op. cit., p. 13) as the "Titano- therium bed." This appears to be the first use of this term, which was afterward, in the form " Titano- therium beds" (now Titanotherium zone), so widely used by geologists and paleontologists. An interval of 15 years in the literature of the sub- ject, from 1854 to 1869, is broken only by Prout's brief notice of an indeterminate molar (now lost) of a titanothere, which he named Leidy otherium. But during this seemingly barren interval Meek and Hayden were making their historical explorations (Merrill, 1906.1, pp. 585-592), which resulted in notable advances in our knowledge of the relations of the geologic deposits of the Rocky Mountains and Great Plains. They also brought back many verte- brate fossils, including specimens of Titanotherium. One of the specimens of titanotheres collected by Meek and Hayden included a nearly complete series of upper teeth. This specimen, which belonged to Prof. James Hall and is now No. 433 of the Hall collection of the American Museum of Natural History, was described and figured by Leidy in his memoir of 1869 (1869.1, pp. 206, 207, pi. 24) and was by far the best spec'men that had been described up to that time. Leidy referred it to his species Titano- therium proutii, but it probably belongs in the genus that Marsh afterward named Brontotherium. This specimen misled Leidy into assigning Titanotherium to the Artiodactyla. "From the form of its lower true molars, which were first discovered," he says, "it was supposed to be more nearly alHed with the Palaeotherium and was hence placed among the uneven- toed pachyderms, or Perissodactyla, but the nearly complete dentition of both jaws, since discovered, appears to indicate its position to be as above stated " — that is, it appeared to be "nearly allied with Chali- cotherium, and, like it, approximates the even-toed pachyderms, or Artiodactyla * * * with the Ru- minantia." In 1870 Leidy (1870.1, pp. 1, 2) described a frag- mentary fossil from Colorado that had been submitted to him by Doctor Hayden. We now know that this specimen consists of the horn cores and attached coossified nasal bones of a titanothere of some sort, but to Leidy, who knew practically nothing of the skull of the titanothere, it proved "singularly puzzling in character." He at first thought it might pertain to Titanotherium, "but in the state of extreme uncer- tainty as to its collocation, it may with equal proba- bility be referred to other genera, perhaps to Megalo- meryx, or it may have been an American species of Sivatherium. Under the circumstances it may be referred to a new genus, with the name of Megacerops color adensis ." This problematical fossil was redescribed and figured by Leidy in his memoir of 1873 (1873.1, p. 239). He states that the specimen "appears to correspond with that portion of the face * * * [of Siva- therium] which comprises the upper part of the nose, together with the forehead and the anterior horn cores." He compares the specimen with the corres- ponding parts of the Sivatherium, the rhinoceros, the tapir, and the mastodon. He decides that the frag- mentary horn core formerly attributed to Titano- therium may perhaps belong to another species of Megacerops. This erroneous determination, together with the previous assignment of Titanotherium to the Artio- dactyla, shows how greatly Leidy, even with all his skill and caution, was deceived by the lack of well- preserved and definitely associated feet and skulls, a lack which is felt to some extent even at the present time. Leidy's description of Megacerops may be regarded as marking the close of the first or pioneer period in the study of the titanotheres, a period characterized by (1) the chance discovery of "Prout's specimen," (2) the exploration of the White River badlands by Evans, Hayden, and others and the resulting knowledge of the general geologic age of the beds, (3) the description of fragmentary remains of titanotheres, chiefly teeth, by Prout, by Pomel, and by Leidy in successive publications, together with the beginnings of the systematic nomenclature, (4) the erroneous reference of Titanotherium to the Anoplotheriidae among the Artiodactyla. TAXONOMIC ARRANGEMENT AND COMPARISON WORK OF MARSH AND COPE (1870-1887) The second period in the study of titanotheres, which may be called the period of systematic descrip- tion, really began before the first period had closed (1873). From 1873 to 1891, inclusive, the literature of the Oligocene titanotheres is dominated almost exclu- sively by the explorations and systematic contribu- tions of Marsh and Cope. During this time Marsh described eight genera and fourteen species as new, and Cope described three genera and twelve species as new. The solution of the exact systematic and phylogenetic interrelations of these genera and species is one of the principal themes of Chapters IV to VII of the present monograph. In 1870 Prof. Othniel C. Marsh (1870.1) headed an expedition sent from Yale College to northern Colo- DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 145 rado, where he not only discovered and explored "an extensive outcrop of the true Mauvaises Terres, or White River formation," but also procured some mag- nificent specimens of titanotheres (including the types of Brontotherium gigas and B. ingens), which he de- scribed and figured three years later. Marsh was also able to solve the problem of the ordinal relationships of the titanotheres (1873.1, p. 486), showing that his Brontotherium gigas was a "true perissodactyl with limb bones resembling those of Rhinoceros. The genus is related to Titanotherium, and the two appear to form a distinct family, which may be called Brontotheridae." He was able in a very few words to throw a flood of light upon the characters of the skeleton, hitherto known chiefly from fragments : It closely resembles that in recent perissodactj^ls but shows some approach to the Proboscidea. The femur has a third trochanter, and its head a pit for the round ligament. The fibula is entire and slender. The astragalus is remarkably short. It has a deep groove on its upper surface, and the articular facets for the navicular and cuboid are nearly equal. In the manus there are four toes of nearly equal size, the first digits being rudimentary or wanting. There were three digits only in the pes, the first and fifth being entirely wanting. The toes were short and thick, as in proboscidians. The meta- carpals and metatarsals are longer than in the elephant, and the phalanges shorter. The foot was also more inclined. The carpal and tarsal bones are very short and form interlocking series. The tail was long and slender. An important point not touched upon in this com- munication was the presence or absence of horns. Prof. Edward D. Cope was not far behind Marsh in contributions to the literature of the titanotheres. Two years after Marsh had made his explorations in Colorado, Cope, in 1872, discovered a number of re- markable skulls (now in the Cope collection of the American Museum of Natural History) which, in bul- letins pubhshed in 1873 and 1874, he made the types of Symhorodon torvus, Megaceratops acer, M. heloceras, Symborodon hucco, S. dltirosfris, S. trigonoceras. He states (1873.2, pp. 2, 3) that "Leidy and Marsh have described two genera of this group, viz, Titanotherium and Brontotherium, but without certain indications of their possession of horns." He regards them as "all true perissodactyls and allied to the Rhinoceros and Palaeotherium." His genus Symhorodon, like Menodus, Titanotherium, and Brontotherium, was "established on mandibular rami only, which can not be certainly associated with crania," the last phrase suggesting one of the most troublesome and obdurate of titanothere problems, which from the first has caused confusion in the systematic nomenclature. Cope regarded the absence of incisors as one of the generic characters that separated Symhorodon from Titanotherium and Brontotherium, thus first raising the problem how far differences in the number of incisors may correspond to true generic differences. The discovery of so many more or less complete skulls enabled Cope to infer specific and generic characters from the variations in form of the horn cores, skull top, nasals, and zygo- matic arches. Thus the discoveries of Cope and Marsh, although they settled the ordinal relationships of the titanotheres, began to complicate the problem of their interrelationships. SUMMARY OF MAESH'S CONTRIBTTTIOKS In Marsh's paper "On the structure and affinities of the Brontotheridae" (1874.1) he developed further the family characters of the group, separating them from the Rhinocerotidae, "apparently their near allies," establishing the number of digits in the fore and hind feet and the general characters of the skull, lower jaw, vertebrae, and limbs. This paper is ac- companied by the first of a series of excellent litho- graphic plates, illustrating some of Professor Marsh's superb specimens of titanotheres from Colorado. Marsh contributed another short but pregnant article on the "Principal characters of the Brontotheridae" in 1876 (1876.1), and after that he published at in- tervals brief descriptions of supposedly new genera and species, not all of them accompanied by illustra- tions, until September, 1891, the date of his last published contribution to the subject. Marsh's most valuable contributions to our knowl- edge of the titanotheres may be summarized as follows: (1) He and his party explored the White River forma- tion in Colorado and collected from it many remark- ably fine specimens; (2) he demonstrated the ordinal position of the group, classifying its members as perissodactyls; (3) he recognized the fact that the titanotheres constitute a distinct family, which he named the Brontotheridae; (4) he made the illuminat- ing observation that his upper Eocene genus Diplaco- don served to connect the Oligocene Brontotheridae with the Eocene "Limnohyidae"; (5) he published many excellent lithographs and woodcuts, showing chiefly the skulls and dentition of titanotheres, but including also (1889) an excellent restoration of Brontops rolustus; (6) he supervised the preparation of a fine series of lithographic plates for the present work; (7) under the auspices of the United States Geological Survey he founded the present series of monographs on fossil vertebrates; (8) he began the preparation of the present monograph, although he left no manuscript for it; (9) he obtained for the National and Yale Museums their superb specimens of titanotheres, most of which were collected by his field assistant J. B. Hatcher, who in turn also made valuable scientific contributions to our knowledge of these animals. Marsh's detailed systematic work on the titanotheres was less fortunate than his broader contributions, owing chiefly to confusion in regard to features of the skull and jaw. After founding the genus and species Brontotherium gigas upon a lower jaw, he referred to the 146 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA same genus as the type of B. ingens, a skull that certainly belongs to another genus (Menodus). In consequence of this initial confusion he erected a new genus (Titanops) for skulls that should have been referred to Brontotherium. Many of his conceptions of the interrelations of the genera and species proposed by him were erroneous. Although recognizing the fact that the genera Brontops, Allops, and Teleodus were all allied to " BrontotTierium" — that is, as repre- sented by the skull of "BrontotTierium [Menodus] ingens" — he nevertheless thought thsLt Diplodonus was related to Titanops (the true BrontotTierium), and he referred to Menops (a near ally of his "BrontotTierium'' ingens) a well-preserved skull that is now known to belong to BrontotTierium proper. In fact, in common with Cope and others. Marsh apparently faUed to recognize the comparatively wide phyletic gap between the true BrontotTierium (his Titanops) and Cope's Symborodon on the one hand and the supposed genera Brontops, Allops, Menops, and Menodus (his "Bronto- tTierium ingens") on the other. Consequently his generic definitions are unsatisfactory, and he was certainly not overconservative in proposing new generic and specific terms. SUMMARY OF COPE'S CONTRIBUTIONS The next year (1874) after publishing his prelimi- nary descriptions of the several species of Symborodon and allied genera Cope (1874.2) gave full descriptions of these forms in his "Report on the vertebrate pale- ontology of Colorado," which was accompanied by eight lithographic plates. He presented a careful review of the general morphology of the skull, includ- ing the brain case and cranial antra, which was fol- lowed by a review of the work of preceding authors and by a tabular analysis and detailed description of the species of Symborodon. He recorded many interesting facts, such as the similarity of the dentition of Sym- borodon to that of Palaeosyops and of CTialicotTierium and the mingling of proboscidian and rhinoceros analogies in the limbs. He considered the indications that Symborodon possessed a short proboscis. In his tabular analysis of species he indicated the differences in the shape of the horns and noted that in S. trigono- ceras and S. Tiypoceras the upper premolars have a strong internal basal cingulum, whereas in S. bucco and S. altirostris the premolars are "without inner basal cingulum." Cope, like Marsh, failed to distinguish the sexes as well as the separate groups or phyla of titanotheres. His "S." trigonoceras , for example, is a Menodus, a member of an altogether different group from his "S." Tiypoceras, which is a BrontotTierium. After an interval of 12 years, in 1886, Cope (1886.1) described the first Canadian species, Menodus angusti- genis, basing it upon fragments discovered by Mc- Connell and Weston for the Geological and Natural History Survey of Canada. Three years later (1889.1, p. 153) he referred this form to a new genus, Hapla- codon, and in the same year (1889.2, pp. 628, 629) he described two other Canadian species. His review (1891.2, p. 17) of these forms and attempted revision of the nomenclature were involved and unsatisfactory. He recognized only two genera, Menodus and Symbo- rodon. The last species of titanothere described by him was his Menodus peltoceras (1891.1), which is probably a female of Marsh's BrontotTierium curtum. EEINTERPRETATION AND PHYLOGENETIC STUDY (OSBOEN, 1887-1919) STUDY OF CERTAIN FEATURES Before Marsh and Cope had ceased naming new or supposedly new genera of titanotheres a turn was given to the trend of study by a paper by Scott and Osborn (1887.1, pp. 157, 158), entitled "Preliminary account of the fossil mammals from the White River formation contained in the Museum of Comparative Zoology." This paper, which was a description of the interesting collection made by Mr. Samuel Garman under the auspices of Prof. Alexander Agassiz, reacted from the polynomial systems of Marsh and Cope and tended toward a mononomial system. In this paper the Perissodactyla were described by Osborn, the Artiodactyla and Carnivora by Scott. Before de- scribing the new titanothere material the authors noted the difficulty in deciding where to draw generic lines, a difficulty that is increased by the fact that the mandibles are seldom found with the skulls. As in Uintatherium, the variability in the various portions of the skull, especially in the region of the horns, is so extreme that no two skulls are found which are exactly alike. But the denti- tion, which is constant among the Dinocerata, here greatly com- plicates the problems of classification. The premolars vary in number, and the incisors, always of relatively small size and fairly constant in number in the upper jaw, vary from three to none in the lower jaw." In all the lower jaws found in Professor Cope's collection of Menodontidae from northern Colorado there are no incisors, and the mandibular symphj'sis is extremely narrow. In the lower jaws of the Cambridge and Princeton collections, which are all from the Nebraska and Dakota exposures, the symphysis is broad, and the incisors, where pre- served, are two in number, while in one of the Cambridge specimens no less than three incisor alveoli may be counted upon one side of the symphysis. We might infer from this that Symborodon can be clearly separated from Menodus by the absence of the lower incisors, accompanied by a narrowing of the symphysis; but Professor Cope has recently described a new species, M. angustigenis, from the Swift Current Creek region (Cope, 1886.1, p. 81c), which combines the narrow type of symphysis with the presence of two incisors. The separation of these genera is rendered still more improbable by the parallelism which exists between the skulls from the Nebraska and Colorado localities, especially in respect to the conformation of the nasal bones and the horns. The genus Symborodon is, however, provisionally adopted at present to include the species with a narrow man- dibular symphysis and no lower incisors. The genus Brontotherium Marsh (that is. Marsh's "Bronto- therium" ingens, not the true Brontotherium) can not be dis- tinguished from Menodus. It rests in part upon the premolar I' One of the Cambridge skulls, M. coloradensis, has but a single upper incisor. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 147 formula |^, in the synopsis given by Professor Marsh (1876.1, p. 339), as distinguished from Menodus, with ?pm J^. One of the lower jaws of the Princeton collection, however, has the premolar formula 57^, demonstrating that the first lower pre- molar is a variable tooth and can not in this case be used in classification. The same rule applies to the second cone upon the last upper molar, the supposed generic character of Dicono- don Marsh. This is found in different species in all degrees of development, from a small prominence upon the basal cin- gulum to a well-developed cone {M. proutii) . From this evidence Osborn draws the following partly erroneous conclusion: Such characters as the invariable absence of lower incisors may subsequently be found to separate one genus of the Meno- dontidae from another; but our present evidence goes to show that they simply characterize the extremes of a closely related series of animals, from the same horizon, of which the inter- mediate forms are represented by numerous species. The safest basis of specific determination seems to be the correla- tion between the developnaent and proportion of the horns and of the nasals, the rule being that where the horns are long the nasals are short, and conversely. The number of the teeth does not at present seem to be absolutely constant, even within the limits of the species. The following determination of the species in the Cambridge collection is, for the above and other obvious reasons, provi- sional. The classification can be finally settled only when the lower jaws and skulls are found in association. Thus the validity of the several genera recognized by Marsh and Cope and of the chief criteria used by them as generic characters was called in question. The species are treated as belonging mainly to the single genus Menodus. Taking up the description of the new material, the authors mistakenly refer to Leidy's Megacerops coloradensis, a well-preserved skull, which at present is referred to Alhps marshi. They then describe two new species — "Menodus" tichoceras, based on a skull, and "Menodus" platyceras, based on a pair of bony horns. Both these species are at present referred to the true Brontotherium or flat-horned genus. The authors conclude their dis- cussion of the "Menodontidae" by presenting the first published restoration of the skeleton, made up of material in several museums, forming a composite animal representing Menodus proutii. In connection with a table of measurements arranged to show pro- gressive and correlated changes in the horns and nasals, they make the following remarks (op. cit., p. 16): The above measurements bring out very clearly the decrease in the proportions of the nasals pari passu with the gradual elongation of the horns. Another very interesting fact is brought out by the comparison of the transverse and longitudi- nal diameters of the horns at the base. As we pass from the short to the long horned types, through M. coloradensis, ticho- ceras, doUchoceras, and platyceras, there is a gradual rotation of the longer axis of the horn section from a fore and aft to a trans- verse plane, the species last named representing the extreme of the transverse type. The fuller development and more or less radical modification of the hypotheses put forward in this paper have been the subject of successive contribu- tions by Osborn, culminating in the present work. GEOLOGIC lEVEIS AND SUCCESSION OF TYPES (HATCHEE, 1886-1893) The work of Marsh and Cope had been exclusively descriptive and systematic. Osborn's observation of the correlated progressive reduction of the nasals and the enlargement and flattening of the horns seems to have been the first important application of evolutionary principles to tlie study of the Oligocene titanotheres. But materials for an exact knowledge of the phyletic succession, resting securely upon a knowledge of the precise geologic levels of a large series of specimens, had hitherto been entirely lacking. This all-important element of the time relations of the different species was largely supplied by the labors and study of J. B. Hatcher. In 1886, 1887, and 1888 Hatcher spent 15 months in the White River beds of South Dakota and Nebraska, collecting material for Professor Marsh's monograph on the Titanotheridae. In an interesting article in the American Naturalist for March, 1893, Hatcher (1893.1, pp. 214, 215) tells us that he collected or purchased "nearly 200 complete skulls and many more or less complete skeletons," a part of which are now on exhibition in the National and Yale Museums. The superb Hatcher collection in the United States National Museum contains skulls and jaws of 157 individuals; as completely listed in the generic sections of this monograph, it furnishes the classic standard of reference. Hatcher writes: Early in the season of 1886 it became apparent that certain forms of skulls were characteristic of certain horizons in the beds. This fact showed the importance of keeping, so far as possible, an exact record of the horizon from which each skull or skeleton was taken. From actual measurement the vertical range of the Titanotheridae was found to be about 180 feet. For convenience in keeping a record of horizons the beds were divided into three divisions of 60 feet each, and each of these three divisions was subdivided into three divisions of 20 feet each. The difi'erent skulls and skeletons, when dug out, were each given a separate letter or number, and this letter or num- ber was placed in that subdivision of the beds from which the skull or skeleton was taken. At present about 60 of these skulls and several more or less complete skeletons have been freed from their matrix. When studied in connection with the horizons from which they were taken, these remains show that a regular and systematic development took place in these animals from the base to the top of the beds. The most noticeable change which took place in the Titanotheridae was a gradual and decided increase in their size from the lowest to the uppermost beds, as is shown by the increase in the size of the skulls, fore and hind limbs, and other portions of the skeleton. Individuals found near the bottom of the beds are little, if any, larger than the living rhinoceros. From this they gradually increase in size as we go up until at the top we find a type described by Professor Marsh as Titanops, rivaling the modern elephant in size. This increase in size from the base to the summit of the beds was attended by a very marked development in certain portions of the skeleton, noticeable among which are the following: A variation in shape and an increase in the size and length of the horn cores as compared with the size of the skulls, attended, near the summit of the beds at least, by a decided shortening of the nasals. 148 TITAJSfOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Hatcher was less fortunate in his observations on the evolutionary changes in the dentition, stating that "the number of incisors, though probably never constant, even in the same species, shows a tendency to decrease in skulls found near the summit of the beds," and concluding that "the number of incisors can hardly be considered as of either generic or specific importance in the Titanotheridae, where they are no longer functional and vary with individuals in the same species and with age in the same individual. The same may be said of the presence or absence of the first premolar." After noting certain other changes rightly believed by him to be progressive, such as (1) the loss of the trapezium, (2) the development of a postero-internal cone on the third upper molar, and (3) the flattening of the horns, Hatcher concludes his paper by giving a tabular paleontologic section of the " Titanotlierium beds," with a general description of the forms char- acterizing the three ascending divisions. As to the number of genera, he gives the impression that he •regards all the various species ascribed by Marsh and Cope to different genera (except Teleodus avus Marsh) as referable to the single highly variable genus Titanotherium Leidy. FIRST EUEOPEAn NOTICE (TOTJIA, 1892) The next important event is the discovery of a titanothere of Oligocene type in Europe, described as Menodus rumelicus by Toula (1892.1). This dis- covery, in connection with that of the Transylvanian BracJiydiastematJierium, described by Bockh and Maty in 1876 (1876.1), extended the known range of the titanotheres to the Old World. DISTINCTIONS OF SEX (OSBOEN AND WOETMAN, 1895) In 1895 Osborn and J. L. Wortman (1895.105) published a corrected restoration of Titanotlierium based upon the fine skeletal material secured by the American Museum field parties in 1892 and 1894. They ventured the conclusion that "it is probable that certain wide differences in the development of the horns, which have been assigned a generic value, are merely sexual characters. " MONOPHYIETIC INTEEPEETATION (OSBOEN, 1898) The extreme development of the erroneous theor}' that all the various species of Oligocene titanotheres belonged to the single genus Titanotherium and were practically monophyletic is worked out in a very elaborate way in Osborn's paper "The cranial evolu- tion of Titanotlierium," published in 1896. This 'was the most comprehensive review of the subject that had hitherto appeared and was illustrated by numerous text figures and several folded plates. The direct observations were based chiefly on the large collection of titanotheres in the American Museum and to a less extent upon figures and descrip- tions previously published. Part I, the systematic introduction, includes a chronologic list of generic and specific terms, with references and a brief history of the progressive complication of the nomenclature, after which the author says (Osborn, 1896.110, p. 162): It is obvious that the only method of clearing up this hetero- geneous list [of nominal genera and species] is first to establish certain laws of cranial development, and second to apply these laws to the distinction of genera and species in chrono- logical order. Examined in this way, the vast array of genera and species is resolved into one or possibly two genera and about fourteen definable species. Accordingly in Part II, "Principles of cranial and dental evolution," we find a study of the differences in size of skull, shape of horns, nasals, zygomatic arches, auditory meatus, cingula on grinding teeth, incisors, canines, second internal cone of last molar, etc., aU considered as indicating either specific or sexual or individual differences within the limits of a single genus, Titanotlierium. This is followed by the "Revision and definition of species," in which some 27 species, including the new T. ramosum, are discussed. The known species from the lower, middle, and upper beds are arranged in a single or monophy- letic series, beginning with the T. heloceras-trigonoceras ingens series, continuing with torvum, rohusfum, doliclioceras, elatum, amplum, acer, and culminating with ramosum and platyceras. This analysis, although wholly wrong in treating all the species as members of a monophyletic series, not only laid the foundation for the present evolution- ary and phylogenetic treatment of the group but established, as it were, the technique of investigation POIYPHYIETIC INTEEPEETATION (OSBOEN, 1902-1919) The reaction against the monophyletic theory was felt by the same author as a result of more extended research. In his paper of 1902 on "The four phyla of Oligocene titanotheres, " after acknowledging the services of the late Professor Marsh and admitting the incorrectness of the monophyletic theory, Osborn says (1902.208, p. 91): This second review is an abstract of a portion of the results obtained for the United States Geological Survey monograph "The titanotheres," now in preparation. It covers practically aU the type material in the Yale, National, American, and Harvard Museums, and advantage has been taken of the invaluable field observations by Hatcher of the levels on which the different skulls in the National Museum collection were discovered. The section method also has been very greatly extended and, taken in connection with the teeth and the detailed structure of the skull, has proved to be a sure criterion of specific and phjdetic character. Four important considerations had led Osborn to give up the monophyletic theory: First, from his phylogenetic studies on the rhinoceroses of Europe and America (Osborn, 1898.143; 1900.192) he had concluded that, contrary to earlier opinions, this DISCOVERY OF THE TtTANOTHERES AND ORIGINAL DESCRIPTIONS 149 group was in a high degree polyphletic, embracing many parallel phyla and having a wide adaptive radiation; second, the principle of dolichocephaly and brachycephaly (Osborn, 1902.207), as interpreted by him in the rhinoceroses and other groups, raised the presumption that similar differences would be found to distinguish genera and phyla among the titano- theres; third, he had learned to realize that the extent to which parallel and convergent evolution had oper- ated in many allied phyla had been but little appre- ciated by earlier writers, who had largely failed also to distinguish between persistent, progressive, and retrogressive characters; fourth, an examination of the titanothere skulls collected by Hatcher and now in the National Museum, which Hatcher had recorded exactly as to level, enabled him, with the aid of prin- ciples just stated, to distinguish several distinct phyla and to foUow them from the lower part through the middle and into the very top of the " Titanotherium beds." The characters of these phyla were summar- ized by Osborn as follows (1902.208, pp. 92-94): THE FOUR GENERA Titanotherium Leidy applies to .long-limbed animals with long skulls, persistently long and broad nasals, short triangular horns placed slightly in front of the e3'es, vestigial incisors ^o' large canine teeth. Known from the base to the summit of the [lower] Oligocene. Megacerops Leidy applies to titanotheres with broad skulls, nasals progressively shortening, short horns rounded or oval in section, shifting anteriorly, one or two pairs of incisor teeth, .|r}, medium-sized canine teeth. Known from the base to the summit of the [lower] Oligocene. Probably related to this are the subgenera of the t3'pes named Allops and Diploclonus by Marsh, differing from the above in horn characters. Known chiefly from the upper beds. Symborodon Cope includes titanotheres with skulls of varying proportion, nasals slender and progressively shortening, horns elongate and peculiar in being placed above the eyes instead of shifting forward, incisors vestigial l^, canines small, approx- imated. Known only from the middle and upper beds. Brontotherium iVlarsh embraces the largest titanotheres, with very broad zygomatic arches, nasals shortening while horns elongate and shift forward; incisors persistent, f in the males, canines stout and obtuse. Representatives of Titanotherium and Megacerops can be now continuously traced from the base to the summit of the [lower] Oligocene. Primitive species of Brontotherium also appear at the base, although the phyletio sequence through the middle to the upper beds is not so clear. Symborodon suddenly appears in the middle beds. Viewed in the light of the foregoing principles, the variations in the horns, nasals, incisors, cingula, etc., took on new meanings — biologic, phylogenetic, and systematic; so that, after more than half a centm-y of research (1846-1902) the systematic problem presented by the Oligocene titanotheres appeared in its main features to be solved. Subsequent research, however, has led to certain regrettable but apparently necessary changes in nomenclature: (a) The name " Megacerops" Leidy, as defined above, has been set aside for Brontops IVIarsh, for the reasons given below; (b) the name Titanotherium Leidy has been abandoned for the prior name Menodus Pomel; (c) the name Symhorodon Cope has been replaced by the prior name Megacerops Leidy. RECENT DISCOVERIES BY LULI, lAMBE, AND OTHERS There remain to be recorded the following contri- butions: (1) The description of Megacerops tyleri by Lull (1905.1), based upon a fine skull and lower jaws with associated limbs, discovered by the Amherst CoUege paleontologic expedition of 1903; (2) the description of Brontotherium hatcheri and Symhorodon copei by Osborn in 1908 (1908.318), based on skulls in the National ]Museum; (3) the description of Mega- cerops primitivus and M. assinihoiensis, based on frag- ments obtained from Saskatchewan, Canada, by Lambe in 1908 (1908.1); (4) W. K. Gregory observed (a) that there is an alliance between Brontops, Allops, and Menodus as these terms are now used by Osborn, indicated by certain intermediate forms between the extremely brachycephalic Brontops roiustus and the dolichocephalic Menodus giganteus, (b) that there is also an alliance between Brontotherium and Megacerops (Symhorodon) in spite of the differences in the incisors. Hence the former group — Brontops, Allops, Menodus — ■ has been called the menodontine group, and the latter group — Brontotherium, Megacerops — has been called the brontotheriine group. Possibly the most valuable general result of the study of the titanotheres has been the fact that it has made possible the close examination of an extensive evolutionary history, stretching from the lower Eocene to the summit of the lower Oligocene. IVTany observa- tions have been made on the precise modes of evolu- tion, especially with regard to the way in which char- acters first appear and subsequently develop. The results of this evolutionary study are set forth in Chapters V, VI, VH, and XI of the present work. THE EOCENE TITANOTHERES PIONEER DISCOVERIES WORK IN THE BRIDGER, WASHAKIE, AND UINTA BASINS BY lEIDY, MARSH, COPE, SCOTT, OSBORN, AND OTHERS (1870-1889) Prof. F. V. Hayden, in the course of his historic explorations in the fossiliferous beds of the Kocky IMountains and Great Plains, obtained at Church Buttes, near Fort Bridger, Wyo., a number of isolated teeth, which were described by Leidy (1870.2) under the name Palaeosyops paludosus. This was the first Eocene titanothere made laiown to science, 24 years after the discovery of Prout's "gigantic Palaeotherium" (Titanotherium) in South Dakota. Although Leidy noted that the lower molar of Palaeosyops "resembles in its constitution those of Palaeotherium, (Jlialicothe- rium, and Titanotherium," he did not classify the new genus with the titanotheres, for the reason that at that time he thought Titanotherium and Chalicotherium were allied to the Artiodactyla. (See p. 247.) Soon 150 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA afterward Leidy (1873.1, p. 27) described a skull, some teeth, and parts of the limb bones of Palaeosyops and, noting the similarities of this species to its supposed allies Tapirus and Palaeoiherium, correctly referred it to the perissodactyls — the odd-toed pachyderms. Three other species (P. major, P. Tiumilis, P. Junius) were also described by him from the Bridger beds upon very fragmentary material. In developing our knowledge of the Eocene titano- theres of the Bridger Basin, as in developing that of the Oligocene titanotheres. Marsh and Cope were not far behind Leidy. The first specimen of an Eocene titanothere described by Marsh, however (1871.2), was not recognized as such by him, as he mistook the isolated second lower premolar of a Bridger Palaeosyops for the fourth upper premolar of a dog and named it "Canis montanus." The next year (1872.1) he de- scribed some well-preserved remains under the name Palaeosyops laticeps and also founded the genus Telmatherium. Marsh's subsequent contributions to our knowledge of middle Eocene titanotheres were not especially significant, but in 1875 he described the very important genus Diplacodon from the upper Eocene Uinta beds of Utah and recognized its inter- mediate position both in time and in structural characters between his "Limnohyidae" (Palaeosyo- pinae) and Brontotherium. Cope's explorations of the Bitter Creek or Wash- akie Basin (middle Eocene) of Wyoming m 1872 led to his describing the species "Palaeosyops" vallidens and "Limnoliyus" laevidens, both represented by imperfect remains. The former is now known to belong to the long-headed genus Dolichorfiinus. The next year, 1873, Cope (1873.5) described the species Limnoliyus ( = Palaeosyops) fonfinalis from the lower levels of the Bridger formation (supposed Bridger A), which is the oldest middle Eocene titano- there yet discovered. His Palaeosyops diaconus, from the upper levels of the Bridger Basin, is prob- ably a synonym of Palaeosyops rohustus (Marsh). DISCOVERY IN HUNGAEY Shortly after these pioneer discoveries in America Bockh and Maty (1876.1) described a large lower jaw from Eocene deposits in Transylvania, in Hun- gary. The animal was supposed to be allied to Palaeoiherium and was named Brachydiastematherium transilvanicum. Its affinities with the Palaeosyops group long remained unnoted, and even to this day it is the only known specimen of its kind in Europe. PEINCETOM AND COPE-WOETMAN EXPEDITIONS The Princeton expeditions sent to the Bridger and Washakie Basins in 1877 and 1878 under Scott, Osborn, and Speir brought to light much valuable material of Palaeosyops and allied genera, especially the types of "Leurocephalus" cultridens and the pecu- liar form which was later described by Earle as Palaeosyops megarMnus. Thus by the end of 1878 re- mains of the genus Palaeosyops and its allies had been discovered in the middle Eocene Bridger and Washakie Basins and in the upper Eocene Uinta Basin. The next year (1879) Dr. J. L. Wortman, who was collecting for Cope, extended the kiiown range of the group into the lower Eocene Wind River formation of Wyoming, where he discovered the very primitive form which Cope in 1880 named Palaeosyops horealis and which is now recognized as approximately ances- tral to the middle Eocene titanotheres. Wortman also discovered a very small form, which was described by Cope in 1880 (1880.1) as Lamhdotherium popo- agicum and recognized as more or less closely allied to the Palaeosyops group. The next important expedition was that made by a Princeton party under Scott and Speir in 1886 into the Uinta Basin (upper Eocene). They collected skeletal material, referred at that time to Diplacodon, which was described by Osborn in 1890 (1890.51) and which demonstrated the intermediate characters of "Diplacodon" {Protitanoiherium) between the Oligo- cene and middle Eocene titanotheres. In the same publication Osborn also described "Palaeosyops" hyognathus, a species based on a jaw that is now known to represent the long-skulled genus DolichorMnus. FIRST SYSTEMATIC AND EVOLUTIONARY REVISION (EARLE, 1889-1891) Although Cope in 1884 (1885.1) had republished and partly extended the original descriptions of his own species, with lithographic figures, no satisfactory revision of the Palaeosyops group was possible at that time or for many years later. In 1889 Charles Earle, at the invitation of Prof. H. F. Osborn, began a careful study of the material in the Princeton Museum and other collections, and in 1892 he published a memoir "On the genus Palaeo- syops Leidy and its allies" (1892.1). Earle gave a very detailed description of the osteology of Palaeo- syops and of the first attempted reconstruction of the skeleton of an Eocene titanothere by Osborn. (See fig. 86.) Owing in part to the lack of sufficient well- associated material, in part to the confusing practice of the earlier writers in designating and founding species upon several specimens of doubtful specific association, Earle's revision of the species and genera was, as he himself recognized, by no means final. He rightly regarded as distinct the genera Lamhdotherium, Limnohyops, Palaeosyops, and "Telmatotherium," but as he showed in his tentative phylogenetic scheme, he, like other paleontologists at that time, did not appreciate the polyphyletic character of groups and consequently referred to a single main line of descent a number of forms that belong to widely separated phyla. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 151 AMERICAN MUSEUM AND OTHER EXPLORATIONS OF THE EOCENE BASINS (1891-1895) The problems relating to Palaeosyops and its allies, which had been barely made evident by the pioneer discoveries and had now been partly formu- lated by Earle, were of course only particular results of the general explorations of the fossil-bearing forma- tions of the West. The early explorations had been in part reconnaissances, and their results were accordingly incomplete as regards both the nature of the material and the records of the stratigraphic levels at which the specimens were found, both absolutely prerequisite to a detailed knowledge of the phylogeny. exhibit a mounted composite skeleton of this animal. Much other material was also collected by the same party. All this material has been used profitably in the present monograph, especially the specimens representing the "prophet-horn stage," to which Doctor Wortman in a letter from the field applied the name Manteoceras. Another American Museum expedition, under Mr. O. A. Peterson, went into the Uinta Basin in 1894 and examined two hitherto unexplored horizons (Uinta B 2 and Uinta B 1 of this monograph), which underlie the true Uinta (Uinta C). This expedition collected many new forms and worked out the faunal sequence of the three horizons indicated. Among the results Figure 86. — Osborn's first restoration of Palaeosyops paludosus Leidy This restoration is a composite one— the skull from the fine specimens in the Academy of Natural Sciences of Philadelphia, and the axial skeleton from the material in the Princeton Museum, twelfth natural size. The fore feet were afterward referred to Mesaiirhinus peterst The founding (in 1890) of the department of verte- brate paleontology in the American Museum of Natural History by Prof. Henry Fairfield Osborn and the consequent establishment of continuous and syste- matic exploration began a new era of exact investiga- tion not only of the titanotheres but of the whole series of vertebrate remains to be found in the Rocky Mountains and Great Plains regions, as well as the stratigraphic horizons at which they occur. The first of these expeditions, led by Dr. J. L. Wortman, procured some important skeletal material of "Palaeosyops" horealis from the Wind River forma- tion. Another expedition, sent out under Doctor Wortman in 1893, procured from the Bridger and Washakie Basins extensive material of the true Palaeosyops, enabling the American Museum to of this expedition, as reported in 1895, were the discus- sion by Osborn and Peterson (Osborn, 1895.98) of the three faunal levels (Uinta B 1, B 2, and C) and the description by Osborn of the specialized and interest- ing titanotheres named " TelmatotJierium" diploconum and T. cornutum. Wortman's "prophet-horn" skulls were referred to " Telmatotherium vallidens," so that animals showing a wide range of form were here erroneously included under a single genus. The very aberrant form SpJienocoelus was also described, but its ordinal and family positions were left "Incertae sedis," on account of the lack of the teeth in the type and the peculiar characters of the base of the skull. In the same year (1894) Mr. J. B. Hatcher, of the Princeton Museum, also went into the true Uinta area and discovered specimens representing the very 152 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA advanced stage which in 1895 (1895.1) he described as Diplacodon ejnarginatus. In a brief postscript to this description he noted the wide range of forms that had been erroneously grouped by Osborn under the genus " TelmatotTierium," and he formally proposed the generic names Manteoceras for the "prophet- horn" and Dolichorhinus for the long-skulled form. Figure 87. — Four stages in the origin and evolution of tlie horns in titanotheres After Hatcher's original plate (1895). A, Palaeosyops laiiceps (= Limnohyops lali- ceps), after Earle; B, Telmatotherium vallidens ( = Manteoceras manteoceras), after Os- born; C, Diplacodon emarginatus (,= PTOtitanotherium emarginatum), after Hatcher; D, Titanotherium varians (= Bronioihermin leidyi), after Marsh. One-eighth natural size. Both these terms, as well as the name Protitanotherium of Hatcher, have proved to be valid. Hatcher's separa- tion of these genera was a very important move toward a correct understanding of their phylogenetic rela- tions. He also figured a series of four stages ranging from the middle Eocene to the lower Oligocene, inclu- sive, showing the origin of the "horns." (See fig. 87.) INVESTIGATIONS AND EXPLORATIONS MADE IN PREPARA- TION FOR THE PRESENT MONOGRAPH (1900-1919) Between 1895 and 1900 no very important work on the Eocene titanotheres was done. By the end of the nineteenth century some 12 nominal genera and 25 nominal species of the titanotheres had been proposed, but many of the real generic limits and phyloge- netic relations were still obscure except for the preg- nant suggestions of Hatcher. In 1900 Professor Osborn, at the invitation of Director Charles D. Walcott, undertook to revise and monograph the Eocene titanotheres in connection with the United States Geological Survey monograph on the Oligocene titanotheres that had been begun by Professor Marsh. The work on the Eocene titanotheres has proved to be by far the most difficult and most extensive part of this task. During the last 28 years Professor Osborn, with the assistance of Dr. W. K. Gregory, has studied the great and growing collection in the American Museum of Natural History and in other institutions and has set forth the results in several prelimmary articles and more fully in the present work. A long series of parties of exploration, beginnLng in 1903, sent out from the American Museum by Osborn (1909.321) and conducted chiefly by Mr. Walter Granger, have carefully examined the various lower, middle, and upper Eocene basins of the West with special reference to the exact succession of species. This very precise work has shown that the Bridger and other formations are divided into a succession of zonal levels characterized by the remains of titano- theres and other mammals in different generic and specific stages of evolution. The stratigraphic rela- tions of the Eocene to the Oligocene deposits have also been in part explored. The results are fully set forth in this monograph. Although this work in the Eocene basins has been carried on chiefly by the Ameri- can Museum of Natural History, the Carnegie and Field Museums have sent expeditions into the Uinta Basin under Douglass (1909.1) and under Riggs (1912.1), which have yielded similar results as to specific and generic succession. The distinction of numerous independent Eocene phyla by Osborn has followed the discovery of the Oligocene phyla, some of which arise from those of the Eocene. Thus have been established secure bases of fact, first, for a general history of the early Tertiary faunas of the West; second, for a demonstration of the evolu- tion of certain phyla of titanotheres through long periods of time; and, third, for a consideration of the modes and factors of evolution of titanotheres in par- ticular and of mammals in general. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 153 SECTION 2. ORIGINAL DESCRIPTIONS OF EOCENE TITANOTHERES FIVE RULES FOE DETERMINING THE NAMES OF TITANOTHERES The systematic revision of the Eocene and Oligo- cene titanotheres was begun in 1900 by the author with the cooperation of W. K. Gregory and has been continued up to the day of the deUvery of the manu- script of this volume to the Geological Survey. The great difficulties and the labor involved in determin- ing the correct prior names for the genera and species have been due to the imperfection of the fossil types, to loose methods of description and comparison, and to the mingling as cotypes of animals belonging to dif- ferent species or even to different genera. Experience has shown that the following five rules are absolutely necessary for future vertebrate paleon- tologic work. Rule 1. Accept the "law of priority," as defined by the International Committee on Nomenclature. In this revision the author has accepted as authori- tative the rules of nomenclature based upon the "law of priority," as defined by the "Code" of the Ameri- can Ornithologists' Union and by the recommenda- tions of the committee on nomenclature of the Twelfth International Congress of Zoology. Special acknowl- egments a,re due to the eminent authority Dr. J. A. Allen for frequent aid in deciding troublesome prob- lems of nomenclature. Rule 2. Determine the geologic level and characters of the type, as the starting point of monographic inquiry. Experience teaches that the characters of the holo- type specimen and the geologic level on which it was found afford the permanent facts to which all questions of nomenclature must be referred as the basis of mono- graphic investigation. Rule 3. Avoid confusion of characters of th& type and cotype or paratype. All the early systematic work on the titanotheres was done without regard to precise discrimination between the certain or permanent nomenclatural value of the holotype specimen and the uncertain value of "specific" characters based on cotype, para- type, and neotype specimens. For example, take the case of the classic species Palaeosyops paludosus Leidy. Leidy used as types the very fragmentary teeth from the lower levels of Bridger B, which first came into his hands; he later erroneously associated with these fragments, practi- cally as cotypes, other more complete specimens, which are now known to belong to two or three different species from higher geologic levels. Subsequently Leidy himself. Cope, Marsh, Scott, Osborn, and Earle all accepted Leidy's erroneous associations, and P. paludosus came to be known by certain of its falsely associated cotype and paratype characters instead of by its true type characters. 101959— 29— VOL 1 13 Thus the entire nomenclature of the subject became a mass of confusion, and the difficulties encountered in clearing it up have been almost insuperable. The rule is that specific definitions must be based on holotypes only, unless there is absolutely no possibility of doubt that the associated types are from the same geologic level and belong to the same species. Rule 4. Distinguish the different values and kinds of types. The use of the terms type (or holotype), cotype, paratype, lectotype, hypotype, neotype has been dis- cussed critically by Oldfield Thomas (1893.1, p. 241), by Schuchert (1905.1, pp. 9-14), and by Osborn (1918.473). The distinctions indicated below should be noted. Type, individual, or holotype. — -A holotype is a particular individual specimen "deliberately selected by the author of a species; or it may be the only example of a species known at the time of original publication. A holotype, therefore, is always a single individual but may embrace one or more parts, as the skin, skeleton, or other portions." (Schuchert, op. cit.) The holotype must usually be determined from the original description. Cotype, coordinate or equivalent type. — The term co- type is applied to specimens when an author's type description refers to remains of two or more individuals without selecting or distinguishing one as the holotype, so that all appear to be equally identified with the specific name given. Lectotype. — "Where the origina' diagnosis is with- out illustrations or is accompanied by figures based on two or more specimens, the first subsequent author is at liberty to select from these cotypes a type for the old species, adhering, so far as can be ascertained, to the intention of the original author. Such a type specimen is to be designated a lectotype ( = a chosen type)." (Schuchert, idem.) The practice of Osborn as to lectotypes in paleontology is either (a) to select the first individual specimen named by the original author, because the second individual specimen may belong to a distinct species, or (&) to select the speci- men to which the specific name obviously refers — for example, Cope's Menodus angustigenis. Hypotype and plesiotype. — As shown by Schuchert (idem), the terms hypotype and plesiotype have been used in two different senses to cover "supplementary types." They may well be dropped. Neotype. — A neotype is defined by Schuchert (idem) as a [new] "supplementary type selected by an [a sub- sequent] author, on which a species is to rest because of the loss of the original type, or where the original material still extant is so poor or fragmentary that from it the characters of the species can not be determined with certainty." Great care must be taken that the neotype comes from the same geologic level as the type. 154 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA Summary. — The usage adopted in this monograph is as follows: Holotype (of original autlior) : The original individual type specimen selected by the author. Cotypes: Different individual specimens rightly or wrongly put together by the author as "types." Paratype (of original author) : Additional individual specimen or specimens noted by the author in the original description and used by him in defining the species. Lectotype (of subsequent author) : The specimen selected by a subsequent author, from among the "cotypes," for pur- poses of subsequent description or redefinition. This may be (a) the specimen first mentioned by the author, or (6) the specimen to which the specific name obviously applies. Neotype (of second or subsequent author) : A new specimen selected in a subsequent description because of the loss or im- perfection of the holotype or type. These five terms are all that are necessary in verte- brate paleontology. The terms plesiotype and hypo- type are discarded in this monograph because they are too indefinite. Monographic revision in the use of above terms. — Leidy founded the species Palaeosyops paludosus upon some isolated teeth from the low levels of Church Buttes. In the original description these teeth, which probably represent more than one in- dividual, were treated as coordinate or equivalent types or "cotypes." Out of this lot the second lower molar (m2), which was the first specimen men- tioned and described by Leidy, has been selected by Osborn in the present volume as the final standard, or "lectotype," of the species. In the same original description by Leidy of P. paludosus a second lot of teeth, from the high levels of Henrys Fork, were mentioned, and the characters, of these teeth entered into Leidy's original conception of the species. These teeth are now called "paratypes." In the present revision, since there is little doubt that Leidy's paratypes are not really conspecific with the specimen first mentioned (lectotype), Osborn has selected from the same geologic level. Church Buttes, a lower jaw in which m2 agrees most clearly with the lectotype m2 and which is to serve as a secondary type, or "neotype." It wUl be seen that cotypes, paratypes, or neotypes may sometimes be wrongly associated specifically with the holotype, in which case the specific name must cling to the holotype and lectotype as the ultimate standard means of identification. The first step toward permanence, therefore, is the settlement of the holotype characters, which is some- times an almost impossible task, owing to the poor quality of the holotype selected — for example, the holotype of Palaeosyops major Leidy, a jaw fragment without teeth; the holotype of P. humilis Leidy, a single deciduous premolar. Rule 5. Avoid mingling as types and cotypes specimens from different geologic levels. The mingling of types and cotypes from different geologic levels has been the second chief source of confusion. To cite a prominent instance. Cope's cotypes of Palaeosyops laevidens were two skulls col- lected at widely separated localities, and in his original description no regard was shown for their possible difl'erence of geologic age. It appears almost certain that the lectotype belongs to a lower level and is perhaps some thousands of years more ancient than the paratype. Similarly we have shown that the lectotype of Leidy's P. paludosus is from Bridger level B 1 or B 2; the paratypes are from level C 2 or C 3, a difference of geologic level representing a very long period of time, in which it is now certain that a very marked progressive evolution took place in teeth, skull, and skeleton. Our geologic leveling of the Bridger formation, described in Chapter II, has therefore not only afforded us the means of determining the evolutionary succession of the species of titanotheres but, if the localities of the types were properly recorded by the authors, it has enabled us to separate many er- roneously associated type specimens. The geologic levels of the materials recently acquired by the American Museum have been ascertained precisely; on the whole, the successive species correspond very closely with the successive levels — that is, in no case have different species in the same line of descent been found at the same level, although species in different lines of descent (that is, in different genera) are found in analogous stages of evolution. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS THE GENERA AND SPECIES OF EOCENE TITANOTHERES 155 The accompanying list shows, in chronologic order, the names assigned to Eocene titanotheres. The numbers in the first column indicate the chronologic order or rank of the systematic names, the roman numerals indicating generic names, the arable numerals specific names. Chronologic list of original descriptions of Eocene titanotheres [Generic names accepted in tliis work as valid are printed In small capitals; abandoned names are inclosed in brackets] I 1 2 3 4 6 6 II 7 Ilia 8 9 Illb 10 11 12 IV 13 V 14 VI 15 16 VII 17 18 19 VIII 20 21 22 23 24 IX 25 X, 26 XI XII 27 28 29 30 31 XIII 32 33 34 35 36 1870 1870 1871 1871 1872 1872 1872 1872 1872 1872 1872 1872 1872 1873 1873 1873 1875 1875 1876 1876 1878 1878 1880 1880 1880 1881 1889 1890 1891 1891 1892 1895 1895 1895 1895 1895 1895 1895 1895 1897 1899 1899 1899 1902 1907 1907 1908 1908 1908 1908 Palaeosyops Palaeosy ops Palaeosyops [Canis] Palaeosyops [Palaeosyops] [Palaeosyops] Telmatherium Telmatheriu m [Limnohyus] [Limnohyus] [Palaeosyops] [Limnohyus] [Limnohyus] [Limnohyus] Palaeosyops DiPLACODON Diplaoodon Bkachydiastbmathebium Brachydiastematherium. [Leurocephalus]_ [Leurocephalu s] . paludosus.. major [montanus]_ [hu mills] Junius laticeps validus. robustus. vaUidens _ laevidens-. fontinalis. . [diaconus]. elatus_ transilvanioum. cultridens. [Palaeosyops] borealis Lambdotherium Lambdotherium [Lambdotherium] [Palaeosyops] LiMNOHYOPS [Palaeosyops] [Palaeosyops] Palaeosyops longirostris [Telmatotherium] diplooonum_ [Telmatotherium] [cornutum]_ Sphenocoelus Sphenoooelus uintensis PrOTITANOTHERIUM ': [Diplaoodon] emarginatus megarhinus. [minor] Manteoceras DoLICHORHINtrS- - [Palaeosyops] [Palaeosyops] [Telmatotherium], [Canis?] Manteoceras EOTITANOPS Lambdotheriu m_ Limnohyops Limnohy ops Limnohyops Palaeosyops Present determination Leidy.- do. do. Marsh. Leidy. . do. Marsh. do. do. do do Cope . Leidy (not Marsh). Cope do do Marsh do Bockh and Maty. do Osborn, Scott, and Spelr. do Cope. .do. popoagicum do__ brownianum do_. hyognathus Osborn.. Marsh. - Earle do._ do_. Osborn.. do_- do.. do.- Hatcher. do.. ultimus manteoceras [diploconum var. minus. [marshii] manteoceras pnmaevum. priscus matthewi monoconus. .! leidyi. do... do... Matthew. do... de- Hay. __ do. Osborn. Loomis. Osborn. do. .do. .do. Palaeosyops Leidy. Palaeosyops paludosus Leidy. Palaeosyops major Leidy. Palaeosyops major? Leidy. Palaeosyops sp. Mesatirhinus Junius (Leidy). Limnohyops laticeps (Marsh). Telmatherium Marsh. Telmatherium validum Marsh. Palaeosyops Leidy. Palaeosyops robustus (Marsh). Dolichorhinus vallidens (Cope). (Preoccupied.) Limnohyops laevidens (Cope). ?Palaeosyops fontinalis (Cope). Palaeosyops robustus (Marsh). Diplaoodon Marsh. Diplaoodon elatus Marsh. Brachydiastematherium Bockh and Maty. Brachydiastematherium transilvani- cum Bockh and Maty. Telmatherium Marsh. Telmatherium cultridens (Osborn, Scott, and Speir). Eotitanops borealis (Cope). Lambdotherium Cope. Lambdotherium popoagicum Cope. Eotitanops brownianus (Cope). Dolichorhinus hyognathus (Osborn) . Limnohyops Marsh. Mesatirhinus megarhinus (Earle). Palaeosyops paludosus Leidy. Palaeosyops longirostris Earle. Rhadinorhinus diploconus (Osborn). Dolichorhinus hyognathus (Osborn) . Sphenocoelus Osborn. Sphenocoelus uintensis Osborn. Protitanotherium Hatcher. Protitanotherium emarginatum Hatcher. Manteoceras Hatcher. Dolichorhinus Hatcher. Telmatherium ultimum Osborn. Manteoceras manteoceras Hay. Metarhinus fluviatilis Osborn. Palaeosyops major? Leidy. Manteoceras manteoceras Hay. Eotitanops Osborn. Lambdotherium primaevum Loomis. Limnohyops priscus Osborn. Limnohyops matthewi Osborn. Limnohyops monoconus Osborn. Palaeosyops leidyi Osborn. 156 TITAJSrOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA Chronologic list oj original descriptions oj Eocene titanotheres — Continued [Generic names accepted in this work as valid are printed in small capitals; abandoned names are inclosed in brackets] Bank Date 37 38 39 XIV 40 • XV 41 42 43 44 45 46 47 48 49 50 XVI 51 52 53 54 XVII 55 56 57 58 59 60 XVIII 61 XIX 62 XX 63 64 65 XXI 66 1908 1908 1908 1908 1908 1908 1908 1908 1908 1908 1908 1908 1909 1909 1909 1909 1912 1912 1912 1912 1912 1912 1912 1913 1913 1913 1913 1913 1914 1914 1914 1914 1914 1916 1919 1919 1919 1919 grangeri copei washakiensis. petersoni. fluviatilis earlei intermedius. ultimum altidens superbum_ _ incisivum uintensis heterodon__ longiceps Palaeosyops Palaeosy ops Manteoceras Mesatirhinus Mesatirhinus Metarhinus Metarhinus Metarhinus Dolichorhinu s Telmatheriu m Telmatherium? Protitanotheriu m [Telmatherium?] Manteoceras Dolichorhinus DoUohorhinus Sthenodectes [Mesatirhinus] Metarhinus Metarhinus Dolichorhinus Rh ADINORHINUS Rhadinorhinus Eotitanops Eotitanops Eotitanops Lambdotherium Lambdotherium ' progressum. [ Diploceras] [Diplooeras] orborni [Heterotitanops] [Heterotitanops] I parvus EOTITANOTHERIUM I [Telmatherium?] j birmanicum Lambdotherium Eotitanops EOMETARHINUS Eometarhinus superior.. riparius cristatus__ fluminalis- abbotti.- gregoryi- princeps. major prisoum. magnum, minimus. huerfanensis _ Osborn. do- .do. .do. .do. .do_ .do_ .do. .do_ .do. .do. do... Douglass. do.._ do._ do.. Gregory. Riggs... do._ .do_ .do. .do. do. Osborn. do. -do. .do- .do. Peterson. do._ do__ .do_ .do. Pilgrim and Cotter . Osborn do .do_ .do_ Present determination Palaeosyops grangeri Osborn. Palaeosyops copei Osborn. Manteoceras washakiensis Osborn. Mesatirhinus Osborn. Mesatirhinus petersoni Osborn. Metarhinus Osborn. Metarhinus fluviatilis Osborn. Metarhinus earlei Osborn. Dolichorhinus intermedius Osborn. Telmatherium ultimum Osborn. Telmatherium altidens Osborn. Protitanotherium superbum Osborn. Sthenodectes incisivus (Douglass). Manteoceras uintensis Douglass. Dolichorhinus heterodon Douglass. Dolichorhinus longiceps Douglass. Sthenodectes Gregory. DoUohorhinus superior (Riggs). Metarhinus riparius Riggs. Metarhinus cristatus Riggs. Dolichorhinus fluminalis Riggs. Rhadinorhinus Riggs. Rhadinorhinus abbotti Riggs. Eotitanops gregoryi Osborn. Eotitanops princeps Osborn. Eotitanops major Osborn. Lambdotherium priscum Osborn. Lambdotherium progressum Osborn. Eotitanotherium Peterson. Eotitanotherium osborni Peterson. ? Metarhinus. ?Metarhinus sp. Eotitanotherium Peterson. Uncertain. Lambdotherium magnum Osborn. Eotitanops minimus Osborn. Eometarhinus Osborn. Eometarhinus huerfanensis Osborn. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 157 OEIGINAI DESCRIPTIONS OF THE SPECIES Palaeosyops Leidy, 1870 Cf. Palaeosyops, this monograph, page 312 Original reference. — Acad. Nat. Sci. Philadelphia Proc, 1870, p. 113 (Leidy, 1870.2). Type species. — Palaeosyops paludosus Leidy. (See p. 319.) Generic characters. — Leidy, in his description of the fragmentary type, very properly refrained from at- tempting to distinguish generic from specific charac- ters. Generic characters are given below. Etymology. — TraXaio?, ancient; am, boar; ih\p, face (appearance). The name was probably suggested by the fact that the "upper true molars exhibit the outer part of a crown composed of a pair of lobes, exactly as in Hyopotamus." (Leidy.) Present determination. — The generic name is a valid one. Palaeosyops paludosus Leidy, 1870 Cf. Palaeosyops paludosus, this monograph, page 319 Original reference. — Acad. Nat. Sci. Philadelphia Proc, 1870, p. 113 (Leidy, 1870.2). Subsequent reference. — Leidy, Extinct vertebrate fauna of the Western Territories, p. 28, pi. 23, figs. 3-6 (fig. 5 lectotype), 1873 (Leidy, 1873.1). Type locality and geologic horizon. — Church Buttes, near Fort Bridger, Bridger Basin, Wyo.; Palaeosyops paludosus-Orohippus zone (Bridger B 1 or Bridger B2). Leidy's cotypes. — M2, p*, m^, m^ (Nat. Mus. 759, 758, 762). (Extinct vertebrate fauna, p. 28, 1873.) "The species Palaeosyops paludosus * * * -nras founded on a number of isolated teeth * * * qI^. tained by Professor Hayden at Church Buttes, Wyo." (Leidy.) (See fig. 88.) Characters. — Leidy (1870.1, p. 113) writes: The crown of a lower true molar [mj of the right side, the Osborn lectotype (fig. 88)] resembles in its constitution those of Palaeotherium, Chalicotherium, and Titanotherium, being com- posed of a pair of fore and aft conjoined pyramidal lobes with crescentic summits. It measures 16 lines anteroposteriorly and 10 lines transversely. Fragments of upper true molars [m^ left, m' right] exhibit the outer part of the crown composed of a pair of lobes exactly as in Hyopotamus. The inner portion of the crown is composed of a pair of simple cones, broad and low, the front one considerably larger than the back one. One of the specimens in the entire condition of the crown measured about 22 lines fore and aft and 18 lines transversely. The crown of an upper premolar [p*] has its outer part composed of a pair of conjoined cones with acute summits and sides. The inner portion of the crown [p* of the opposite side] consists of a single broad, simple cone embraced in front and behind by a basal ridge. The anteroposterior diameter of the crown externally measures 9}/2 lines; the transverse diameter is an inch. Leidy's cotypes. — The first lot of specimens from Church Buttes (Bridger B 1), upon which the species was originally established, consist of a second lower molar (ma, Nat. Mus. 759; see Leidy, 1870.1, p. 113; 1873.1, pi. 23, fig. 5); "of an upper fourth premolar nearly unworn" (p*, Nat. IVIus. 762; see Leidy, 1873.1, pi. 5, fig. 5); of the anterior half of a second upper molar (m^ Nat. IVIus. 758; see Leidy; 1873.1, pi, 23, fig. 6); and of the inner side of a premolar (p*) of the opposite side. This lot constitutes the cotypes, which are here refigured. Of these, the second lower molar agrees with the specimens described in this monograph as P. paludosus. The upper teeth do not certainly belong to the same animal; it appears necessary, therefore, to base the genus and species on the first specimen described in the original description, namely, the second lower molar, which may be taken as the lectotype. Leidy's paratypes. — Specimens of a second lot, from Henrys Fork, belonging to a much older individ- ual, were treated practically as paratypes of this species in the original notice; they were described in Leidy's memoir of 1873 (1873.1, pp. 29, last line, and 30), were figured in Plate 5, Figures 4, 6, 7, 8, 9, and are Figure 88. — Leidy's cotypes of Palaeosyops paludosus Specimens upon which the species was originally established. Hayden's collection of 1870. After Leidy, 1873; Nat. Mus. 758, 759, 762. Natural size. The second lower molar (Nat. Mus. 759) is the lectotype. preserved in the United States National IVTuseum. These are the specimens that Cope, IVEarsh, Osborn, Earle, and others may have taken for the types, but they are from a higher geologic level and may pertain to P. major or P. leidyi. A third lot of specimens, from Grizzly Buttes, included the "facial portion of a skull containing nearly all the molars and the canines of both sides." This specimen was treated virtually as a paratype by Leidy (1873.1, pp. 30-34, pi. 18, fig. 51, and pi. 4, fig. 3) and was described at length by him. It is probably but not certainly conspecific with the lectotype m2. Osiorn's neotype. — The determination of P. palu- dosus therefore rests positively on the second lower molar alone. To supplement this lectotype the present author has selected as a neotype a lower jaw (PI. LVI, B; fig. 268, C) with dentition, Am. IVIus. 1 1680, in which m2 agrees closely with the lectotype and with the measurements given by Leidy for P. paludosus (1873.1, p. 57 and pi. 5, figs. 10, 11). The locality 158 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA (Millersville) is about 10 miles distant from that of the holotype (Church Buttes), but the geologic level is believed to be identical, namely, Bridger B 1 . Etymology. — paludosus, marshy, dwelling in the marshes, probably because the remains were found in one of the supposed "ancient lake basins." Present determination. — Palaeosyops paludosus Leidy is a valid species, but the "P. paludosus" of other authors refers to related species of more recent geologic age (P. major, P. leidyi, P. rolustus). Palaeosyops major Leidy, 1871 Cf. Palaeosyops major, this monograph, page 321 Original reference. — Acad. Nat. Sci. Philadelphia Proc, 1871, p. 229 (Leidy, 1871.1). Subsequent reference. — Leidy, Extract vertebrate faima of the Western Territories, p. 45, pi. 20, fig. 8, 1873 (Leidy, 1873.1). Figure 89. — Leidy's type (.holotypej of Palaeosyops major in the collection of the Academy of Natural Sciences of Phila- delphia Part of the right ramus of a lower jaw. After Leidy, 1873. One-half natural size. "The specimen is somewhat swollen and altered from disease and is one of those upon which the species was first indicated. Discovered by Dr. Carter at Grizzly Buttes." (Leidy.) Type locality and geologic liorizon. — Grizzly Buttes, Bridger Basin, Wyo.; Palaeosyops paludosus-OroTiip- pus zone (Bridger B 2 or B 3). Dr. J. Van A. Carter, collector. Holotype. — "A jaw fragment with the retained fragments of the true molars." This type is now in the collection of the Philadelphia Academy of Natural Sciences. (See fig. 89.) Characters (Leidy). — Size apparently "much larger than P. paludosus." The true molars occupied a space of 4J^ lines. The last molar measured IJ^ inches fore and aft and an inch trans- versely in front. " In his "Extinct vertebrate fauna" of 1873 (1873.1, pp. 45, 46) Leidy gives a fuller description of the very imperfect holotype and figures it on Plate 20, Figure 8. He believed the jaw specimen to be in some degree abnormal in form, due to inflammation or some other affection connected with the second molar tooth. * * * In its proportions the jaw, in a normal condition, would appear to be of more robust character than in Palaeo- syops paludosus. * * * In its present state the base is more convex fore and aft than in the latter, and the alveolar border more ascending posteriorly. The remains of the molar fangs at the entrance of the alveoli appear to indicate teeth of the same form and construction as in Palaeosyops paludosus, for which reason the fragment was referred to the same genus. The true molars appear to have occupied a space of 4^ inches, though this is probably some- what exaggerated, as the interval occupied by the last inter- mediate molar appears proportionately somewhat too large. The crown of the last molar, which was clearly trilobate as in Palaeosyops paludosus, had an antero-posterior diameter of 2 inches. Leidy's paratype, "consisting of the left ramus of the lower jaw, containing six molar teeth," was obtained by Doctor Carter "in Dry Creek Canyon, 40 mUes from Fort Bridger" (Bridger Basin, Bridger C) and together with a second similar specimen from the same locality is described by Leidy (1873.1, p. 46, pi. 23, fig. 1; second specimen, fig. 2). The holotype, it is important to note, is from the low level (probably Bridger B 2) of Grizzly Buttes, but Leidy's paratype, which has the characters of the more progressive Palaeosyops leidyi Osborn, is from the higher level (Bridger C) of Dry Creek. The paratype is thus certainly not conspecific with the holotype. Osborn's neotype. — In order to supplement the characters of Leidy's imperfect holotype, the present writer has selected as a neotype a lower jaw (fig. 268, C) with dentition (Am. Mus. 12181) from Cotton- wood Creek and from about the same level (B 3) as the holotype, with which it agrees closely. (See Chap. V.) Etymology. — major, in allusion to the larger size as compared with P. paludosus. Present determination. — The species P. major is believed to be a valid one. Canis montanus Marsh, 1871 Cf. Canisf marshii Hay, below {Palaeosyops major?), page 178 Original reference. — Am. Jour. Sci., 3d ser., vol. 2, p. 123, August, 1871 (Marsh, 1871.2). Type locality and geologic liorizon. — Grizzly Buttes, Bridger Basin, Wyo.; Palaeosyops paludosus-Oro- hippus zone (Bridger B, probably B 2). Marsh's cotypes. — "A last upper premolar tooth in good preservation, a canine, wanting most of the crown, and a number of the larger bones of a skeleton, all apparently of the same species, but pertaining to three individuals, differing somewhat in size " (Marsh). Of these materials the "last upper premolar" (first lower premolar) alone is described and measiu'ed, and it is also the first specimen mentioned. It should therefore be taken as the lectotype (Yale Mus. 11770). Characters. — "The last upper premolar * * * is robust, has a short compressed crown. The princi- pal cusp is conical, with subacute edges, the anterior being about twice the length of the posterior. Behind DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 159 Figure 90. — Leidy's type of Palaeosyops humilis the main cusp there is a large triangular tubercle, with its apex exterior to the fore and aft axis of the crown." (Marsh.) Anteroposterior diameter of last upper pre- molar 9 lines [19 mm.] Greatest transverse diameter of same 4.25 lines [8 mm.l Height of main cusp 6 lines [12.7 mm.] Height of posterior tubercle 3.75 lines [7 mm.] Synonym. — Canis? marsMi Hay was proposed in place of 0. montanus IVIarsh, name preoccupied by 0. montanus Pearson (Hay, 1899.1). Etymology. — montanus, dwell- ing in the mountains — that is, from the Eocky Mountain region. Present determination. — The type specimen of "Canis mon- specimen in the collection of tdnus" is a first lowcr premolar the Academy of Natural of somc Undetermined member S c i e n c e s of Philadelphia, <■ ,i n v After Leidy, 1S73. Natural 01 t^ie gCnUS FttlaeOSyOpS, pOSSl- size. Regarded by Leidy as jjly PaltteOSyOpS paludoSUS Or P. "A last upper molar of the . rm p , /-y • leftside. • • • Found by mc-jor. ihe reference to Cams Doctor Corson on the buttes ^^Q,s, doubtless made by rcason of Dry Creek Canyon." r ,i i . • ii n of the deceptive resemblance oi one of the lower premolars to the upper carnassial tooth of a dog. Palaeosyops humilis Leidy, 1872 Cf. Palaeosyops major, this monograph, page 321 Original reference. — Acad. Nat. Sci. Philadelphia Proc, 1872, p. 168 (Leidy, 1872). Subsequent reference. — Leidy, Extinct vertebrate fauna of the Western Territories, p. 58, pi. 24, fig. 8, 1873 (Leidy, 1873.1). Type locality and geologic Tiorizon. — "Valley of Dry Creek 40 miles from Fort Bridger (Wyo.)." Doctor Corson, discoverer. "Buttes of Dry Creek Canyon," Bridger Basin; horizon probably Bridger C (Uinta- therium- Manteoceras- MesatirMnus zone) . Holotype. — "An upper molar." (See fig. 90.) Characters (Leidy). — "An upper molar tooth of this animal measures three-foiu-ths of an inch in diame- ter." In his later description Leidy recognized that the specimen belonged to the milk series. Etymology. — Tiumilis, lowly, small; in allusion to the small size in comparison with P. paludosus. Present determination. — This milk tooth probably pertains to the genus Palaeosyops, but comparison with P. major and P. leidyi leaves the species unde- termined. Palaeosyops Junius Leidy, 1872 Cf . Mesatirhinus Junius (Leidy) , this monograph, page 388 Original reference. — Acad. Nat. Sci. Philadelphia Proc, 1872, p. 277 (Leidy, 1872.3). Subsequent reference. — Leidy, Extinct vertebrate fauna of the Western Territories, p. 57, no figure, 1873 (Leidy, 1873.1). Type locality and geologic horizon. — Fort Bridger, Bridger Basin, Wyo.; level not recorded. Dr. J. Van A. Carter, collector. Holotype. — -"Doctor Carter recently sent the writer several small fragments of the right side of a lower jaw, together with a sketch of a larger fragment of the left side, containing the last premolar and the succeeding molars." Of this type material only p4 (right) and the posterior half of ma (right) were located (1906) in the collection of the Academy of Natural Sciences of Philadelphia. Characters. — Leidy writes: Intermediate in size to P. paludosus and P. humilis. Founded on portions of a lower jaw agreeing in character with the cor- responding parts of P. paludosus but smaller. Space occupied by the last premolar and the true molars, 4 inches. Antero- posterior diameter of last premolar, 8 lines; of last molar, 173^ hnes. In the fuller description in his memoir of 1873, Leidy says: The specimens * * * appear to indicate a small species of Palaeosyops, though it is not improbable that they pertain to a small variety of P. paludosus. The parts agree closely with the corresponding parts of the lower jaw and teeth of the latter, except in size. They have been viewed as representatives of a species with the name of Junius. Figure 91. — Leidy's cotypes of Palaeosyops Junius Specimens in museum of Academy of Natural Sciences of Philadelphia; Bridger B(?), level doubtful. A, Eight fourth lower premolar (pO; B, posterior part of third lower molar Cms) . Natural size. The measurements of the teeth (fig. 91) in comparison with those of P. paludosus are as follows: Space occupied by the last pre- molar and molars Space occupied by the molars Breadth [anteroposterior] of last premolar Thickness [transverse] of last premolar Breadth [anteroposterior] of first molar Breadth [anteroposterior] of sec- ond molar Breadth [anteroposterior] of third molar Thickness [transverse] of third molar at middle 39J^ 8 10 12 17 7 [Milli- meters] [102] [94] [17] [12] [21] [25] [10] [14] 55 46 6M 12J^ 15 19 93^ [MUli- meters] [116] [96] [19] [12] [38] [32] [39] [19] 160 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Etymology. — Junius, younger, in allusion to its small size. Present determination. — From the two teeth (p4 and part of ma) preserved it appears that this species probably pertains to the genus Mesatirhinus. It is smaller than Mesatirhinus megarhinus. No other material has been certainly identified with it. (See p. 388.) Palaeosyops laticeps Marsh, 1872 Cf. Limnohyops laticeps Marsh, this monograph, page 311 Original reference. — Am. Jour. Sci., 3d ser., vol. 4, p. 122, August, 1872, dated "July 18, 1872" (Marsh, 1872.1). Type locality and geologic horizon.— NesiT Marsh's Fork, about 15 miles from Fort Bridger, Wyo. A. H. Ewing, discoverer. Level not recorded. Holotype. — "A__nearly complete skeleton" (Yale Mus. lioOO). Etymology. — latus, broad; caput (in compounds ceps), head; in allusion to the width across the zygomata. Present determination. — Marsh's accurate diagnosis of this excellent type was made before the generic characters of Palaeosyops were fully known. The species was subsequently chosen by Marsh as the type of the genus Limnohyops Marsh, and both the genus and the species are valid. Telmatherlum Marsh, 1872 Cf. Telmatherium, this monograph, page 340 Original reference. — Am. Jour. Sci., 3d ser., vol. 4, p. 123, August, 1872 (Marsh, 1872.1). Type species. — Telmatherium validum Marsh. (See pp. 160, 344 of this monograph.) Generic characters. — See T. validum (p. 340). Etymology. — reXfia, a pool, marsh (cf. "paludosus"); dripiov, beast. Figure 92. — Marsh's type of Palaeosyops laticeps Natural size. Characters. — Marsh writes : The teeth in this specimen have apparently the same general structure as those in the type of P. paludosus but differ in being nearly smooth, and this is not the result of age, as this individual was younger than the original of the larger species. The proportions, moreover, given for the molar described (Leidy, 1870.2, p. 113), "22 lines fore and aft and 18 trans- versely," would not apply to any of the series in the present specimen. The last upper molar of the latter has two well- developed internal cones. * * * The upper teeth form a complete series. The canine is large and broadly oval at its base. The outer incisor is the largest, and at its posterior edge the premaxillary is subtriangular in transverse section. The sagittal and occipital crests are strongly developed, and the coronoid process of the lower jaw is short and recurved. Measurements [Marsh] ^ Millimeters Length of entire upper molar series 155 Anteroposterior extent of three true upper molars 94 [90] Anteroposterior diameter of last upper molar 36 [33] Transverse diameter [protocone to mesostyle] 40 Anteroposterior diameter of upper canine at base [alveolar portion 28] 29 Transverse diameter 22 Present determination. — The generic term as re- defined in the present monograph is a valid one. Telmatherium validus Marsh, 1872 Cf. Telmatherium validum, this monograph, page 344 Original reference. — Am. Jour. Sci., 3d ser., vol. 4, p. 123, August, 1872; dated "July 18, 1872" (Marsh, 1872.1). Type locality and geologic horizon. — "Near Henrys Fork of the Green River in Wyoming." (Bridger Basin, level C or D.) J. F. Quigley, discoverer. Holotype. — "The greater portion of a skull, with teeth" (Yale Mus. 11120). (See fig. 93.) Characters. — Marsh writes: The dentition of this genus, so far as known, appears to be similar to that of Palaeosyops; but the two may readily be dis- tinguished by the anterior portion of the skull, which in the present genus has the premaxillaries compressed, with an elongated median suture. The zygomatic arch is also much less strongly developed, and the squamosal portion of it is com- paratively slender. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS Limnohyus Marsh, 1872 The upper molar teeth have the inner cones more elevated and more pointed than in Palaeosyops, and the basal ridge is well developed. The last upper molar has but a single internal cone. The upper canines are large, pointed, and have strong cutting edges. The outer incisors are the largest, and all these teeth have a strong inner basal ridge. The roof of the mouth is deeply excavated between the premolars. The nasals are de- curved laterally and much compressed. Cf. Palaeosyops, this monograph, page 331 Original reference. — Am. Jour. Sci., 3d ser. p. 124, August, 1872 (Marsh, 1872.1). Type species. — Limnohyus rohustus Marsh. 161 vol. 4, Figure 93. — Marsh's type of Telmatherium validus Natural size. Measurements [Marsh] Millimeters Extent of upper molar series : 224 Extent of upper true molars 130 Anteroposterior diameter of last upper molar 54 Anteroposterior diameter of last upper premolar 28 Transverse diameter 33 Anteroposterior diameter of upper canine at base 27 Transverse diameter 22 Etymology. — validus, strong, stout; perhaps in allu- sion to the large size of the upper canines. Generic characters (Marsh). — The term Palaeosyops is restricted to those specimens which, like P. paludo- sus, possess two inner cones on m^ The other specimens have but a single internal cone on the last upper molar, and for the genus thus represented the name Limnohyus is proposed. These genera may be distinguished from Telmatherium by the premaxillaries, which are short, stout, and depressed, with a small median suture. Etymology. — Xi^ufi?, a marshy lake; Cs, boar. Present determination. — Since the type species Lim- nohyus rohustus is now believed to be congeneric with Figure 94. — Marsh's type of Limnohyus rohustus Natural size. Present determination. — This is a valid genus and species. The name Telmatherium was amended to Telmatotherium by Marsh in 1880 (1880.1) in his "List of genera established by Prof. O. C. Marsh, 1862-1879," and the amended form was accepted by Earle, Osborn, Hatcher, and later writers, but accord- ing to the rules of nomenclature now generally ac- cepted the amended form has no standing and the original form Telmatherium should be used. Leidy's Palaeosyops paludosus, Marsh's genus Limno- hyus becomes a synonym of Palaeosyops. Limnohyus rohustus Marsh, 1872 Cf. Palaeosyops rohustus (Marsh), this monograph, page 331 Original reference. — Preliminary description of new Tertiary mammals: Am. Jour. Sci., 3d ser., vol. 4, p. 124, August, 1872; dated "July 18, 1872" (Marsh, 1872.1). 162 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Type locality and geologic horizon. — Near Henrys Fork, Wyo.; Uintatherium- Manteoceras- Mesatirhinus zone (Bridger C or D). F. Mead, jr., collector. Holotype. — A fragmentary skull including nasals and palate with teeth (Yale Mus. 11122). Characters. — Marsh writes: The present species may be distinguished from those above described [Palaeosyops laticeps Marsh, Tdmatherium validum Marsh], especially by the strong basal ridge of the molars. On the last lower molar it extends entirely around the posterior lobe. The first of the upper true molars has the two inner cones nearly of the same size. The small intermediate median FiGTJRE 95. — Cope's cotypes of Palaeosyop. After Cope, 1884. Ai, Lett mandibular ramus, superior view Oeetotype, Am. Mus. 5098); Aj, internal view otsame; B, left upper premolars and molars (Am. Mus. 5097); Ci, third left superior molar, crown view; Ct, the same, buccal view (Am. Mus. 5099); Di, fourth superior premolar, crown view; Dj, the same, buccal view (Am. Mus.). tubercles are well developed on the upper molars, and all the teeth are strongly rugose, even in fully adult animals. The nasal bones contract anteriorly and are rounded in front. The outer margin is decurved and thickened. The premaxil- laries unite by a very short median suture, similar to that in Palaeosyops laticeps. The zygomatic process of the squamosal is stout but much compressed, thus differing widely from both the species already described. Measurements [Marsh] Millimeters Anteroposterior extent of last three upper molars 110 Anteroposterior diameter of last upper molar 41 Transverse diameter 43. 5 Anteroposterior diameter of last lower molar 51 Etymology. — robustus, robust; in allusion to the stout skull and dentition. Present determination. — The species is probably a valid one, referable to the genus Palaeosyops. Limnohyus Leidy (not Marsh), 1872 Cf. Palaeosyops, this monograph, page 155 Original reference. — Acad. Nat. Sci. Philadelphia Proc, 1872, pp. 240-242; published December 17, 1872 (Leidy, 1872.1). ^ As we have seen above, Marsh's genus Limnohyus is simply a synonym of Palaeosyops, which had been defined by Leidy as having "but a single lobe to the inner part of the crown " of the "last upper molar." In 1872 Leidy, after pointing out this fact, says that the name Limnohyus "might with propriety be applied to the animal with molars like those of Palaeo- syops except that the last upper one has two inner cones to the crown." This doubtless sug- gested Marsh's subsequent term Limnohyops. Lim- nohyus Leidy is thus preoccu- pied by Limnohyus Marsh, which is a sj'^nonym of Palaeosyops. Etymology. — Xifivrj, a marshy lake; Cs, boar. Palaeosyops vallidens Cope, 1872 Cf. DoKchorhinus vallidens (Cope), this monograph, page 401 Original rejerence. — -Pal. Bull. No. 7, dated "Aug. 22, 1872"; Am. Philos. Soc. Proc, vol. 12, p. 487, 1873 (Cope, 1872.1). Subsequent reference. — Ter- tiary Vertebrata, p. 699, pis. 51, fig. 1; 52, fig. 3; 53, fig. 1; 36, figs. 10, 10a, 11, 11a, 1884 [1885] (Cope, 1885.1). Type locality and geologic horizon. — -"Mammoth Buttes, southwestern Wyoming, near the headwaters of Bitter Creek," Washakie Basin; Eobasileus-Dolichorhinus zone (Washakie B 2). Characters. — Cope writes: Represented by the dentition of one maxillary bone with other bones of one individual [Cope, Am. Mus. 5097]; a portion of the same dentition of a second [No. 5099]; with both rami of the mandible with complete dentition of a third [No. 5098]. The species is distinguished by the details of the dental struc- ture and by the superior size. It exceeds, in this respect, the Palaeosyops major Leidy; while the three posterior lower molars measure 4.5 inches in length, the same teeth of the present animal measure 6.25 inches. The last superior molar of an- other specimen measures 2 inches in length; in the third the DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 163 first true molar is 1.5 inches in length, while the last inferior molar is 2.25 inches long. The pecuharity in the structure of the superior molars consists in the existence of two strong transverse ridges, which connect the inner tubercle with the outer crescents, inclosing a pit between them. These are most marked on the premolars, where also is found the peculiarity of the almost entire fusion of the outer crescents into a single ridge. These united crescents are narrower than in P. major, and the summits of all the crescents are relatively more ele- vated. The number of inner tubercles is the same as in that species; all the teeth have very strong basal cingula, which rise up on the inner tubercle. The last inferior molar is rela- tively narrower than in P. major, and the posterior tubercle is larger and longer and is an elevated cone. From the foregoing description it will be seen that Cope based his specific description upon three speci- mens (cotypes) without designating any one of the three as being more typical than the others. (See fig. 95.) The name vallidens, however, from vallum, a wall, seems to refer to the peculiarity in the struc- ture of the superior molars, which consists in the existence of two strong transverse ridges, which connect the inner tubercle with the outer crescents. * * * These ridges are most marked on the premolars, where also is found the pecuharity of the almost entire fusion of the outer crescents into a single ridge. If we had nothing further to guide us we would thus be led to infer that the upper dentition (Am. Mus. 5097), which best shows these peculiarities alluded to in the name vallidens, should be regarded as the most typical of the three specimens and should be chosen as the lectotype. But in his "Tertiary Vertebrata" Cope (1885.1, p. 700) says: The bones containing the maxillary and mandibular teeth were not found together in any instance, so that it is possible that the different series may represent different species. No other species of the genus was, however, found in the locaUties to which the respective parts could be referred. Should these prove not to pertain together, the lower jaws may be regarded as typical of the species. As Cope was the "first reviser" of the species there seems to be no escape from the conclusion, if modern rules of nomenclature are to be followed, that the lower jaw (Am. Mus. 5098) must be treated as Cope's lectotype. Etymology. — vallum, wall or redoubt; dens, tooth; allusion as explained above. Present determination. — This little-known species is allied to but probably specifically distinct from Doli- cJiorTiinus Tiyognathus of Washakie B and Uinta B. It is also more primitive than that species (see below). Limnohyus laevidens Cope, 1873 Cf. Limnohyops laevidens (Cope), this monograph, page 305 Original reference. — Pal. Bull. No. 11 ("issued Jan. 31, 1873"); Am. Philos. Soc. Proc, vol. 13, pp. 35, 36, 1873 (Cope, 1873.5). Subsequent references. — Cope, On the extinct Verte- brata of the Eocene of Wyoming, observed by the expedition of 1872: U. S. Geol. and Geog. Survey Terr. (Hayden) Sixth Ann. Kept., p. 591, 1873 (Cope, 1873.6); Tertiary Vertebrata, p. 701, cotype skull, pi. 50, figs. 1, 2 (holo type) , fig. 3 (paratype), 1884 [1885] (Cope, 1885.1). Type locality and geologic horizon. — Type ("No. 1 "), Cottonwood Creek, Bridger Basin, Wyo.; Palaeosyops paludosus-Orohippus zone (Bridger B). Cotype ("No. 2"), Bitter Creek, Washakie Basin, Wyo.; horizon uncertain. Cope's cotypes: "A cranium lacking the posterior part of one side and the lower jaw," from Cottonwood Creek ("No. 1," now Cope collection. Am. Mus. 5104). Also "a nearly complete cranium with dentition from Figure 96. — Cope's cotypes of Limnohyops laevidens After Cope, 1885. One-fourth natural size. A, Am. Mus. 5104, lectotype: Ai, " Cranium lacking posterior part of one side and lower jaw, from Cottonwood Creek" (Cope), "No. 1"; As, upper teeth of the same. B, Am. Mus. 6105, now retered to Palaeosyopst copei, right maxilla, p^m'. Bitter Creek" ("No. 2," now Cope collection. Am. Mus. 5105). (See fig. 96.) Cope's lectotype: Cope's first-mentioned specimen is the one from Bitter Creek (Washakie B?) (Am. Mus. 5105), now referred to Palaeosyops? copei. But the "No. 1" of Cope's description and measurements and the specimen to which the name "laevidens" refers is unquestionably the skull Am. Mus. No. 5104, from Cottonwood Creek (level Bridger B), Bridger Basin, now referable to Limnohyops. Furthermore, in the "Tertiary Vertebrata" (Cope, 1885.1, pp. 701-703, pi. 50, figs. 1, 2) Cope definitely selects, describes, and figures this specimen as the type, again referring to the Washakie specimen as "No. 2" and admitting that its specific association with the other specimen was doubtful. We therefore follow Cope in regarding 164 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA "No. 1," the Bridger specimen (Am. Mus. 5104), as the lectotype. Characters.- — Cope writes: This species is one of the larger forms of the group originally represented by Palaeosyops, and which has turned out to be so numerous in species. [This statement refers apparently to "No. 2."] The anterior median small tubercle of the first true molar is wanting. The last true molar has but one interior cone. [All these statements apply evidently to " No. 1," the Bridger or Cot- tonwood Creek specimen.] The canine tooth is powerful and bearhke; the outer incisor is the largest. The premaxillary bones are short, and the side of the face elevated and plane to the convex nasal bones. Zygo- matic arch massive. Am. Mus. 5107. Figure 97.— After Cope, 1885. Ai, natural A3 -Cope's type (holotype) of Limnohyus fontinalis Young skull seen from above, one-half natural size: As, the same, right side, one-half size; A3, right maxilla with dp<, m', m', natural size. The molars have the general form of those of L. robustus, but the second superior premolar has but one outer tubercle. The cingula are much less developed than in that species, those between the inner cones of the molars being entirely absent. These cones are low and, with the rest of the crowns of all the teeth, covered with smooth and shining enamel. Measurements [Cope, condensed and corrected] IVtillimeters Length of molar series (No. 1) 141 Length of true molars 84 Length of crown canine (anteroposterior) 20 Length of crown last molar (anteroposterior) 30 Width of crown last molar (transverse) 34 DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 165 Etymology. — laevis, levis, smooth, shining; dens, tooth; apparently in allusion to the "smooth and shining enamel." Present determination. — This is a valid species refer- able to the genus Limnohyops. Limnohyus fontinalis Cope, 1873 Cf. fPalaeosyops fontinalis Cope, this monograph, page 317. Original reference. — Pal. Bull. No. 11, "issued Jan- uary 31, 1873"; Am. Philos. Soc. Proc, vol. 13, pp. 35, 36, 1873 (Cope, 1873.5). Subsequent references. — Cope, On the extinct Verte- brata of the Eocene of Wyoming, observed by the expedition of 1872: U. S. Geol. and Geog. Survey Terr. (Hayden) Sixth Ann. Kept., p. 594, 1873 (Cope, 1873.6); Tertiary Vertebrata, p. 707, pi. 49, fig. 9; pi. 50, fig. 4; pi. 58a, figs. 4, 5, 1884 [1885], (Cope, 1885.1). Type locality and geologic Tiorizon. — "Found by the writer on a bluff on Green River, near the mouth of the Big Sandy, Wyoming." ("Isolated patch lying northeast of the badlands." Probably Eometarliinus- Trogosus-Palaeosyops fontinalis zone (Bridger A).) Holotype. — A young, fragmentary skull (Cope col- lection. Am. Mus. 5107, retaining dp*, m', - - m^ of the right side. (See fig. 97.) Characters. — Cope writes: A small species agreeing with the P. paludosus in the two interior cones of the last superior molar. It is represented especially by a consider- able part of the cranium of an individual in which the last superior molar is not quite protruded, but with the other molars and last premolar of the permanent dentition in place. The enamel of these teeth is in accordance with the age, delicately rugose, and while the cingulum is present fore and aft, it is wanting internally and externally. The anterior median tubercle is present on all the true molars, and the bases of the acute inner cones are in contact. The sagittal crest is truncate, and the squamosal portion of the zygoma very stout. The nasal bones are together very convex in transverse section. Palaeosyops diaconus Cope, 1873 Cf. Palaeosyops robustus (Marsh), this monograph, page 331. Original reference. — Pal. Bull. No. 12, p. 4, "pub- lished March 8, 1873" (Cope, 1873.1). Subsequent references. — Cope, On the extinct Verte- brata of the Eocene of Wyoming observed by the expedition of 1872: U. S. Geol. and Geog. Survey Terr. (Hayden) Sixth Ann. Kept., p. 593, 1873 (Cope, 1873.6); Tertiary Vertebrata, p. 706, pi. 51, fig. 3, 1884 [1885] (Cope, 1885.1). Type locality and geologic horizon. — Henrys Fork of Green River, Wyo.; Uintatherium- Manteoceras- Mesa- tirhinus zone (Bridger C or D). Holotype. — "Represented by parts of the two maxUlary bones, which present the crowns of the third and fourth premolars, and of the second and third true molars, with the bases of the other molars and premolars." (Cope collection. Am. Mus. 5106.) (See fig. 98.) Characters. — Cope writes: Belonging to the genus Palaeosyops as understood by Marsh — that is, with two cones on the inner side of the last superior molar. The species is as large as the Limnohyus major of Leidy but differs in the relative proportions of the teeth. Measurements [Cope] Millimeters Length of true molar series (2.75 inches) 67 Length of last molar 25 Width of last molar 26 Etymology. — fontinalis, of or from a spring, hence original; in allusion to the primitive characters. Present determination. — Cope was in error in inter- preting the teeth of this skull, which belong to a very juvenUe animal, the teeth exposed being the last upper mUk tooth, dp*, the first and second molars, m*, m^. The cranial characters, too, are very juve- nUe. So far as they serve to guide us, the animal probably belongs to the genus Palaeosyops, and also probably to a distinct species, from a low geologic level, possibly Bridger A. Figure 98. — Cope's type (holotype) of Palaeosyops diaconus Left upper teeth. Am. Mus. 5106. After Cope, 1885. One-half natural size. Thus the last three molars have the same anteroposterior length, while the space occupied by four premolars is shorter. The anterior and posterior cingula of the true molars are very strong, but it is not weU marked on the inner side between the cones. The latter are acutely conic, and the median anterior tubercle is strongly developed. Although the wearing of the teeth indicates maturity, the enamel is coarsely and obtusely rugose. The fourth premolar differs from that of L. major in its smaller size relatively and absolutely and in the presence of a prominent vertical tubercle on the outer face, rising to the angle of the deep notch between the lobes. The third premolar is as wide as the fourth and about as large as the corresponding tooth in L. major, but different from it in the absence of tubercle and ridge that mark its external face. The first premolar has two roots, and the canine is large and short. Measurements [Cope] Millimeters Length of entire molar series 171 Length of true molars 106 Length of last molar (crown) 42 Width of last molar (crown) 43. 7 In comparison with Marsh's description of his P. laticeps, the measurements are all larger, and the enamel is as rugose as in L. major, instead of smooth. The shortening of the pre- molar series is greater in P. diaconus; thus in P. laticeps the two sets of molars are related as 94 to 61 millimeters; in the present one, as 106:65; were the proportions similar, the length of the premolar series should be 69 millimeters. 166 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA Etymology. — 5t?, double; kcows, cone; because the third upper molar had two mner cones. Present determination. — The name P. diaconus Cope is probably a synonym of Palaeosyops rohustus (Marsh), as explaiaed in Chapter V of this mono- graph. Diplacodon Marsh, 1875 Cf. Diplacodon Marsh, this monograph, pages 155, 439 Original reference. — Am. Jour. Sci., 3d ser., vol. 9, p. 246, March, 1875, "dated February 20, 1875" (Marsh, 1875.1). Type species. — Diplacodon elatus Marsh. (See p. 439.) Generic characters. — Marsh writes: The genus here established presents characters in some respects intermediate between Limnohyus and Brontotherium. It agrees with the former in its complete dentition (44 teeth) and in the general form of the incisors, canines, and true molars. It resembles the latter still more closely in the premolar and molar teeth, and parts of the skeleton, especially in the verte- brae, and bones of the extremities. From the Eocene Limno- hyidae, already described, this genus is sharply distinguished by the last upper premolar, which has two distinct inner cones, thus agreeing essentially with the first true molar. This char- acter, which has suggested the name of the genus, is one step ^9 Characters. — The specific characters were not for- mally separated from the generic characters above given under Diplacodon. Measurements [Marsh] Millimeters Extent of upper molar series 242 Extent of upper true molars 152 Anteroposterior diameter of last upper molar 60 Transverse diameter 59 Anteroposterior diameter of [upper] canine, at base 32 Height of crown 27 Etymology. — elatus, lofty; apparently in allusion either to the large size or to the advanced stage of evolution. Present determination. — This important genus and species was based upon an excellent type. The genus and species are vahd. (See p. 439.) Brachydiastematherium Bockh and Maty, 1876 Cf. Brachydiastematherium Bockh and Maty, this monograph, page 382 Original reference — Mittheilungen aus Jahrb. K. k. geol. Anstalt, Band 4, pp. 125-150, 1876 (1876.1). Type species. — Brachydiastematherium transilvani- cum Bockh and Maty. (See p. 382.) Figure -Marsh's type of Diplacodon elatus One-third natuial size. toward the modern type of perissodactyl dentition. The dental formula of the genus is the same as Limnohyus, viz, incisors f , canines \, premolars |, molars f. In other respects the teeth most resemble those of the Brontotheridae. From this family Diplacodon differs widely in its dentition and the absence of horns. Etymology. — SlwXoos, double; aK-q, a point; 65ovs, tooth; because the upper premolars had two inner cones. Present determination. — This genus is certainly valid so far as it applies to the type species. (See p. 439.) Diplacodon elatus Marsh, 1875 Cf. Diplacodon elatus Marsh, this monograph, page 439 Original reference. — Notice of new Tertiary mam- mals: Am. Jour. Sci., 3d ser., vol. 9, p. 246, March, 1875; dated "February 20, 1875" (Marsh, 1875.1). Type locality and geologic horizon. — "Upper Eocene beds of Utah"; horizon probably Diplacodon-Proti- tanotherium-Epihippus zone (Uinta C, true Uinta for- mation). Holotype. — A palate with dentition nearly complete, parts of the skull and skeleton (Yale Mus. No. 11180). Generic characters. — The generic characters mingled with the specific characters are given below. (See also p. 382.) Etymology. — jSpaxvs, short; didaTrnxa, an interval; dr]piov, beast; in allusion to the short diastema between the lower canines and first premolars. Present determination. — The type of this genus is an animal closely similar in size and in stage of evolution to the Protitanotherium superhum of the upper Eocene of Utah but differs in certain characters, which are probably of generic value. (See p. 382.) Brachydiastematherium transilvanicum Bockh and Maty, 1876 Cf. Brachydiastematherium transilvanicum Bockh and Maty, this monograph, pages 382, 941 Original reference. — -Mittheilungen aus Jahrb. K. k. geol. Anstalt, Band 4, pp. 125-150, pis. 17, 18, 1876 (1876.1). Cf. Toula, Akad. Wiss. Wien Sitzungsber. , Band 101, pp. 612 et seq., 1892 (1892.1). Type locality. — Andrashaza (Siebenbtirgen), Hun- gary (Transylvania, eastern Hungary, about 150 miles northeast of Belgrade). Collected in 1871 by Dr. Alex. Pavay. DISCOVERY OP THE TITANOTHEEES AND ORIGINAL DESCRIPTIONS 167 Geologic level. — The specimen, according to Pro- fessor Koch (Bockh, 1876.1, p. 149), was found in "buntes Thongebilde" of "lower" Eocene age, but the assignment of a form of this advanced stage to a level so low appears unwarrantable, and later evidence indicates that the age of this specimen is more prob- ably upper Eocene. (See p. 382.) Holotype. — Anterior part of lower jaw, containing incisors, canines, four premolars, and one molar. Originally described and defined by Bockh (1876.1) as a palaeotherioid. (See fig. 100.) Generic and specific characters. — Bockh and Maty (p. 148) write: einer dreieckigen Emailzunge. An der Krone sammtlicher Zahne sind die Reste einer diinnen cementartigen Kruste zu sehen. The following measurements are taken from the original figures: Millimeters I3, anteroposterior 22 I3, transverse 20 C, maximum anteroposterior diameter (horizontal measure- ment near base) 38 C, maximum transverse 31 C, lieight of crown (estimated) 40 Postcanine diastema (at top) 12 Pi-p4, anteroposterior 107 Pi, anteroposterior 18 Figure 100. — Type (holotype) lower jaw of Brachydiastematherium transilvanicum After Bockh and Maty, 1876. A', Side view; K', inner side; A', rear view of mi; A<, front view of mi; A^, top view of jaw; A', outer view of right lower canine; A', section of root of right lower canine; A^, fragment of right lower incisor. Two-fifths natural size. I3, mit flachliegelformiger Krone, welche mit warziger Emailwulst versehen ist; Ci, mit kegelformiger Krone, welche gleichfalls eine warzige, starke Emailwulst besitzt; seine Wurzel ist iiberaus stark, lang und gerade. Die Zahnliicke ist sehr kurz; pi, deren erster am kleinsten, und seine nur eine Wurzel besitzende Krone stellt nur einen einfachen Kegel dar; die iibrigen drei wachsen gradatim und die warzige Wulst der Basis fehlt an der inneren Seite dieser letzteren. Die drei letzten Praemolare ahmen wohl die Form der entsprechenden Zahne der echten Palaeotherien nach, wirkliche Halbmonde an der Oberflache seiner abgewetzten Krone zeigt indessen nur der vierte Praemolar; an den demselben vorangehenden zwei Zahnen kann die Verzierung noch niclit als Halbmond bezeich- net warden. Die hintere Bucht des vierten Praemolares 1st durch eine Scheide in zwei Theile abgetheilt, und heizu ist der Keim auch schon beim dritten Praemolar zu bemerken; m (?)3, die innere Seite des ersten echten Molares zeigt gleich- falls keine Emailwulst, an der Mitte der hinteren Seite des hinteren Halbmondes vereinigt sich indessen die Wulst mit P2, anteroposterior 26 P2, transverse (through anterior lobe) (estimated) 17 P3, anteroposterior 31 P3, transverse (estimated) 22 P4, anteroposterior 38 P4, transverse (estimated) 28 Ml, anteroposterior 50 Ml, transverse (estimated) 30 Etymology. — transilvanicum, Transylvanian. Present determination. — The species is probably valid. Leurocephaius Osborn, Scott, and Speir, 1878 Cf. Telmatherium Marsh, this monograph, page 341 Original reference. — ^E. M. Mus. Geol. and Arch. Princeton Coll. Contr. No. 1, p. 42, pi. 4, 1878 (Osborn, Scott, and Speir, 1878.3). 168 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Type species. — LeurocepTialus cultridens Osborn, Scott, and Speir. (See p. 341.) Generic characters. — Scott and Osborn write: Upper incisors acute, with strong posterior ridges, lower in- cisors compressed and laniariform, canines compressed, with serrated cutting edges; first upper premolar with rudimentary anterior lobe, last upper molar with rudimentary postero- internal cusp. Molars constructed as in Palaeosyops but higher, with sharper cones and more erect external lobes. Internal median valley very much deeper. Little or no depression at the forehead; zygomatic arch round, comparatively straight and does not project outward, and with obscure postorbital process. Premaxillaries short and straight. Mandible with nearly straight lower margin and shallow masseteric fossa; mental foramen single. Etymology. — Xeupos, smooth; Ke(j>a\r], head; in allusion to the smooth texture of the bone. Present determination. — LeurocepTialus is a synonym of Telmatherium Marsh. Figure 101. — Type (holotype) of Leurocephalus cultridens Right prematiUa, maxilla, and mandibular ramus. Princeton Mus. IQ027. Atter Osborn, 187S. One-third natural size. Leurocephalus cultridens Osborn, Scott, and Speir, 1878 Cf. Telmatherium cultridens (Osborn, Scott, and Speir), this monograph, page 341 Original reference. — E. M. Mus. Geol. and Arch. Princeton Coll. Contr. No. 1, p. 42, pi. 4, 1878 (Osborn, Scott, and Speir, 1878.3). Subsequent reference. — Earle, A memoir on the genus Palaeosyops Leidy and its allies: Acad. Nat. Sci. Phil- adelphia Jour., 2d ser., vol. 9, pp. 343-348, pi. 10, tig. 3, 1892; type (Earle, 1892.1). Type locality and geologic horizon. — Henrys Fork divide, near Fort Bridger, Wyo.; Vintatherium- Manteoceras- Mesatirhinus zone (Bridger C or D). Holotype. — "Established on specimen having a nearly complete dentition and portions of the cran- ium" (Princeton Mus. 10027). (See fig. 101.) Specific characters. — The specific and generic char- acters were not distinguished in the original descrip- tion. Etymology. — culter, a knife; dens, tooth; in allusion to the sharp-edged reciurved canines. Present determination. — This genus is a synonym of Telmatherium Marsh. The species is valid. (See p. 341.) Palaeosyops borealls Cope, 1880 ' Cf. Eotitanops horealis (Cope), this monograph, pages 156, 292 Original reference. — Am. Naturalist, vol. 14, p. 746, 1880 (Cope, 1880.1). Subsequent reference. — Cope, Tertiary Vertebrata, p. 703, pi. 58a, fig. 3, 1884 [1885], (Cope, 1885.1). Type locality and geologic horizon. — "Badlands in the upper drainage basin of the Big Horn River in western- central Wyoming"; Wind River formation, horizon not determined, probably Lambdotherium-Eotitanops- Coryphodon zone (Wind River B). J. L. Wortman, collector. Holotype. — "Founded on a portion of the right maxillary bone, which supports the three true molars and one premolar" (Cope collection. Am. Mus. 4892). (See fig. 102.) Characters. — Cope writes : Size of Limnohyus fontinalis, or much smaller than P. laevidens. Anterior median tubercle well developed; anterior and pos- terior cingula strong, not rising to inner cones. A low ridge extending outward and forward from posterior cone. Enamel smooth. Differs from P. junior Leidy in the presence of the intermediate tubercle and crest and in the weak ex-ternal cin- gulum. Length of true molar series 63 [mil- hmeters]; diameters of first true molar, anteroposterior, 19; transverse, 20. Etymology. — borealis, relating to Boreas; in allusion to the Wind River formation. Present determination. — The species is valid but generically distinct from Palaeosyops. It is the type of the genus Eotitanops Osborn. (See p. 289 .) Lambdotherium Cope, 1880 Cf . Lambdotherium Cope, this monograph, page 279 Original reference. — Am. Naturalist, vol. 14, p. 746, 1880 (Cope, 1880.1). Subsequent reference. — Cope, Tertiary Vertebrata, p. 710, 1884 [1885] (Cope, 1S85.1). Figure 102. — Type (holotype) of Palaeosyops borealis Right upper part of right maxilla with p<-m3. Am. Mus. 4892. After Cope, 18S5. Natural size. Type species. — Lambdotherium popoagicum Cope. (Seep. 281.) Generic characters. — Cope writes : Dentition much as in Limnohyus, excepting that there is a diastema in front of the second inferior premolar. Presence of first inferior premolar not ascertained. Fourth inferior pre- molar without posterior cusps. Superior molars with an DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 169 angular ridge extending inward from each inner cusp. Last inferior molar with heel. * * * The V-shaped crests of the inferior molars separate it from Hyracotherium. Etymology. — Xa/i/35a, lambda; dTjplov, beast; in allusion to the A-shaped crests of the lower molars. Present determination. — The genus is valid and is now referred to the Eocene titanotheres. Lambdotherium popoagicum Cope, 1880 Cf. Lambdotherium popoagicum Cope, this monograph, page 281 Original reference. — Am. Naturalist, vol. 14, pp. 746, 747, 1880 (Cope, 1880.1). Subsequent reference. — Tertiary Vertebrata, p. 710, pi. 58b, figs. 7 and 7a, 1884 [1885] (Cope, 1885. 1). Type locality and geologic Tiorizon. — Badlands of Wind Elver, western-central Wyoming; Lambdo- iherium-Eotitanojps-Coryphodon zone (Wind River B). Type. — A lower jaw with dentition (Am. Mus. 4863). (See fig. 103.) Specific cliaracters. — Cope writes : The heels of the second and third premolars have a median keel; the third only has an anterior tubercle. The crest of the heel of the fourth forms an imperfect V. Heel of the last true molar small. No cingula; enamel smooth. Length of molar series 80 [millimeters]; of true molars 44; of last molar 19; depth of ramus at first premolar 21; at last molar 31. * * * About the size of Hyrachyus agresiis. Etymology. — popoagicum, in allusion to Popo Agie River, a tributary of Wind River. Present determination. — The species is valid. (See p. 283.) Lambdotherium brownianum Cope, 1881 Cf. EoHtanops brownianus (Cope), this monograph, page 292 Original reference. — U. S. Geol. and Geog. Survey Terr. Bull., vol. 6, p. 196, 1881 (Cope, 1881.2). Subsequent reference. — Cope, Tertiary Vertebrata, p. 709, pi. 56a, fig. 10 (not the type), 1884 [1885] (Cope, 1885.1). Type locality and geologic horizon. — Badlands of Wind River, western-central Wyoming; Lambdo- therium- Eotitanops-CorypJiodon zone (Wind River B). Holotype. — "The greater part of a lower jaw," with p^, m'-m' (Cope collection. Am. Mus. 4885). (See fig. 104.) Characters. — Cope writes: Considerably larger than the L. popoagicum and about equal to the Tapirus ierreslris. The greater part of a lower jaw represents the species, and on this, unfortunately, only one of the premolar teeth remains. The three premolars are all two-rooted, and the posterior lobe of the last true molar is well developed. The inferior part of the e.xternal side of the ramus contracts or retreats rather abruptly posteriorly, below the last molar. It presents a slight external convexity below the second and third premolars. The alveolar line rises rapidly 101959— 29— VOL 1 14 posteriorly, so that the last true molar is quite oblique. The second (first) premolar has a considerable heel, which is narrow and elevated on the middle line. The principal cusp is large and compressed but obtuse and has no anterior basal tubercle. Measurements [Cope] Millimeters Length of six molars 90 Length of true molars 55 Diameters of second (first) premolar: Vertical 9 Anteroposterior 12 Transverse 6 Length of base of first true molar 15 Width of base of first true molar 9 Length of base of third true molar 23 Width of base of third true molar 11 Depth of ramus at second premolar 30 Depth of ramus at ma: At front of tooth 39 At end of tooth 47 Etymology. — "Dedicated to my friend Arthur E. Brown, superintendent of the Philadelphia Zoological Garden" (Cope). FiGUEE 103. — T3'pe (holotype) of Lambdotherium popoagicum Left mandibular ramus, with pj-ms. Am. Mus. 4863. After Cope, 1885. Natural size. Present determination. — The species is valid, generic reference is to Eotitanops. (See p. 292.) The Palaeosyops hyognathus Osborn, 1889 Cf. Dolichorhinus hyognathus (Osborn), this monograph, page 409 Original reference. — Am. Philos. Soc. Trans., new ser., vol. 16, p. 513, 1890 [author's reprint issued Aug. 20, 1889; O. P. Hay] (Scott and Osborn, 1890.51). Subsequent reference. — Earle, A memoir upon the genus Palaeosyops Leidy and its allies: Acad. Nat. Sci. Philadelphia Jour., 2d ser., vol. 9, pi. 11, figs. 10, 11 [type], 1892 (Earle, 1892.1). Type locality and geologic horizon. — Washaliie, White River, northeastern Utah; Washakie B. 170 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Eolotype and specific characters. — Scott and Osborn write : In the Washakie beds is found a large species, about the same size as P. vallidens Cope, which is provisionally referred to Palaeosyops (P. hyognathus, sp. nov., Princeton collection. No. 10273). This is represented by a lower jaw seven-eighths as large as the type mandible of Diplacodon. [See fig. 105.] ;-->;x..i4:: Figure 104. — Cope's type of Lainhdotherium brownianum One-half natural size. As in the latter, the incisors form a close procumbent series; the tips forming a gently arched line when seen from above. The symphj'sis is extremely long (11 centimeters) and shallow; the canines are rather small and semiprooumbent. The molar- premolar series measures 24.5 centimeters, the last molar measures 6.5 centimeters, the transverse measurement outside of the canines is 9.6 centimeters; in Diplacodon elalus the same measurement is 10 centimeters. Unfortu- nately, the premolar crowns are broken; it is probable that one or two of the premolars will be found to be like the molars. The characters of the chin and sym- physis are significant of close relationship to Dipla- codon elatus. Etymology. — vs, boar; yvados, jaw; in allusion to the forward-pointing lower incisors and shallow mandibular symphasis. Present determination. — The species is valid. The generic reference is to DolicliorTiinus. (See p. 409.) Liitinohyops Marsh, 1890 Cf. Limnohyops Marsh, this monograph, page 303 Original reference. — Am. Jour. Sci., 3d ser., vol. 39, p. 525, 1890 (Marsh, 1890.1). Type species. — Palaeosyops laticeps Marsh (Am. Jour. Sci., 3d ser., vol. 4, p. 122, 1872). (Seep. 311.) Generic characters. — Marsh says: In 1872 the writer described a large mammal from the Eocene of Wyoming under the name of Palaeosyops laticeps. As the name Palaeosyops has since been restricted, this species must be regarded as representing a distinct genus, which may be called Limnohyops. In this form the last upper molar has two inner cones, and in Palaeosyops, as now defined, there is only one. Etymology. — Xiixfrj, shore; vs, boar; &}//, face. Present determination. — This is a valid genus and species. For fuller descriptions, see page 303. Palaeosyops megarhinus Earle, 1891 Cf. Mesatirhinus megarhinus (Earle), this monograph, page 388 .--''" "~\ Original reference. — Am. Naturalist, vol. 25, No. 289, pp. 45-47, 1 fig., Jan- ;-''' uary, 1891 (Earle, 1891.1). \__,,— ,^ Subsequent reference. — Earle, A mem- oir on the genus Palaeosyops Leidy and its allies: Acad. Nat. Sci. Philadelphia Jour., 2d ser., vol. 9, pp. 320-329, pi. 10, fig. 2; pi. 11, figs. 4, 5, 1892 (1892.1). Type locality and geologic horizon. — Washakie Basin of Wyoming; level unde- termined, probably Uintatherium- Man- teoceras- Mesatirhinus zone (Washakie A). Type.— "A fine skull (No. 10008) in the Princeton collection" (Earle). (See fig. 106.) Paratype. — Earle writes: There is also another portion of a skull (No. 10041), probably belonging to this species, with the occiput well preserved, from the Bridger proper [Earle, 1891.1, p. 45]. This paratype probably belongs to a more advanced species of this genus. (See p. 388.) Figure 105. — Type (holotype) of Palaeosyops hyognathus Incomplete lower jaw. Princeton Mus. 10273. After Earle, 1892. One-fifth natural size. Specific characters. — Earle writes: Cranium: The characters of this skull are quite unique and depart widely from any of the species of the family that I have examined. The general form of the skull is broad and depressed. Its dorsal contour is very like that of Palaeotherium crassum — namely, there is no frontal depression, which is so characteristic of Palaeosyops paludosus, and the occipital DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 171 region is only slightly higher than the frontal. The temporal fossae are not deeply excavated, and the occipital crests are weakly developed when compared to this region of the slcull in Limnohyops. The occiput itself is high and rather narrow. The foramen magnum is wide, bordered by very large condyles. The auditory processes are widely separated. The post- tympanics are broad and heavy. The postglenoid is peculiar in form; it is very short and thick; its form is very different from other species in the collection. An internal glenoid process is present in this species. The mastoid was probably exposed. The form of the zygomatic arch is striking; it is very light, nearly straight, with the temporal portion strongly compressed. The malar portion is also peculiar; the malar insertion is very abrupt and strongly depressed, With the external part very broad, thin, and shelf-like. The infra- orbital foramen is not exposed. The form of the malar in this species is totally different from all other allied forms that I have examined. The orbit is very small, termi- nates anteriorly above the anterior border of the second superior molar; the postorbital processes are well marked. The facial region of the skull is very short, compared to the total length of the cranium. The nasals are very long and heavy; their distal portion is expanded and broader than the middle part. The nasal notches are very deep and high. The premaxillaries are triangular in outline; their symphysis is short and nar- row, with a prominent anterior keel. The canine al- veolus is very prominent. The palate is long and nar- row, the roof of the same being strongly arched. The posterior termination of the palate is at the second superior molar. The incisive foramina are not divided. Teeth: The crowns of the teeth in this skull are badly damaged, but enough remains to give the total measurements and the characters of the last molar. The superior molars in this species form a continuous series, being not interrupted by a diastema. The sections of the incisors are very small. The canines are also very small and diverge widely. Only the second and third molar of each side are partially pre- served. They have a square form with low crowns; externally they are totally without a cingulum. The external V's are rather wide and angular, in this respect approaching that of Telmalotherium. The last molar is without any intermediate conules. Measurements Millimeters Length of skull, from premaxillary symphysis to end postglenoid ' 285 Length from orbit to premaxillary symphysis 125 Length from orbit to postglenoid 160 Depth of nasal notch 84 Length of nasals 100 Entire molar series 148 Last superior molar: Anteroposterior 37 Transverse 39 Etymology. — fie-yas, great, pis, nose; in allusion to the length of the nasal bones. Present determination. — This is a valid species which has been made the type of the genus MesatirJiinus by Osborn. (See p. 388.) Palaeosyops minor Earle, 1891 Cf. Palaeosyops paludosus, this monograph, page 319 Original reference. — Acad. Nat. Sci. Philadelphia Proc. for 1891, p. 112, issued March 31, 1891 (Earle, 1891.2). Subsequent reference. — Earle, A memoir upon the genus Palaeosyops Leidy and its allies: Acad. Nat. Sci. Philadelphia Jour., 2d ser., vol. 9, pp. 269, 331, 332, 1892 (1892.1). Earle's cotypes. — In his original description Earle says (1891.2, p. 112), "P. minor embraces specimens which Leidy erroneously described as P. paludosus, pi. 4, figs. 3-6, of Leidy's report for 1873." In his memoir (1892.1, pp. 269, 330) Earle refers again to Leidy's Plate 4, Figures 3-6, as the types of P. minor, but on page 332 he says, "We may consider as the type specimen" the "beautifully preserved mandible fig- FiGURB 106. — Type (holotype) of skull of Palaeosyops megarhinus Princeton Mus. 10008. After Earle, 1892. No scale given. ured by Leidy" (Leidy, 1873.1, pi. 5, figs. 10, 11); and again on page 387 he states that the specimen figured in his (Earle's) Plate 12, Figure 14, is "the type of this species and is in the collection of the Academy of Natural Sciences of Philadelphia." But this specimen is apparently the same one figured in Leidy's Plate 4, Figure 5. (See fig. 107.) Specific characters. — Earle writes: Second superior premolar with two external lobes, external lobes of last superior premolar equal. Intermediate conules of true molars reduced, a strong external cingulum present. Etymology. — minor, in allusion to the relatively small size. Present determination. — Of the first-mentioned speci- mens (Leidy, 1873.1, pi. 4, figs. 3-6) Figures 3 and 4 represent an upper dentition, which is probably con- specific with P. paludosus as determined in this mono- graph; hence if this is taken as Earle's type P. minor becomes a synonym of P. paludosus. 172 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA On the other hand, Leidy's Plate 4, Figures 5 and 6, represent an upper dentition of uncertain specific ref- erence. The "beautifully preserved mandible" (Leidy's pi. 5, figs. 10, 11) is probably referable to P. paludosus. Hence we may regard P. minor as a syno- nym of P. paludosus. Type locality and geologic horizon. — Cottonwood Creek, Bridger Basin, Wyo.; Palaeosyops paludosus- Orohippus zone (Bridger B). Holotype. — "A jaw, No. 10275 [Princeton Mus.], associated with a well-preserved radius, ulna, and two metacarpals." Ci FiGUKJs 107. — Earle's ootypes of Palaeosyops minor in the collection of the Academy of Natural Sciences of Philadelphia Ai, Lett maxilla with root of canine and premolar-molar series. After Leidy, 1873. Two-thirds natural size. Aj, The same; outer view of premolar-molar series. B, Another specimen; left upper premolar-molar series. After Leidy, 1873. Two- thirds natural size. (A reversed view of this specimen, which is of uncertain specific reference, was figured by Earle as the type (Earle's pi. 12, fig. 14).) Ci, Left mandibular ramus with p3-m3. After Leidy, 1873 (pi. 5, fig. II). One-halt natural size. C2, The same, pj-ma; crown view. After Leidy, 1873 (pi. 5, fig. 10). One-half natural size. The last two specimens are referable to Palaeosyops paludosus. Palaeosyops longirostris Earle, 1892 Cf. Palaeosyops longirostris Earle, this monograph, page 319 Original reference. — Acad. Nat. Sci. Philadelphia Jour., 2d ser., vol. 9, p. 338, 1892 (Earle, 1892.1). Characters. — Earle writes: The type jaw of this species, with the parts of the skeleton associated with it, was referred by Scott and Osborn [Osborn, 1878.3, pp. 37, 38] to our P. minor (equal, in part, to P. palu- dosus Leidy). After comparing Leidy's type specimen [prob- DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 173 ably the specimen figured in Leidy's memoir of 1873, pi. 5, fig. 11] with this jaw, I find that there is such a marked difference in some of its characters that I have to give it a specific rank. The following characters distinguish it from Leidy's type: (1) The great posterior extension of the jaw behind the last molar (this is a unique character of this jaw — I have not observed it in any other species of this subfamil}') ; (2) the symphysis is much more elongated than in P. minor; (3) the lower border is straighter and less inflected than in P. minor; (4) the posterior en'f Figure 108. — Earle's type of Palaeosyops longirostris Princeton Mus. 1027-5. One-foarth natural size. tubercle of the last inferior molar is much larger than in the last named species; (5) the V's of premolar 4 are not so well developed as in P. ininor, and there is also a well marked dif- ference in the size of the first molars of the two species. In this jaw the first true molar is con- siderably smaller than in P. minor. The canine is very large and semiprooumbent, its position in the jaw resembling that of T. hyognathus. Etymology. — longus, loia^g; rostrum, bill, snout (hence, in this instance, jaw); in allusion to the great posterior extension of the j aw behind the last molar. (Earle.) Present determination. — This prob- ably valid species is certainly refer- able to the Palaeosyopinae and probably to Palaeosyops. (See p. 319.) Telmatotherium diploconum Osborn, 1895 Cf . Rhadinorhinus diploconus (Osborn) , this monograph, page 431 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 7, p. 85, fig. 6, 1895 (Osborn, 1895.98). Type locality and geologic horizon. — NoTthe&sternlJtah;" Telmatotherium cornutum" beds, Eohasileus-Doli- chorhinus zone (Uinta B). Holotype. — "The type is a skull (No. 1863) [Am. Mus.] in which the nasals are wanting and the mid- region of the cranium was ci'ushed." (See fig. 109.) Characters. — Osborn writes: Superior premolar-molar series, 174 millimeters. A large hypocone upon last upper molar. Nasofrontal without horn. Long sagittal crest. Canines small, rounded. This species differs from T. megarhinum in the absence of the infraorbital shelf and in the presence of a large hypocone upon the last upper molar. The premolar-molar dentition is similar in size and form to that of T. cultridens, but there are the following important general differences: (1) Canines small and circular in section; (2) a very short diastema, if any, behind the canine; (3) a large hypocone upon m'; (4) the infraorbital foramen close beneath the anterior border of the molar [malar]. [Com- parisons with T. cultridens follow.] Etymology. — StxXoos, double; Kcofos, cone; in allu- sion to the presence of two internal cones on the third upper molar. Present determination. — The species is valid; it is now referred to the genus Rhadinorhinus. (See p. 431.) Telmatotherium cornutum Osborn, 1895 Cf. Dolichorhinus hyogiiathus (Osborn), this monograph, page 409 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 7, p. 90, figs. 10, 11, 1895 (Osborn, 1895.98). Type locality and geologic horizon. — Northeastern Utah; "Telmatotherium cornutum beds," Eohasileus- Dolichorhinus zone (Uinta B 2). Holotype and paratypes. — Osborn writes: The type of this species is a fine skull (No. 1851) [Am. Mus.], while several other well-preserved skulls from the same levels give us all the cranial characters and the superior dentition (Nos. 1850, 1847, 1848, 1852, 1837). [See fig. 110.] Figure 109. — Type (holotype) of Telmatotherium diploconum Superior and lateral views of skull. The nasals are broken off. Am, Mus. 1863. After Osborn, 1895. One-fourth natural size. Characters. — Osborn writes: Incisors f . Premolar-molar series, 208 millimeters. A nar- row diastema. Upper canines lanceolate. Long premaxillary symphysis. A well-developed nasofrontal protuberance. Top of cranium completely flattened. No sagittal crest. An infraorbital process upon malar. This species is remarkable for its very long flat-topped cranium and its incipient knoblike osseous horns borne chiefly upon the nasals but partly upon the frontals. These horns project laterally and rise slightly above the general surface, and are best 174 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA seen in the anterior view, Figure 110. Ttiese characters and the absence of the frontoparietal and interparietal sutures all point well toward Titanotherium, but the premolars are still absolutely simple, showing no trace of the postero-internal cusps which characterize Diplacodon elatus. Other striking peculiarities are the upward-arching mid- cranial region, the extremely long, narrow, and laterally de- curved nasals; the strong infraorbital shelf upon the molars [malars] (seen also in T. megarhinum) , the slender zygomatic arch, the low occiput, the backward extension of the posterior nares by the palatines, and the partial inclosing of the roof of the pharynx by the pterygoids. Sphenocoelus Osborn, 1895 Cf. Sphenocoelus Osborn, this monograph, page 417 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 7, pp. 98-102, 1895 (Osborn, 1895.98). Generic characters. — Osborn writes: The distinctive features of the skull may therefore be summed up as follows: Deep paired pits in the aUsphenoids, and orbito- sphenoids upon either side of the thin presphenoid [basisphenoid] ; a long alisphenoid canal; foramen ovale widely separated from Figure 110. — Type (holoty Side, front, and top views of skull. Am. Mus. This general description of character was followed by a more detailed description. Etymology. — cornutus, horned; in allusion to the osseous "horns." Present determination. — Comparison of the lower jaw with the type of Palaeosyops hyognathus Osborn indicates that the species T. cornutum is a synonym of P. hyognathus, a species which is now referred to the genus Dolichorhinus. pe) of Tebnatotherium cornutum 1851. After Osborn, 1S95. One-fourth natural size. 1 for. lac. medium; condjdes very broad; foramen magnum large; occipital crest extending anteriorlj' into a short sagittal crest with convex sagittal ridges; skull apparently long and narrow. Etymology. — riv, a wedge; koIXos, hollow; in allusion to the paired cavities in the basisphenoid bone. Present determination. — This is a valid genus of Eocene titanotheres related to the long-skulled Dolichorhinus. (See p. 417.) DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 175 Sphenocoelus uintensis Osborn, 1895 Cf. Sphenocoelus uintensis Osborn, this monograph, page 419 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 7, pp. 98-102, figs. 12-15, 1895 (Osborn, 1895.98). Type locality and geologic horizon. — Northeastern Utah; Metarhinus zone (Uinta B 1). Holotype. — "Represented by the posterior portion of a slvull" (Am. Mus. 1501). (See fig. 111.) convex sagittal ridges. The occiput is rather broad, and below it are two widely set occipital condyles which are directed obliquely downward and backward. On either side of these the exoccipitals extend down into obtuse paroccipital processes, which are closely joined to the post-tympanics. The external auditory meatus is open inferiorly. In front of this the post- glenoid process faces somewhat inward; the glenoid facet is L-shaped, two narrow arms extending out upon the squamosal, and a broad arm descending upon the postglenoid. The dis- tinctive feature of the zygoma is the presence of a deep depres- sion just behind the lateral arm of the glenoid facet. FiGUKE 111. — Type (holotype) of Sphenocoelus uintensis Posterior half of cranium. Am. Mus. 1601. After Osborn, 1895. , Basal view; Aa, top view; As, occipital view; natural size. fiew of left side. One-third Specific characters. — Osborn writes: This new genus is represented by the posterior portion of a skull, which is distinct from any cranium known to the writer. Its most distinctive feature is the presence of a pair of pits in the floor of the skull upon either side of the narrow presphenoid [basisphenoid]. These pits were at first mistaken for the for. lac. media, but more careful investigation shows that they are roofed over by bone and apparently do not communicate at all with the cranial cavity. The pit on the right side is per- fectly preserved and clearly exhibits these characters. The pits are 42 millimeters long, 14 millimeters wide, and' 2 milli- meters deep. The skull has a long, narrow cranium surmounted posteriorly by a sagittal crest, which diverges anteriorly into two decidedly Skull measurements Millimeters Width across zygomatic arches 230 Height of occiput 142 Breadth 117 Breadth of occipital condyles 130 Basioccipital to top of sagittal crest 114 The foramina of the skull are related to those of the Peris- sodactyla, for there is a long alisphenoid canal, upon the outer side of the anterior opening of which is the foramen. Just behind the posterior opening of the canal is the foramen ovale, and between these foramina are the two pits above mentioned. This foramen is separated by a very wide plate of bone from the for. lac. medium, which is partly filled by the periotic mass. 176 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Etymology. — uintensis, from Uinta, in allusion to the Uinta Basin. Present determination. — This is a valid genus and species. (See p. 419.) Protitanotherium Hatcher, 1895 Cf. Protitanoiheriiim Hatcher, this monograj)h, page 374 Original reference. — Am. Naturalist, vol. 29, p. 1084, December, 1895 (Hatcher, 1895.1). would seem that Professor Marsh's conclusion is entirely con- jectural, since his material does not show whether there were horns or not. The present skull has a well-developed pair of frontonasal horns, and, since it agrees in all the characters known to that genus, I have preferred to refer it to that genus rather than to propose for it a new one on the strength of this purely conjectural character ascribed to Diplacodon by Pro- fessor Marsh. Should future discoveries show that there are hornless forms with the same dental characters as Diplacodon, it will then be necessary to establish for the present specimen a new genus, which may be called Protitanotherium. FiGUKE 112. — Type (holotype) of Diplacodon emarginatus Facial part of skull and anterior part of mandible. Princeton Mus. 11242. After Hatcher, 1895. Ai, Az, Aj, Side, top, and front views of skull; Bi, B2, B3, side, top, and front views of mandible. One-fourth natural si:e. Type species. — Diplacodon emarginatus Hatcher. Generic characters. — Hatcher writes: In referring this skull to Diplacodon, I have been compelled to ignore certain characters ascribed to that genus by Professor Marsh. That author, in speaking of the relations of this genus to the Titanotheriidae (Brontotheridae) , in his original descrip- tion of the type specimen, says (Marsh, 1875.1, p. 24) : "From this family, Diplacodon differs widely in its dentition and the absence of horns." In describing Diplacodon as hornless, it Etymology. — pro, before; Titanotherium — that is, forerunner of Titanotherium. Present determination. — It is not yet settled whether Diplacodon elatus Marsh had horns or not, but it is now believed that even if this character is set aside D. elatus is generically distinct from D. emarginatus, and we may therefore regard Hatcher's Protitanothe- rium as a valid genus. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 177 Diplacodon emarginatus Hatcher, 1895 Cf. Protitanotherium emarginatum Hatcher, this monograph, page 377 Original reference. — Am. Naturalist, vol. 29, pp. 1084-1087, pi. 38, figs. 1-4, December, 1895 (Hatcher, 1895.1). Type locality and geologic horizon. — Found by J. B. Hatcher "near the base of the Diplacodon elatus beds [Uinta C of Osborn], in the upper Eocene or Uinta of Marsh. The locality is about 8 miles north of White River and 25 miles east of Ouray Agency, Utah, and is locally known as Kennedy's Hole." Holotype. — A skull with lower jaw (Princeton Mus. 11242). The anterior part only of the skull is well preserved. (See fig. 112.) Characters. — Hatcher writes: The present species is at once clistinguished from D. elatus by its greater size, as is shown by a comparison of the length of the premolar and molar series, which is* 310 millimeters in the former and 242 in the latter. In general appearance the cranium of D. emarginatus is re- markably like some of the smaller forms of Titanotherium. Etymology. — emarginatus, referring to the emargi- nate form of the distal end of the nasals. Present determination. — The species is valid but generically distinct from Diplacodon Marsh and is now referred to ProtitanotTierium Hatcher. (See p. 377.) Manteoceras Hatcher, 1895 Cf. Manteoceras Hatcher, this monograph, page 362 Original reference. — Am. Naturalist, vol. 29, p. 1090, 1895 (Hatcher, 1895.1). Type species. — By designation Telmatotherium valli- dens (of Osborn, not Palaeosyops vallidens Cope) = Manteoceras manteoceras Hay ex Osborn, MS., 1902. Hotelier's description — Hatcher writes: The genus Telmatotherium as it now stands should be divided, since it embraces at least three distinct forms. The type of T. vallidens should be removed from that genus and made the type of a new genus. This new genus may be called Man- teoceras, as suggested by Wortman from the field; it would be distinguished from Telmatotherium by the absence of the infra- orbital shelf, the stronger and more expanded zygomata, and the concave superior aspect of the skull and incipient fronto- nasal horns. In the above passage the reference to "the type of T. vallidens" if taken by itself would lead one to regard "Palaeosyops" vallidens Cope as the type of the genus Manteoceras Hatcher. But a careful study of Hatcher's full text and a knowledge of the history of the subject proves that Hatcher had in mind the "Telmatotherium vallidens" of Osborn, not of Cope: because (a) Hatcher refers to his Plate 29, Figure 2, as "Telmatotherium vallidens," and this figure is copied from Osborn's " Telmatotherium vallidens," Figure 7; (6) these figures represent Wortman's original "prophet horn" skull, to which he had applied the name Manteoceras "in a letter from the field" (Osborn); (c) the generic characters assigned by Hatcher refer most clearly to this skull and are utterly inapplicable to Telmatotherium {"Palaeosyops") valli- dens Cope, in which only the dentition and not the skull is known. Thus the type of the genus Manteoceras Hatcher is Telmatotherium vallidens of Osborn not Cope, which is equivalent to Manteoceras manteoceras Hay ex Osborn MS. The generic name can not be credited to Wortman, because he never published it, although Osborn (1895.98), mentions it as a manuscript name. Etymology. — juavrtj, prophet; Ktpas, horn; in allu- sion to the incipient "horns" above the orbits. Present determination. — This valid genus is fully de- scribed on page 362. Dolichorhlnus Hatcher, 1895 Cf. DoKchorhinus Hatcher, this monograph, page 396 Original reference. — Am. Naturalist, vol. 29, p. 1090, 1895 (Hatcher, 1895.1). Type species. — Telmatotherium cornutum Osborn. Characters. — Hatcher writes: The genus Telmatotherium as it now stands should be divided, since it embraces at least three quite distinct forms * * * The type of T. cornutum should also be made the type of a new genus which may be called Dolichorhinus; it would be dis- tinguished from Manteoceras and Telmatotherium by the reduced number of inferior incisors, presence of incipient horns, presence of infraorbital shelf, and position of posterior nares. Etymology . — SoXixos, long; pis, nose. Present determination. — This is a valid genus. p. 396.) (See Palaeosyops ultimus Matthew, 1897 (ex Osborn MS.) Cf. Telmatherium ultimum Osborn, 1908, this monograph, page 345 Original reference. — Am. Naturalist, vol. 31, pp. 57-58, 1897 (Matthew, 1897.1). Subsequent reference. — Bibliography and catalogue of the fossil Vertebrata of North America: U. S. Geol. Survey Bull. 179, p. 631, 1902 (Hay, 1902.1). Doctor Matthew had no intention of describing a new species. He merely stated incidentally that P. ultimus, as established in manuscript by Osborn, and P. paludosus both have a short-necked astragalus. No type was mentioned, and the single character given does not separate the species from P. paludosus. Hence "Palaeosyops ultimus Matthew" (cited by Hay, 1902, p. 631) remained a nomen nudum until the type was fixed by Osborn in 1908. (See p. 345.) Etymology. — ultimus, last, latest; in allusion to the relatively late geologic horizon and to the apparent extinction of the race. Palaeosyops manteoceras Matthew, 1899 (ex Osborn MS.) Cf. Manteoceras manteoceras Hay, this monograph, page 395 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 12, p. 47, 1899 (Matthew, 1899.1). 178 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA In this faunal list the present specific name is merely mentioned. No type is specified, and no characters are given, so that Palaeosyops manteoceras Matthew was a nomen nudum until the type was designated by Hay in 1902. (See p. 365.) Etymology. — /xavris, prophet; Ktpas, horn; in allusion to the incipient "horns" above the orbits. Telmatotherium diploconum var. minus Matthew, 1899 (Nomen nudum) Original reference. — Am. Mus. Nat. Hist. Bull., vol. 12, p. 50, 1899 (Matthew, 1899.1). In Matthew's faunal list " Telmatotherium diploconum var. minus" is recorded but not defined, and no type is specified. Hence Telmatotherium diploconum minus is a nomen nudum. Etymology. — minus, less; because smaller than the tyjDical T. diploconum. Canis? marshii Hay, 1899 Of. Palaeosyops major? Leidy, this monograph, page 321 Original reference. — Science, new ser., vol. 10, p. 253, 1899 (Hay, 1899.1). Founded on "Canis montanus" Marsh (see p. 158), which was preoccupied by Canis montanus Pearson. Etymology. ^-'Named in honor of Prof. O. C. Marsh. Present determination. — As explained above, the type of Canis montanus Marsh (not Pearson) and Canis? marshii Hay is a second lower premolar of an Eocene titanothere, probably Palaeosyops paludosus or P. major. Canis? marshii Hay is therefore either indeterminate or a synonym of P. paludosus or P. major. Manteoceras manteoceras Hay, 1902 (ex Osborn MS.) Cf . Manteoceras manteoceras Hay, this monograph, pages 365-370 Original reference. — U. S. Geol. Survey Bull. 179, p. 632, 1902 (Hay, 1902.1). Lectotype. — A skull (Am. Mus. 1569) lacking the dentition, described and figured by Osborn as "Telma- totherium vallidens" (Osborn, 1895.98, pp. 87-90, figs. 7-8). (See fig. 113.) Paratype. — An incomplete skull (Am. Mus. 1570) with dentition (op. cit., fig. 9) from the same locality and level. Type locality and geologic horizon. — Washakie Basin, Wyo.; discovered by J. L. Wortman, of the American Museum Bridger expedition of 1893, "in a brown layer of sandstone 3 miles north of the base of Haj''- stack Mountain, upon Bitter Creek" (op. cit., p. 87). Uintatherium- Manteoceras- Mesatirhinus zone (Wash- akie A). . Hay's type. — We have seen above that the name Palaeosyops manteoceras Matthew (ex Osborn MS.) was a nomen nudum, because no type had been desig- nated. The type was for the first time clearly indi- cated by Hay (1902.1, p. 632), who refers to this species Hatcher's (1895.1) Plate 39, Figure 2 (p. 368, this monograph) and Osborn's (1895.98) Figures 7-9 (pp. 366, 368). These are clearly the same two "prophet horn" skulls (Wortman's first "Manteoceras" speci- mens) that had been at first erroneously referred by Osborn to "Telmatotherium vallidens Cope." Of these two skulls. Am. Mus. 1569 — that is, Osborn's Figures 7, 8 and Hatcher's Figure 2 (copied from Osborn's fig. 7) — may be taken as the lectotype. The generic name Manteoceras and the specific name manteoceras were first brought together by Hay in the reference now under consideration. Specific characters. — In Osborn's original descrip- tion (Osborn, 1895.98, p. 87) these skulls were errone- ously identified as conspecific with the type of Palaeo- syops vallidens Cope, under the name "Telmatotherium vallidens Cope." The specific characters given by Osborn were as follows: Superior premolar-molar series, 184-220 millimeters. A narrow diastema. Molar cusps less elevated. A rudimentary nasofrontal tuberosity. Premaxillary symphysis short. Top of cranium flattened; very short bifid sagittal crest. Etymology. — fiavrLs, prophet; Kepas, horn; in allu- sion to the incipient "horns" above the orbits. Present determination. — The species is a valid one and is fully described on pages 365-370. Lambdotherlum primaevum Loomis, 1907 Cf . Lambdotherium -primaevum Loomis, this monograph, page 283 Original reference. — Am. Jour. Sci., 4th ser., vol. 23, p. 363, fig. 2, May, 1907 (Loomis, 1907.1). Type locality and geologic horizon. — Buffalo Basin, near Meeteetse, Wyo. "Wasatch beds of the Big Basin." Horizon regarded by Loomis as equivalent to the base of the Wind Eiver formation — that is, the Heptodon-Coryphodon-Eohippus zone (Wind River A). Holotype. — Amherst Mus. 254, "consisting of upper molars 1 and 2 of the right side and lower molars 1, 2, and 3 from the same side, the specimen being from the Buffalo Basin, near Meeteetse, Wyo. This species is fau'ly abundant at this horizon and is intermediate in size between L. hrownianum and L. popoagicum." (See fig. 114.) Characters. — Loomis writes: On the upper molars the parastyle, though strong, is not so well developed as in the foregoing forms; the paraoonule is well developed, but the metaconule is so annexed to the metaoone as to appear like a buttress of this cusp. The second molar measures 12 milUmeters transversely [anteroposteriorly] by 17 millimeters lengthwise [transversely]. The robust lower molars have the protoconid markedly bifid, while the paraconid and hypoconid are each high crescents. The heel of the last molar is a high shallow basin completely surrounded by an outer rim. The three molars occupy 41 milhmeters. The brackets above indicate that in the foregoing description the measurements of the molar teeth have been inadvertently transposed. The description DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 179 should read: "Second superior molar, antei'oposterior, 12 millimeters; transverse, 17 millimeters." Etymology. — primaevus, earliest in age; in allusion to the supposedly low geologic horizon. Present determination. — Provisionally recognized as a valid species. Mus. Nat. Hist. Bull., vol. 24, pp. 600, 601, 1908 (Osborn, 1908.318). Type species. — Palaeosyops horealis Cope. Generic characters (Osborn, 1908.318, p. 601). — Superior molars subquadrate and rounded in form; conules reduced, sublophoid; m'-m^ 63 millimeters Figure 113. — Cotypes of Manteoceras manteoceras {Telmalotheriam vallidens) After Osborn. Ai, Composite Am. Mus. 1569, 1570; side view otslsull; As, Am. Mus. 1669 (lectotype), superior view ot slcull; B, Am. Mus. 1570, superior view of slcull. All one-fourth natural size. Eotitanops Ogborn, 1907 Cf. Eotitanops Osborn, this monograph, page 289 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 23, p. 242, 1907; type species designated (Osborn 1907.294). Subsequent reference. — Osborn, New or little-known titanotheres from the Eocene and Oligocene: Am. (estimated). Inferior molars without metastylids. Hypoconulid of m' subconic. Fii'st inferior premolar present. Manus tetradactyl, functionally tridactyl with a tendency to mesaxonic structure. From Wind River formation. Etymology. — ^cbs, dawn; Tltclv, a titan; w^, face — ■ that is, first of the titanotheres. Present determination. — This genus is valid. (See p. 289.) 180 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Limnohyops priscus Osborn, 1908 Cf. Limnohyops priscus Osborn, this monograph, page 306 Original reference. — Am. Miis. Nat. Hist. Bull., vol. 24, pp. 601-602, fig. 5, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Grizzly Buttes, Bridger Basin, Wyo.; Palaeosijops paludosus-Oroliippus zone (Bridger B 2). A Figure 114. — Type (holotype) of Lambdotherium primaevum Amherst Mus. 254. After Loomis, 1907. A, Right upper molars 1 and 2; B, right lower molars (mi-ms). Natural size. Holotype. — A crushed skull with excellent dentition (Am. Mus. 11687), discovered by the American Museum expedition of 1903. (See fig. 115.) Figure 115. — Type (holotype) of Limnohyops priscus Am. Mus. 11687. Pi-m' left. After Osborn, 1908. One-half natural size. Characters. — Osborn writes: P'-m', 148 (type) to 161 millimeters. Distinguished from the contemporary Limnohyops laevidens Cope by its larger size and by the more progressive character of pm^-pm'. Second superior premolar obliquely elongate with a very rudi- mentary tritocone. Large hypocone on m'. Etymology. — priscus, ancient, in allusion "to the low geological level and primitive characters of this species." (Osborn.) Present determination. — The species and generic reference are probably valid. (See p. 306.) Figure 116.- -Tj'pe (holotype) skull of Limnohyops matthewi Am. Mus. 11684. After Osborn, 1908. One-fourth natural size. Limnohyops matthewi Osborn, 1908 Cf. Limnohyops matthewi Osborn, this monograph, page 308 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 602, fig. 6, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Grizzly Buttes, Bridger Basin, Wyo. ; Palaeosyops paludosus-Orohippus zone (Bridger B 2). Holotype. — A skull (Am. Mus. 11684) lacking the anterior portion and dentition. Discovered by the American Museum expedition of 1903. (See fig. 116.) Specific characters. — Osborn writes: Intermediate in size between L. laevidens and L. monoconus. M' of small size with large hypocone and quadrate inner half Occiput very high and narrow. Cranial portion of skull greatly abbreviated, bringing post-tympanic and postglenoid processes into broad union. Temporal openings subcircular as defined by zygomatic arches. Etymology. — Named "in honor of Dr. W. D. Matthew, of the American Museum staff." (Osborn.) Present determination. — The species is probably valid. (See p. 308.) Figure 117. — Type (holotype) skull of Limnohyops monoconus Am. Mus. 11679. After Osborn, 1908. One-fourth natural size. Limnohyops monoconus Osborn, 1908 Cf. Limnohyops monoconus Osborn, this monograph, page 309 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 603, fig. 7, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Grizzly Buttes, Bridger Basin, Wyo. ; Palaeosyops paludosus-Orohippus zone (Bridger B 2). Holotype. — A crushed skull with dentition (Am. Mus. 11679). Discovered by Mr. Quackenbush, of DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 181 the American Museum expedition of 1903. (See fig. 117.) Specific characters. — Osborn writes: M^ without hypocone, roundly triangular in form, with broadly extended ectoloph and parastyLe. P^-m' 150, p'-m^ 163 milUmeters. Condyle to incisive border 510. Occiput very high, cranium relatively elongated, with space (4 miUi- meters) between post-tympanic and postglenoid processes. Temporal openings as defined by zygomatic arches elongate. Figure 118. — Type (holotype) skull of Palaeosyops leidyi Inferior view. Am. Mus. 1544. After OsbDrn, 1908. One-fourth natural size. Etymology. — yibvo's, single; kccws, cone; named "in reference to the presence of but a single cone on the inner side of the third superior molar, an exceptional condition in the genus Limnohyops." (Osborn.) Present determination. — The specific and generic references are probably valid. (See p. 309.) Palaeosyops leidyi Osborn, 1908 Cf. Palaeosyops leidyi Osborn, this monograph, page 323 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 604, fig. 8, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Henrys Fork, Bridger Basin, Wyo.; Uintatherium- Manteoceras- Mesa- tirliinus zone (Bridger C 2 to C 4). Discovered by the American Museum expedition of 1893, under Dr. J. L. Wortman. Holotype. — A well-preserved skull (Am. Mus. 1544) associated with considerable portions of the skeleton. This specimen, which is associated with a considerable portion of the postcranial skeleton, is now mounted in the American Museum, the missing parts having been supplied from other individuals. (See p. 323; Pis. XXVII, L, LXI; and fig. 118.) Specific characters. — Osborn writes: Of larger size; total length of skull 415 millimeters; p'-m^, 158; P2-m3, 168; diastema behind canines; p-', p^ superior, with mesostyles. Barely defined sweUings representing the rudi- ments of osseous frontonasal horns. Etymology. — Named "in honor of Joseph Leidy, the discoverer of the family and [founder] of the genera Palaeosyops, Titanotherium, and Megacerops." (Osborn.) Present determination. — The species is probably valid. Palaeosyops granger! Osborn, 1908 Cf. Palaeosyops grangeri Osborn, this monograph, page 335 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 604, fig. 9, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Twin Buttes, Bridger Basin, Wyo. ; Uintatherium- Manteoceras- Mesa- tirhinus zone (Bridger C 1). Holotype. — A palate and grinding teeth with por- tions of the lower jaw and skull (Am. Mus. 12189), American Museum expedition of 1904. (See fig. 119.) Specific characters. — Osborn writes: Exceeding P. robustus in certain dental proportions; p^-m^, 165 millimeters. Fourth superior premolar enlarged (trans- verse, 31 mm.). Molars with extremely prominent parastyles and oblique ectolophs. Etymology. — Named "in honor of Mr. Walter Gran- ger, of the American Museum staff, whose explora- tions have transformed our knowledge of the Bridger animals. " (Osborn.) Present determination. — The species is probably ^alid. (See p. 335.) Figure 119. — Tj'pe (holotype) of Palaeosyops grangeri ; maxillary with p'-mi. Am. Mus. 12189. After Osborn, 1908. One-half natural size. Palaeosyops copei Osborn, 1908 Cf. Palaeosyops copei Osborn, this monograph, page 336 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 606, fig. 10, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Lone Tree Hen- rys Fork, Bridger Basin, Wyo.; Uintatherium- Man- teoceras-Mesatirhinus zone (Bridger D 3). Holotype. — A series of superior grinding teeth (Am. Mus. 11708). (See fig. 120.) 182 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Specific characters. — Osborn writes: Of more diminutive size (p'-m', 153 mm.), but the miost progressive species of Palaeosyops known in the evolution of its superior premolars and molars. Heavy oingula embracing the inner sides of the crowns. A rudimentary tetartooone on p^. Etymology. — Named "in honor of the late Prof. E. D. Cope, the describer of Lambdotherium, 'Palaeo- FiGTjRE 120. — Type (holotype) of Palaeosyops copei P'-ms, right. Am. Mus. 11708. After Osborn, 1908. One-half natural size. syops' horealis, and other species of Eocene titano- theres." (Osborn.) Present determination. — The species is probably valid. Manteoceras washakiensis Osborn, 1908 Cf . Manteoceras washakiensis Osborn, this monograph, page 371 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 607, fig. 11, 1908 (Osborn, 1908.318). Type locality and geologic Tiorizon. — Base of Hay- stack Mountain, Washakie Basin, Wyo.; summit of UintatJierium- Manteoceras- Mesatirliinus zone (Washa- kie A). Holotype. — A well-preserved skull with dentition (Am. Mus. 13165). Discovered by Mr. Paul Miller, of the American Museum expedition of 1906. (See fig- 121.) Specific characters. — Osborn writes: Distinguished from M. manteoceras of a somewhat lower geological level by its more progressive characters, as follows: Canines short, obtuse, recurved; internal lobes of pm^, pm^ broadening, with shelf for development of deuterocone; p^ (ap. 19 mm., tr. 17) with marked external convexities and a re- duced external cingulum; p^ (ap. 19, tr. 25) exhibits the tetartocone fold somewhat more conspicuously than in the most progressive Bridger level D speci- mens. ?■* (ap. 24, tr. 30) is progressive in transverse measurement and in the development of the tetar- tocone shelf. The molars are progressive in their large size (m^ ap. 42, tr. 48), in the strong develop- ment of the internal cingulum, and in the elongate ectolophs. Etymology. — washaJciensis; "so named be- cause it is a more recent phase, probably characteristic of the Washakie rather than of the Bridger." (Osborn.) Present determination. — The species and the generic reference are valid. (See p. 371.) Mesatirhinus Osborn, 1908 Cf. Mesatirhinus Osborn, this monograph, page 387 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 608, 1908 (Osborn, 1908.318). Type species and geologic horizon. — Palaeosyops megarhinus Earle. Bridger Basin, Wyo., levels Bridger C and D; Washalcie Basin, Wyo., levels Washakie A and base of Washakie B. Specific characters. — Osborn writes: Titanotheres of small size (skull length 354^425 mm.) typically mesaticephalic, persistent or progressing to dolicho- cephalic. The horns when present incipient or rudimentary, chiefly borne on the nasals. An infraorbital shelf. Cranium with a sagittal crest. Humerus relatively abbreviated — that is, with refeYence to Palaeosyops — carpus and tarsus narrow, astragalus with elongate neck, the sustentacular distal and ouboidal facets continuous and forming a reversed L (j) ; meta- podials slender. Etymology. — fiiaaros, middle; pis, nose; because the length of the snout is moderate compared with that in the allied genus Dolichorhinus. Present determination. — The genus is valid. (See p. 387.) Mesatirhinus petersoni Osborn, 1908 Cf. Mesatirhinus petersoni Osborn, this monograph, page 389 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 608, fig. 12, 1908 (Osborn, 1908.318). Holotype. — A skull with dentition (Am. Mus. 12184) from Cattail Spring, Bridger Basin, Wyo., levels Bridger D 3 and C 3. (See fig. 122.) The species is also recorded from Washalde Basin, Wyo., level Washakie A. Specific characters. — Osborn writes: Pm'-m^, 156 millimeters; m'-m', 90. Skull length, pre- maxillaries to condyles 412 (estimated) ; preorbital facial region more elongate (217). Other characters as in Mesatirhinus megarhinus — that is, broad occipital condyles, broad infra- orbital shelf on malar, etc. Comparison of this animal with the type of M. megarhinus can leave no doubt that we have to do here with a much more advanced stage of evolution. The skull is longer, the pre- orbital region especially. The grinding teeth occupy more space, and there is an average advance in all the rectigradations which proves that these differences in form and size are not merely due to fluctuations of size or differences of sex. ffK?- Figure 121. — Type (holotype) skull of Manteoceras washakiensis Left side. Am. Mus. 13165. After Osborn, 1908. One-flfth natural size. Etymology. — "The species is named in honor of Mr. O. A. Peterson, now of the Carnegie Museum, whose titanothere collections in the Uinta formation greatly extended our knowledge." (Osborn.) Present determination. — The species and generic ref- erence are valid. (See p. 389.) DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 183 Metarhinus Osborn, 1908 Cf. Metarhinus Osborn, this monograph, page 420 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 609, 1908 (Osborn, 1908.318). Type species and geologic liorizon. — Metarhinus flu- viatilis Osborn. Washakie Basin, Wyo., level Washa- kie B; Uinta Basin, Utah, levels Uinta B 1 and B 2. Specific charac- t er s . — Osborn writes : Small tltanotheres (skull length 355 to 440 mm.), persistently mes- aticephalic. Narrow, abbreviated preorbital region, premaxillary symphysis greatly elongated, and anterior narial openings deeply recessed in side view. Infraorbital shelf pres- ent, or wanting (M. diploconus) ; occipital condyles narrow. Grinding teeth sub- hypsodont; premolars progressive; hypoco- nulid of ms small, conic. Etymology. — iiera, after; rJiinus (that i s , MesatirMnus) . "The name alludes to the somewhat later geological ap- pearance of this genus as compared with MesatirMnus." (Osborn.) Present determination. — The genus is valid. (See p. 420.) Metarhinus fluviatilis Osborn, 1908 Cf. Metarhinus fluviatilis Osborn, this monograph, page 421 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 609, fig. 13, 1908 (Osborn, 1908.318). Figure 122. — Type (holotype) skull of MesatirMnus petersoni Top view. Am. Mus. 12184. After Osborn, 1908. One-fourth natural size. Figure 123. — Type (holotype) skull of Metarhinus fluviatilis Lett side. Nasals broken off. Am. Mus. 1500. natural size. After Osborn, 1908. One-fourth Type locality and geologic liorizon. — Uinta Basin, Utah; Metarhinus zone (Uinta B 1). Holotype. — A skull (Am. Mus. 1500) discovered by the American Museum expedition of 1894 in horizon B 1 of the Uinta Basin. (See fig. 123.) Specific characters. — Osborn writes: Pm'-m^ — 144 millimeters. A relatively short (355 mm., estimated), broad (200 mm., estimated) skull. Eye sockets small and very prominent. Premaxillary symphysis elongate, grinding teeth subhypsodont, m' with a cingulum-hypocone in the type. Etymology. — fluviatilis, fluviatile. "The name is given in allusion to the possibly river-living or am- phibious habits of the animal." (Osborn.) Present determination. — The species and the generic reference are valid. For fuller specific distinctions see page 421. Metarhinus earlei Osborn, 1908 Cf. Metarhinus earlei Osborn, this monograph, page 426 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 610, fig. 14, 1908 (Osborn, 1908.318.) Figure 124. — Type (holotype) .skull of Metarhinus earlei Top view. Am. Mus. 13166. After Osborn, 1908. One-fourth natural size. Type locality and geologic horizon. — North side of Haystack Mountain, Washakie Basin, Wyo.; Meta- rhinus zone (Washakie B 1 ) . Type. — A skull (Am. Mus. 13166) lacking the nasals, American Museum expedition of 1906. (See fig. 124.) Specific characters. — Osborn writes: Pmi-ni' = 167 millimeters. Skull proportions, length 380, breadth 230. Narrow occipital condyles. Extremely elongate premaxillar}' symphysis. A short sagittal crest. No hypocone on m^. This animal is readily distinguished from M. diploconus by (1) the infraorbital shelf of the malars; (2) the elongate premaxil- 184 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA lary; (3) the absence of a double cone on m^ In many other respects it resembles M. diploconus, especially in its proportions. It is distinguished from M. megarhimis by (1) the elongate premaxillary symphy- sis, correlated with the long, narrow facial re- gion; (2) the narrowness of its occipital condyles. It is distinguished from M. fluviatilis by (1) its greatly superior size and (2) the lesser prom- inence of the orbits. Etymolo g y. — Named "in honor of Charles Earle, the first monographer of the genus Palaeo- syops and its allies." (Osborn.) Present determina- tion. — The species is probably valid. (See p. 426.) Dolichorhinus interme- dius Osborn, 1908 Cf. Dolichorhinus inter- niedius Osborn, this monograph, page 405 Original refer- ence. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 611, fig. 15, 1908 (Osborn, 1908.318). Type locality and Top view. Am. Mus. 1837. After Osborn, 1908. neoloqic llOrizOn. — One-fourth natural size. .^t. , -r» • i.^ Umta Basin, north- eastern Utah; Eobasileus - Dolicliorhinus zone (Uinta B 2). Hohtype. — A skull with dentition (Am. Mus. 1837), discovered by the American Museum expedition of 1894. (See fig. 125.) Specific characters. — Osborn writes : Distinguished from D. hyognathus Scott and Osborn by (1) its inferior size (pm'-m^ 179, m'-m' 109 mm.); (2) premolars less progressive, with subconic deuterocones; (3) all oingula less robust; (4) nasals more pointed and less expanded distally; (5) infraorbital shelf of malar relatively narrow. Etymology. — "The name 'intermedins' is given because in some characters this species is intermediate between Mesati- rhinus petersoni and DolichorMnus Tiyo- gnathus, although on the whole it is Figure 126. — Type (holotype) skull of Telmatheriiim ultimum much more nearly allied to the latter." side ™w. Am. Mus. 2O6O. After Osbom, 19O8. One-flfth natural size. The skull has been some- //-j V s what deformed by pressure. Present determination. — The generic reference ap- I pointed. P,, pz laterally compressed, nonmolariform; ps, p4 pears certain; the species is probably valid. (See submolariform; dolichocephalic, anterior portion of face p. 405.) I elongate. Figure 125. — Type (holotype) skull of Dolichorhinus inter medius Telmatherium ultimum Osborn, 1908 Cf . Telmatherium ultimum Osborn, this monograph, page 345 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 613, fig. 17, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Uinta Basin, northeastern Utah; Diplacodon-Protitanotherium-Epi- hippus zone (Uinta C, lower levels). Holotype. — A well-preserved skull with dentition (Am. Mus. 2060). Discovered by Mr. Peterson, of the American Museum expedition of 1895. (See fig. 126.) Synonymy. — This species was mentioned by Mat- thew as Palaeosyops ultimus Osborn MS. (see p. 177), but as no type was indicated or specific diagnosis given the name remained a nomen nudum until a type was designated and a diagnosis given by Osborn in 1908. Specific characters. — Osborn writes: P'-m*, 226 mm. Lateral superior incisors greatly en- larged, caniniform. Pm^- ^. * with Internal subcrescentic deuterocone ridges, with faint rudiments of tetartocones posteriorly. Ectolophs of premolars elevated and biconvex. Etymology. — ultimus, latest. "The specific name is given because this appears to be the last representative of the Palaeosyops-Limnohyops-Telmatherium group." (Osborn.) Present determination. — This species is certainly a valid one. The grounds for regarding it as allied to the genus Telmatherium are given on page 345. Telmatherium? altidens Osborn, 1908 Cf . Telmatherium altidens Osborn, this monograph, page 351 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 614, fig. 18, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — Uinta Basin, northeastern Utah; Diplacodon-Protitanotherium-Epi- hippus zone (Uinta C). Holotype. — A lower jaw with dentition (Am. Mus. 2025) discovered by the American Museum expedition of 1895. (See fig. 127.) Specific characters. — Osborn writes: Pmj-mj, 330 milUmeters; a wide diastema (70 mm.) behind the canines. Canines in male exceptionally elevated (76) and DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 185 The specific characters are more fully given on page 351 of this monograph. Etymology. — "The specific name refers to the high- crowned piercing canine." Figure 127. — Type (holotype) of Telmaiherium? altidens Lower jaw. Am. Mus. 2025. After Osborn, 1908. One-sixth natural size. Present determination. — The species is probably valid. The generic reference is somewhat less certain. (Seep. 351.) Protltanotherium superbum Osborn, 1908 Cf. Protitanotherium superbum Osborn, this monograph, page 379 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, p. 615, fig. 19, 1908 (Osborn, 1908.318). Type locality and geologic Jiorizon. — Uinta Basin, northeastern Utah; Diplacodon-Pi'otitanotherium-E'pi- Jiippus zone (Uinta C) ; probably higher levels. Holotype. — A well-preserved lower jaw with denti- tion (Am. Mus. 2501). (See fig. 128.) Specific characters. — Osborn writes: Pi-m3, 318 millimeters. Canines in males very robust; pi double fanged; postoanine diastema abbreviated; premolar series relatively abbreviated; p2 with very large talonid and crescentic protoconid; ps, p4 with talonid heavy and promi- nent — that is, submolariform — but no entoconid. Ma with hypooonulid sharply constricted off at base. Etymology. — superbum, haughty, arrogant. "The name is given in reference to the great size and pre- sumed power of this Uinta titanothere, which con- siderably exceeds that of the smaller [lower] Oligocene titanotheres." (Osborn.) Present determination. — The species is probably valid. The generic reference is somewhat less certain. (See p. 379.) Telmatherium? incisivum Douglass, 1909 Cf. Sthenodecies incisivus (Douglass), this mono- graph, page 354 Original reference. — Carnegie Mus. Annals, vol. 6, No. 2, pp. 305-307, text figs. 1, 2, 3, pi. 13, fig. 1, 1909; "issued November 6, 1909" (Douglass, 1909.1). Type locality and geologic horizon. — Uinta Basin, Utah, about 3 miles northeast of well 2, from "a thick deposit of sandstone and small gravel evidently of stream origin, near the mid- dle of horizon B." Near the summit of Eohasileus- Dolichorhinus zone (Uinta B 2). Discovered by Mr. J. F. Goetschius. 101959— 29— VOL 1 15 Type. — A skull, lacking the ends of the nasals (Carnegie Mus. 2398). (See figs. 129, 130.) Specific characters. — Douglass writes: I think that this skull represents a different genus from Telmatherium, but I prefer to place it provi- sionally here rather than establish another genus. The skull is broad and short, but not high. The forehead is broad and flat. ' The premaxillaries are oblique, not transverse. The face is short and concave. Apparently there are vacuities anterior to the orbits. Beneath these there is a rounded angle on the malar, but there is no flattened shelf beneath the orbit. The zygomatic arch is spreading and moderately heavy. The sagittal crest is quite high and thin. The superior wings of the occiput are also thin. The brain case^is small; the outward-projecting zygomatic proc- esses of the squamosals shelf -like and broad anteropos- teriorly. The paroccipital processes extend laterally and are continuous with the paramastoid processes posterior to the external auditory meatus and the postglenoid process. The anterior portion of the opening of the posterior nares is between the anterior portions of the last molars. The teeth increase quite regularly in size from p2 to m'. The premolars have heavy cingula. The deuterocones on p" and p^ are oblong anteroposteriorly, while that on p* is high and conical. Measurements [Douglass] Millimeters Length of skull, basal 490 Width of skull 330 Length of dental series 295 Length of molar-premolar series 212 Transverse diameter of i' 21 Anteroposterior diameter of i' 22 Transverse diameter of i^ 27 Anteroposterior diameter of i^ 25 Transverse diameter of i' 22 Anteroposterior diameter ofi^ 25 Transverse diameter of canine 24 Anteroposterior diameter of canine 27 Transverse diameter of p2 22 Anteroposterior diameter of p^ 20 Transverse diameter of p^ 30 Anteroposterior diameter of p^ 24 Transverse diameter ofp* 37 'T\ Anteroposterior diameter of p"" 27 Figure 128. — Type (holotype) of Protitanotherium superbum Lower jaw. Am. Mus. 2501. After Osborn, 1908. One-sixth natural size. Anteroposterior diameter ofm' 44 Transverse diameter of m^ 53 Anteroposterior diameter of m' 46 Transverse diameter of m^ 53 Anteroposterior diameter of m' 46 186 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Etymology. — incisivum, provided with incisors; in allusion to the great size of the superior incisors. Present determination. — The species probably repre- sents a genus distinct from Telmatherium (see p. 353) named Sthenodectes by Gregory. Type. — A skull lacking the posterior portion (Carnegie Mus. 2888). (See figs. 131 and 132.) Specific characters. — Douglass writes: The skull is high, the forehead broad, and the zygomatic arches spreading. The premaxillary region as seen from the front is broad, though the incisors are only moder- ately large. The canines are directed outward. The free nasals are short and moderately broad. Appar- ently the infraorbital foramen is not excessively large. The malar is rounded beneath the orbit and has no protuberance or shelf. The zygomatic arch is not very heavy and is only moderately deep anterior to the glenoid articular surface. It is not nearly so heavy as in Telmatherium uliimum. The opening of the posterior nares extends forward to the middle of the second molars. Their border is rounded and thickened. The incisors are moderately large but not cupped. They are arranged in an oblique line about halfway between a transverse and anteroposterior direction. The crowns of i' and i^ are low. The anterior faces are very convex. There are two posterior flattened surfaces separated by a rounded ridge. There are no cups, but the posterior portion forms a kind of ledge or keel. P is higher and is directed more down- ward. The posterior portion is flattened, and there is a low flat ledge behind the conical cusp. The canine has a moderately high curved crown, on which there are antero-internal and postero-external ridges, pass- ing downward from the base to the apex. There is also a narrow postero-internal ledge. Unless the skull is more crushed laterally than it appears to be, there is a sudden contraction posterior to the canine, so that the first two premolars are much nearer to the median line of the palate than are the canines. The diastema between the canine and p' is about 3 centimeters in length. P' is a simple oblong conical tooth, which has a small antero-internal depression, and a small ridge passes backward from the apex to the posterior por- tion of the rudimentary keel. P 2, 3, and 4 have low cusps. The teeth increase nearly uniformly in width and size from p^ to the last molar. The two outer elements in each are well defined and are subequal in size, although the anterior cusp is slightly the larger. The internal cusp on p^ is small, oblong anteroposteriorly, and is placed far back. The inter- nal cusp on p' is much larger and is crescent-shaped. On p* it is more nearly conical. There are rudi- mentary cingula on the inner faces of the last three premolars. The postero-internal cusp on m' is repre- sented by a low crescent-shaped ridge. FiGUBE 129. — Type (holutype) skull of Telmatherium? incisivum palatal view; A3, Carnegie Mus. 2398. After Douglass, 1906. Ai, Superior view; view. One-fifth natural size. Manteoceras uintensis Douglass, 1909 Cf. Manteoceras uintensis Douglass, this monograph, page 372 Type reference. — Carnegie Mus. Annals, vol. 6, No. 2, pp. 307-310, text figs. 4, 5, pi. 13, fig. 4, 1909; "issued November 6, 1909" (Douglass, 1909.1). Type locality and geologic horizon. — Uinta Basin, Utah, about 5 miles northeast of well 2, from "gray sandstone in red Uinta beds. Lower portion of horizon C." Diplacodon-Protitanotherium-Epihippus zone (Uinta C) . Measurements [Douglass] Millimeters Is, lateral Lgjjgth of skull, anterior portion to glenoid 430 Length of dental series 356 Length of molar-premolar series ^-_-- 247 Length of premolar series 106 Length of molar series 141 Transverse diameter of i' 16 Anteroposterior diameter ofii 18 Transverse diameter ofi^ 16 Anteroposterior diameter of i^ 18 Transverse diameter of i' 20 Anteroposterior diameter ofi^ 22 Transverse diameter of canine 22 Anteroposterior diameter of canine 26 DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 187 Figure 130. — Type (holotype) of Telmatherium? incisivum Lett upper teeth, crown view. After Douglass, 1909. One-half natural size. Millimeters Transverse diameter of p' 12 Anteroposterior diameter of p' 22 Transverse diameter of p^ 21 Anteroposterior diameter ofp^ 28 Transverse diameter of p^ 28 Anteroposterior diameter ofp' 27 Transverse diameter of p* 33 Anteroposterior diameter ofp' 30 Transverse diameter of m' 44 Anteroposterior diameter of m' 40 Transverse diameter ofm.^ 63 Anteroposterior diameter ofm^ 55 Transverse diameter ofm^ 56 Anteroposterior diameter ofm^ 51 Width of palate between canines? 68 Width of palate between first premolars 54 Width of palate between last molars 83 Etymology. — uintensis, in reference to the Uinta Basin. Present determination. — The generic refer- ence to Manteqceras appears to be correct. The species is a valid one. Dolichorhinus heferodon Douglass, 1909 Cf. Dolichorhinus heterodon Douglass, this monograph, page 416 Original reference. — Carnegie Mus. Annals, vol. 6, No. 2, pp. 310-311, text figs. 6, 7, pi. 13, fig. 3, 1909; "issued November 6, 1909" (Douglass, 1909.1). Type locality and geologic horizon. — Uinta Basin, Utah, 6 or 7 miles northeast of well 2; from "upper part of horizon B or lower part of horizon C"; Eohasileus-DolicJiorJiinus zone (Uinta B 2). Type. — A skull lacking the front teeth and both zygomatic arches (Carnegie Mus. 2340). (See figs. 133 and 134.) Discovered by Mr. J. F. Goetschius. The infraorbital foramen is large. The infraorbital shelf is represented by a protuberance, which is thickened on the free Figure 132. — Type (holotype) of Manteoceras uintensis Upper teeth. Carnegie Mus. 2388. After Douglass, 1909. One-third natural size. Specific characters. — Douglas writes: The skull is long, narrow, and moderately high. The face is short and the brain case long. The free nasals are long, the posterior opening of the anterior nares extending well backward toward the orbit. The lower borders of the nasals approach each other, but this is probably in part due to lateral crushing. Figure 131. — Type (holotype) skull of Manteoceras uintensis Carnegie Mus. 2388. After Douglass, 1909. Ai, Palatal view; Aj, view of right side. One-fifth natural size. outer surface. If there were horn cores above the orbit they ■v\'ere very small. The long brain case was apparently arched from before backward, the posterior de- scent to the crest of the occiput being very steep, though this may be somewhat ex- aggerated by crushing. The occipital con- dyles are very large. The median portion of the occiput above them is convex, while above this there is a large concavity. The postglenoid processes are not excessively large. The premolars are small, the last being very decidedly smaller than the first molar. The first premolar is not pre- served, but it was evidently a simple tooth. In the last three premolars there is a lobe or buttress on the antero-external portion of the tooth, which makes the anterior margin oblique. 188 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The inner cusps (deuterocones) are low with rounded summits. They are more nearly opposite the postero-external than the antero-external cusi5. There are inner cingula on p' and p*. The antero-internal cusp in m^ is quite high and m' conical. The postero-internal cusp is due simply to an increase in height of the cingulum. Measurements Millimeters Total length of top of skull 500 From anterior orbit to front of nasals 160 Width of occiput 128 Height of occiput 140 Length of molar-premolar series 190 Length of premolar series 75 Length of molar series 115 Length of p2 20 Width of p2 16 Dolichorhinus longiceps Douglass, 1909 Cf. Dolichorhinus longiceps Douglass, this monograph, page 406 Original reference. — Carnegie Mus. Annals, vol. 6, No. 2, pp. 312-313, text fig. 8; pi. 13, fig. 2; pis. 14, 15, 1909; "issued November 6, 1909" (Douglass, 1909.1). Type locality and geologic horizon. — Uinta Basin, Utah, "about 1^ miles east of well No. 2," from Figure 133. — Type (holotype) skull of Dolichorhinus heterodon Carnegie Mus. 2340. After Douglass, 1909. Ai, Palatal view; As, right lateral view. One-fifth natural size. Millimeters Length of p3 21 Width of p3 20 Length of p< 24 Width of p^ 27 Length ofm' 34 Width of ml 35 Length ofm^ 46 Width of m2 42 Length ofm^ 48 Width of m3 42 Figure 134. — Type (holotype) of Dolichorhinus heterodon Upper premolar series. Carnegie Mus. 2340. After Douglass, 1909. Slightly less than one-half natural size. "the lowest level at which fossils were found in horizon 'B' of the Uinta, about 700 feet below the bottom of the Uinta red beds (horizon 'C')." Eohasileus- DolicJiorJiinus zone (Uinta B 2). Type. — A skull lacking the incisors, part of the dentition, and the basioccipital region (Carnegie Mus. 2347). (See figs. 135 and 136.) Specific characters. — Douglass writes: Phis skull in general outline is very much like that of Dolichorhinus hyognathus, though broader. In describing it I prefer to point out the char- acters which distinguish it from that species. Apparently it is somewhat broader proportionally than that of D. hyognathus. The skull is some- what crushed, but it evidently was not flattened on top. The present specimen had no heavy protuberances or horn cores, though there may Figure 135. — Type (holotype) skull of Dolichorhinus longiceps Top view. Carnegie Mus. 2347. After Douglass, 1909. One-sixth natural size. Etymology. — crepos, difl^erent, or various; bbobs, tooth. Allusion not clear; name possibly given because no two teeth in the superior premolar-molar series are alike. Present determination. — The form is closely allied to D. intermedins, of which it may be the successor. Its specific separateness is somewhat doubtful. have been the slightest beginning of such. There is a rather narrow shelf, or lateral expansion of the malars, with rounded outer borders, beneath the anterior portion of the orbit, but it is not like the infraorbital process of D. hyognathus. The postorbital hook does not appear to have been long or prominent. Evidently the zygomatic arches extend laterally outward more than in the last-named species; the postglenoid processes are not nearly so heavy; the palate is broader; the top of the cranium, though there is no zygomatic arch, becomes narrower anterior to the crest of the occiput. DISCO'V'EEY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 189 The teeth are very similar to those of Dolichorhinus heterodon, so much so that, if only the teeth were known, they might be referred to that species. They, as well as the skul), are larger. Measurements [Douglass] Millimeters Length of top of skull 590 Length of free nasals 150 Length of skull posterior to anterior portion of orbit 393 Width of skull at glenoid articular surface 267 Width at infraorbital shelves 247 Millimeters Length of p' 24 Width of p3 ■. 25 Length of p^ 27 Width of p* 31 Length ofm' 30 Width of m', about 37 Length of m^ 37 Width of m2 44 Length of m^, about • 41 Width of m3, about 43 A2 FiGUKE 136. — Type (holotype) of Dolichorhinus longiceps Carnegie Mus, 2347. After Douglass, 1909. Ai, Palatal view of skull, somewhat less than one-third natural size; Aj, left lateral view of skull, somewhat less than one-third natural size; A3, crown view of right upper premolar series, one-half natural size. Length of molar-premolar series 192 Length of premolar series 88 Length-of molar series 112 Length of p' 15 Width of pi 11 Length of p^ 20 Width ofp2 20 Etymology. — longiceps, in allusion to the long skull. Present determination. — For the reasons stated above it appears that this form is connected with the typi- cal D. Jiyognathus by a skull of intermediate char- acters. Its status as a distinct species is therefore somewhat doubtful. 190 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Sthenodectes Gregory, 1912 Cf. Sthenodectes, this monograph, page 353 Original reference. — Science, new ser., vol. 35, No. 901, p. 545, April, 1912 (Gregory, 1912.1). Subsequent reference. — Riggs, New or little known titanotheres from the lower Uinta formations: Field Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. ^8, June, 1912 (Riggs, 1912.1). Figure 137. — Type (holotype) skull of Mesatirhinus superior Field Mils. 12188. After Riggs, 1912. Side, top, and palatal views. Less than one-fourth natural size Type species. — Telmatherium? incisivum Douglass. Generic characters. — Gregory writes: This, genus is distinguished from Telmatherium ultimum Os- born by the following assemblage of characters: (1) The in- cisors are far larger and more advanced in evolution, i' being closely appressed to its fellowr in the median line, with anterior face elongate, antero-internal tip blunt, median basin large, posterior wall or cingulum very massive, i^ i^ extremely large with low recurved tips and very heavy posterior cingula. (2) The postcanine diastema is reduced or absent. (3) Superior premolars 2, 3, 4 are much more advanced than in T. ultimum, having very heavy internal cingula, pronounced external cin- gula, high slender internal cusps (deuterocones) ; p^ especially is in a relatively advanced stage as compared with T. ultimum. (4) The least tranverse diameters of p* and of the anterior lobe of m' are greater, that of m^ much less, than in T. ultimum. (5) The basicranial region differs in many details, such as the apparent junction of the postglenoid and post-tympanic proc- esses below the auditory meatus. (6) The occiput is low, with a sharp, long sagittal crest. (7) The forehead is relatively wide. (8) The nasals taper dis- tally. From Manteoceras (especially M. uinten- sis) the genus under consideration is dis- tinguished by (1) the form and size of the incisors and canines, (2) the much more advanced stage of evolution of the premo- lars, (3) the shorter anteroposterior diam- eter of m^, (4) the reduction of the post- canine diastema, (6) the arched and spreading zygomata, etc. From Dolichorhinus and Mesatirhinus it is separated by the shortness and relative breadth of the skull, the great size of the incisors, the relatively heavy zygomata, and many other details. Etymology. — adho^, strength, driKT-qs, a biter; in allusion to the great power and development of the in- cisors and canines. Present determination. — A valid genus, offshoot of the typical Telma- therium phylum. Mesatirhinus superior Riggs, 1912 Cf. Dolichorhinus superior (Riggs), this monograph, page 405 Original reference. — Field Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. 26, pi. 6, June, 1912 (Riggs, 1912.1). Type locality and geologic hori- zon. — White River divide, north- eastern Utah; upper " Metarhinus sandstones," summit of Metarhinus zone (Uinta B 1). (See fig. 137.) Holotype.— A skull (Field Mus. 12188). Specific characters. — Riggs writes: Skull 485 by 255 millimeters, molar series 182 millimeters, nasals free to a point over last premolar, infra-orbital process present, arches slender anteriorly, nasals infolded at margins, sagittal area expanded, canines small, p^ and p' oblique to axis of series. Molars relatively small, strong hypocone on m^, pos- terior nares opening opposite the anterior margin of last molar. Etymology. — superior, in allusion to its large size and high stage of evolution. Present determination. — This is a valid stage im- mediately ancestral to the Dolichorhinus stage. DISCOVERY OF THE TITANOTHEEES AND ORIGINAL DESCRIPTIONS 191 Metarhinus riparius Riggs, 1912 Cf. Metarhinus riparius, this monograph, page 429 Original reference. — -Field Miis. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. 28, pi. 7, fig. 1, June, 1912 (Eiggs, 1912.1). Type locality and geologic horizon. — White River canyon and divide, northeastern Utah; "entire upper Metarhinus beds," base of Metarhinus zone (Uinta B 1). Figure 138. — Type (holotype) skull of Metarhinus riparius Field Mus. 12186. After Eiggs, 1912. About one-fourth natural size. Holotype.— Skull (Field Mus. 12186). (See fig. 138.) Paratype {"cotype"). — "Lower jaws" (Riggs, pi. 7, figs. 2, 3). Specific characters. — Riggs writes: Skull long and narrow (405 by 210 mm.). Anterior cranial region expanded, sagittal crest short. Interorbital region relatively narrow and rounded, rudimentary horn cores above orbits, canines large, molar series short (88-93 mm.), hypocone usually present on m', mandible straight in the ramus, lower canine long and recurved. Etymology. — cristatus, crested; in allusion to the high sagittal crest. Present determination. — A valid stage in the Meta- rhinus fluviatilis phylum. Dolichorhinus fluminalis Riggs, 1912 Cf. Dolichorhinus fluminalis, this monograph, page 417 Original reference. — Field Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. 33, pi. 10, figs. 1-3, June, 1912 (Riggs, 1912.1). Type locality and geologic horizon. — Uinta Basin, northeastern Utah; "Amy- nodon sandstone," summit of Eohasileus- Dolichorhinus zone (Uinta B 2). Holotype. — A fine skull. Field Mus. 12205; collector M. G. Mehl. (See fig. 140.) Specific characters. — Riggs writes: Skull small and narrow (520 by 230 mm.), facial region much shorter than cranial, nasals narrow and slightly tapering, posterior nares opening between hamular processes, postorbital process of jugal back of the last molar, molar- premolar series 171 millimeters; canines short and recurved, in- cipient horn cores in the form of high, narrow ridges. * * * The skull is slender, light and complex in structure as com- pared with the massive and rounded D. cornutus. The molar teeth are no longer in the crown than those of Metarhinus Etymology. — riparius, riparian, in allu- sion to the nature of the habitat. Present determination. — A valid species in the Metarhinus phylum. Metarhinus cristatus Riggs, 1912 Cf . Metarhinus cristatus, this monograph, page 429 Original reference. — Field Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. 28, pi. 9, fig. 3, June, 1912 (Riggs, 1912.1). Type locality and geologic horizon. — White River canyon, northeastern Utah; "upper Metarhinus beds," lower section of Metarhinus zone (Uinta B 1). Holotype. — A skull, lacking the muzzle (Field Mus. 12194). (See fig. 139.) Specific characters. — Riggs writes: Skull length approximately 380 millimeters, molar series 94 millimeters. Frontal region broad, sagittal crest long and high, molars short-crowned, no hypocone on m', arches rela- tively heavy. Represented by a single skull lacking the nasals and the premaxillaries. Figure 139. — -Type (holotype) skull of Metarhinus cristatus Field Mus. 12194. After Eiggs, 1912. One-third natural size. earlei. The jugal process of the maxillaries arises at a point back of the last molar rather than beside it as in Z). longiceps. There is no offset in the palate between the last molars, though the primary position of the posterior narial opening is marked by a slight rugosity. D. fluminalis is most nearly related to D. intermedins. The skull exceeds in length the type of that species in the ratio of 520:465 millimeters. The molar teeth are proportionately much smaller; the series measures relatively 99:109 millimeters. 192 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The position of the posterior narial opening is the most distinc- tive character, appearing much farther back in D. fluminalis than in any other described species. The two forms agree more closely in the tapering form of the nasals and in the narrow recess separating them, from the maxillaries. Etymology. — fluminalis, pertaining to rivers; in allusion to the habitat. Present determination. — A stage in the DolichorTiinus phylum, not very clearly distinguished specifically from other progressive stacc?. ,to» Figure 140. — Type (holotype) skull of Dolichorhinus fluminalis Field Mus. 12205. After Eiggs, ] 912. Side, top, and palatal views. About one-fifth natural size Rhadlnorhinus Riggs, 1912 Cf. Rhadinorhinus, this monograph, page 430 Original reference. — Field Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. 36, June, 1912 (Riggs, 1912.1). Type species. — Rhadinorhinus ahbotti Riggs. Generic characters. — Riggs writes : Titanotheres with slender skulls, nasals deeply recessed later- ally and tapering, molars long-crowned, p-- ^- * subrectangular, a wide median area between the incisors, no infra-orbital process. The name Rhadinorhinus alludes to the tapering nasals which characterize this genus. Etymology. — pa8iv6s, slender; pis, nose. Present determination. — Probably a valid stage, an extreme offshoot of the Metarhinus phylum. (See p. 17, fig. 15.) Rhadinorinus abbotti Riggs, 1912 Cf. Rhadinorhinus ahbotti, tliis monograph, page 430 Original reference. — Field Mus. Nat. Hist. Pub. 159, Geol. ser., vol. 4, No. 2, p. 36, pi. 11, figs. 2, 3, June, 1912 (Riggs, 1912.1). Type locality and geologic horizon. — Northeastern Utah; "upper Meta- rhinus beds," center of Metarhinus zone (Uinta B 1). Holotype. — A fine skull (Field Mus. 12179). (See fig. 141.) Specific characters. — Riggs says: Length of skull 435 millimeters, molar- premolar series 168 millimeters, nasals shorter than premaxillaries, thickened at suture, and tapering toward a terminal rugosity. Arches slender, posterior nares open opposite middle of m2. Sagittal crest long and narrow. Hypocone of m' vestigial, diastema short. Etymology. — Named in honor of Mr. J. B. Abbott, of the Field Museum of Natural History. Present determination. — A valid spe- cific stage. Eotitanops gregoryi Osborn, 1913 Cf. Eotitanops gregoryi, this monograph, page 291 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 32, p. 407, fig. 1; p. 411, fig. 4B, September 2, 1913 (Osborn, 1913.400). Type locality and geologic horizon. — Type from Wind River Basin, Wyo., 100 feet above Alkah Creek "red stratum . ' ' Lamhdotherium-Eotitanops- Coryphodon zone (Wind River B, "Lost Cabin"). Type. — An incomplete lower jaw, containing the right lower premolar- molar series (pa-ms), also fragments of left maxilla containing m", m^ (Am. Mus. 14889). (See fig. 142.) Specific characters. — Osborn writes: Of inferior size. P2-m3, 78.4 millimeters; mi_3, 49; P2-3 with the internal cusps, paraconid and metaoonid, consisting of rectigradations of most rudimentary stage; hypoconulid of ms very small; m^ with a single internal cone, no hypocone. This very sharply defined species may represent a persistent primitive stage, because its recorded (Granger) geologic level, 100 feet above the Alkali Creek "red stratum," is higher than that of the DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 193 typical and relatively progressive E. horealis. Its primitive condition is shown in the comparison of the premolars with the same teeth in E. horealis (Cope) and E. princeps Osborn. The third inferior premolar is seen to be much less progressive than in E. princeps or even in Lamhdo- FiGURE 141. — Type (holotype) skull of Rhadinorhinus abboiti Field Mus. 12179. After Riggs, 1912. About one-fourth natural size. tJierium; the other premolars are also very primitive. P2 short, compressed, with a very rudimentary hypo- conid; ps laterally compressed, hypoconid distinct, paraconid, metaconid, and entoconid extremely rudi- mentary rectigradations. In the molar teeth, mi_3, the metastylid and entostylid are also in an extremely rudimentary or rectigradational stage. In ms the hypoconulid is small, subconic, external in position. Etymology. — Named in honor of Dr. W. K. Gregory, of the American Museum of Natural History, the colleague of the author in the preparation of this monograph. Present determination. — A valid specific stage. Eotitanops princeps Osborn, 1913 ('f. Eotitanops princeps, this monograph, page 295 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 32, pp. 410-411, fig. 4E, September 2, 1913 (Osborn, 1913.400). Type locality and geologic horizon. — Wind River Basin, Wyo.; Lamidotherium-Eotitanops-CorpJiyodon zone (Wind River B, "Lost Cabin," exact level not recorded). J. L. Wortman, collector. Type. — Am. Mus. 296, including lower jaw, femur, humerus, right manus, one cervical, three dorsal, and one caudal vertebrae. (See figs. 143, 144.) Specific cJiaracters. — Osborn writes: Of still larger size, pa-ms 105 millimeters (estimated). Inferior premolar teeth some- what more complicated, as shown in the type specimen. P2 with elevated, distinct, but very rudimentary paraconid and metaconid; ento- conid very rudimentary; talonid narrow. P3, paraconid quite distinct, elevated; metaconid small, distinct; entoconid rudimentary ; talonid broad. P4, talonid broad; entoconid distinct. Hypoconulid of ma rounded, more robust. Ramus, larger and more robust. The more advanced development of the premolar rectigradations, the increased size of the teeth and of the jaw, the larger size of the hind feet in the referred specimen (Am. Mus. 4902) combine to distinguish this specimen as a mutation or subspecific stage between E. horealis and E. maJQr. Etymology. — princeps, chief; in allu- sion to its comparatively large size. Present determination. — A valid spe- cific stage. Eotitanops major Osborn, 1913 Cf . Eotitanops major, this monograph, page 296 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 32, pp. 412-413, figs. 5D, 6, September 2, 1913 (Osborn, 1913.400). Type locality and geologic horizon. — • From Alkali Creek, Wind River Basin, Lambdoiherium-Eotitanops-CorypTiodon zone Wyo.; Figure 142. — Type (holotype) teeth of Eotitanops gregoryi Am. Mus. 14889. After Osborn, 1913. A, Left m'-m'; B, right lower premolar series (P2-ms). Natural size. (Wind River B, "Lost Cabin"; exact level unre- corded). Type. — Am. Mus. 14894, a left median metatarsal; also the distal end of the tibia. (See fig. 145.) 194 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Specific cJiaracters. — Osborn writes: Of superior size, Mts III 104 millimeters longitudinal, 16 transverse, index 15. This ill-defined species indicates the existence in Wind River times of a relatively large, short-footed titanothere, v^hich is Figure 143. — Lower jaws of Lambdotherium and EoHlanops A, Lambdothenum popoagicum; B, Eotitanops gregoryi (holotype); C, Eoiiianops browniamis; D, Eotitanops boreatis; E, Eotitanops princcps (type). .One-fourth natural size. After Osborn, 1913. possibly ancestral to some of the short-footed middle Eocene types. The comparative measurements with the median metatarsal of E. borealis are as follows: E. borealis E. major Mm. 86 13 15 21 Mm. 104 Width of shaft - 16 15 25 Etymology. — major, larger; in allusion to the supe- rior size of this animal compared with others of the same genus. Present determination. — A valid specific stage. Lambdotherium priscum Osborn, 1913 Cf. Lambdotherium priscum, this monograph, page 286 Original rejerence. — Am. Mus. Nat. Hist. Bull., vol. 32, pp. 413-414, figs. 7A, 9A, September 2, 1913 (Osborn, 1913.400). Type locality and geologic horizon. — -Wind River Basin, 3 miles east of Lost Cabin, Wyo.; Lambdo- therium-Eotitanops-Ooryphodon zone (Wind River B). Granger, American Museum expedition, 1905. Type. — Am. Mus. 12822, anterior portion of jaw with P2-P4, nil of right side, also ps, mi, m2 of left side. Rami fragmentary. (See fig. 146.) Specific cJiaracters. — Osborn gives the following description: P2-P4, 25 millimeters. Second and third lower premolars extremely simple, with rudimentary paraconid. Metaconid of p3 rudimentary, placed very low upon slope of protoconid; talonid narrow, depressed, with cingular rudiment of entoconid. The extremely simple or primitive structure of the second lower premolar clearly distinguishes this stage. A referred specimen (Am. Mus. 14908) is slightly more advanced in the structure of the second lower premolar, but is still much more primitive than the type of L. popoagicum. This specimen was found in the Wind River Basin, Dry Muddy Creek, 18 miles up (Granger, Am. Mus. expedition, 1909). The measurements of these two specimens are: Type (No. 12822) Mm. Second to fourth premolar, inclusive 25 Third premolar, anteroposterior 8 Third premolar, transverse 5 Fourth premolar, anteroposterior 9 Fourth premolar, transverse 6. 5 First molar, anteroposterior 11.5 First molar, transverse 7. 5 First to third molar, inclusive Beferred specimen (No. 14908) 10 7 37 Etymology. — priscus, ancient; in allusion to the primitive character of the species. Present determination. — A valid specific stage. Lambdotherium progressum Osborn, 1913 Cf. Lambdotherium progressum, this mongraph, page 286 Original rejerence. — Am. Mus. Nat. Hist. Bull., vol. 32, p. 415, fig. 8, September 2, 1913 (Osborn, 1913.400).- Type locality and geologic horizon. — Wind River Basin, Wyo. (Alkali Creek, Buck Spring); Lambdo- therium- Eotitanops- Cor yphodon zone (Wind River B). Granger, American Museum expedition, 1909. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 195 Type. — Am. Mus. 14917. Right ramus and sym- physis of jaw containing ps-mz of right side, also left canine. (See fig. 147.) Specific characters. — Osborn writes: P2-P4 16.5 millimeters. Second, third, and fourth lower premolars progressive. Rudiment of metaconid on p2. Pswith Figure 144. — Type (holotype) of Eotitanops Left lower grinding teetii. Am. Mus. 296. After Osborn, 1913. elevated metaconid subequal with protoconid, broad talonid with rudimentary entoconid. P4 with bifid metaconid and distinct entoconid. This species is readily distinguished from both L. priscum and L. popoagicum by the advanced con- dition of p3, which may be described as submolariform. pnnceps Natural size. AM 14894^ Figure 145. — Type (holotype) of Eotitanops major Metatarsal (A) and fragment of tibia (B). Am. Mus. 14894. After Osborn, 1913. A, Median metatarsal: A', posterior view; A', anterior; A', distal; A*, projdmal. B', Distal end of left tibia, anterior view; B^ the same, distal view. All one- half natural size. Measurements of type Millimeters Second to fourth lower premolar, inclusive 26 Second premolar, anteroposterior 8 Second premolar, transverse (trigonid) 4. 8 Third premolar, anteroposterior 9 Third premolar, transverse 6 Fourth premolar, anteroposterior 9. 3 Fourth premolar, transverse 7. 3 First molar, anteroposterior 11.5 First molar, transverse 8. 5 Second molar, anteroposterior 12. 5 Second molar, transverse 9. 5 Etymology . — progressum , progressive . Present determination. — A valid specific stage. Diploceras Peterson, 1914 Cf. Eolitanolherium, this monograph, page 435 Original reference. — Carnegie Mus. Annals, vol. 9, Nos. 1-2, pp. 29-52, text figs. 1-15, pis. 6-10, 1914; "issued August 17, 1914" (Peterson, 1914.1). Type species. — Diploceras oshorni. Generic characters. — Peterson writes: Dentition: I|, C^, P-J, M|; premolar series proportionally long; p5 with two distinct internal tubercles; horn cores well developed; limbs relatively long and slender; tibial trochlea not extended back on the calcaneum. Astragalus high, with long neck, cal- caneal and cuboidal facets laterally located. Etymology. — SittAoj, double ; Kepai, horn. Present determination. — The name Diploceras being preoccu- pied, Eotitanotherium was later substituted. (See below.) The genus itself is probably related to the typical Diplacodon Marsh. Diploceras osborn! Peterson, 1914 Cf. Eotitanotherium oshorni, this monograph, page 435 Original reference. — Carnegie Mus. Annals, vol. 9, Nos. 1-2, pp. 29-52, text figs. 1-15, pis. 6, 7, 1914; "issued August 17, 1914" (Peterson, 1914.1). Type locality and geologic horizon. — On Duchesne River near Myton, Uinta County, Utah; Eohasileus- Dolichorhinus zone (upper levels of Uinta B 2). Type. — Front of skull, lower jaws, portion of pelvis, atlas, portion of axis, fragments of scapula and foot bones, No. 2859 (Peterson, figs. 2, 3, 4, 7, 12; pis. 6, 7, 10). (See figs. 148, 149.) Paratypes. — Front of skull, No. 2858; vertebral column, fragments of ribs, bones of limb and foot, No. 2860; crowns of two upper molars, No. 2860a; hu- merus, No. 2861; tibiae. No. 2862 (Peterson, figs. 1, 5, 6, 8, 9, 10, 11, 13, 14, 15; pi. 8). Figure 146.- -Type (holotype) of Lamhdotherium priscum Am. Mus. 12822. Ai, Anterior part of lower jaw; As, inner view of right pj-p (reversed). After Osborn, 1913. Natural size. Specific characters. — Peterson writes: Alveolar borders of the premaxillaries extending well in front of the canines; nasals long and relatively thin, their anterior portion abruptly turned downward and convex on the anterior border; incisors well in front of the canines and relatively sub- equal in size; canines proportionally small. 196 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Etymology. — Named in honor of Prof. H. F. Osborn. Present determination. — The genus is doubtfully sep- arable from Diplacodon Marsh, but the species differs in the more advanced development of the third upper premolar. FiGUKE 147. — Type (holotype) of Lambdotherium progressum Lower jaw. Am. Mus. 14917. After Osborn, 1913. Natural size. Heterotitanops Peterson, 1914 Cf. Metarhinus, this monograph, page 420 Original reference. — Carnegie Mus. Annals, vol. 9, Nos. 1-2, pp. 53-57, text figs. 1, 2; pi. 11, "issued August 17, 1914" (Peterson, 1914.2). Type species. — Heterotitanops parvus Peterson. Generic characters. — Peterson writes: Dentition: If?, C-}-?, P|?, M|. Deciduous dentition: If?, C-r, Mf?. Rapid increase in size of the deciduous upper cheek teeth from first to last tooth. D * with perfectly formed internal tubercles (proto- and hypocones) and the antero- external angle very greatly developed. Molars hypsodont. Ml with large conical proto- and hypocones, the external faces of the ectoloph less emarginated anteroposteriorly than in the titanotheres generally and the median vertical ridge of the ectoloph projecting forward to a greater degree. Etymology. — erepoj, other, different; Ttrdi', Titan; ajf, face; in allusion to its supposed possible relationship to such forms as Eoti- tanops. Present determination. — According to Dr. W. K. Gregory, who has studied the type specimen of Heterotitanops parvus, the animal probably represents a very young individual of Metarhinus or Rhadinorhinus. Heterotitanops parvus Peterson, 1914 Cf. Metarhinus sp. or Rhadinorhinus sp., this mono- graph, page 198 Original reference. — Carnegie Mus. Annals, vol. 9, Nos. 1-2, pp. 53-57, text figs. 1, 2, pi. 11, 1914; "issued August 17, 1914" (Peterson, 1914.2). ^"'""'° Type locality and geologic horizon. — White River, Uinta County, Utah; base of Metarhinus zone (Uinta B 1). The type specimen "was found articulated in a hard sandstone concretion, and lower down in horizon A ['^J of the Uinta sediment than any mam- " The upper or fosslliferous part of Uinta A of previous reports is Uinta B 1 of this monograph. malian remains hitherto described from that forma- tion." (Peterson.) Type. — Skull, lower javrs, vertebral column, ribs, limb bones, calcaneum, and astragalus of young indivi- dual (Carnegie Mus. 2909). (See figs. 150, 151, 152, 360.) Specific characters. — Not determined. Etymology. — parvus, poor, small. Present determination. — According to Dr. W. K. Gregory the type specimen probably represents a very young individual of an undetermined species of one of the previously described genera of Uinta Basin titanotheres, probably of Metarhinus. Eotitanotherium Peterson, 1914 (To replace Diploceras Peterson, 1913, preoccupied) Cf. Eotitanotherium, this monograph, page 435 Original reference. — Carnegie Mus. Annals, vol. 9, p. 220, September 12, 1914 (Peterson, 19U.4); Eotitano- therium, a new generic name to replace Diploceras Peterson. (See Peterson, 1914.1.) In my article entitled "A new titanothere from the Uinta Eocene" I employed the generic name Diploceras, having overlooked the fact that this name is already preoccupied, having been employed by Conrad as early as 1844 to designate a genus belonging to the Mollusca. For this name I now sub- stitute the name Eotitanotherium, which, after a diligent search of the literature, I believe is not preoccupied. (Peterson.) Etymology. — ^cos, dawn; Ttrav, a Titan; drjpiov, a beast. FiGUEE 148. — Type of Diploceras osborni I lower jaw. Carnegie Mus. 2859. After Peterson, 1914. One-fourth natural size. Present determination. — The genus is doubtfully separable from Diplacodon Marsh. Telmatherium? birmanicum Pilgrim and Cotter, 1916 Cf. Telmatherium f birmanicum, this monograph, pages 196-199 Original reference. — India Geol. Survey Records, vol. 47, pt. 1, pp. 72-74, pi. 5, figs. 9-11, 1916 (Pilgrim and Cotter, 1916.1). DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 197 Type locality and geologic Jiorizon. — Myaing Town- ship of the Pakokku district, Burma; Pondaung sand- stone (upper to middle Eocene). Cotypes. — Pilgrim and Cotter write: This species is represented by five fragments of upper molars, two of which are ahnost identical in shape and com- prise the antero-internal quarter of two of the upper molars probably occupying successive positions in the maxilla and ably more behind the level of the paracone than is the case in the Chalicotheriidae; thirdly, because in pm^ there is a single large rounded and isolated inner cusp — the protocone, which is totally unconnected with the two main outer cusps — a condi- tion which never occurs in any chalicotheroid. In that family the protocone in the premolars is connected to the outer cusps either by a single or by a double crest. In addition to these specific differences, the general structure of the tooth is unlike that of any chalicotheroid that is known to us. Figure 149. — Type of Diploceras {Eolitaiiotherimn) osborni Palatal view. Carnegie Mus. 2859. After Peterson, 1914. One-half natural size. being either m^ and m' or m' and m', two other portions of the wall of the external crescents, and another an isolated proto- cone. A sixth fragment consists only of the internal half of what we take to be the last upper premolar. Three of these pieces are figured in Plate 5, Figure 11 [9-11]. (See fig. 153.] Systematic characters. — Pilgrim and Cotter write: It is obvious that these are not chalicotheroid; first because there is no trace of a protoconule, which in the Chalicotheriidae is always present between the protocone and the paracone, being invariably united to the latter by a transverse crest; secondly, because the protocone in our specimens lies consider- On the other hand, it approximates so nearly to that of many of the Titanotheriidae that we have no hesitation in assigning these fragments to that family. A careful comparison with the various known species of the Titanotheriidae convinces us that the Burmese fragments belong to a new species, but whether this is to be referred to one of the known genera of that family or whether it belongs to a new genus is a point which the material at our disposal is insufficient to enable us to deter- mine. We shall therefore do no more than indicate its prob- able affinities, leaving a definite conclusion to the future, when we may hope that more abundant material may come to fight. 198 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA One of the most crucial points which has presented itself to us for decision in connection with the material belonging to this species is the position in the jaw of the tooth (G. S. I. No. C. widening which we must assume to have taken place in m' of this species. Again the faint V-ing of the line which connects the two external crescents points to these being more closely FiGUEE 150. — Type (holotype) skeleton of Heterotitanops parvus Carnegie Mus. 2909. After Peterson, 1914. One-fourth natural size. 315) figured in Plate 5, Figure II. Although in some respects this specimen reminds us of the last upper molar in some of the FiGtTBE 151. — Type (holotype) skull of Heterotitanops parvus Carnegie Miis. 2909. After Peterson, 1914. One-half natural size. upper Eocene members of the Palaeosyopinae, yet its small size as compared with the two other specimens of the upper connected than is the case in the last upper molar of a titano- there. On the other hand these features are such as the last upper premolar of that family would present, the only peculiar- ities being the rounded nature of the inner cone and the highly de- veloped cingula on the anterior and posterior margins of the frag- ment, dying away internally and apparently also on either side of the two main external cusps. It is evident that this simple struc- ture of pm* prohibits the possibility of this species being one of theTitan- otheriinae of the Oligocene, while on the other hand the increased development of the cingulum and the absence of an intermediate tubercle point to its representing one of the latest develop- mental stages of the Eocene subfamily of the Palaeosyopinae. A similar indication is afforded by the fragmentary upper Figure 152. — Type (holo- type) of Heterotitanops parvus Upper and lower teeth. Carnegie Mus. 2909. After Peterson, 1914. 1, Deciduous upper pre- molars, first permanent molar; 2, permanent mi. One-half natural size. Figure 153. — Cotypes of Telmatherium? birmanicum In the collection of the Geological Survey of India. After Pilgrim and Cotter, 1916. Naturalsize. A, "The antero-internal portion of a right upper molar, surface view"; B, "e-xternal portion of an upper molar, showing the gently rounded median fold, external view"; C, "internal portion of last upper premolar, surface view." molars militates against this view. Further, the almost rectangular shape of the inner portion of the tooth, which alone is preserved to us, is inconsistent with the external molars, in which the protocone is rather lofty and the only vestige of a protoconule is the presence of a minute row of fringing the protocone between it and the paracone. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 199 These start from the prominent cingular protostyle and cul- minate in a more elevated portion some 13 millimeters to the rear, diminishing again behind this point. Attention may also be called to the presence in one of the specimens of a broad, gently rounded median fold iu the center of the external paraconal wall of the tooth, although in the other specimen no such fold is visible. According to Earle such a median rib is characteristic of all the early titano- theres, tending to vanish in the upper Eocene and being entirely absent in the Oligocene subfamily of the Titano- Measurements of inferior teeth Pi-m^ and superior teeth m'-m^ Millimeters P2-m3: Huerfano A. L. priscum (ref.), Am. Mus. 17526. 67 Wind River B. L. popoagicum (type), Am. Mus. 4863 69 Wind River B. L. progressum (type), Am. Mus. 14917 (estimated) 71 Huerfano A. L. magnum (type), Am. Mus. 17527. 74 Mi-m^: Huerfano A. L. priscum (ref.), Am. Mus. 17529. 21. 5 Huerfano A. L. priscum (ref.). Am. Mus. 2688.. 22. 5 Wind River B. L. popoagicum (ref.), Am. Mus. 14902 25 Huerfano A. L. progressum (ref.), Am. Mus. 17530 23.5 Wind River B. L. magnum (ref.). Am. Mus. 15600 27.5 These measurements show that there is not a great range in size between the smaller and the larger animals referred to this genus. Etymology. — magnum, large. Present determination. — A valid specific Figure 154. — Type (holotype) of Lambdotherium magni Lower jaw. Am. Mus. 17527. After Osborn, 1919. Natural size. theriinae. In any case the external lobes are broad and flat and considerably elevated, hli;e those of the latest members of the Palaeosyops-Diplacodon phyla. Perhaps taking everything into consideration the present species shows greater affinities with Telmatherium than any other known titanotherid genus. Etymology.- — Mrmanicum, relating to Burma. Present determination. — Position uncertain. The very close beading and massive cones of the single grinding tooth figured suggest comparison with Palaeosyops, a progressive species like P. copei. These teeth might belong to a chalicothere, such as Macrotherium or Moropus, but the resemblance is not close. Lambdotherium magnum Osborn, 1919 Cf. Lambdotherium magnum, this monograph, page 288 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 41, p. 562, fig. 3, 1919 (Osborn, 1919.494). Type locality and geologic Jiorizon. — Lower horizon of the Huerfano formation (Huerfano A) of Colorado. Specific characters. — Osborn writes: Exceeding in size any other known lambdothere is the type jaw (Am. Mus. 17527) from the Garcia Canon, lower Huerfano, containing a complete inferior series, p2-m3 of both sides, represented in Figure 3. (1) These teeth exceed in length over all (74 mm.) those of the type of L. popoagicum, in which the same teeth measure 69 millimeters. (2) P3 has a rudimentary metaconid and paraconid, in the same stage of evolution as in L. popoagicum. (3) Of similar large size is a referred specimen. Am. Mus. 15600, from the Big Horn, west end of Tatman Moun- tain. "These_ referred grinders, m', m^, coincide closely in size with the type of L. magnum and may be regarded as a paratype. [See fig. 154.] stage. Eotitanops minimus Osborn, 1919 Cf. Eotitanops minimus, this monograph, page 296 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 41, p. 564, fig. 4, A, A\ 1919 (Osborn 1919.494). Type locality and geologic Jiorizon. — Two miles north of Gardner, Huerfano Basin, Colorado; from the lower level of the upper horizon of the Huerfano formation (Huerfano B). Specific characters. — Osborn writes: In reference to the fact that it is the smallest true titanothere known, these type lower molar teeth, pi-ms. Am. Mus. 17439 (fig. 4, A, A'), * * * are assigned a new specific name on the following grounds: (1) The measurement of p4-m3 (53 mm.) is much less than that (58) of the corresponding teeth E.minlmus, Type Figure 155. — Type (holotype) of Eotitanops minimus Lower teeth. Am. Mus. 17439. After Osborn, 1919. A, Lingual or internal view; A^, crown view. Natural size. in E. gregoryi; (2) the other characters are so similar to those of E. gregoryi as to suggest that this is a related form. [See fig. 155.] The accompanying figures (fig. 4, A, B, C) exhibit the dimensional proportions of the above species of Eoiilaiiops. It has been found from the large number of measurements of Eocene titanotheres that no single species exhibits so great a range of size. Etymology. — minimus, small. Present determination. — ^A valid specific stage. 200 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Eometarhinus Osborn, 1919 Eometarhinus huerfanensis Osborn, 1919 Cf. Eometarhinus, this monograph, page 419 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 41, pp. 568, 569, 1919 (Osborn, 1919.494). Generic characters. — Osborn writes: Small; ancestral to Metarhinus; with rudimentary frontonasal horn; nasals elongate; overhanging premaxillaries, decurved as Cf. Eometarhinus huerfanensis, this monograph, page 420 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 41, pp. 567-569, fig. 6, 1919 (Osborn, 1919.494). Type locality and geologic horizon. — Huerf ano-Muddy divide, 3 miles west of Gardner, Huerfano Basin, Colo.; Huerfano formation, 205 feet below top (Huerfano B). Figure 156. — Type (holotype) skull of Eometarhinus huerfanensis 11. Mus. 17412. After Osborn, 1919. A, Nasals, superior view; Ai, Aj, sections; B, skull, view of left side; C, right upper jaw and teeth. One-half natural size. in Metarhinus; no infraorbital shelf; characters apparently in- termediate between those of the Metarhinus and Mesatirhinus phyla. Etymology. — rjcos, dawn; Metarhinus, a genus of the middle Bridger beds; indicating an ancestral form of Metarhinus. Present determination. — This genus appears to be ancestral to the Dolichorhinus phylum. Type. — Anterior portion of skull (Am. Mus. 17412). (See fig. 156.) Specific characters. — Inferior in all measurements to Mesatirhinus megarhinus. Premolars with small deu- terocone. p'-m', 124 millimeters ; p'-p*, 53; m'-m', 72. Etymology. — huerfanensis, in allusion to type locality. Present determination. — A valid specific stage. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 201 SECTION 3. ORIGINAL DESCRIPTIONS OF TYPES OF OLIGOCENE TITANOTHERES IIST OF GENERA AND SPECIES The following list shows that 49 species of Oligocene titanotheres in North America and Europe have been described and made the types of 17 different genera, of which seven are regarded as valid. The types come from many geologic levels. In finally determining the genera wo are reluctantly compelled to adopt Menodus Pomel in preference to Titanotherium Leidy, to adopt Megacerops Leidy although it is based on a poor type, and to reject Symhorodon Cope, because the genotype species belongs to Menodus. The genera that rest on the genotypic specimens are Brontotherium Marsh and Brontops Marsh. Diplacodon Marsh is the least soundly determined. It is close to Brontops and may represent a sport. Teleodus Marsh represents an in- ferior stage of the Brontops phylum, transitional to Protitanotherium. Chronologic list of the genera and species of Oligocene titanotheres [Generic names accepted in this work as valid are printed in small capitals; abandoned names are inclosed in brackets.] Date 1846 I 1849 1 1849 2 1850 II 1852 3 1852 4 1852 III 1853 6 1854 IV 1860 V 1870 6 1870 VI 1873 7 1873 VII 1873 8 1873 VIII 1873 9 1873 10 1873 11 1873 12 1873 13 1873 14 1873 1.5 1873 16 1874 IX 1875 17 1875 X 1876 18 1886 19 1887 20 1887 21 1887 XI 1887 22 1887 23 1887 XII 1887 24 1887 XIII 1887 25 1887 26 1887 XIV 1887 Present determination ["Gigantic Palaeotheri- um."] Menodus Menodus [Palaeotherium(?)] giganteus. proutii [Titanotheriu m] [ Palaeothieriu m] [Rhinoceros] [Eotherium.] (Type Rhi- noceros americanus Leidy.) [Palaeotherium] [Leidyotherium] Megacerops Megacerops Brontotherium Brontotherium [Symborodon] [Symborodon] [Miobasileus] [ Miobasileus] Megaceratops [ Megaceratops] [Symborodon] [Symborodon] [Symborodon] [Brontotherium] [Symborodon] [ Anisacodon] [Anisaoodon] [Diconodon (not Anisa- codon).] [Menodus] [ Menodus] [maximum], [americanus] [giganteum]. coloradensis. gigas torvus. [ophryas] acer heloceras bucco [altirostris] . _ trigonoceras- [ingens] hypoceras [montanus]. angustigenis. tichoceras [Menodus]- [Menodus]_ dolichoceras _ platyceras 101959 Brontops Brontops Brontops [M'enops] [Menops] [Titanops] [Titanops] [Titanops] Allops —29— VOL 1 16 robustus- dispar curtus.. [elatus]_ Prout- Pomel _ do_ Owen, Norwood, and Evans. Leidy do _do_ -do. do_ Prout_. Leidy- . do_ Marsh. do_ Cope.- do. .do_ .do. _do. .do. _do. .do_ do. Marsh. Cope.- Marsh. do. do. Cope Scott and Osborn. .do. Marsh. do. do. .do. .do. .do. .do. -do. .do. Menodus Pomel. Do. Menodus giganteus Pomel. Menodus proutii (Owen, Norwood, and Evans). Do. (Indeterminate.) Do. Subfamily Menodontinae, genus in- determinate. (Indeterminate.) Do. Megacerops Leidy. Megacerops coloradensis Leidy. Brontotherium Marsh. Brontotherium gigas Marsh. Menodus Pomel. Menodus torvus (Cope). (Indeterminate.) Do. Megacerops acer Cope. Menodus heloceras (Cope). Megacerops bucco (Cope). Megacerops acer Cope. Menodus trigonoceras (Cope). Menodus giganteus Pomel. Brontotherium hypoceras (Cope). (Indeterminate.) Menodus giganteus Pomel. Menodus giganteus? Pomel. ?Brontops angustigenis (Cope). Brontotherium tichoceras (Scott and Osborn) . Brontotherium dolichoceras (Scott and Osborn). Brontotherium platyceras (Scott and Osborn) . Brontops Marsh. Brontops robustus Marsh. Brontops dispar Marsh. Menodus Pomel. Menodus varians (Marsh). Brontotherium Marsh. Brontotherium curtum (Marsh). Brontotherium gigas Marsh. Allops Marsh. 202 TITANOTHERES OP ANCIENT "WTOMING, DAKOTA, AND NEBRASKA Chronologic list of the genera and species of Oligocene titanotheres — Continued [Generic names accepted in ttiis work as valid are printed in small capitals; abandoned names are inclosed in brackets.] Present determination 27 XV 28 29 XVI 30 XVII 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 1887 1S89 1889 1889 1890 1890 1890 1890 1891 1891 1891 1891 1892 1896 1902 1902 1902 1902 1905 1908 1908 1908 1908 1913 1916 1916 serotinus. selwynianus . syceras amplus. AUops [Haplacodon] " [Menodus] [Menodus] DiPLOCLONUS Diploclonus Teleodus Teleodus Allops Brontops [Titanops] [Menodus] [Menodus(?)] [Titanotherium] [ Megacerops] [ Megacerops] [Megacerops] Brontotherium [Megacerops] Brontotherium [Symborodon] [ Megacerops] j primitivus — Megacerops assiniboiensis [Titanotheriu m] | [bohemicum] _ Allops ' walcotti Megacerops j riggsi Marsh. Gope_- do_ avus crassicornis [validus] medius [peltoceras] rumelicus ramosum braehycephalus - bicornutus marshi leidyi tyleri hatcheri copei do_ Marsh. do- .do. .do. -do. do_ do_ Cope.. Toula.. Osborn. do. .do. .do. .do. LuU... Osborn. do- Lambe.- do.. Kiernik. Osborn.. do_. Allops serotinus Marsh. Allops sp. Diploclonus selwynianus (Cope). ? Megacerops syceras (Cope). Diploclonus Marsh. Diploclonus amplus Marsh. Teleodus Marsh. Teleodus avus Marsh. Allops crassicornis Marsh. Brontops dispar Marsh. Brontotherium medium (Marsh). ? Brontotherium curtum (Marsh). ? Brontotherium rumelicum (Toula). Brontotherium ramosum (Osborn). Brontops braehycephalus (Osborn) . ?Diploclonus bicornutus (Osborn). Allops marshi (Osborn). Brontotherium leidyi Osborn. ? Diploclonus tyleri (Lull). Brontotherium hatcheri Osborn. Megacerops copei (Osborn). Teleodus primitivus (Lambe). Megacerops assiniboiensis Lambe. Menodus giganteus Pomel. AUops walcotti Osborn. Megacerops riggsi Osborn. « Genotype Menodus angastigenis, upper teeth only. See No. 18, above. PROUT'S DESCRIPTIONS OF A FRAGMENTARY lOWER JAW, THE FIRST TITANOTHERE MADE KNOWN TO SCIENCE "Gigantic Palaeotherium," Prout, 1846 Original reference. — Am. Jour. Sci., 2d ser., vol. 2, pp. 288-289, 1 fig., 1846 (Prout, 1846.1). Subsequent references. — Leidy, Description of the remains of extinct IVIammalia and Clielonia from Nebraska Territory, in Owen, Report of a geological survey of Wisconsin, Iowa, and JMinnesota, p. 551, 1852 [Tab. 9, figs. 3, 3a, is not Prout's specimen] (Leidy, 1852.1); The ancient fauna of Nebraska, pp. 72, 114, pi. 16, fig. 1, 1853 (Leidy, 1854.1). Original description. — Dana and Silliman write: Gigantic Palaeotherium. — We have recently received infor- mation from Mr. H. A. Prout, of his discovery of the remains of a Palaeotherium in the Tertiary near St. Louis, and we are also indebted to him for a cast of the jaw, a view of the pos- terior tooth of which is represented below. Mr. Prout is pre- paring a memoir on the subject; and in the meantime we state the following facts from his letter. This fossil was found in the great northwestern Tertiary belt, which is deflected from the north by the Black Hills and which crosses the Missouri River at about latitude 43°. It was accompanied by several Baculites compressus, an Inocera- mus concentricus, a vertebra of a large fish, and some crystallized gypsum. [As noted later by Prout these were from the Creta- ceous and from another locality.] The entire jawbone, judg- ing from the decrease in size of the teeth, must have been at least 30 inches long, which far exceeds in size the Palaeotherium magnum. The face of the posterior tooth is 4^ inches in length; and from the posterior side of the last tooth to the anterior side of the antepenultimate molar of the same side the distance in the specimen is 11 inches. [See fig. 157.] This is the aggregate length, in the line of the jaw, of but three out of seven teeth; and with the most liberal allowance for decrease of size in the other four the whole of the seven could not have measured less than 16 or 18 inches, which is about one-half larger than in the P. magnum. Remarlcs. — This specimen was "the first of the many mammalian remains which have been brought to the notice of the scientific world from the vast Eocene cemetery of Nebraska" (Leidy, 1852.1, p. 551). It was the subject of Prout's second article cited below and was the type of Menodus giganteus Pomel and one of the cotypes of Palaeotherium? proutii Owen, Nor- wood, and Evans (1850.1) and of Titanotherium, proutii. " Fossil maxillary bone of a Palaeotherium," Prout, 1847 Original reference. — Am. Jour. Sci., 2d ser., vol. 3, pp. 249, 250, 1 fig., 1847 (Prout, 1847.1). Subsequent references. — (See p. 204.) Prout's description. — The following notice, written by Dr. Prout himself, is a full description of the same lower jawbone mentioned in his letter of the preceding year: The palaeotherial bone here described was sent to me some time ago by a friend residing at one of the trading posts of the St. Louis Fur Co., on the Missouri River. From information since obtained from him, I learn that it was discovered in the DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 203 Mauvais Terre, on the White River, one of the western confluents of the Missouri, about 150 miles south of St. Pierre, and 60 east of the Black Hills, at a point which would very nearly The fifth and sixth molars (first and second true molars) re- semble the one described, except that they want the third lobe, and the dentine area on the crown of each lobe is much larger. Figure 157. — "Vertical view of the posterior tooth belonging to the lower jaw of Mr. Prout's Palaeotherium" After Prout, 1846. Natural size. The sixth is 33^ inches from front to posterior side. The posterior lobe is 2 inches from the outer to the inner surface and 1% inches long in the line of the jaw. The whole distance on the jaw occupied by the three teeth is 11 inches. In the correspond with the intersection of latitude 43" with longitude 26° west of Washington. The Baculites and the Inoceramus which accompanied it and which I at first supposed belonged to the same locality were found in another formation — probably the Cretaceous- distant about 100 miles, and included in the Grande Detour or Great Bend of the Missouri River. This fossil bone is a fragment of the inferior maxillary of the left side, consisting of the posterior part of the bone, together with the last three molar teeth. The ramus is much fractured and presents an irregular sur- face; yet the general direction of its outline may be made out. The length of this fragment is 15 inches, its depth from the liighest point of the ramus (a) to the lowest (h) is 9K inches: it narrows regularly forward so as to measure only 3}4 inches from the lower sur- face of the bone at (d) to the alveolar process of the antepenultimate tooth at (c). The inner surface of the bone is more uniform, being marked merely by depres- sions for the attachment of muscles. The alveolar por- tion is here very prominent and well rounded, the teeth being planted more than an inch from a vertical line which is tangential to the inner surface of the bone. It is covered in places with a concretionary matter which could not be removed without injury to the specimen; on analysis, this was found to consist chiefly of carbonate of lime, with some alumina, and a small proportion of silex. The last molar tooth has the three lobes of the Pa- laeotheria, as shown in Figure 2. The inner surface is nearly smooth and flat and shows no trace of lobes. The size of the tooth from posterior to anterior sides is 4}/^ inches, of which 1^ inches belong to the anterior lobe, the same to the middle, and 134 inches to the posterior. In an upper view the two larger lobes have a deltoid form, with the sides somewhat convex, and a rounded outer angle. The thickness through from the outer to the op- posite side is 15^ inches. The enamel of the inner side folds over the surface, covering nearly a semicircular space and leaving between it and the edge of the posterior en- amel a subcrescent-shaped space (deltoido-lunate) of den- tine, somewhat concave, which is nearly seven-eighths of an inch broad at its widest part. These crescent-shaped Figure 158. — Original figures of Prout's "gigantic Palaeotherium," the areas of the two lobes are connected by a continuous tract first titanothere discovered of dentine, nearly IJ^ lines wide at the narrowest part; and the same tract continues from the middle lobe to the posterior; upon the latter it does not widen over the in- terior, as the reflexed inner enamel covers the whole of the crown, excepting a narrow space adjoining the posterior enamel. The prominent points of the crown between the lobes project about half an inch; and probably much more in the perfect tooth. After Prout, 1847. A, "Fragment of the inferior maiillary of tlie left side," one-fourth natural size; B, last lower molar on the left side, four-fLIths natural size. largest Palaeotherium hitherto described, the P. magnum, the same teeth occupy a space scarcely one-third that of the Mis- souri animal. St. Louis, December 10, 1846. 204 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA POMEL'S GENUS MENODUS, BASED ON PROUT'S DESCRIPTION AND FIGURE OF THE FRAG- MENTARY LOWER JAW Menodus Pomel, 1849 Cf. Menodus, this monograph, page 522 Original reference. — Bibliotheque universelle de Geneve (Supp.) Arch. sci. phys. nat., vol. 10, pp. 73-75, January, 1849 (Pomel, 1849.1). Type species. — Menodus giganteus Pomel. Original description. — Pomel writes : Ce fossile a 6t6 dScouvert k Mauvais-Terre sur la Riviere Blanche a 43° latitude nord et 26° longitude ouest de Wash- ington, sur le versant occidental du bassin du Missouri. C'est un fragment de mandibule portant les deux dernieres molaires et I'alveole de I'antepenultieme, qui montrent tous les caracteres du genre palaeotherium. La derniere molaire, la mieux oon- servee, indique une espece plus voisine des vrais palaeotherium (dont les P. magnum, medium, etc., sont les types) ou du sous- Malheureusement on ignore I'age du terrain ou ce fossile remarquable a 6t6 decouvert, quoiqu'il soit probable que c'est dans la serie des formations de I'epoque alluviale qu'il faudra le ranger. Cette difference d'age entre ce palaeothere et ceux de I'Europe ocoidentale, ne doit pas etonner, puisque Ton trouve dans I'Amerique du sud, dans des formations de meme age, un animal de la meme tribu (on pourrait dire du meme grand genre), le macrauchenia qui, lui aussi, est d'une taille superieure aux esptices d' Europe. On salt, du reste, que sa derniere molaire inf^rieure n'a que deux collines, comme dans le paloplotherium, et que ses membres sont assez greles, tandis qu'il est probable qu'un animal aussi gigantesque que ce nouveau palaeotherium a ete assez trapu. Nous proposons de designer cette forme animale fossile sous le nom de Menodus giganteus, en la consi- derant comme un sous-genre des palaeotherium. Etymology. — fxrivrj, the moon; 65ovs, tooth; in allu- sion to the crescents of the lower molars. Present determination. — Pomel proposed Menodus as a subgenus of PalaeotJierium, using the latter term in a very comprehensive sense, as later authors would genre plagiolophus, que des anchitherium et des paloplotherium, en ce que la troisieme coUine est bien d^veloppee, et forme un troisieme croissant k la couronne; les autres croissants sont un peu anguleux (croissants deltoides, dit I'auteur). La base de la couronne est entouree d'un petit bourrelet comme dans les palaeotherium d'Europe; mais si le dessin est exact, la maniere dont les croissants principaux se reunissent indiquerait quelque rapport avec ce qui existe chez les anchitheriums et les paloplo- theriums, cette partie dtant plus 6paissie. II serait n^cessaire d'en connaitre une molaire superieure pour fixer sa veritable place; nous serions porte a presumer toutefois, que ce palaeothe- rium est le type d'un sous-genre particulier; car independam- ment de la brievetiS du fAt de la couronne des molaires, sa taille est trop au-dessus de ceUe de nos plus grandes espfeoes euro- p^ennes, pour qu'on puisse admettre sans hesitation son identity subg^nerique avec ceUes-ci. En effet, I'arriere-molaire du palaeotherium magnum est k peine le tiers de celle de I'espece americaine: aussi cette derniere est-elle r^ellement colossale, mesurant 0m,116, dont 0m,032 appartiennent k la troisieme coUine; son ^paisseur est 0m,045. L'os mandibulaire est, comme on devait s'y attendre, tres-robuste; il a 0m,112 de diametre vertical entre les deux arriere-molaires; il s'elargit consid^rablement k la partie du bord inf^rieur situ^e sous la branche montante. Figure 159. — Type of Menodus giganteus Prout's original specimen. After Leidy, 1854. One-tlurd natural size. speak of a family. In 1873 Marsh (1873.1, p. 486) rejected the name Menodus on the ground that it was essentially the same word as Menodon Meyer, 1838, a genus of reptiles (Palmer, 1904.1, p. 410); but, as the two names are spelled differently, according to the modern rules of nomenclature' Menodus Pomel can not be rejected on that ground. As shown below, the type species Menodus giganteus rests upon Prout's specimen, of which an excellent figure was given later by Leidy (1854.1, pi. 16, fig. 1). Menodus giganteus Pomel, 1849 Cf. Menodus giganteus, this monograph, pages 530, 535 Original reference. — See genus Menodus, above. Type specimen. — As noted above, the species rests upon Prout's original specimen, which was figured by Prout in 1847 (1847.1, p. 249, and 1 fig.) and by Leidy under the name Titanotherium proutii in 1854 (1854.1, pi. 16, fig. 1 only). The type may have been destroyed in the "great fire" of St. Louis. DISCOVEKY OF THE TITANOTHEEES AND ORIGINAL DESCRIPTIONS 205 Neotype (Osborn). — A carefully made model, based on Leidy's figures and measurements of the lower jaw, was compared with various specimens of Menodus untn an upper dentition was found (in a skull, Am. Mus. 505) which appears to fit very well the lower teeth of the type. Hence the skull (Am. Mus. 505) has been selected as a neotype of Menodus giganteus. Specific characters. — Not separated from the generic characters in Pomel's description. (See p. 530.) Etymology. — giganteus, gigantic ; because larger than the Palaeotherium magnum. Present determination. — Although Prout's original specimen, the type of Menodus giganteus Pomel, has been lost, Leidy's carefully executed figure of this specimen, together with his measurements and descrip- tions, reveals generic and specific identity with the dolichocephalic titanothere which Osborn in 1902 designated (1902.208, p. 96) Titanotherium ingens Marsh. Titanotherium ingens is therefore to be regarded as a synonym of Menodus giganteus Pomel. Type. — From a study of the foregoing references it is evident that Owen, Norwood, and Evans intended the name Palaeotherium? proutii to cover both Prout's original specimen and "Owen's specimen," discovered by Evans, the lower jaw which was figured by Leidy in 1852 (1852.1, pi. 9, figs. 3, 3a) and is still preserved in the United States National Museum (No. 113; our fig. 160). Prout's specimen is the type of Meno- dus giganteus Pomel; hence, by the method of elimina- tion, Owen's specimen becomes the type of Palaeothe- rium? proutii Owen, Norwood, and Evans. Etymology. — Named in honor of Dr. Hiram Prout. Present determination. — "Owen's specimen" (Nat. Mus. 113) appears to represent a Menodus, of a stage slightly smaller than M. trigonoceras. (See p. 528.) Titanotherium Leidy, 1852 Cf. Menodus, this monograph, page 522 Original reference. — "Palaeotherium? proutii Owen, Norwood, and Evans," Owen, Eeport of a geological Y 'pvs, eyebrow*; possibly in allusion to the form of the orbit. Present determination. — Owing to the loss of the type and the uncertain character of the description, this genus and species is indeterminate. Megaceratops Cope, 1873 Original reference. — Pal. Bull. No. 15, p. 4, "issued August 20, 1873" (Cope, 1873.2). Present determination. — This name Megaceratops was not proposed in order to denominate a new genus but was merely an emendation on etymologic grounds of Leidy's term Megacerops, of which it must be regarded as a synonym. Megaceratops acer Cope, 1873 Cf. Megacerops acer, this monograph, page 545 Original reference. — Pal. Bull. No. 15, p. 4, "issued August 20, 1873" (Cope, 1873.2). Subsequent reference. — Cope, Report on the verte- brate paleontology of Colorado, p. 488, pi. 7; pi. 8, fig. 3, 1874 (Cope, 1874.2). Type locality and geologic Jiorizon. — Horsetail Creek, northeastern Colorado; Chadron formation {Titano- therium zone), level not ascertained. Type. — "A single cranium without under jaw." Am. Mus. 6348. (See figs. 167, 170.) Figure 166. — Type (lectotype) jaw of Symhorodon torvus One-sixth natural size. Specific cJiaracters. — Cope writes: Top of head flat, forming a narrow plane between the temporal fossae; latter produced backward. Orbit not inclosed behind, an overhanging superciliary ridge. Nasal exceedingly short and massive, each supporting a large acute horn core, which is connected with its fellow by a ridge at the base and diverges widely from it with an outward and forward curve to the acutely compressed apex. Each horn core about 1 foot long. The top of the head is plane between the orbits, and little concave fore and aft. The zygoma is very deep, and the post- glenoid process well developed. End of nasal bones short and thick but flat. Measurements Millimeters Length of cranium (35 inches) 895 Length from posterior rim temporal fossa to middle of super- ciliary ridge 345 Width front between eyebrows 210 Length horn core on inner side (10 inches) 254 The elemental origin of the horn cores is probably different in this genus from that which exists in Miobasileus. Etymology. — acer, fierce, in allusion to the somewhat ferocious appearance. Present determination. — This valid species, which pertains to the genus Megacerops, is fully described on page 545. 212 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Megaceratops heloceras Cope, 1873 Cf. Menodus heloceras, this monograph, pages 524, 681 Original reference. — Pal. Bull. No. 15, p. 4, "issued August 20, 1873" (Cope, 1873.2). Subsequent reference. — Cope, Report on the verte- brate paleontology of Colorado, pp. 487-488, 1874 (Cope, 1874.2). Type locality and geologic horizon. — Horsetail Creek, northeastern Colorado; Chadron formation {Titano- therium zone), level not ascertained. Type. — "A cranium * * * with nearly com- plete maxillary dentition," anterior teeth and part of frontals wanting. Am. Mus. 6360. (See fig. 168.) Specific characters. — Cope writes: There is a prominent horizontal superciliary ridge without horns, and two short obtuse horn cores on the muzzle. These Figure 167. — Type (holotype) skuU of Megaceratops acer Am. Mus. 6348. After Cope, 1874. One-sixth natural size. diverge outward, the outer sides being flattened and the sum- mits contracted and truncate. They are mere rudiments of the horns seen in M. aceronsor [sic], M. coloradoensis. The molar teeth do not exhibit the T-shaped cross ridges seen in Miobasileus, and the two outer crescents are continuous with each other. Measurements Millimeters Length from posterior rim of temporal fossa to middle of osseous eyebrow 472 Least width of parietal plane 104 Superciliary width 260 Elevation of horn core 50 Etymology. — ^Xos, wart; xepas, horn; in allusion to the wartlike appearance of the horn. Present determination. — The species referable to the genus Menodus. (See p. Symborodon bucco Cope, 1873 Cf. Megacerops bucco, this monograph, page 544 Original reference. — Synopsis of new Vertebrata from the Tertiary of Colorado, p. 11, 1873 (Cope, 1873.3). is valid 524.) but Subsequent reference. — Cope, Report on the verte- brate paleontology of Colorado, pp. 484, 485, pis. 2-4. 1874 (Cope, 1874.2). Type locality and geologic horizon. — Horsetail Creek, northeastern Colorado; Chadron formation {Titano- therium zone), level not ascertained. Cotypes. — In the original description Cope says the species is represented "by an imperfect cranium; by a cranium almost perfect, including very probably both mandibular rami, with entire dentition; a frag- mentary skeleton, including parts of cranium, teeth, and vertebrae; and by a series of cervical and dorsal vertebrae." Which of these cotype individuals thus mentioned shall we select as the lectotype? If we should take the first specimen mentioned, namely, the imperfect cranium (known to be Am. Mus. 6346), we find that since it consists of only the posterior portion it lacks most of the characters given in the specific description, except the single one of possessing ex- panded zygomata (hence the name hucco). On the other hand the "cranium almost perfect" (Am. Mus. 6345) also has expanded zygomata and was evidently the chief specimen, since it furnished most of the specific characters and measurements given in the origi- nal description; moreover, in Cope's fuller report (1874.2) it was figured in Plates 2, 3, 4, imder the name Symhorodon hucco, and in the key to the species (p. 484), in which S. hucco is contrasted with S. altirostris, the diagnostic characters (referring to the horns, premolars, nasals, de- pressed cranium) are evidently from the "cranium almost perfect" (No. 6345) rather than from the "im- perfect cranium." Lectotype. — From these clear indi- cations of the author's intention the skull (Am. Mus. 6345) may therefore be regarded as the lectotype. (See figs. 169, 170.) Specific characters. — Cope mentions especially the enormous buccal expansion of the zygomata, the char- acters of the horns, nasals, skull top, orbits, etc. Specific characters are fully given on page 544. Etymology. — hucco, one having extended cheeks. Present determination. — This species is provisionally regarded as a valid one. Symborodon altirostris Cope, 1873 Cf. Megacerops acer, this monograph, page 545 Original reference. — Synopsis of new Vertebrata from the Tertiary of Colorado, p. 12, 1873 (Cope, 1873.3). Subsequent references. — Cope, Report on the verte- brate paleontology of Colorado, p. 486, pis. 5, 6, 8, fig. 1, 1874 (Cope, 1874.2); The Perissodactyla, pi. 33, fig. a, opposite p. 1062, 1887 (Cope, 1887.1). DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 213 Type locality. — Cedar Creek, Logan County, Colo. Type. — A cranium with premolar-molar teeth, zygo- matic arches fractured (Am. Mus. 6350). (See figs. 170, 171.) Characters of type (summarized from Cope). — Nasal bones very short, broad, obtuse, massive, and "stand- ing on a plane above that of the front." Orbit very far forward. Horns straight, with approximated bases Present determination. — As shown (p. 545), there are reasons for regarding the type of S. altirostris as repre- senting a female skull of Megacerops acer. Symborodon trigonoceras Cope, 1873 Cf. Menodus trigonoceras, this monograph, page 528 Original reference. — Synopsis of new Vertebrata from the Tertiary of Colorado, p. 13, 1873 (Cope, 1873.3). 1^ ^^5 ^^^^^N^^^H H Hk ';^^v^H 1 and moderately divergent, subcylindrical at base and compressed inward and forward at the narrow apex. "The first premolar and two incisors are very insig- nificant; canines with short stout crowns." The pre- molars with two smooth cones. Many other charac- ters are given. Etymology. — altus, high; rostris, beak, snout; in allusion to the high position of the nasals. FiGUEE 168. — Type (holotype) skull of Megaceratops heloceras Am. Mus. 6360. After Cope. One-flfth natural size. Subsequent references. — Cope, Eeport on vertebrate paleontology of Colorado, 1874, p. 488, 1874 (Cope, 1874.2); The Perissodactyla, p. 1065, figs. 29, 30, 1887 (Cope, 1887.1). Type locality and geologic Tiorizon. — ^Horsetail Creek, northeastern Colorado; Chadron formation {Titano- therium zone), level not ascertained. 214 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Cotypes. — Skull (Am. Mus. 6355) lacking all the teeth except m^; Am. Mus. 6356, anterior-inferior por- tion of skull, including horns, nasals, right zygoma, and teeth. Of these two cotypes we may regard No. 6355 as the lectotype. (See fig. 172.) Specific characters (summarized from Cope). — A strong basal cingulum, on the inner side of the pre- molars, which is continued in a less prominent form Present determination. — This is a valid described on page 528, referable to Menodus. species, Brontotherium ingens Marsh, 1873 Cf. Menodus giganteus, this monograph, page 530 Original reference. — Am. Jour. Sci., 3d ser., vol. 7, pp. 85, 86, pis. 1, 2, January, 1874; "published Dec. 30, 1873" (Marsh, 1874.1). ■"-^fS^ ^ Figure 169. — Type (lectotype) skull of Symborodon bucco Am. Mus. 6345. After Cope, 1874. One-ninth natural size. The mandible in the upper figure probably does not belong with the skull. between the bases of the cones of the molars. Bases of cones of premolars strongly plicate. Horns tri- quetrous, dii-ected outward and upward. Squamosals not expanded, nasals elongate transversely plane. Etymology. — rpis, three; yuvla, angle; Kepas, horn; in allusion to the three-sided section of the horn. Subsequent reference. — Marsh, The principal char- acters of the Brontotheridae, p. 335, text figs. 1, 2, pis. 10, 11, 1876 (Marsh, 1876.1). Type locality and geologic horizon. — Colorado; Chad- ron formation {Titanotherium zone); exact locality and level not recorded. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 215 Type. — A complete skull; premaxillaries with in- cisors and canines wanting; nasals and horns partly restored. Yale Mus. 2010. (See fig. 173.) Characters. — Marsh says: to the median line. The upper part of the horn cores is rugose, and the base contains large air cavities. The free extremities of the nasals are coossified and much elongated. They are rounded in front, slightly decurved, and the surface at the end is rugose. [Many other characters are listed.] Figure 170. — Type skulls of Symhorodon altirostris (1), S. bucco (2), and Megacerops acer (3) Front views. After Cope, 1874. One-sixth natural size. The most striking peculiarity of this cranium is the pair of huge horn cores on the nasals. They are about 8 inches in length and extend upward and outward. They are triangular at the base, with the broadest face external. The two inner faces of each core are separated by a ridge, which is continued | Millimeters Length of skull from occipital condyles to end of nasals (36 inches) 915 Distance on median line from occipital crest to end of nasals 762 216 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Millimeters Expanse of zygomatic arches 558 Least distance across vertex 157 Space occupied by four upper premolars 162 Space occupied by three upper true molars 266 Space occupied by molar-premolar series 428 Etymology. — ingens, vast. Present determination. — The species is a synonym of Menodus giganteus Pomel. Symborodon hypoceras Cope, 1874 Cf. Brontotherium hypoceras, this monograph, page 562 Original reference. — U. S. Geol. and Geog. Survey Terr. Ann. Rept. for 1873 (Hayden), p. 491 [no figure], 1874 (Cope, 1874. 2). cores of very different shape described below. (See fig. 174.) Specific characters. — Infraorbital foramen "fiat with a wide external face, instead of being a cylindric col- umn as in S. acer, altirostris, bucco, and ophryas." One of the horn cores "consists of the extremital part. * * * j^g section is a compressed oval narrowed in front; its profile with parallel outlines and a little recurved and not very rugose. Its size as compared with the rest of the skull is the smallest in the genus, and not more than half the proportions of the S. altirostris." Another fragment Cope deter- mined as a portion of the frontal bearing a "large osseous tuberosity, which consists of a mass of bone FiGTJRE 171. — Type (holotype) skull of Symborodon altirostris Am. Mus. 6350. After Cope, 1874. One-sixth natural size. Type locality. — ?Cedar Creek, Logan County, Colo. Type. — Cope writes: This species reposes on a fragmentary cranium only, which embraces nasal, maxillary, frontal, malar bones, etc., both zygomata, premolar, and parts of molar teeth. These frag- ments were taken out of the matrix by the writer and were found in juxtaposition. They represent parts of the same skuU and, as no other was found in the same bank, are prob- ably without admixture. The only remains representing this type which are now preserved in the American Museum of Natural History (Am. Mus. 6361) include two portions of the malar bones, a fragment of the orbit and infraorbital canal, a fragment of the alveolar region, and two horn coossified with the upper surface as in the horn of the girafi'e." Cope concluded that "it is probable that this species possessed two pairs of osseous proc- esses or cores on each side, the one on the nasal, the other on the frontal bone." The name "hypoceras" doubtless referred to the supposed presence of the second horn core (the rounded tuberosity) behind and below the oval-sectioned horn on the nasals. Cope gives 14 measurements, including the following: Millimeters Length from front of orbit to glenoid fossa (axial) 365 Depth of malar below orbit 20 Length of molars and last three premolars 293 Length of last three premolars 110 Diameter of horn core, transverse 38 DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 217 Fixation of ledotype. — Cope's conclusion that the above-mentioned fragments "are probably without admixture" appears open to doubt. The "frontal tuberosity" referred to is shown by comparison with well-preserved material to be the horn core of the left side of an immature individual resembling Allops marshi, a reference favored by Cope's observation of the wide bridge over the infraorbital foramen, which contrasts with the narrow columnar bridge in Bron- totheriwn and Symhorodon. The oval-sectioned horn core which Cope supposed to be borne on the nasals is a right horn core of very different shape, agreeing closely with that in skull No. 4702, U. S. National Museum, which Osborn selected (1902.208, p. 106) as the neo- type of this species. The oval-sectioned horn core may, therefore, be regarded as the ledotype. Etymology. — invb, below; /cepas, horn; in allusion to the supposed presence of a low horn swelling on the frontal, behind the one on the nasal. Present determination. — As thus interpreted, hypo- ceras is a valid species of the genus BrontotJierium. Anisacodon Marsh, 1875 Cf. Menodus, this monograph, page 522 Original reference. — Am. Jour. Sci., 3d ser., vol. 9, p. 246, March, 1875 (Marsh, 1875.1). Type species. — Anisacodon montanus Marsh (see below). Generic characters (Marsh). — "Dentition: Incisors-?; canines y; premolars f ; molars f. No superior dias- tema. Strong inner basal ridge on upper pre- molars. Last upper molar with two inner cones. No postorbital process." Etymology. — iivtcT-os, unequal; (xktj, point; 66ous, tooth. Possibly in allusion to the unequal develop- ment of the two inner cones on the third upper molar. Present determination. — In view of the strong similarities to Menodus in the vestigial condition of the incisors, in the strong internal cingulum in the premolars, in the shape of the nasals, and in the second internal cone of the third molar, this genus is now regarded as a synonym of Menodus. Anisacodon montanus Marsh, 1875 Cf. Menodus giganteus?, this monograph, page 537 Original reference. — ^Am. Jour. Sci., 3d ser., vol. 9, p. 246, March, 1875 (Marsh, 1875.1). Type locality and geologic horizon. — "Northern Nebraska" (Big Badlands, White River, S. Dak.); Chadron formation {Titanotherium zone); exact local- ity and level not recorded. Type. — A fragmentary skull including the maxil- laries and fragmentary molar teeth. Yale Mus. 10022. (See fig. 175.) Specific characters. ^Mursh. writes: This species is especially distinguished by the emargination of the extremity of the nasals, the short premaxillaries, and 101959— 29— VOL 1 17 the rectangular form of the last upper molar. The inner pos- terior cone of this molar is smaller than the one in front, and quite distinct from the posterior basal ridge. Measurements [selected from Marsh] Millimeters Width of nasals above end of premaxillaries 95 Anteroposterior diameter of last upper premolar 43 Anteroposterior diameter of penultimate upper molar 77 Anteroposterior diameter of last upper molar 84 Etymology. — montanus, dwelling in the mountains. Exact allusion uncertain, unless the badland topogra- phy of South Dakota is thought of as mountainous. \ Figure 172. — Type (holotype) skviH of Sy7nborodontrigonoceras Am. Mus. 6355. One-ninth natural size. Present determination. — In the form of its premolars and third molar as well as in its vestigial incisors this animal resembles Menodus giganteus; the emarginate nasals with processes on either side of the median notch also recall female Menodus skulls. Anisacodon (Diconodon) montanus is probably referable to Meno- dus cf. M. giganteus. "Diconodon non Anisacodon" Marsh, 1876 Cf. Menodus giganteus, this monograph, page 530 Original reference. — Am. Jour. Sci., 3d ser., vol. 11, p. 339, April, 1876 (Marsh, 1876.1). In this paper Marsh gives diagnosis of four genera of Brontothe- ridae. No. 4 is called "Diconodon Marsh (Anisaco- 218 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 173. — Type (holotype) skull of Brontotherium {=Menodus) ingens Yale Mus. 2010. After Marsh, 1S74. About one-sixth natural size. Figure 174. — Type (lectotype) of Symborodon {= Brontotherium) hypoceras Am. Mus. 6361. One-half natural size. Fragment of right horn core: A', front view; A^, rear view; A', top view. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 219 Ann. don). * * * Type D. montanus Marsh." The term Anisacodon had been preoccupied by Anisacodon Marsh, 1872, a genus of insectivores. Etymology. — Sis, double; kuvos, cone; 65ovs, tooth. Present determination. — See remarks under Anisaco- don, above. FIRST NOTICE OF CANADIAN TITANOTHERES BY COPE, 1886 Menodus angustigenis Cope, 1886 Cf. Megacerops angustigenis, this monograph, page 482, fig. 176, Ci Original reference. — Canada Geol. Survey Kept., new ser., vol. 1, p. 81, 1886 (Cope, 1886.1). Subseguent references. — " Haplacodon angustigenis," The Vertebrata of the Swift Current River, II, p. 153, 1889 (Cope, 1889.1); On Vertebrata from the Tertiary and Cretaceous rocks of the Northwest Territory, I, p. 13, pi. 5, figs. 1, 2; pi. 6, figs. 2, 2a; pi. 7, figs. 1, la, la [bis], 1891 (Cop, 1891.2). Type locality and geologic liorizon. — Swift Current River, Assiniboia, Canada; Cypress Hills beds, level not determined. McConnell and Weston, collectors. Cope's cotypes. — Cope writes: This large mammal is represented by numer- ous specimens. I select for present description two maxillary bones from the same skull [fig. 176, A] [Cope, 1891.2, pi. 5, figs. 1, 2], each of which contains the first [fourth] premolar and the true molars; and two lower jaws from second and third individuals [fig. 176, B]. One of these [op. cit., pi. 7, figs. 1, la, la [bis], our fig. 176 C, now regarded as the lectotype] consists of little more than the symphysis. The other [op. cit., pi. 5, fig. 2; pi. 6, figs. 2, 2a] includes part of the symphysis and the left ramus, which con- tains all the molar teeth except the first and last. [See fig. 176.] Lectotype. — Of these semingly coequal types or cotypes, which is to be regarded as the lectotype? The one mentioned first is "the two maxillary bones from the same skull," but the mandibular symphysis (op. cit., pi. 7, figs. 1, la, la [bis]), from which the species evidently takes its name (meaning narrow chin) , is certainly to be selected as the lectotype. Specific cTiaracters. — Cope's description is too long to quote here. He compared Menodus angustigenis with " Symhorodon trigonoceras" and other species and gave numerous measurements. Among the chief characters noted are the contracted shape of the mandibular symphysis and the square outline of the molars. Etymology. — angustus, narrow; gena, chin. Present determination. — As defined from the lecto- type the species is provisionally referred to Megacerops, although its generic reference is uncertain. The maxilla with the dentition belongs to a very different animal. It is apparently referable to Allops sp. (See below.) The lower jaw appears to be referable to Menodus cf . M. proutii. SPECIES DESCRIBED BY SCOTT AUD OSBORN IN 1887 Menodus tichoceras Scott and Osborn, 1887 Cf. Brontotherium tichoceras, this monograph, page 565 Original reference. — Mus. Comp. Zoology BuL., vol. 13, No. 5, p. 159, text figs. 3, 2; 5, 2; 6, 2, 1887 (Scott and Osborn, 1887.1). Figure 175. — Type (holotype) of Anisacodon montanus Yale Mus. 10022. A, Third right upper molar; B, fourth upper premolar and first and second molars; C, alveoli of upper canines and incisors; Di, nasals, top view; Da, nasals, front view. All one-half natural size. Type locality and geologic Tvorizon. — Big Badlands, South Dakota; exact locality and horizon not recorded. S. Garman, collector. Type.^&coit and Osborn describe the type as "a large skull with the dentition complete, lacking the upper part of the horns and the crest of the occiput." Now in the Museum of Comparative Zoology at Cam- bridge, Mass. (See fig. 177.) 220 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Characters (abbreviated from Scott and Osborn). Dentition: I 2, C 1, P 4, M 3. The skull is described as 29 inches [736 mm.] in length; with a narrow and elevated anterior portion; nasals of medium length, with short, obliquely placed horns, zygomatic arch very massive, presenting a bulge in the posterior half which is much less prominent than in S. iucco. Etymology. — relxos, wall; Kipas, horn; possibly in allu- sion to the high connecting crest. Type locality and geologic horizon. — South Dakota; Chadron formation {TitanotJierium zone); exact local- ity and level not recorded. Type. — A skull incomplete in the supraoccipital region; zygomatic arch fragmentary; maxillary, pala- tine, and basioccipital regions much distorted. Now in the Museum of Comparative Zoology, Harvard University. (See fig. 177.) Characters. — Scott and Osborn write: Figure 176. — Cope's cotypes of Menodus angustigenis A, Right maxilla (subsequently made the type of Haplacodon angustigenis), three-sixteenths natural size; B, left halt of a lower jaw (now referred to Menodus sp.) , three-sixteenths natural size; C, symphysis mandibulae (leetotype), one-third natural size (Ci, front; Ci, right side; C3, under side). Present determination. — This species is provisionally referred to Brontotherium, but its exact position in that phylum is uncertain. (See p. 565.) Menodus dolichoceras Scott and Osborn, 1887 Cf. Brontotherium dolichoceras, this monograph, page 572 Original reference. — Mus. Comp. Zoology Bull., vol. 13, No. 5, p. 160, figs. 3, 3; 5, 3; 6, 3, 1887 (Scott and Osborn, 1887.1). Dentition: I ?, C^-, P^, M^. Upper premolars with a faint internal cingulum. Nasal bones extremely short and obtuse. Horns extremely long and powerful, directed obliquely forward and outward, projecting beyond the nasals in side view. The section is suboval at the base, with the long axis obliquely transverse. Cranium very broad and saddle-shaped above the orbits, narrowing somewhat posteriorly. A prominent and overhanging superciliary ridge. Postglenoid and post-tym- panic processes united for a short distance. The skull which we have made the type of this species is much larger and more powerful than Professor Cope's type of jS. acer. The horns are DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 221 longer and more widely divergent at the base. The angle of inclination of the horns and the diminutive proportions of the nasals, as well as the form of the top of the cranium, all bring this specimen near S. acer and separate it from other known species. Unlike S. acer, the horns are not united by a ridge. [This is an error.] The specimen is incomplete in the supra- occipital region, the zygomatic arch is fragmentary, and the maxillary, palatine, and basioccipital regions are much dis- torted. Menodus platyceras Scott and Osborn, 1887 Cf. Brontotherium platyceras, this monograph, page 578 Original reference. — Mus. Comp. Zoology Bull., vol. 13, No. 5, pp. 160, 161, fig. 4, 1887 (Scott and Osborn, 1887.1). Subsequent reference. — The cranial evolution of Tita- notherium, p. 186, fig. 7A, 1896 (Osborn, 1896.110). The specimen figured is not the type. Figure 177. — Anterior part of skulls of (1) " Megacerops colorodensis" (not the type), now referred to Allops marshi; (2) Menodus tichoceras (type) (present determination, Brontotherium tichoceras) ; and (3) Menodus dolichoceras (type) (present determination, Brontotherium dolichoceras) Specimens in the Museum of Comparative Zoology, Harvard University. After Scott and Osborn, 1887. Greatly reduced in size. A, Side views: B, front views, showing the variations in the horns, nasals, and anterior nares; O, top views, showing the nasals and horns, and sections of the bases of the horns. Revised measurements IVIilUmeters Occipital condyles to nasal tips 690 Free length of nasals 46 Free breadth of nasals 90 Outside measurement of horns 310 Anteroposterior diameter of horns 85 Transverse diameter of horns 125 Etymology. — 56\ixoi, long; Kepas, horn. Present determination. — As shown in Chapter VI the present species probably pertains to Brontotherium rather than to Symhorodon. Type locality and geologic horizon. — Big Badlands, South Dakota; Chadron forma'tion (Titanotherium zone, Chadron C); exact locality and level not re- corded. S. Garman, collector. Type. — A pair of horns with the nasal bones at- tached. Now in the Museum of Comparative Zoology at Cambridge, Mass. (See fig. 178.) Neotype.SknW (Am. Mus. 1448). Characters. — Scott and Osborn write: Nasal bones e.xtremely short and obtuse, as in M. dolicho- ceras and M. acer. The inner [posterior] contour of the horns 222 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA is concave; they are greatly flattened anteroposteriorly, with a ridgelilie outer margin, and connected by a well-raised median Figure 178. — Type (holotype) horns of Menodus platyceras In the collection of the ]Museum of Comparative Zoology, Harvard University. After Scott and Osborn, 1887. Greatly reduced. A, Front view; B, cross section: C, side view. ridge. The posterior face is nearly plane, the anterior is con- vex, so that the section of the horn is plano-convex from base SPECIES DESCRIBED BY MARSH IN 1887 Brontops Marsh, 1887 Cf. Brontops, this monograph, page 482 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 326, October, 1887 (Marsh, 1887.1). Type species. — Brontops rohustus Marsh. (See below.) Generic characters. — Marsh writes: The present genus is quite distinct from any of the forms previously described. * * * xhe skull is large and massive, with widely expanded zygomatic arches, and short and robust horn cores, projecting well forward. In general form it re- sembles the skull of Brontotherium but may be readily dis- tinguished from it by the dental formula, which is as follows: Incisors xi canines-}-; premolars |; molars |. The presence of four premolars in each ramus of the lower jaw is a distinctive feature in this genus. This character, with the single, well-developed incisor, marks both the known species [B. robusius, B. dispar]. FitiUHE 179. — Tj'pe (holotype) skeleton of Brontops, robusius Yale Mus. 12048. After Marsh, 1889. One twenty-fourth natural size. to tip. In side view the horns completely overhang the nasals and are slightly recurved. The long axis of the horn section is [almost or quite] directly transverse. Measure7nents Millimeter.'! Outside length of horns 315 Transverse diameter of horns 125 Anteroposterior diameter of horns . 67 The type probably belongs to a young male in which the horns are not fully developed, because the horns increase in width and flatness and the basal section becomes more truly transverse, with age. Etymology. — irXarvs, flat; /cepas, horn. Present determination. — This valid species, described on page 578, belongs in the genus Brontotherium. Etymology. — Brontotherium; &^, face, "having the face or appearance of"; resembling Brontotherium. Present determination.— In 1902 Osborn (1902.208) treated Brontops as a synonym of Megacerops Leidy, but renewed examination of Leidy's figure of M. coloradensis indicates that it is not congeneric with Brontops, which is here regarded as a valid genus. Brontops robustus Marsh, 1887 Cf. Brontops robustus, this monograph, page 492 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 326, October, 1887 (Marsh, 1887.1). Subsequent references. — Restoration of Brontops ro- iustus: Am. Jour. Sci., 3d ser., vol. 37, pp. 163-165, pi. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 223 6, 1889 (Marsh, 1889.1); skeleton and restoration, this monograph. Plates XCVI-CIII, CXCV-CCXXIX. Type locality and geologic horizon. — "Near the White River in northern Nebraska. " "The geological horizon is in the upper part of the Brontotheriwn beds [Chadron formation, Titanotherium zone] " (Marsh). "Upper levels of middle beds at least 60 feet below the top of the upper beds" (Hatcher, 1901). Figure 180. — Type (holotype) lower jaw of Brontops dispar Nat. Mas. 4941. After Marsh, 1887. One-eighth natural size. Type. — A skull and skeleton, Yale Mus. 12048. (See fig. 179.) Specific characters. — Marsh did not formally sepa- rate the specific from the generic characters. He records the fact that the skull is large and massive, with widely expanding zygomatic arches and stout, robust horn cores, projecting well forward. Etymology. — roiustus, robust (that is, strong as an oak, rohur). Present determination. — The genus and species are valid. The species is described also on pages 492-499. Brontops dispar Marsh, 1887 Cf. Brontops dispar, this monograph, page 488 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, pp. 327, 329, figs. 7, 8 (jaw); not figs. 5, 6 (skull), October, 1887 (Marsh, 1887.1). Type locality and geologic horizon. — Found on Hat Creek, Sioux County, Nebr., by J. B. Hatcher, May 14, 1886; Chadron formation {Titanotherium zone), middle level. Type. — "A nearly complete skull with lower jaws and entire dentition." (Marsh.) Nat. Mus. 4941 (skull D). (See fig. 180.) Characters. — Marsh writes: "The skull is less mas- sive and proportionately more elongate than in the type species, and the lower jaw more slender." In the same brief passage Marsh described a young skull (Nat. Mus. 4258) as belonging to the same species; this is a somewhat more primitive type (Brontops brachycephalus) belonging to a younger individual (p. 483). Etymology. — dispar, uneven, probably in allusion to the asymmetrical distortion of the type skull. Present determination. — The species is valid and is now referred to Brontops. Menops Marsh, 1887 Cf. Menodus, this monograph, page 522 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 328, October, 1887 (Marsh, 1887.1). Type species. — Menops varians. (See below.) Generic characters. — Marsh writes: The present genus is most nearly related to Diconodon and in its molar teeth agrees with that form. It differs in the presence of two upper incisors on each side. The superior dentition is as follows: Incisors, 2; canine, 1; premolars, 4; molars, 3. Etymology. — Menodus; &\p,i ace; resembling Menodus (cf. Brontops, above). Present determination. — The incisors are vestigial, the alveoli being very small. The skull presents re- semblance to both Menodus and Allops. The generic reference is to Menodus. Menops varians Marsh, 1887 Cf. Menodus varians, this monograph, page 535 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 328, fig. 9, October, 1887 (Marsh, 1887.1). Type locality and geologic horizon. — " Brontotherium beds of Dakota" (Chadron formation, Titanotherium zone); exact locality and level not stated. George A. Clarke, collector. FiGUKE 181. — Type (holotype) skull of Menops varians Yale Mus. 120G0. Front view. One-eighth natural size. Type. — A well-preserved skull (Yale Mus. 12060). (See fig. 181.) Specific characters.— Not separated by Marsh from generic characters. (See above.) Etymology. — varians, variant; allusion doubtful, but possibly to the somewhat aberrant character of the type skull. Present determination. — The species is valid and is referred to Menodus. 224 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Titanops Marsh, 1887 Cf. Brontolherium, this monograph, page 555 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 330, October, 1887 (Marsh, 1887.1). Type species. — Titanops curtus. (See below.) Figure 182. — Type (holotype) skull of Titanops curtus Front view. Yale Mus. 12013. After Marsh, 1887. One-eighth natural size. Generic characters. — Marsh writes: This genus contains the largest members of the Brouto- theridae and some of the last survivors of the group. They are distinguished from all the other known types by the long, narrow skulls, lofty, flat horn cores, and short nasals. The upper dentition corresponds nearly to that of Brontotherium, but the upper molars have all two inner cones. * * * The nasals are the shortest known in the group. Etymology. — Titanotlierium; u^/, face — that is, like Titanotlierium. Present determination. — The genus is a synonym of Brontotherium Marsh. Titanops curtus Marsh, 1887 Cf. Brontotherium curtum, this monograph, page 574 Original reference. — Am. Jour. Sci. 3d ser., vol. 34, p. 330, fig. 11, October, 1887 (Marsh, 1887.1). Type locality and geologic horizon. — Colorado; e.xact locality not stated but recorded by Hatcher (1901) as from the upper Titanotlierium zone [of Chadron formation]. Type. — A complete skull with teeth (Yale Mus. 12013). (See fig. 182.) Specific characters. — Not separated from generic characters by Marsh. Etymology. — curtus, short; in allusion to the short nasals. Present determination. — The species is valid and is referred to Brontotherium. Titanops elatus Marsh, 1887 Cf. Brontotherium gigas, this monograph, page 567 Original reference. — -Am. Jour. Sci., 3d ser., vol. 34, p. 330, fig. 12, October, 1887 (Marsh, 1887.1). Type locality and geologic horizon. — "Upper Titano. therium zone, South Dakota" (Chadron formation). Type.— A skull and jaw (Yale Mus. 12061). (See fig. 183.) Specific characters. — Marsh writes: The nasals are much longer, and the occipital crest much higher, than in the type species [T. curtus]. The zygomatic arches are unfortunately wanting, but the lower jaw is present, nearly in place. It shows no marked characters different from that of Brontops. Etymology. — elatus, lofty; possibly in allusion to the high stage of specialization. Present determination. — The species is synonymous with Brontotherium gigas Marsh. Allops Marsh, 1887 Cf. Allops, this monograph, page 506 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 331, October, 1887 (Marsh, 1887.1). Type species. — Allops serotinus. (See below.) Generic and specific characters. — Marsh writes: This skull in its general form resembles that of Brontotherium, but differs in having only a single upper incisor, and the last molar has the posterior inner cone more strongly developed. The superior dentition is as follows: Incisor, 1; canine, 1; premolars, 4; molars, 3. In the tj'pe specimen the canine is small, extending but httle below the premolars. There is no diastema. The upper premolars have a very strong inner basal ridge. The nasals are wide, expand forward in the free portion, and are notched in front. The entire length of the skull is 31 inches (79 centi- meters), the distance across the zygomatic arches 21 inches (53 centimeters), and the length of the horn cores about 10 inches (25 centimeters). Figure 183. — Type (holotype) skull of Titanops elatus Front view. Yale Mus. 12061. After Marsh, 1887. One-eighth natural size. Etymology. — aXXos, strange; ciiA, face. Present determination. — Allops is intermediate be- tween Menodus and Brontops and is here regarded as a valid genus. (See p. 506.) DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 225 Allops serotinus Marsh, 1887 Cf. Allops serotinus, this monograph, page 515 Original reference. — Am. Jour. Sci., 3d ser., vol. 34, p. 331, October, 1887 (Marsh, 1887.1). Type locality and geologic horizon. — Quinn Draw, South Dakota, "near the top of the Brontotherium beds," Chadron formation {TitanotTierium zone). Figure 184. — Type (liolotype) skull of Allops serotinus After Marsh. Nat. Mus. 4251. One-seventh natural size. Type. — "A well-preserved skull and various other remains." U. S. Nat. Mus. 4251. J. B. Hatcher, collector. (See fig. 184.) Specific characters. — Not separated from generic characters in original description. Etymology. — serotinus, from sero{1), to bind, connect; possibly because the characters appeared to be more or less annectant with those of other species. Present determination. — The species is valid. It is described on page 515. CANADIAN SPECIES DESCRIBED BY COPE IN 1889 Haplacodon Cope, 1889 Cf. Allops, this monograph, page 506 Original reference. — Am. Naturalist, vol. 23, p. 153, March, 1889 (Cope, 1889.1). (See p. 202.) Type species. — Menodus angustigenis Cope. The genus was founded on the characters of one of the several "types" of Menodus angustigenis, namely, a maxilla containing the fourth upper premolar and the three molars. Generic characters. — Cope writes: It differs from all the genera of the Menodontidae in the presence of but a single internal cusp of the first (posterior) superior premolar, a fact which renders it highly probable that the premolars which precede it in the maxillary bone were similarly constituted. It differs from all other genera of Lambdotheriidae and also from Diplacodon, to which it is allied, in the presence of but two inferior incisors on each side. It is not certain whether it possesses horns or not. Comparative measurements of the type of "Haplacodon" Cope, in millimeters M'-m' P', ap. by tr__ M', ap. by tr_ M\ ap. by tr_ M^, ap. by tr_ 187 38X62 50X52 66X62 65X62 169 35X51 45X51 61X61 60X61 Etymology. — awXoos, simple; aKri, cone; 65ous, tooth; in allusion to the "single internal cusp" of the fourth upper premolar. Present determination. — The upper teeth agree closely in general characters with those of Allops and are intermediate in size between Allops walcotti and Allops marshi. A2 A3 Figure 185. — Type of Menodus selwynianus Coossifled nasal. Ai, Left side; Aj, upper side; A3, under side. Three-eighths natural size. Menodus selwynianus Cope, 1889 Cf. Diploclonus selwynianus, this monograph, page 502 Original reference. — Am. Naturalist, vol. 23, p. 628, July, 1889 (Cope, 1889.2). Subsequent reference. — On Vertebrata from the Tertiary and Cretaceous rocks of the Northwest Territory, I, p. 17, pi. 5, figs. 3, 3a, 3b, 1891 (Cope, 1891.2). 226 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Type locality and geologic horizon. — Swift Current River, Assiniboia, Canada; Cypress Hills beds, level not recorded. Type. — Coossified nasal bones detached from skull. Ottawa Mus. (See fig. 185.) Figure 186. — Type of Menodus syceras Coossified nasal and left horn eoie. After Cope. Ai, Leftside; A2, front; A3, section of left horn. One-half Characters of type. — Cope writes: Represented by a nasal process, which consists of the coossi- fied nasal bones, of peculiar form. They are elongate as com- pared with their width and are vaulted. The lateral borders are nearly parallel, and the extremity viewed from above is rounded. Owing to the thickness of the body, the profile descends abruptly at the extremity, and the convex surface is roughened as though for the attachment of some fixed body, tegumentary or muscular. From this tuberosity the surface descends steeply to a thin border. A short distance posterior to the extremity the lateral margins are decurved, forming the lateral walls of a deep longitudinal median gutter-like nasal meatus, which is deeper than in any other species. The horns are broken off, but the median inferior surface is so little recurved laterally that it is evident that the former were not only small but laterally placed. Length of fragment above, millimeters, 130; length of nasal border, 70; width at nasal notch, 80; width near extremity, 65; depth at apical tuberosity, 26. Additional ohservations. — The lower surface of the horns in the type exhibits a portion of the frontal sinus. The nasals are shorter than in the type of M. coloradensis. The measurements are as follows: Millimeteis Free length of nasals 80 Free width of nasals 101 Outside measurement of horns 67 Anteroposterior measurement of horns 79 Etymology. — "This species is dedi- cated to Dr. A. R. C. Selwyn, the accomplished Director of the Survey of Canada." (Cope.) Present determination. — The species is probably allied to Diploclonus hicornutus (Osborn). Menodus syceras Cope, 1889 Cf. Megacerops syceras, this monograph, page 549 Original reference. — Am. Natu- ralist, vol. 23, pp. 628-629, July, 1889 (Cope, 1889.2). Subsequent reference. — Cope, On Vertebrata from the Tertiary and Cretaceous rocks of the Northwest Territory, I, p. 18, pis. 7, fig. 2; 8, figs. 4, 5, 1891 (Cope, 1891.2). Type locality and geologic hori- zon. — Swift Current River, Assiniboia, Canada; Cypress Hills beds, level not recorded. The nasal bones of three individuals present the characters above given." Of these we may select as the lectotype the specimen figured by Cope (1891.2, pi. 8) that shows the character from which DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 227 the name syceras is derived, in reference to the approxi- mation of the horns at their bases. Portion of right frontal, coossified nasals, and right horn. (See fig. 186.) Characters of type. — Cope writes: It differs from the two species of that group now known, the M. proutii Leidy and the M. Hchoceras S. and 0., in the very close approximation of the basis of the horns and the pres- ence of a strong angle or ridge connecting them, so that the nasal bones are in a different plane from that of the front. The entire width of the skull at the basis of the horns is not greater than the length of each horn above the nasal notch. The horns are not long, and the section of their base is a longitudinal oval, flattened on the external side. Summit subround. The nasal bones are fiat, with broadly rounded extremitj', and are much wider than long. The width of the nasals at the base of the horns is 116 milli- meters; length of do. from do., 70; diameters of bases of horns; anteroposterior, 94; transverse, 67; length of horn from nasal notch, 160; width of muzzle at bases of horns inclusive, 160. Etymology. — aiiv, together; /cepas, horn; because the horns were set very near to each other at the base. Present determination. — M. syceras is at present indeterminate or possibly a synonym of M. angustigenis, both are provisionally referred to the genus Megacerops. LAST FIVE SPECIES DESCRIBED BY MARSH, 1890-91 Diploclonus Marsh, 1890 Cf. Diploclonus, this monograph, page 499 Original reference. — Am. Jour. Sci., 3d ser., vol. 39, p. 523, June, 1890 (Marsh, 1890.1). Type species. — Diploclonus amplus. (See below.) Characters. — Marsh writes: One of the most marked features is seen in the horn cores, which are high, compressed transversely, and have a prominent knob on the inner superior margin about one-third of the dis- tance to the summit. Seen from the front the horn cores thus appear to be branched. It is probable that in life this feature was still more evident, and the covering of the horn core may have shown an actual division, but this can not be determined from the present specimen. There is a sharp ridge at the base of the horn cores on the outside. The nasals project but very little in front of the horn cores. The zygomatic arches are especially strong and widely expanded. The posterior nares have their front margin opposite the back of the last upper molars. There were apparently but two upper incisors — that is, one on each side — and no diastema exists behind the canines. The premolars have a strong inner basal ridge, and the last upper molar has two inner cones. This genus appears to be most nearly related to T'iianops, but the horn cores will distinguish it readily from all known forms of the Brontotheridae. Etymology. — 5t7rX6os, double; kXwv, a twig; in allusion to the branched appearance of the "horn core." Present determination. — The genus is now regarded as valid by Osborn. Diploclonus amplus Marsh, 1890 Cf. Brontops amplus, this monograph, page 504 Original reference. — Am. Jour. Sci., 3d ser., vol. 39, p. 523, June, 1890 (Marsh, 1890.1). Type locality and geologic horizon. — South Dakota; " Brontotherium beds" (= Chadron formation, or Titan- otherium zone). Type. — "Nearly complete skull, in good preserva- tion, but without the lower jaws." Yale Mus. 12015a. (See fig. 187.) Specific characters. — Marsh writes: The skull measures 28 inches from the front of the nasals to the back of the occipital condyles and 24 inches in greatest width across the zygomatic arches. The space occupied by the upper dental series is 13J^ inches, and by the true molars 8 inches. Etymology. — amplus, broad, in allusion to the great breadth of the skull. FiGUEE 187. — Type skull of Diploclonus amplus After Marsh. One-eighth natural size. A', Side view; A', front view Present determination. — This species is cither an aberrant stage in the evolution of Brontops — that is, a sport — or a lateral phylum of Brontops. Teleodus Marsh, 1890 Cf. Teleodus, this monograph, page 481 Original reference. — Am. Jour. Sci., 3d ser., vol. 39, p 524, June, 1890 (Marsh, 1890.1). Type species. — Teleodus avus Marsh. (See below.) Generic characters. — Marsh writes: The present genus is allied to Brontotherium but differs from it in having six lower incisors instead of four. It has the same number of inferior premolars and molars, and these teeth are similar in the two genera. From Diplacodon of the upper 228 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Eocene the present genus may be distinguished by having only three lower premolars on a side instead of four. * * * Of the three lower incisors in place on each side, the middle one is the largest. There is a short diastema behind the lower canine, but no first premolar. The dental formula of the lower jaws is as follows: Incisors, 3; canine, 1; premolars, 3; molars, 3. The space occupied by the lower dental series is 143^ inches, and by the last three molars 8H inches. Etymology. — reXeos, distant; dSovs, tooth; in allusion to the peculiar character of the incisors. Present determination. — The genus may either be valid or synonymous with an early stage in the evolu- tion of Brontops. Teleodus avus Marsh, 1890 Cf. Teleodus avus, this monograph, page 481 Original reference. — Am. Jour. Sci., 3d ser., vol. 39, pp. 523, 524, June, 1890 (Marsh, 1890.1). Type locality and geologic horizon. — " BrontotJierium beds of Dakota" ( = Chadron formation, or Titano- therium zone) ; exact geologic level not re- corded but probably lower beds (Chad- ron A). Type . — A lo wer j aw. Yale Mus. 10321. (See fig. 188.) Specific cliaracters. — Not separated by Marsh from the gen- eric characters. (See Type ol Teleodus avus p. 481.) Etymology. — avus, grandfather; in allu- sion to the primitive character of the animal. Present determination. — The species is probably valid. Allops crassicornis Marsh, 1891 Cf. Allops crassicornis, this monograph, page 517 Original reference. — Am. Jour. Sci., 3d ser., vol. 42, p. 268, September, 1891 (Marsh, 1891.1). Type locality and geologic horizon. — "BrontotJierium beds of South Dakota" ( = Chadron formation, or Titanotherium zone). Geologic level as recorded by J. B. Hatcher, collector, is the lower portion of the upper Titanotherium zone (Chadron C). Type. — A "nearly perfect skull of an adult but not old animal." Nat. Mus. 4289. (See fig. 189.) Specific characters. — Marsh writes: The skull is of medium size, with the zygomatic arches moder- ately expanded. The nasal bones do not project beyond the premaxillaries. The horn cores are very short and massive, with rounded summits, and thus form one of the striking fea- tures of the skull. The dentition is complete and in fine pres- ervation. The single incisor is quite small and situated close Figure 188.- Lower incisors and canines, natural size. to the canine. The latter is of moderate size and projects but little above the rest of the dental series. There is no diastema between the canine and the first premolar, which is small and has its inner face on a line between the canine and the second premolar. The second, third, and fourth premolars are large and have a strong inner basal ridge. The last molar has its anterior margin somewhat in advance of the front border of the posterior nares. The length of this skull on the median line is about 30 inches, and the width across the zygomatic arches 23 inches. The width across the horn cores is 14 inches. The extent of the superior dental series is 16 inches. Etymology. — crassus, thick; cornus, horn. Present determination. — The species is valid. It is fully described on page 517. Brontops validus Marsh, 1891 Cf. Brontops dispar, this monograph, pages 230, 488 Original reference. — Am. Jour. Sci., 3d ser., vol. 42, p. 269, September, 1891 (Marsh, 1891.1). Type locality and geologic horizon. — The geologic level as recorded by J. B. Hatcher, collector, is the "middle portion of the middle Titanotherium beds, White River, S. Dak." (Chadron formation, horizon Chadron B). Type. — A "skull in fine preservation." Nat. Mus. 4290 (skull K). (See fig. 190.) Specific characters. — Marsh writes: [The skull] agrees in its main characters with the other species of the genus but is particularly short and robust. The zygomatic arches are widely expanded, almost as much as in any skull of this group. The nasal bones have only a moderate extension in front and do not reach the end of the premaxil- laries. The free portion is broad and massive. The horn cores are of moderate size, nearly round in section, and have their obtuse summits directed somewhat backward. The occipital crest slopes forward and is expanded transversely. The length of this skull on the median line is about 26 inches. The great- est transverse diameter across the zygomatic arches is 22 inches, and across the summits of the horn cores 14 inches. Etymology. — validus, stout, brave. Present determination. — As shown on page 202, this species is probably synonymous with Brontops dispar. Titanops medius Marsh, 1891 Cf. Broniotherium medium, this monograph, page 573 Original reference. — Am. Jour. Sci., 3d ser., vol. 42, p. 269, September, 1891 (Marsh, 1891.1). Type locality and geologic horizon. — "Near the top of the Brontotherium beds of South Dakota" (Chadron formation, Titanotherium zone). J. B. Hatcher, col- lector. Type. — "One skull in fair preservation with the horn cores and dentition complete." Nat. Mus. 4256. (See fig. 191.) Specific characters. — Marsh writes: The free portion of the nasals is very small and projects but slightly beyond the anterior line of the horn cores. The latter are compressed anteroposteriorly and project laterally nearly DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 229 at right angles to the median line of the skull. The two in- cisors on each side are quite small and separated from each other and from the canine. There is a slight diastema behind the canine. The first premolar is small and triangular in out- line. The second premolar is of moderate size, and the third and fourth premolars have only an incomplete inner basal ridge. The width of this skull across the horn cores is 23 inches, and the distance from the end of the nasals to the front of the XAST SPECIES DESCRIBED BY COPE, 1891 Menodus peltoceras Cope, 1891 Cf. Brontotherium curtum, this monograph, page 574 Original reference. — Am. Naturalist, vol. 25, p. 48, January, 1891 (Cope, 1891.1). FiGUKE 189. — Type skull of Allops crassicornis Palatal view. Nat. Mus. 4289. After Marsh. One-fifth natural size. posterior nares is 16 inches. The extent of the upper dental series is 17 inches. Etymology. — medius, middle; in allusion to the in- termediate character (between the species elatus and curtus) of this form. Present determination. — The species is probably valid and is referable to Brontotherium. Type locality and geologic horizon. — " Titanotherium beds of northern Nebraska" (Chadron formation). Type. — "Represented by the nasal region and the horn cores; the apex of one of the latter being broken away." Am. Mus. 10719. Dr. Hobart Hare, col- lector, Nebraska. Presented by the Museum of the University of Pennsylvania. (See fig. 192.) 230 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Specific characters. — Cope writes: • The peculiarity of the species consists in the immense trans- verse extent of the horn cores and their complete fusion into an osseous wall which extends across the muzzle, forming a huge plate or shield. The superior border of this shield is moderately concave, a protuberant angle on each side representing the apex of each horn core. The nasal bones form a flattened protuber- ance much wider than long, which overhangs the nares. * * * Measurements: Elevation of horn-core plate at middle line behind, 180 millimeters; do. at lateral apex, 190 millimeters; total width of do. at middle, 300 millimeters. Projection of nasal bones beyond lateral base of horn-core plate, 20 milli- meters; width of nasal meatus at base of nasal bones, 65 milli- mieters; width of base of horn-core plate outside of nasal meatus, 90 millimeters. Anteroposterior diameter of base of , Figure 190. — Type (holot3'pe) skull of Brontops validus After Marsh. Nat. Mus. 4290. One-eighth natural size, horn core above side of and parallel to nasal meatus, 85 milli- meters. This species is nearest the M. platyceras S. and O., which has transverse compressed horn cores. They are, how- ever, distinct from each other, and not nearly so expanded transversely as in the present form. The M. pelioceras, in fact, carried a transverse shield on the end of its nose, which must have given it an extraordinary appearance. Etymology. — we\Tri, small shield; Kepa^, horn; be- cause the bases of the horns formed together a "huge plate or shield" extending across the muzzle. Present determination. — The type specimen (fig. 192) possibly represents a female of one of the long-horned species of Brontotherium, perhaps B. curtum, B. platy- ceras, or B. ramosum. The species is therefore practi- cally indeterminate at present. FIRST EUROPEAN OLIGOCENE SPECIES, DESCRIBED BY TOULA, 1892 Menodus? rumelicus Toula, 1892 Cf. Brontothermm rumelicum, this monograph, pages 660, 941 Original reference. — Akad. Wiss. Wien, Math.-nat. Classe, Sitzungsber., Band 101, Abt. 1, pp. 608-615, 1 pi., May, 1892 (Toula, 1892.1). Subsequent reference. — Ueber einen neuen Rest von Leptodonf (Titanotherium?) rumelicus Toula spec, pp. 922-924, 1896 (Toula, 1896.1). Type locality and geologic horizon. — Near the railroad on the Jambol line near Kajali, northwest of Burgas, eastern Rumelia. Level, lower Oligocene (?). The formation from which the type was re- corded was correlated by Toula with the " Belvedereschotter." On account of the extreme rarity of titano- theres in Europe it seems important to note the published evidence concerning the provenience of the type and referred speci- mens of this species. According to Toula the specimens were received from his friend G. N. Zlatarski in Sofia. Toula does not state that Zlatarski himself collected the specimens. He states only that they must have come from near the railroad at Kajali, from the great heaps of material which had been dug up in the search for usable rubble ("taugli- chem Schotter"), and that these "Schotter- massen" should correspond at best with that isolated remnant of a formation at Lidscha, northwest of Burgas, of which he had already spoken in his first report on the geology of the eastern Balkans. He writes : "I have referred to these 'Schotter' as Belvedereschotter, and I believe, from the condition of preservation of the specimens from Kajali, and especially from the rusty sand grains still adhering to them, that they must be referred to the same kind of rock." Besides the specimens of titanotheres Toula records a lower molar and a canine of a "middle-sized rhinoceros" from the same locality. Later he received from the same locality, this also from Zla- tarski, a fragment of the lower jaw of a titanothere that included the symphyseal region (Toula, 1896.1, pp. 922-924). But Toula has not disproved the possibility that these specimens may have been im- ported from America, perhaps by laborers returning home from the western United States. (See p. 560.) Lectotype. — Third right lower molar and part of the right ramus of the lower jaw. (See fig. 193.) Paratypes. — A second right lower molar and a canine. Referred specimen. — The symphyseal region of the jaw with the roots of pi, p2, Ps, and the worn p4, in place. DISCOVERY OF THE TITANOTHEEES AND ORIGINAL DESCRIPTIONS 231 Specific characters. — Toula's description is too long to quote here. The principal characters revealed by his figures are, symphysis massive, canines (?) large, four lower premolars, lower molars with faint external cingula, hypoconulid of ms without strong internal crest. Etymology. — rumelicus, in allusion to Rumelia, the region in Hungary where the type was discovered. Present determination. — The species is probably valid, and its generic reference is probably to Brontotherium. SPECIES DESCRIBED BY OSBOEN IN 1896 AND 1902 Titanotherium ramosum Osborn, 1896 Cf. Brontotherium ramosum, this monograph, page 577 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 8, p. 1941, pi. 4, text fig. 13, 1896 (Osborn, 1896.110). Type locality and geologic horizon. — "Upper Titanotherium beds, South Dakota." Chadron for- mation, Quinn Draw, Big Badlands, S. Dak. Type. — A complete male skull lacking incisive border. Am. Mus. 1447. (See fig. 194.) Characters of type. — Osborn writes: The distal spreading or branching of the horns is the character by which this species is designated. It differs from T. elatum in this character, but more especially in the great depth of the "connecting crest" and the ex- treme flattening of the horns, the section, as shown in diagram 1, being intermediate between that of the T. elatum and of T. plaiyceras. It is remarkable that the teetli in this large skull are relatively of 3mall size; the last upper molar has no second cone. Etymology. — ramosum, branched, in allusion to the "distal spreading or branching of the horns." Present determination. — The species is probably valid. Megacerops brachycephalus Osborn, 1902 Cf. Brontops brachycephalus, this monograph, page 483 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 16, pp. 97-98, fig. 3 (not the type), 1902 (Osborn, 1902.208). Type locality and geologic horizon. — Big Bad- lands, S. Dak.; Chadron formation, Chadron A, base or level A of lower Titanotherium zone. Type. — A complete skull (Nat. Mus. 4261, skull a), collected by J. B. Hatcher in 1887. (See fig. 195.) Specific characters. — Osborn writes: The type of this species is No. 4261, U. S. Nat. Mus. It includes very small, broad-skulled titanotheres with very rudi- mentary second internal cones upon the upper premolars; nasals elongate, narrowing anteriorly, as in Palaeosyops. Horns of anteroposterior oval section placed above orbits. It is represented in the National Museum by numerous skulls besides the type, all collected and recorded by Hatcher. One of these skulls was provisionally referred by him to Teleodus avus, from which this species is quite distinct. Etymology. — /3paxi's, short; Ki4>a\i), head, in allusion to the brachycephalic form of the skull. Present determination. — The species is probably valid. Megacerops bicornutus Osborn, 1902 Cf. Diploclonus bicornutus, this monograph, pages 234, 501 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 16, p. 99, fig. 5, 1902 (Osborn, 1902.208). Figure 191. — Type (holotype) skull of Titanops medius After Marsh. Nat. Mus. 4256. One-eightli natural size. Type locality and geologic horizon. — Quinn Draw, White River, S. Dak.; exact level not recorded. Col- lected by J. W. Gidley, 1896. Type. — Skull and lower jaws (Am. Mus. 1476). (See fig. 196.) Paratype, skull (Am. Mus. 1081). One of these skulls (No. 1081) was first described by Osborn (1896.110, p. 176) as Titanotherium torvum ox rohustum. Specific characters. — Osborn writes: Horns directed anteriorly. Hornlets upon the inner and anterior midportion of the horn. Basal section of the horn slightly oval, subtransverse. Nasals narrow and relatively elongate. Sharp malar bridge in front of orbit. Orbit large. 232 TITANOTHERES OF AKCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 192. — Type (holotype) nasofrontal shield of Menodus peltoceras Am. Mus. 10719. One-fourth natural size. FiGORB 193. — Cotypes of Menodus? rumelicus After Toula, 1892. Two-thirds natural size. The right lower molar (two upper figures) is the leetotype. Figure 194. — Type (holotype) skuU of Tiianoiherium ramosum After Osborn, 1896. Am. Mus. 1447. Ai, Side view, one-twelfth natural size; A2, top view, one- thirteenth natural size; A3, front view, one-thirteenth natural size. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 233 This animal stands nearest M. selwynianus, though dis- tinguished by the greater size and slightly greater width of the nasals. The sharp malar bridge is the most abso- lute character. The two hornlets are possibly variations. Type locality and geologic horizon. — -Big Badlands (probably Cheyenne River badlands), S. Dak.; Chadron formation (TitanotJierium zone), exact level not determined. FiGTjHE 195. — Type skull of Megacerops brachycephalus Nat. Mus. 4261. One-fourth natural size. Etymology. — lis, twice; cornutus, horned; in allusion to the presence of small accessory horn swellings. Present determination. — The species is probably valid. Megacerops marshi Osborn, 1902 Of. Allops marshi, this monograph, pages 511-515 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 16, pp. 100-101, fig. 6, 1902 (Osborn, 1902.208). 101959— 29— VOL 1-^18 Type. — A complete skull (Am. Mus. 501). (See fig. 197.) Paratype. — Skull (Am. Mus. 1445). Collected by American Museum expedition, 1892. Specific characters. — Osborn writes: Type, skull No. 501; cotype, skull No. 1445, Am. Mus. Nasals elongate and square distaUy, horns short, of oblique oval basal section, overhanging the maxillae, or projecting forward or outward. Incisors, ?f . Canines short, tetartocones 234 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA of premolars moderately developed. These skulls were pre- viously confused by the writer with T. trigonoceras, from which they are readily separated by the horn section, which relates them to some of the primitive types of M. hrachycephalus and equally to M. robustus. The canines are more obtuse than in Figure 196. — Type (holotype) skull and lower jaw of Megacerops bicornutus Am. Mus. 14/6. After Osborn, 1902. One-eighth natural .size. M. dispar, and the superior incisors resemble those in Bronto- iherium rather than in M. robustus. Etymology. — Named in honor of the late Prof. O. C. Marsh, who estabUshed the remarkable collections of titanotheres in the Yale and National Museums, pro- posed the family name Brontotheridae, gave names to many of the genera and species, and projected the present monograph. Present determination. — The species is probably valid. Brontotherium leidyi Osborn, 1902 Cf. Bronlhotherium leidyi, this monograph, page 558 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 16, pp. 105-106, figs. 9, 10, 1902 (Osborn, 1902.208). Type locality and geologic horizon. — Big Badlands, S. Dak.; Chadron formation, lower levels of lower Titanotherium zone (Chadron A). Type.— A complete skull (Nat. Mus. 4249, skull R) collected by J. B. Hatcher in 1887. (See figs. 198, 199.) Specific characters. — Osborn writes: Nasals elongate, narrowing anteriorly. Horns very short, slightly recurved, of transverse oval section. Canines stout and blunt. Premolars noncingulate, with rounded contours and weU-developed tetartocones. Incisors?^. Etymology. — Named in honor of Joseph Leidy, the first of the three great founders of American vertebrate paleontology, describer of Titanotherium, Megacerops, Palaeosyops, author of "The ancient fauna of Ne- braska" and of "The extinct mammalian fauna of Dakota and Nebraska." Present determination. — The species is probably valid. SPECIES DESCRIBED BY LULL IN 1905 Megacerops tyleri Lull, 1905 Cf. Diploclonus tyleri, this monograph, page 502. Original reference. — Jour. Geology, vol. 13, No. 5, pp. 443-456, pis. 3, 4, August, 1905 (Lull, 1905.1). Type locality and geologic horizon. — North side of Spring Draw Basin, about 10 miles from the mouth of Bear Creek, a tributary of Cheyenne River, S. Dak. Type specimen found 35 feet above the base of 200 feet of the Chadron formation {Titanotherium zone) lying upon Pierre deposits, "hence in the upper part of the lower division," as defined by Hatcher in 1893 (1893.1, p. 218). Type. — Skull, limbs, and many vertebrae of a single individual (Amherst Mus. 327). (See figs. 200 and 201.) Found by T. C. Brown, of the Amherst College paleontologic expedition of 1903. Specific characters. — Lull writes: Horns well in front of orbits, directed somewhat forward and outward, an elongate oval in basal section with the long axes in line, rounded oval at the summit. Hornlets quite conspicuous, on the inner face of the horns midway between the base and summit. Con- necting crest low and inconspicuous. Nasals broad, well rounded in front, and but sHghtly arched beneath. Zygomata expanded and deep, with a well-rounded outer face. Dentition: Superior incisors represented by the deep and well-defined median alveoli and by the lateral teeth, which remain in place and which have hemispherical crowns which show little sign of wear. The canines are lanceolate, with a well-developed postero-internal cingulum. There is a short diastema in front of, and a longer one behind, the canine. Premolars with a smooth internal cingulum, less pronounced in the middle of the tooth, and with no external cingulum. The deuterocone is well developed, while the tetartocone, especially of premolar 4, is inconspicuous. The jaw is deep and robust, with the alveoli of two incisors, probably of the second and third, deep and distinct. There is Figure 197. — Type skull of Megacerops marshi After Osborn, 1902. Am. Mus. 501. One-tenth natural size. The lower jaw (Am. Mus. 516) figured with this skull does not belong with it. It is probably referable to Brontotherium leidyi. 'no space between the lateral incisors and the canine, though between the two median alveoli a considerable gap occurs. There seems to have been a small diastema behind the lower canines, which are lanceolate, though with a less prominent cingulum, and not so strongly recurved as the upper ones. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 235 Etymology. — Named in honor of Prof. John M. Tyler, of Amherst College, "a teacher of men, who, by his earnest efforts, as well as by his own generosity, was mainly instrumental in maldng possible the ex- pedition which secured the specimen" (Lull). Present determination. — This species is probably valid. It is discussed on page 502 of this monograph. Figure 198. — Type (holotype) skull of Brontotherium leidyi After Osborn, 1902. Nat. Mus. 4249. One-eighth natural size. The side view of this skull was figured by Marsh (Am. Jour. Soi., October, 1887) as Menops varians. SPECIES DESCRIBED BY OSBORN IN 1908 Brontotherium hatched Osborn, 1908 Cf. Brontotherium hatcheri, this monograph, page 563 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, pp. 615-616, fig. 20, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — South Dakota; Chadron formation, middle Titanotherium zone (Chad- ron B), lower levels. J. B. Hatcher, collector. Type. — A nearly complete skull (Nat. Mus. 1216, skull a) lacking the premaxillaries and anterior por- tion of the maxillaries. (See fig. 202.) Specific characters. — Osborn writes: If, Pf. Nasals moderately long (97 mm.), thin at the edges. Horns 250 -f millimeters, two-thirds the length oj B. gigas horns. Skull length (pm.x-condyles) , 710 (estimated), width across zygomata, 530 (estimated) . This species appears to represent an early phase of evolution of B. gigas. The horns are very round or convex in section and have a well-defined malar ridge on the lower outer portion. The connecting crest is relatively shallow, and the nasals are thin. The premolars are well advanced, the tetartocone of p' being well rounded and quite distinct. Etymology. — Named "in honor of the late J. B. Hatcher, who discovered many of Professor Marsh's titanothere types, brought together the great collec- tion of titanotheres ia the National and Yale Mu- seums, and placed the stratigraphic succession of the species upon a secure basis." (Osborn.) Present determination. — The species is probably valid. Symborodon copei Osborn, 1908 Cf. Megacerops copei Osborn, this monograph, page 548 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 24, pp. 616, 617, fig. 21, 1908 (Osborn, 1908.318). Type locality and geologic horizon. — South Dakota, Big Badlands, Indian Draw; Chadron formation, level probably middle Titanotherium zone (Chadron B). J. B. Hatcher, collector. Type. — A complete skull (Nat. Mus. 4711, skull V), collected by J. B. Hatcher, 1888. (See fig. 203.) Specific and generic characters. — Osborn writes: Incisors (type) persistent but greatly reduced; canines very small, reduced (28 mm.) ; premolars with cingula reduced or absent; tetartocones connected with deuterocones by a longitudinal ridge. Skull: nasals thin, short and broad in pro- portion, 80 by 125 millimeters; horns, S , 300, no connecting crest, transverse oval near summit; buccal processes of zygomata t? stout and conve.x; malar in front of buccal process very deep, beneath postorbital process stout, convex; occipital pillars not greatly expanded at the summits. Etymology. — Named in honor of the late Prof. E. D. Cope, prolific author of "The Vertebrata of the Tertiary formations of the West," original describer of Symborodon, founder of the "Cope collection," now in the American Museum of Natural History. Present determination. — The species is probably valid. CANADIAN SPECIES DESCRIBED BY LAMBE IN 1908 Megacerops primitivus Lambe, 1908 Cf. Teleodus primitivus, this monograph, page 482 Original reference.— Contr. Canadian Paleontology, vol. 3, pt. 4, pp. 49-51, pi. 6, figs. 4, 5, 1908 (Lambe, 1908.1). FiauRE 199.^Upper premolars of type skull of Brontotherium leidyi After Osborn, 1902. Nat. Mus. 4249. One-halt natural size. Type locality and geologic horizon. — "Oligocene deposits of the Cypress Hills," Saskatchewan. Col- lector, L. M. Lambe, 1904. Type. — Both halves of the lower jaw, with the denti- tion of the left side complete. Ottawa Museum. (See fig. 204.) Specific characters. — Lambe writes: Incisors, in three pairs, with a space between the inner pair; canines, of small diameter, apparently short; a diastema 236 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 200. — Type (holotype) skull of Megacerops iyleri After Lull. Amherst Mus. 327. A, Dorsal aspect of skull; B, lateral aspect of skull and jaw; C, anterior aspect of horns and nasals; all about one-eighth natural size. D, Upper dentition (incisor, canine, and premolar series), one-fourth natural size. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 237 Figure 201. — Right manus and right hind limb of the type of Megacerops tyleri Alter Lull, 1905. Amherst Mus. 327. A, Pro.dmal row of carpals, proximal aspect; B, distal row of carpals, proximal aspect (sc. /., scaphoid facet, lu.f., lunar facet, en. /., cuneiform facet); C, right manus; all one-fourth natural size. D, Right hind limb, one-eighth natural size. 238 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA between the canine and the first premolar; first premolar small; third premolar becoming molariform; fourth premolar molariform; symphysis long; symphyseal surface between canines narrow; jaw contracted at the diastema; external cingula moderately developed; internal cingula wanting; mental foramen beneath the second premolar; coronoid process short. Megacerops avus (Marsh), from the Oligocene of South Dakota, has three pairs of inferior incisors but only three pre- molars below on each side, and there is a short diastema behind P ^ ^B r T'tif' ^^ ^^H L_ _ r^^ ^^ Jr- , "^^B i^Tr^^H n W'^^^P" ^H F 1 B^ H^^' PI f 1 ^m kw ~ 1 i^m 1 : -rf»r-"^ l^^l iH mn H Figure 202. — Type (holotype) skull of Bronlo- therium hatcheri Top view. Nat. Mus. 1216. After Osborn. 1908. One-tenth natural size. the lower canine. Its dimensions are greater than those of M. primitivus. These two species are apparently the only ones of the Oligocene titanotheres in which there are three pairs of incisors in the lower jaw. In the Cypress Hills specimen the crowns of the incisors are of a depressed spherical shape, with a tendency to come to a rounded central point above. The second incisor is the largest, and the first is slightly smaller than the third, which is the most upright. The first is more procumbent than the second. Between the inner pair is a very decided interval, leaving a space of 6.5 millimeters between the crowns of the two teeth. The crowns of the canines are broken ofi' (that of the right tooth being restored in fig. 5 of pi. 6) and the right first premolar is lost from its alveolus. * * * Keeping in mind the differences due to sex in titanotheres generally and the apparent variability, both specific and indi- vidual, of certain dental characters, such as the degree of devel- opment of the cingula, the presence or absence of the first premolar, the size of the canines, and the number of the incisors, M. primitivus is apparently a well-marked species, characterized principally, so far as known at present, by the breadth of the mandible anteriorly (as compared with M. angustigenis) and the presence of the fuU number of teeth, with a comparatively long diastema behind the canines. This species, for which the name primitivus is used, is regarded as representing a rather early stage in the development of the titanotheres. The general character of the dentition suggests the appropriateness of referring the species to the genus Megacerops. Measurements Millimeters Length of ramus 475 Depth of same at posterior end of fourth premolar 74 Depth of same at posterior end of second molar 81 Depth of same from tip of coronoid process to lower border 247 Maximum thickness of same beneath third molar 46 Length of symphysis 144 Distance apart of inside surface of base of canines" 31 Length of premolar series 103 Length of molar series 183 Diameter of canines at base: Anteroposterior 18 Transverse 16 Diameter of second premolar: Anteroposterior 26 Transverse 18 Diameter of third premolar: Anteroposterior 32 Transverse 23 Diameter of fourth is premolar: Anteroposterior 35 Transverse 27 Etymology. — primitivus, primitive; in reference to the presence of three lower incisors. Present determination. — The species is probably valid. It is probably referable to Teleodus. Megacerops assiniboiensis nom. prov., Lambe, 1908 Cf. Brontotherium curium, this monograph, page 574 Original rejerence. — Contr. Canadian Paleontology, vol. 3, pt. 4, pp. 51-53, pi. 5, fig. 6, 1908 CLambe, 1908.1). Figure 203. — Type (holotype) skull of Symborodon copei After Osborn, 1908. Nat. Mus. 4711. One-tenth-naturai size. Type locality and geologic horizon. — Oligocene de- posits of the Cypress Hills, Saskatchewan. Collection of 1904. " In the mandible of M. angustigenis (No. II) figured by Cope, op. cit. [1891.2] this measurement is about 18 millimeters, and in the symphysis of the jaw (No. I, also figured) a like measurement given, by the same authority, as 27 millimeters , should be 22 millimetei s. >' First premolar in Cope's description of M. angustigenis. DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 239 Type. — "A robust, short left mandibular ramus," lacking the posterior end. The three molars and the the fourth premolar and the first molar. The bone is massive and heavy throughout. The mental foramen is placed beneath the posterior root of the third premolar, farther back than in M. ■primiiivus. The cingula are very slightly developed. The external cingu- lum is present for a short distance only, on the anterior face of each of the four teeth, and in the third molar in advance of the heel. The only trace of an internal cingulum is to be seen in the third molar on the posterior slope of the heel. Figure 204. — Type (holotype) jaw of Megacerops primiiivus In the collection or the Ottawa Museum. After Lambe, 1908. A, Superior aspect, one-half natural size; B, lateral aspect, one-third natural size. fourth premolar are preserved, as well as part of the symphyseal region. Ottawa Museum. (See fig. 205.) The fourth premolar is fully molariform. The teeth are stout and of about the size of the corresponding ones in M. Figure 205. — Type (holotype) jaw of Megacerops assiniboiensis In the collection of the Ottawa Museum. After Lambe, 1908. One-third natural size. Characters. — Lambe writes: The jaw is much deeper, thicker, and relatively shorter than in angustigenis and primiiivus, and the teeth are much larger than in these species. It is narrow anteriorly, and the sym- physis extends back to a point in line with the division between marshi Osborn, but the jaw is relatively shorter than in this species. From the material available, the species, for which the provisional name assiniboiensis is proposed, can not be defi- nitely characterized. 240 TITAJSrOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements of ramus {type) Millimeters Depth of ramus at posterior end of fourth premolar 80 Depth of ramus at posterior end of third molar 156 Thiclcness of ramus above lower border beneath posterior end of first molar 55 Vertical thickness of symphysis a little in advance of its posterior termination 53 Vertical thickness of symphysis in line with front root of third premolar 31 Space occupied by fourth premolar and the molars 260 Diameter of fourth premolar: Anteroposterior 41 Transverse 31 Diameter of first molar: Anteroposterior 55 Transverse 36 SECOND EUROPEAN OIIGOCENE SPECIES, DESCRIBED BY KIERNIK, 1913 Titanotherium bohemicum Kiernik, 1913 Cf. Menodus giganteus, this monograph, page 530 Original reference. — Acad. sci. Cracovie Bull., ser. B, vol. lOB, pp. 1211-1225, pi. 63, 1913 (Kiernik, 1913.1). Type locality. — Uncertain. The specimen, a frag- ment of the lower jaw containing the third right lower molar, was received with a lot of fossils from the dilu- vium near Prague. It was supposed to have come from the lime pits of Podbaba, near Prague, and to have been sold by one of the workers in the lime pits C D Figure 206. — Type of Titanotherium bohemicutn Kiernik Fragment of a lower jaw with third right lower molar. After Kiernik. A, Outer side view; B, inner side view; C, top view, showing the grinding surface of ms; D, front view, showing the exposed posterior roots of mj. Ahout one-fourth natural size. Diameter of second molar: Anteroposterior 71 Transverse 41 Diameter of third molar: Anteroposterior 99 Transverse 43 Space occupied by roots of third premolar (anteroposterior) 34 Space between fourth premolars (twice the distance of fourth premolar from vertical plane through symphysis) - 60 Etymology. — assiniboiensis , in allusion to the geo- graphic occurrence of the type. Present determination. — This species apparently be- longs in the Brontotheriinae. It is smaller than Brontotherium Tiatcheri. The nasals doubtfuUy referred by Lambe to this species suggest those of Bronto- therium curtum. to Herr Baumeister Kuchta (died 1910). He gave it, along with other prehistoric specimens, to Herr EoJanek, who in turn gave it to Herr Jira, who pre- sented it to the Institute for Comparative Anatomy at Prague. After carefully considering the possi- bility that the specimen might have been of American provenience the author, Herr Kiernik, inclines rather to the view that it really came from Bohemia, al- though not from Pfodbaba, but from the fresh-water Tertiary deposits of Tuchofitz (northwestern Bo- hemia). The well-known fauna of Tuchofitz is, however, of lower Miocene facies. Type. — A lower jaw fragment containing the third right lower molar. (See fig. 206.) DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 241 Characters. — Kiernik carefully compares the frag- ment with the types of Brachydiastemafherium tran- sihanicum Bockh and Maty, Menodus rumelicus Toula, and Titanotherium proutii Leidy. He shows that the third lower molar is much larger than that of either Brachydiastematherium or Menodus rumelicus, but that it is nearer in its measurements to the type of Titanotherium proutii, as indicated in the following table : Measurements of Titanotherium hohemicum, T. -proutii, and Menodus rumelicus, in millimeters T. bohemicum M. rumelicus Total length of the wear- ing surface Breadth of the first section (lobe) of the tooth Breadth of the second sec- tion (lobe) of the tooth... 27 T. proutii The author concludes that this species is widely distinct from the Ivnown European forms but that possibly it may eventually prove to be identical with either Titanotherium proutii or another species of the same genus. This, however, he considers unlikely, in view of its [supposed] European origin, so that he thinks he is quite justified in retaining the name Titanotherium hohemicum. Etymology. — hohemicum, in allusion to the country where the specimen was supposedly found. Present determination. — According to Dr. W. K. Gregory, who has compared a cast of the type of Titanotherium hohemicum with various American ti- tanotheres, the type specimen is closely similar to one in the American Museum of Natural History referred to Menodus giganteus (Am. Mus. 1007). It differs chiefly in the greater width of the anterior lobe of m3. It appears indeed to be specifically referable to Menodus giganteus, and it seems possible that it is in reality an American specimen which became mixed with the collection of fossils from Podbaba, near Prague. (Cf. pp. 230, 560, 941.) Measurements of Menodus bohemicus and M. giganteus, in millimeters M3, total length (estimated) M3, breadth of first lobe at base M3, breadth of second lobe M3, breadth of third lobe Center of protooonid to center of hypoconid Center of metaconid to center of entoconid Depth of jaw below front edge of ms-. Depth of jaw just behind ms 108 52 47 33 39 39 + 111 152 M. giganteus (trigonoceras). Am. Mus, 1007 109 47 47 111 143 FINAI OLIGOCENE SPECIES DESCRIBED BY OSBOEN IN 1916-1919 Allops walcotti Osborn, 1916 See page 509 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 35, pp. 721, 722, fig. 1, 1916 (Osborn, 1916.433). Type locality and geologic horizon. — "Big Badlands," S. Dak., probably Corral Draw; Chadron formation {Titanotherium zone), lower levels (Chadron A). Type. — A nearly complete skull in the National Museum (No. 4260, skull Q). (See fig. 207.) Figure 207. — Type (holotype) skull of Allops walcotti Nat. Mus. 4260. After Osborn, 1916. One-eighth natural size. Specific characters. — Osborn writes: Premolars with small tetartocones; p'-m^ 285 millimeters. Incisors f . Horns elongate oval, no connecting crest. Mesa- ticephaUo. Nasals elongate, broad. Face relatively elongate. The type skull of this species (U. S. Nat. Mus. 4260) from level A is narrow and elongate, partly owing to lateral crushing. This feature conceals its resemblance to Allops marshi, which is apparent in other features — namely, (1) primitive, long nasals, (2) horns primitively short and obhquely oval, (3) large lateral incisor (ij) and small first (ij) or median incisor, (4) premolars accelerated, tetartocones more advanced than in Brontops robustus of level C. 242 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Observations on the measurements oj AUops wal- cotti. — The type and only known specimen of this species exhibits the following comparison in measure- ments with skulls of B. hrachycephalus and Menodus Jieloceras, which show that the type of AUops walcotti has relatively large premolars and small molars. Measurements of AUops walcotti, Menodus heloceras, and Brontops brachycephalus, in millimeters A.walcotti, Nat. Mus. 4260 (type) M. helo- ceras. Am. Mus. 14576 B. brachycephalus Nat. Mus. 4940 Nat. Mus. 42S1 Pi-m3 Pi-p* Mi-m3. 285 112 169 640 105 100 35X51 60X61 265 170 603 132 70 265 101 160 280 " 104 178 Pmx-condyles 680 102 32X51 62X70 85 33X53 68X73 Etymology. — -"The species is named in honor of the Secretary of the Smithsonian Institution, Charles D. Walcott." (Osborn.) Present determination. — The skull is 'crushed later- ally but probably had a low zygomatic index — that is, it was mesaticephalic. While its reference to AUops requires confirmation, its nearer affinities appear to be with this genus rather than with Brontops or Menodus. The external cingula of the premolars are not as sharply defined as in other primitive members of the menodontine group. Megacerops riggsi Osborn, 1916 See page 550 Original reference. — Am. Mus. Nat. Hist. Bull., vol. 35, p. 723, fig. 2, 1916 (Osborn, 1916.433). Type locality and geologic horizon. — Northeastern Colorado, Horsetail Creek; Chadron formation (Titano- therium zone), upper (?) levels. Type. — A nearly complete lower jaw in the American Museum (No. 6364). E. D. Cope, collector. (See fig. 208.) Specific characters (Osborn). — Of small size, smaller than any known individual of Megacerops or Bronto- therium. Very massive jaw with a small coronoid FiGUEE 208. — Type (holotype) jaw of Megacerops riggsi Am. Mus. 6364. After Osborn, 1916. One-sixth natural size. process and a very short symphysis. Premolar series greatly abbreviated (85 mm.). Premolars and molars with reduced external cingula. Measurements of type Millimeters Symphysis to condyle (estimated) 465 Premolar-molar series (pi-ms) 282 Premolar series (pi-pi) 85 Molar series (mi-ma) 194 Etymology. — Named "in honor of Mr. E. S. Riggs, of the Field Museum of Natural History, in recogni- tion of his discoveries of Eocene titanotheres." (Os- born.) Present determination. — The type of this species is a jaw in the Cope collection (Am. Mus. 4636), which was wrongly referred by Cope to his species " Symhorodon" acer. It represents a highly specialized and small form of Megacerops. Note. — For descriptions of upper Eocene and lower Oligo- cene titanotheres from MongoUa described by Osborn in 1923 see appendix; also the final opinion regarding the titanotheres of eastern Europe, page 941. CHAPTER IV SYSTEMATIC CLASSIFICATION OF THE TITANOTHERES SECTION 1. PHYLETIC VERSUS LINNAEAN SYSTEM OF CLASSIFICATION NEO-LINNAEAN SYSTEMATIC DIVISIONS (ZOOLOGIC) AND EVOIUTIONARY PHYIA (PALEONTOIOGIC) As explained in the introduction, the Linnaean system was based on the theory of the special creation of all systematic divisions coinciding in geographic space, so that its application to our modern paleonto- logic phyla, which succeed one another over long periods of geologic time, is beset with great difficulties and has led to different uses of systematic terms by different authors. The present monograph employs a phyletic system which has been used by the author since 1892 in the classification of the Perissodactyla (Osborn, 1892.67, pp. 90-94). The taxonomic principle is that ancestral affinity is stronger than contemporary resemblance. Thus an animal that is directly ancestral to the titanotheres is placed in the family Brontotheriidae; an animal that is directly ancestral to BrontotJierium is placed in the subfamily Brontotheriinae; a series of ascending species in the same line are placed in the genus BrontotJierium; a series of "ascending mutations" may be placed within the single species BrontotJierium gigas. Such a vertical or phyletic application of the Lin- naean system involves, it is true, a departure from the traditional Linnaean methods, but in the author's opinion it is far preferable to the introduction of a new systematic terminology. If necessary the author's system may be distinguished as neo-Linnaean. It is an adaptation of the Linnaean system to phylogeny as revealed by paleontology. The degrees or steps in the evolution of neomorphic and heteromorphic characters, or rectigradations and allometrons, afford the real basis of our division of the great family tree of the titanotheres into branches and subbranches as follows: Family, a branch of the Perissodactyla having a large num- ber of similar characters and similar tendencies of evolution. Subfamily, a branch of the main family embracing one or more genera retaining certain similar characters and developing certain peculiar evolutionary tendencies. Genus, a branch of a subfamily or a stage of a subfamily distinguished by the prominent position of certain distinctive characters, which may be in widely different stages of develop- ment — for example, Brontotherium leidyi, B. platyceras. Species and subspecies, divisions distinguished by certain gradations in the development of characters common to the genus, also by certain rectigradations and allometrons. Ascending mutations, divisions distinguished by various intermediate stages of development of rectigradations and allometrons. These principles of phyletic classification as devel- oped and adopted in this monograph are also fully explained in Chapter I. Classification is simply a convenient and condensed expression of our knowledge of hereditary lines of descent. It is constantly shifting and changing with discovery. The final classification can be attained only after we have worked out all the lines of descent of this great family. In the meantime we may review the history of the successive attempts at classification made up to the present time. SUPEEFAMILY NAMES PROPOSED BY OSBORN (1898) AND HAY (1902) Superfamily Titanotherioidea Osborn, 1898 Original reference. — Am. Mus. Nat. Hist. Mem., vol. 1, pt. 3, p. 79, 1898 (Osborn, 1898.143). Osborn divided the Perissodactyla into five super- families : I. Titanotherioidea ("including the single family Titano- theriidae"), understood by Osborn to include both Eocene and Oligocene titanotheres. II. Hippoidea, including Equidae and Palaeotheriidae. III. Tapiroidea, including Tapiridae and Lophiodontidae. IV. Rhinocerotoidea, Including Hyracodontidae, Amyno- dontidae, Rhinocerotidae. V. Chalicotherioidea, Chalicotheriidae. Present determination. — Superfamily names are formed by adding oidea to the stem of the family name, and as Brontotheriidae is now regarded as valid, it appeared necessary to Doctor Hay to substitute for Osborn's term Titanotherioidea the term Bronto- therioidea, first used by Hay in 1902. Superfamily Brontotherioidea Hay, 1902 Original reference. — U. S. Geol. Survey Bull. 179, p. 629, 1902 (Hay, 1902.1). The content of this term is as follows: Brontotherioidea : Brontotheriidae: Lambdotheriinae (Eocene titanotheres) . Brontotheriinae (Oligocene titanotheres). The content of the term Brontotherioidea Hay, 1902, is thus the same as that of Titanotherioidea Osborn, 1898. FAMILY NAMES PROPOSED OR ADOPTED BY MARSH (1873), FLOWER (1875), COPE (1879-1889), AND OSBORN (1889) Family Brontotheridae Marsh, 1873 Original reference. — Am. Jour. Sci., 3d ser., vol. 5, p. 486, 1873 (Marsh, 1873.1). Included genera. — Titanotherium Leidy and Bronto- therium Marsh. 243 244 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Family characters. — Not distinguished, but state- ment is made that Brontotherium was a " true perisso- dactyl with hmb bones resembling those of RM- noceros." Marsh gave the famUy characters fully in a paper entitled "On the structure and affinities of the Brontotheridae." He writes (Marsh, 1874.1, p. 82): Among the more marked characters of the Brontotheridae, which readily distinguished them from the Rhinocerotidae, apparently their near allies, may be mentioned the following: There are four short and thick toes in the manus, and three in i the pes. The skull supports a pair of large horn cores, placed transversely, as in modern artiodactyls.^' There are well- i developed canine teeth in both jaws. The molar teeth, above and below, are not of the Rhinoceros type but resemble those of Chalicotherium. Present determination. — As long as Brontotherium was regarded as a synonym of Titanotherium the term Brontotheriidae had no standing, but since Bronto- therium has been shown to be a good genus the term Brontotheriidae must be held valid. Family Limnohyidae Marsh, 1875 Cf. Palaeosyopinae, this monograph, page 298 Origin^ reference. — Am. Jour. Sci., 3d ser., vol. 9, p. 246, 1875 (Marsh, 1875.1). Present determination. — In defining the genus Dipla- codon, Marsh says: "From the Eocene Limnohyidae, already described, this genus is sharply distiuguished." The name Limnohyidae does not occur in Marsh's previous descriptions, and so far as one can judge the famUy had not been defined. As Limnohyus is a synonym of Palaeosyops the family name is invalid. Family Titanotherlidae Flower, 1876 Cf. Brontotheriidae Marsh, this monograph, page 279 Original reference. — Nature, vol. 13, p. 328, 1876 (Flower, 1876.1). Present determination. — Flower regarded Bronto- therium as synonymous with Titanotherium and so naturally called the family Titanotherlidae; but siace Brontotherium is now regarded as valid, Flower's term becomes a synonym of Brontotheriidae Marsh. Family Chalicotherlidae Cope, 1879 Original reference.- — U. S. Geol. and Geog. Survey Terr. Bull., vol. 5, p. 228, 1879 (Cope, 1879.1). Included genera. — "Limnohyus Leidy [ = Limnohyops Marsh], Palaeosyops Leidy, ' Leurocephalus S., O. & S.' [= Telmatherium cultridens], Menodus Pomel, Sym- lorodon Cope, Daeodon Cope, Chalicotherium Kaup, Nestor itherium Kaup." Present determination. — The titanotheres should never have been included in the same family with Chalicotherium. 19 Ehinoceros pleuToceTOS Duv., from the Miocene of France, has a transverse pair of small horn cores on the nasals, not unlike those in DiTtoceras. R. mxnutua Cuv. has somewhat similar processes. Menodontidae Cope, 1881 Cf. Brontotheriidae Marsh Original reference. — Am. Philos. Soc. Proc, vol. 19, pp. 378, 379, 397, 1881 (Cope, 1881.1). Present determination. — The name Menodontidae as applied to the Ohgocene titanotheres is invalid because antedated by Brontotheriidae Marsh. Family Lambdotheriidae Cope, 1889 Cf. Lambdotheriinae, this monograph, page 279 Original reference. — Am. Naturalist, March, 188 9 p. 153 (Cope, 1889.1). Included genera. — From Cope's description it is plain that he intended to refer to the Lambdotheriidae not only the type genus Lambdotherium but all titanotheres with "but a single internal cusp on the first (posterior) superior premolar." He thus con- trasts the Lambdotheriidae with the Menodontidae ( = Brontotheriidae). Cope then also referred to the family Lambdotheriidae an Oligocene genus "Hapla- codon" (= Megacerops angustigenis) . Synonymy. — The term Lambdotheriidae as used by Nicholson and Lydekker (1889.1, vol. 2, p. 1371) had the same connotation. It was apparently first limited to the genera Lambdotherium, Palaeosyops, and "Lim- nosyops" { = Limnohyops) by Flower and Lydekker (1891.1, p. 413) in 1891. Later authors, as Earle in 1892 (1892.1) and Zittel in 1893 (1893.1, p. 300), used the term Palaeosyopidae or Palaeosyopinae to include the same genera. Present determination. — In this monograph the group under consideration is treated as a subfamily Lambdo- theriinae of the Brontotheriidae. Family Titanotherlidae Osborn, 1889 (1890?) Cf. Brontotheriidae Marsh, this monograph, page 279 Original reference. — Am. Philos. Soc. Trans., new ser., vol. 16, p. 514, 1889 (1890) (Scott and Osborn, 1890.1). Included genera. — Osborn writes: Palaeosyops has hitherto been referred to the ChaUcothe- riidae, but the discovery of the footbones of Chalicotherium by Filhol shows that the genera are widely separated. The discovery of the skeleton of Diplacodon, however, links Palaeo- syops very closely to Titanotherium. * * * It seems best to group the three genera [Palaeosyops, Diplacodon, and Titanotherium] in the single family Titanotherlidae. Present determination. — This was the first descrip- tion which included the true titanotheres of the Eocene and Oligocene without extraneous elements (Chalicotherium). The term is nevertheless pre- occupied by Titanotherlidae Flower, 1876, which is in turn a synonym of Brontotheriidae Marsh, 1873. SYSTEMATIC CLASSIFICATION OF THE TITANOTHERES 245 SUBFAMILY NAMES AND PHYLA PROPOSED BY STEIN- MANN AND DODERIEIN (1890), EARLE (1892), AND RIGGS (1912) Subfamily Falaeosyopinae Steinmann and Doderlein, 1890 Original reference. — Elemente der Palaontologie, p. 777, 1890 (Steinmann and Doderlein, 1890.1). Included genera. — The authors divide the Chali- cotheriidae into three subfamihes — Falaeosyopinae, Brontotheriinae, Chalicotheriinae. The Falaeosyo- pinae include the genera Palaeosyops, "Limnohyus" {Limnohyops) , Diplacodon. Present determination. — Falaeosyopinae Steinmann and Doderlein, 1890, thus has priority over Falaeo- syopinae Earle, 1892. Subfamily Falaeosyopinae Earle, 1892 Cf. Falaeosyopinae Steinmann and Doderlein Original type reference. — Acad. Nat. Sci. Fhila- delphia Jour., 2d ser., vol. 9, pp. 272 et seq., 1892 (Earle, 1892.1). Included genera. — Lamidotherium, Limnohyops, Palaeosyops, Telmatherium, Haplacodon [Megacerops angustigenis]. Present determination. — Earle gives a detailed and accurate description of the subfamily characters (pp. 274-276). The term is preoccupied by Falaeo- syopinae Steinmann and Doderlein, 1890, and in its content is preoccupied by Lambdotheriidae Cope, 1889. Subfamily Dolichorhinae Riggs, 1912 Cf. Dolichorhininae Original reference. — Field Mus. Nat. Hist. Fub. 159, Geol. ser., vol. 4, No. 2, p. 25, June, 1912 (Riggs, 1912.1). Included genera. — Middle Eocene titanotheres having nasals elongate and deeply recessed laterally, face shorter than cranium, an infraorbital process more or less developed, and molars only moderately expanded. This group is proposed in order to designate those long-nosed titanotheres which evidently sprang from a common stock and form a natural and homogeneous group. It includes the genera MesatirJiinus, Meta- rJiinus, Dolichorhinus, and RhadinorJiinus. DIVISION OF THE OLIGOCENE TITANOTHERES INTO FOUR CONTEMPORARY PHYLA, OSBORN (1902) Original reference. — Am. Mus. Nat. Hist. Bull., vol. 16, pp. 91-109, February 18, 1902 (Osborn, 1902.208). Included genera. — Osborn writes: The Oligocene titanotheres consisted of at least four contem- porary phyla, to which the prior generic names Titanotherium, Megacerops, Symborodon, and Brontotherium may be applied. They represent an adaptive radiation for different local hab- itat, different modes of feeding, fighting, locomotion, etc., which took origin, in part at least, in the middle or upper Eocene. Europe and Asia also may have shared in this radiation, since titanotheres are now definitely known in the Balkan region. The main phyletic characters are analogous to those recently (Osborn, 1900, p. 231) determined among rhinoceroses; the great antiquity of the lines leading to the existing species of rhinoceroses necessitated the revival of a number of discarded generic names to distinguish them. Similarly the separateness of four of the titanothere phyla, throughout the Ohgocene and possibly from the Eocene, renders it desirable to revive certain generic names which in my first review I considered undefinable. Radiation involved three main sets of characters, two of which were correlated: First, doliohocephaly and brachycephaly, associated with numerous changes in the skull and teeth and, in at least two phyla, with longer and shorter limbs. Second, four distinct types in the shape and position of the horns, correlated with the structures of the nasals and frontals and indicative of different modes of combat among the males. (See fig. 209.) Third, canines of different form; and, finally, the presence of one or two pairs of functional incisor teeth, or the total degeneration of these teeth. Titanotherium Leidy applies to long-limbed animals with long skulls, persistently long and broad nasals, short triangu- MegaceropSy Upper Beds. Diplodonus, Upper Beds. Syiiiborodoti. Upper Beds, .Brontotkcriuv!, Upper Beds, Figure 209. — Characteristic basal sections of horns of Oligocene titanotheres l.ar horns placed slightly in front of the eyes, vestigial incisors, ^:?, large canine teeth. Known from the base to the summit of the Oligocene. Megacerops Leidy applies to titanotheres with broad skulls, nasals progressively shortening, short horns rounded or oval in section, shifting anteriorly, one or two pairs of incisor teeth, ^, medium-sized canine teeth. Known from the base to the summit of the Oligocene. Probably related to this are the subgenera of the types named Allops and Diploclonus by Marsh, differing from the above in horn characters. Known chiefly from the upper beds. Symborodon Cope includes titanotheres with skulls of vary- ing proportion, nasals slender and progressively shortening, horns elongate and peculiar in being placed above the eye instead of shifting forward, incisors vestigial, |^, canines small, appro.ximated. Known only from the middle and upper beds. Brontotherium Marsh embraces the largest titanotheres, with very broad zygomatic arches, nasals shortening while horns elongate and shift forward; incisors persistent, f in the males, canines stout and obtuse. Representatives of Titanotherium and Megacerops can now be continuously traced from the base to the summit of the Oligocene. Primitive species of Brontotherium also appear at 246 TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA the base, although the phyletic sequence through the middle to the upper beds is not so clear. Symborodon suddenly appears in the middle beds. The names of three of the genera thus recognized were subsequently changed, for the reasons given, as follows: For " Titanotherium Leidy" was substituted Menodus Pomel; for " Ilegacerops Leidy" was substi- tuted Brontops Marsh; for "Symborodon Cope" was substituted Megacerops Leidy; " Brontotherium Marsh" was permanently accepted. The phyla subsequently were called subfamilies. (See below.) RECLASSIFICATION OF THE EOCENE AND OLIGOCENE SUBFAMILIES BY OSBORN (1914) Original reference. — Geol. Soc. America Bull., vol. 25, pp. 403-405, Sept. 15, 1914 (Osborn, 1914.409). Reasons for reclassification. — Osborn makes the following statement : Recent discoveries have modified the author 's earlier opinions as to the lines of descent of the titanotheres, and still further changes are anticipated with increase of knowledge of the connections between upper Eocene, or Uinta, titanotheres and those of the lower Oligocene, or White River. The main lines of division are indicated in the proportions of the limbs, whether cursorial, mediportal, or graviportal; the proportions of the skull, whether mesaticephalic, brachy- cephalic, or dolichocephalic; the development of frontonasal horns, whether accelerated or retarded; the molarization of the premolar teeth, whether accelerated or retarded; the presence or absence of incisor teeth; the abbreviate or elongate, the tri- angular or oval form of the frontonasal horns as developed in Oligocene times. The new arrangement. — With these criteria the vari- ous phyla were distinguished in 1914 as follows: A. Wind River titanotheres, face longer than cranium: I. Lambdotheriinae, light-limbed, cursorial: Lambdotherium. II. Eotitanopinae, medium-limbed, mediportal: Eotitanops. B. Bridger and succeeding titanotheres, cranium longer than face: III. Palaeosyopinae, short-limbed, brachycephalic : Palaeosyops, Limnohyops. IV. Telmatheriinae, mesaticephalic to dolichocephalic: Telmatherium, Sthenodecies. V. Diplacodontinae, dolichocepahlic, with accelerated molarization of the premolars, imperfectly known: Diplacodon. VI. Manteoceratinae, mesaticephalic to brachycephalic, accelerated development of the horns, mediportal: Manteoceras, Protitanotherium. VII. Dolichorhinae, mesaticephalic to dolichocephalic, limbs, so far as known, abbreviate: Dolichorhinus, Mesatirhinus, Sphenocoelus, Meta- rhinus, Rhadinorhinus. VIII. Menodontinae, mesaticephalic to dolichocephalic, with abbreviate, triangular horns, with incisor teeth reduced or wanting, feet and limbs elongate: Menodus { = Titanotherium), Allops. IX. Brontopinae, brachycephalic, horns abbreviated, rounded, or oval, incisors persistent: Brontops {= Megaceratops^") , Diploclonus. " Error; should have been Megacerops. B. Bridger and succeeding titanotheres — Continued. X. Megaceropinae, mesaticephalic to extreme brachy- cephalic, horns elongate, vertically placed, no in- cisor teeth: Megacerops {^Symborodon). XI. Brontotheriinae, mesaticephalic to brachycephalic, horns elongate, transversely flattened and diver- gent: Brontotherium. The free use of subfamily divisions to express the distinct phyletic series is similar to that which the author adopted in the phylogeny of the rhinoceroses. More conservative usage would have divided the titano- theres into four subfamilies only. Of these names of phyla those assigned to Nos. II, IV, V, VI, VIII, IX, X, and XI had apparently not hitherto been pub- lished, and those assigned to Nos. I, III, and VII, although they had been used in previous publications, mostly by other authors, were now used in a more restricted sense. Other subfamilies awaited further study and the discovery of connecting forms, namely : Diplacodontinae = ancestors of Menodontinae or Bronto- theriinae. Eotitanopinae = ancestors of Palaeosyopinae. Rhadinorhininae = ancestors of Megaceropinae. Each subfamily name is carried back as far as possi- ble — that is, to the point, even very remote, where the subfamily characters and tendencies of evolution are first clearly and unmistakably manifested. SPECIES WRONGLY REFERRED TO THE TITANOTHERES Palaeosyops minor Marsh, 1871 (=Anchippodus minor) Original reference. — Am. Jour. Sci., 3d ser., vol. 2, p. 36, 1871 (Marsh, 1871.1). Type. — "A molar tooth, from the right lower jaw, and probably by some other less characteristic re- mains" from Grizzly Buttes, Bridger Basin, Wyo. Present determination. — This specimen was wrongly referred to Palaeosyops, as was recognized by Marsh, Cope, and others. The specimen pertains to the order Tillodontia. Helotherium procyoninum Cope, 1872 Original reference.— Fed. Bull. No. 2, p. 466, 1872 (Cope, 1872.2). Synonymy. — LambdotTierium procyoninum Cope, Tertiary Vertebrata, pp. 631, 711, pi. 24, fig. 22, 1884 [1885] (Cope, 1885.1). "Syn.? of OroUppus pumilis," Hay (1902.1, p. 612). Hyracotherium procyoninum Matthew, Am. Mus. Nat. Hist. Bull., vol. 12, p. 45, 1899 (Matthew, 1899.1). OroMppus sp. Granger, Am. Mus. Nat. Hist. Bull., vol. 24, p. 227, 1908 (Granger, 1908.1). Daeodon shoshonensis Cope, 1878 Original reference. — Pal. Bull. No. 30, "December 3, 1878" (Cope, 1878.1). SYSTEMATIC CLASSIFICATION OF THE TITANOTHERES 247 Type and geologic horizon. — "The terminal portion of the lower jaw of a huge mammal " (Am. Mus. 7387), from the Miocene of Oregon. Present determination. — The genus and species be- long in the family Entelodontidae (Peterson, 1909.1, p. 63). SECTION 2. CLASSIFICATION OF THE TITANOTHERES ADOPTED IN THIS MONOGRAPH SYNOPSIS OF THE CLASSIFICATION The natural classification or ancestral tree of the titanotheres is based on the characters of the skull and teeth, as set forth in Chapters V and VI, com- bined with those of the limbs and feet, as set forth in ' Chapter VII. The full definitions of the family and of the 12 subfamilies into which the titanotheres are now divided are presented in Chapters V and VI, of which the following classification is a synopsis. It should be compared with the phylogenetic tree given m Chapter X (p. 769). Each of the chief phyla has H subfamily name. A. Wind River titanotheres, face longer than cranium: I. Lambdotheriinae, light-limbed, cursorial: Larnhdoiherium. II. Eotitanopinae ( = ?Palaeosyopinae), medium- limbed, mediportal: Eoiitano-ps. B. Bridger and succeeding titanotheres, cranium longer than face: III. Palaeosyopinae ( = ?Eotitanopinae), short-limbed, brachycephalic: Palaeosyops, Limnohyops. IV. Telmatheriinae, mesatioephalic to dolichocephalic: Telmatherium, Sthenodectes. V. Manteoceratinae ( = Brontopinae), mesaticephalic to brachycephalic, accelerated development of the horns, mediportal: Manteoceras, Protitanoiherium, Brachydiasie- ntatherium. VI. Dolichorhininae, mesaticephalic to dolichocephalic; limbs, so far as known, abbreviate; facial region downturned: Eomeiarhinus, Dolichorhinus, Mesatirhinus, Sphenocoelus, Metarhinus. VII. Rhadinorhininae ( = ?Megaceropinae), mesatice- phalic, facial region cyptocephalic, upturned: Rhadinorhinus. VIII. Diplacodontinae ( = ?Menodontinae, =7Bronto- theriinae), dolichocephalic, with accelerated molarization of the premolars, imperfectly known: Diplacodon, Eotitanoiherium. IX. Brontopinae (= Manteoceratinae), brachyce- phalic, horns abbreviated, rounded or oval, incisors persistent; premolars retarded: Teleodus, Brontops { = Megacerops), Diplo- clonus. X. Menodontinae ( = ?Diplacodontinae), mesatice- phalic to dolichocephalic, with abbreviate, tri- angular horns, with incisor teeth reduced or wanting, feet and limbs elongate, premolars accelerated: Menodus {= Titanotherium) , Allops. B. Bridger and succeeding titanotheres — Continued. XI. Megaceropinae ( = ?Rhadinorhininae), mesatice- phalic to extreme brachycephalic, horns elon- gate, vertically placed, no incisor teeth: Megacerops {=^ Symhorodon) . XII. Brontotheriinae ( = ?Diplacodontinae), mesatice- phalic to brachycephalic, horns elongate, trans- versely flattened and divergent, premolars accelerated: Bronlotherium. Suggestions as to resemblance or the affinity between subfamilies are given above in parentheses, and the families are arranged according to the general geologic sequence. One of these suggestions of ancestral affinity is now apparently well established, namely, that the Manteoceratinae are ancestors of the Bron- topinae. I. TITANOTHERES OF LOWER EOCENE TIME (Face elongate) Group I. Hornless: 1. Subfamily Lambdotheriinae Osborn. "Lamb- dotheres." (Lower Eocene titanotheres. Long-headed, very small; body and limbs slender and cursorial; face longer than cra- nium, slender.) Pages Genus Lambdolherium Cope 168,279,690 Species priscum Osborn 194, 286, 590 primaevum Loomis 178, 283, 590 popoagicum Cope 168, 281, 590 progressum Osborn 194, 286, 590 magnum Osborn 199,288,590 2. Subfamily Eotitanopinae (— ?Palaeosyopinae) Osborn. " Eotitanopines. " (Lower Eocene titanotheres of intermediate size. Head of medium length; body and limbs less slender and cursorial than in the lambdotheres; gait submediportal; face longer than cranium.) Genus Eoiitanops Osborn 179, 289, 591 Species gregoryi Osborn 192, 291, 593 brownianus (Cope) 169,292 borealis (Cope) 168,292,593 princeps Osborn 193,295,593 major Osborn 193, 296, 597 minimus Osborn 199,296 II. TITANOTHERES OF MIDDLE AND UPPER EOCENE TIME (Face abbreviate) Group II. Retarded horn rudiments: 3. Subfamily Palaeosyopinae (=? Eotitanopinae) Steinmann and Doderlein. "Palaeosyo- pines." (Titanotheres larger than tapirs. Broad-headed, skull and limb proportions be- coming stout; skull broad; zygomata progres- sively brachycephalic; grinders small; nasals tapering distaUy; face shorter than cranium; feet abbreviate, brachypodal; gait gravi- portal.) Pages Genus Limnohyops Marsh (mesaticephalic to brachycephalic) 170,303,612 Species prisons Osborn 180, 306 laevidens (Cope) 163,305 matthewi Osborn 180, 308 monoconus Osborn 180, 309, 614 laticeps Marsh 160, 311, 618 248 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA Group II. Retarded horn rudiments — Continued. 3. Subfamily Palaeosyopinae — Continued. Pages Genus Palaeosyops Leidy (bracliycephalic, hyperbrachy cephalic) 157, 312, 619 Species ffoniinalis Cope 165,317 longirostris Earle 172,319 paludosus Leidy 157, 319 major Leidy 158, 321, 620 grangeri Osborn 181, 335 leidyi Osborn 18 1, 323, 620 robustus (Marsh) 161, 331, 626 copei Osborn 181, 336, 629 4. Subfamily Telmatheriinae Osborn. "Telma- theres." (Middle and upper Eocene titano- theres of larger size. Heads of medium length, with large cingulate incisors and heavy, sub- lanceolate canines; grinders large; mesatice- phaUc {Telmatherium) or subbrachycephalic (Sthenodectes) ; of mediportal gait.) Genus Telmatherium Marsh (mesaticephalic, narrow sagittal crest) 160, 340 Species cuUridens (Osborn, Scott, and Speir) 168, 341 validum Marsh 160, 344 altidens Osborn 184, 351 ultimum Osborn 184, 345 Genus Sthenodectes Gregory (mesaticephalic to subbrachycephalic) 190, 353 Species incisivus (Douglass) 185, 354 Group III. Accelerated horn rudiments : 5. Subfamily Manteoceratinae (Brontopinae) Osborn. " Manteoceratines " (prophet-horn), "bronto- pines." (Precociously horned titanotheres, of the same stock as the Dolichorhininae. Skull mesaticephalic, face abbreviate; feet abbre- viate, brachypodal, gait graviportal; premolars retarded, incisors rounded; ancestral or related to the Brontops phylum of the Oligocene.) Genus Manteoceras Hatcher (horns rudi- mentary) 177, 362, 631 Species manteoceras Hay 177, 365, 631 washakiensis Osborn 182,371 uintensis Douglass 186, 372 Genus Protitanotherium Hatcher (horns elon- gate, oval, more prominent than in Man- teoceras) 176, 375 Species emarginatum Hatcher 177, 377 superhum Osborn 185, 379 Genus Brachydiastematherium Bockh and Maty (large size; upper Eocene of Tran- sylvania) 166, 382 Species transilvanicum Bockh and Maty 166, 382 6. Subfamily Dolichorhininae Riggs. "DoUcho- rhines" (long-snouted). (Middle and upper Eocene titanotheres. Typically dolichocepha- lic and dolichopic; nasals typically long and expanding distally; precocious horn rudiments; infraorbital shelf usually conspicuous.) Genus Eometarhinus Osborn (ancestral to Metarhinus, primitive, mesaticephalic). 200, 419 Species huerfanensis Osborn 200, 420 Group III. Accelerated horn rudiments — Continued. 6. Subfamily Dolichorhininae — Continued. Pages Genus Mesatirhinus Osborn (ancestral to Dolichorhinus; subdolichocephalic) . 182,387,636 Species Junius (Leidy) 159, 388 megarhinus (Earle) 170, 388 peiersoni Osborn 182, 389, 641 Genus Dolichorhinus Hatcher (extremely dolichocephalic, cyptocephalic; becoming extinct) 177, 396, 645 Species superior (Riggs) 190, 395, 405 longiceps Douglass 188,406,651 vallidens (Cope) 162, 401 heterodon Douglass 187,416 intermedins Osborn 1 84, 405 hyognathus (Osborn) _ 169, 173, 409, 646 Jluminalis Riggs 191, 417 Genus Metarhinus Osborn (dwarfed, aber- rant, mesaticephalic) 183, 420, 648 Species earlei Osborn 183, 420 fluviatilis Osborn 183, 421 cristatus Riggs 191, 429 riparius Riggs 191, 429 Genus Sphenocoelus Osborn (little known; (?) branch of Mesatirhinus) 174, 417 Species uintensis Osborn 175,419 7. Subfamily Rhadinorhininae ( = ?Megaceropinae) Osborn. " Rhadinorhines " (slender - nosed) . (Middle Eocene titanotheres. Mesaticephalic, cyptocephalic; infraorbital shelf reduced.) Genus Rhadinorhinus Riggs (nasals short, pointed; possibly ancestral to the Oligo- cene Megacerops; cyptocephalic) 192, 430 Species abbotti Riggs 192, 430 diploconus (Osborn) 173, 431 Group IV. Short-horned: 8. Subfamily Diplacodontinae (=?Menodontinae, Brontotheriinae) Osborn. " Diplacodonts. " (Upper Eocene ancestors of the Oligocene sub- family Menodontinae. Heads probably mesa- ticephalic; grinding teeth foreshadowing the menodont type.) Genus Diplacodon Marsh (horns well devel- oped) 166,439 Species eZa^Ms Marsh 166,439 Genus Eoiitanotherium Peterson (horns well developed) 196, 435, 656 Species osborni Peterson 195, 435, 656 in. TITANOTHERES OF LOWER OIIGOCENE TIME (Face extremely abbreviate) Group I. Short-horned: 9. Subfamily Brontopinae (Manteoceratinae) Osborn. " Brontopines. " (Lower Oligocene and possibly middle to upper Eocene titanotheres. Progres- sively brachycephalio, with short-crowned teeth and moderately short feet; horns short, sub- oval; incisor teeth persistent, rounded crowns, one or two pair; premolars with retarded tetartocones.) Pages Genus Teleodus Marsh (with three lower incisors; basal Oligocene) 227,481 Species avus Marsh 228, 481 primitivus (Lambe) 235, 482 SYSTEMATIC CLASSIFICATION OF THE TITANOTHERES 249 Group I. Short-horned — Continued. Pages 9. Subfamily Brontopinae — Continued. Genus Brontops Marsh (with two or one lower incisors; lower Oligooene) . 222, 482, 664-676 Species brachycephalus (Osborn) 231, 483, 675, 676 dispar Marsh 223, 488, 664 robustus Marsh i___ 222, 492, 666 fangustigenis (Cope) 219,482 Genus Diploclonus Marsh (with internal branching horns; lower Oligocene) 227, 499, 675-678 Species ftyleri (Lull) 234, 502, 675 fbicornutus (Osborn) 231,501 amplus Marsh 227, 504 selwynianus (Cope) 225,502 10. Subfamily Menodontinae ( = ?Diplacodontinae) Osborn. "Menodonts." (Lower Oligocene and possibly upper Eocene titanotheres. Heads of medium width, progressively elongating (Menodus) or broadening (Allops); horns short, trihedral in section; incisor teeth vesti- gial; grinding teeth long-crowned with promi- nent cingula; premolars with accelerated tetartocones.) Genus Menodus Pomel {— Titanolherium Leidy) 204, 522, 681 Species heloceras (Cope) 212, 524, 681 iorvus (Cope) 210,525 proutii (Owen, Norwood, and Evans) 205,526 trigonoceras {Cope) 213,528,683 varians (Marsh) 223,535 giganieus Pomel 204, 530, 687 Genus Allops Marsh 224, 506, 678 Species walcotli Osborn 241, 509 marshi (Osborn) 233,511,678 serotinus Marsh 225, 515 crassicornis Marsh 228,517,679 Group II. Long-horned: Pages 11. Subfamily Megaceropinae ( = ?Rhadinorhininae) Osborn. "Megaceropines," "symborodonts." (Relatively small, long-horned titanotheres, possibly descended from Rhadinorhinus. Of lower Oligocene age. Horns precociously evolved, with little or no connecting crest; head mesaticephalic to brachycephalic, oypto- oephalic; narrow-lipped; premolars with pre- cocious tetartocones; grinding teeth without cingulum; vestigial incisor teeth.) Genus Megacerops Leidy { = Symborodon Cope) (horns rounded, erect) 208,541,691 Species riggsi Osborn 242, 550 assiniboiensis Lambe 239, 549 copei (Osborn) 235, 548 acer Cope 211, 545 bucco (Cope) 212, 544 coloradensis Leidy 208, 544 ?syceras (Cope) 226, 549 12. Subfamily Brontotheriinae ( = ?Diplacodontinae) Osborn. "Brontotheres." (Lower Oligocene titanotheres. Primitively dolichocephalic, pro- gressively mesaticephalic and brachycephalic, slightly cyptocephalic; broad-lipped; very pre- cocious development of the horns; accelerated development of internal cones of superior pre- molars; prominent cingulate incisor teeth in males.) Genus Broniotherium Marsh (horns progres- sively elongate, nasals abbreviate; lower Oligocene) 209, 555, 690 Species leidy i Osborn 234, 558, 690 hypoceras (Cope) 216,562 hatcheri Osborn 235, 563, 695 tichoceras (Scott and Osborn). 219, 565 gigas Marsh 209, 567 dolichoceras (Scott and Osborn). 220, 572 medium (Marsh) 228,576 curtum (Marsh) 224, 574 ramosum (Osborn) 231,577 platyceras (Scott and Osborn). 221, 578 frumelicum (Toula) 230, 560 Note. — Additional species are described and classified in the appendix, including Mongolian, east European, and Burmese titanotheres. -29— VOL 1 19 CHAPTER V EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES SECTION 1. GENERAL PRINCIPLES OF THE STUDY OF THE CHARACTERS OF THE SKULL AND TEETH PROPORTION CHARACTERS AND TENDENCIES OF EVOLU- TION DISTINGUISHED BY ANALYSIS AND SYNTHESIS The key to the evolution of the titanotheres is afforded by the analysis and synthesis of the separate characters of which the individuals in each line of ascent are composed — characters large or small, single or multiple — in correlation with one another and in correlation with the individual as a whole; characters progressing or retrogressing in successive generations; characters evolving rapidly or evolving slowly: such is the composition of each individual titanothere, as well as of each phylum. Out of an almost infinite number of characters that are independently evolving we select a few that are visible and measurable. In a few individuals we observe the origin of new characters, but generally we observe the changes of form and of proportion in existing characters, which make up the greater part of the transformation of the individuals composing the family. In heredity each character is a separate unit, completely separable from all others; in adap- tation it is correlated with other characters of the individual, as is fully explained in Chapter IX. DISTINCTIONS BETWEEN PROPORTION CHARACTERS AND NEW RECTIGRADATION CHARACTERS Methods employed. — The present chapter explains how the characters of titanotheres have been observed, examined, and measured, partly by new methods, largely devised especially to solve the problems that have arisen in the task of working out the genealogy of this family, and partly by old methods, which have been in use by paleontologists and systematists. To distinguish the characters of the teeth, skull, and limbs, which are generally but fragments, necessitates very refined and precise systems of measurement and comparison, because the individual members of differ- ent lines of descent may be very close to each other in certain characters yet readily separable in others. Animals that the zoologist would readily distinguish as species and subspecies by their external coloring, bodily form, or habits of life may be extremely simi- lar in skeletal characters, yet the close methods of measurement and analysis that we have been com- pelled to adopt prove that every character has dis- tinctions that may be revealed by minute and precise observation. Researches on proportion characters. — The chief papers on the principles of evolution of the mammaJian skull and teeth which the author has published in the investigation of the titanotheres are the following: 1896.110. Th • cranial evolution of Titanotherium: Am. Mus. ra . Hist. Bull., vol. 8, art. 9, pp. 157-197, July 31, 1896. 1902.207. Dolichocephaly and brachycephaly in the lower mam- mals: Am. Mus. Nat. Hist. Bull., vol. 16, art. 7, pp. 77-89, Feb. 3, 1902. 1902.208. The four phyla of Oligocene titanotheres: Am. Mus. Nat. Hist. Bull., vol. 16, art. 8, pp. 91-109, Feb. 18, 1902. 1907.301. Evolution of mammalian molar teeth to and from the triangular type, 250 pp., New York and Lon- don, Macmillan Co., September, 1907. 1912.368. Skuh measurements in man and the hoofed mammals: Science, new ser., vol. 35, No. 902, p. 596, Apr. 12, 1912. 1912.372. The continuous origin of certain unit characters as observed by a paleontologist (Harvey lecture) : Am. Naturalist, vol. 46, No. 544, pp. 185-206, April, 1912; No. 545, pp. 249-278, May, 1912; Harvey Soc. Volume, 7th ser., pp. 153-204, No- vember, 1912. 1912.382. Craniometry of the Equidae: Am. Mus. Nat. Hist. Mem., new ser., vol. 1, pt. 3, pp. 57-100, figs. 1-17, June, 1912. 1915.416. Origin of single characters as observed in fossil and living animals and plants (Presidential address before the Paleontological Society of America, Dec. 31, 1914) : Am. Naturalist, vol. 49, No. 580, pp. 193-239, AprO, 1915. Proportion characters defined hy indices and ratios. — Many specific, generic, and subfamily characters of animals can be best expressed in mathematical ratios and indices, for these figures record most precisely the movements or tendencies of development that lead from species to species. In all ascending series of titanotheres every measurable character is in a state of movement either progressively or retrogressively. Significance of tendencies in proportion. — A tendency or trend to evolve in a certain proportional direction is found to be a phyletic distinction of prime im- portance, which leads us through all the stages of mutative, specific, generic, and subfamily characteris- tics. For example, certain titanotheres become more and more broad-headed from lower to higher geologic levels; progressive brachycephaly thus becomes a phyletic character of taxonomic value. One genus may be defined as "progressively brachycephalic," whereas a related genus, in which the tendency to become long-headed prevails, may be defined as " progressively dolichocephalic." Taxonomic value oj proportion tendencies.- — Thus the true relations of most of the lines of descent among the Eocene and Oligocene titanotheres have been grad- ually discovered, partly by the old methods of descrip- tive anatomy, used by Leidy, Marsh, and Cope, and partly by the new methods which have been developed since 1900 in the preparation of this monograph. It 251 252 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA has been found that the changing proportions of the various parts of the slvull, of the individual grinding teeth, of every part of the skeleton, especially the limbs, are highly distinctive systematic and phyletic characters. Five distinctions of 'phyla. — Each line of descent is distinguished by five different methods: First, by the presence or absence of certain characters; second, by the new proportions of certain characters; third, by the tendencies or directions in which proportions are being changed; fourth, by the rates of change in proportion characters, whether retarded or acceler- ated; fifth, by the appearance of new rectigradation characters. Numerous extinct iranclies or phyla. — The distinc- tions in characters multiply with the multiplication of the phyla. In 1914 no less than 20 branches of the titanothere famdy were known, and probably many more existed that had not yet been discovered. Throughout Eocene time titanotheres continued to migrate into the mountain region of the Bridger and Washakie Basins of Wyoming. Allowing for certain branches that drop out, we find that the number of their known branches constantly increases from lower to higher levels, as shown below. Oligocene : White River group 7-8 Summit of upper Eocene: Lower part of Uinta C (true Uinta formation), Uinta Basin, Utah 4 Upper Eocene: Uinta B 2 of Uinta Basin, Utah; Washakie B 2 of Washakie Basin, Wyo 6 Upper Eocene: Washakie B 1 of Washakie Basin, Wyo.; and Uinta B 1 of Uinta Basin, Utah 8 Middle Eocene: Bridger C and D of Bridger Basin, Wyo.; Wasliakie A of Washakie Basin, Wyo 5 Middle Eocene: Bridger A and B of Bridger Basin, Wyo.; Huerfano B, Huerfano Park, Colo 2 Lower Eocene: Wind River formation, Wind River Basin, Wyo.; Huerfano A, Huerfano Park, Colo 2 Universal change oj form. — No characters in any genus or phylum are stationary. During the long intervals of geologic time the members of each of these branches were constantly diverging in some characters and converging in others and becoming more and more unlike one another both as a whole and, so far as we can observe, in each one of their single characters. ATLometrons and rectigradations. — The term allome- trons (Osborn, 1912.372, pp. 249-278) designates characters that arise through continuous changes of size or proportion in old features — that is, purely quantitative changes — such as may be expressed in differences of measurement as well as in indices and ratios. Rectigradations are new characters that tend to evolve in a definite direction — the earliest "rudi- ments" or discernible stages of absolutely new forms. In 1889 Osborn called such characters "definite vari- ations" (Osborn, 1907.301, p. 239). Six points in the distinction between allometrons and rectigradations may be readily grasped: (1) When the shadowy beginning of a new cusp on the grinding teeth or the rudiment of a horn is first dis- cernible as a new character it appears as a "rectigrada- tion"; (2) when this same rudiment of a cusp or horn takes on a new shape the change of form appears as an " allometron " ; (3) in the hard parts of a titano- there, as of any other mammal, the rectigradations — the numerically new characters of any kind — are com- paratively few and uncommon, but the allometrons — the transformations of existing characters — comprise the larger number of changes; (4) both allometrons and rectigradations are distinctly heritable characters; (5) in the genesis of rudiments (rectigradations) of new cusps or of horns all the branches or phyla of titano- theres sooner or later tend to resemble one another — that is, to develop the same cusps and the same horn swellings — and thus to become convergent; (6) on the other hand, in changes in the proportions (allometrons) of the skull, the different phyla may differ widely from one another and through dissimilar allometrons may become divergent. (See fig. 210.) STEPS IN TRANSFORMATION OF CHARACTERS So far as we have observed, all absolutely new char- acters that we have traced to their very beginnings in titanotheres arise gradually and continuously; there is no evidence of sudden leaps from mutation to muta- tion or from species to species. This continuous mode of evolution is more fully considered in Chapter IX. The addition (rectigradation) or the modification (allometron) of a single character is theoretically the first step in transformation, but as a matter of fact all characters are being simultaneously more or less modified, and in the individual as a whole new char- acters are constantly being added. Only when fully developed after the lapse of many generations does a rectigradation or an allometron become of sufficient systematic value to define the mutation or the species. None the less each of these changes forms one in a series of steps in the transformation of species. One by one the characters, either rectigradations or allometrons, in many parts of the titanothere are inde- pendently changed until the changes build up what paleontologists call an "ascending mutation" in the sense in which the German invertebrate paleontologist Waagen defined this term in 1869. An ascending mu- tation is a stage in a continuous evolutional ascent in one or more characters from one species to another; there is no evidence that it is a saltation or "mutation" in the sense of that word as used by De Vries. Finally these rectigradations and allometrons attain by accumulation sufficient importance to enable us to call a stage a "species" in the Linnaean sense or a "subspecies" in the modern sense. The divergence between the several branches of the titanothere family therefore actually consists of the sum total of changes in an almost infinite number of single characters, only a few of which are measurable. These changes are of the following principal lands: EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITAN0THERE3 253 V w m I Recit^ra^daiionS NetM cliarcioiers, similar, hu-i ept Dnylu.) jillomeiroyvs New proporiions disii-noiiz>e oj ea-ch phylum. Dijjeveni -ow alt oiher phyla. t,n evey xj phyL FiGUBE 210. — Skulls showing different numerical and proportional characters in five separate phyla of titanotheres Similar numerical characters (rectigradations, A, B) and dissimilar proportional characters (allometrons, C, D), all arising independently in descendants of the same ancestors. Each of the five phyla (I-V) exhibits similar rectigradations of the premolar teeth and in the osseous horn rudiments (H) but dissimilar allometrons of the sliull ( C) and of the foot bones {D). pa'', mes^, New cuspules on the teeth! H, rudi- diments of the newly arising horns. I, Eotitanops, a dolichocephalic ancestral form; II, Palaeosyops, brachycephalic; III, Telmatherium, mesaticephalic; IV, Manteoceras, mesaticephalic; V, Dolichorhmus, dolichocephalic. 254 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 1. Loss of old characters (paleomorphs) : Absolute loss of character is rare in the titanotheres. The tra- pezium is the only bone known to be lost duriag the recorded history of the family, whereas in the horses many bones disappear. The incisor teeth disappear in several phyla. The grinding teeth lose certain cusps. 2. Appearance of new characters (neomorphs), in- cluding rectigradations : A large number of new cusps appear on the premolar grinding teeth, and a few new features appear on the molar grinding teeth. A pair of new hornlets appear on the frontonasal region of the skull. 3. Increase or diminution of size as a whole (hetero- morphs) : Increase of size is the prevailing tendency among the titanotheres, but in some phyla size is per- sistent or is even arrested, as in the dwarf Metarhinus; or it is reduced, as in the species Palaeosyops copei. 4. Change in proportions of different regions of the skeleton (heteromorphs, including allometrons) : Such changes are expressed in ratios — for example, length of tibia length of femur These ratios are extremely significant. Thus one phy- lum may become short limbed or brachymelic, another long limbed or dolichomelic ; one short footed or brachypodal, another long footed or dolichopodal; one phylum may become large headed or macrocephalic, another small headed or microcephalic. 5. Change in proportions of single parts of the cranial skeleton (allometrons) : Such changes are best expressed in indices, such as width of skull length of skull Some series become long headed or dolichocephalic, others broad headed or brachy cephalic; some become long faced or dolichopic, others become short faced or brachyopic. The manner in which these changes of proportion (allometrons) and the successive addition of rectigra- dations serve to distinguish the genera of Eocene titanotheres from one another is clearly shown in the following descriptive characterizations of ten Eocene genera : Lambdotherium: Small, long headed, long limbed, without horns. Eotitanops: Larger, long faced, limbs somewhat heavier, horn- less. Limnohyops: Still larger, broad headed, short faced, light limbed, broad footed, hornless. Palaeosyops: Massive, broad headed, short faced, heavy limbed, short footed, rudiments of horns (rectigradations). Telmatherium: Large, long headed, short faced, light limbed. rudiments of horns (rectigradations). Manteoceras: Large, medium headed, short faced, medium limbed, short footed, small, distinct horns (rectigradations). Mesatirhinus: Of medium size, narrow headed, short faced, light limbed, long footed, small horns quite distinct. Metarhinus: Very small, medium headed, short faced, light limbed, horns not very distinct. Dolichorhinus: Large, extremely long headed, short limbed, short footed, horn rudiments very prominent. Rhadinorhinus: Medium size, medium headed, light limbed, horn rudiments indistinct. The degrees of change among the "species" con- stituting each of these "genera" are exemplified in the "standard measurement tables" that accompany the description of every genus in Chapters V and VI. In these tables it is demonstrated, first, that the Linnaean lines of division between species do not exist ; second, that occasionally the type and paratype spec- imens of a single species selected by the pioneer pale- ontologists belong to separate stages because they were found at different geologic levels. Some of these ancient specific names have historic value and are retained for convenience, although some that were applied to forms on the border line between two specific stages are very inconvenient. PROPORTIONS AND FLEXURES OF THE SKULL The skull is the chief center of evolution movement in the titanotheres, both in the transformation of its pro- portions and in the development of horns, and with the teeth it furnishes a complete key to the evolution, relationship, and ascent of these mammals, although the proportions of the skeleton and the feet also furnish valuable indications. The forms of the feet, which are evolving so rapidly and are so significant in the horses, are relatively stationary in the titanotheres. The chief principles in the transformation of the skull through changes of proportion are the following: 1. Elongation and narrowing of the skull as a whole — that is, dolichocephaly. 2. Abbreviation of the cranium and elongation of the face — that is, proopic dolichocephaly or dohchopy, as in Equus. 3. Abbreviation of the face and elongation of the cranium. — that is, postopic dolichocephaly or brachyopy, as in all Oligocene titanotheres. 4. Abbreviation and broadening of the skull as a whole — that is, brachycephaly. 5. Flexure of the facial upon the cranial region — that is, cyptocephaly. The principal measurements of the skull in the titanotheres differ somewhat from those employed in the craniometry of the Equidae. (Osborn, 1912.382.) They are listed below and are illustrated in Figure 211. Direct measurements of skull 1. Cephalic or basilar length from incisive border to occipital condyles, inclusive. 2. Facial length from postorbital process to incisive border, inclusive (projected on the basilar Une). 3. Cranial length from postorbital processes to occipital condyles, inclusive (projected on the basilar line). 4. Facial breadth, or frontal width, as measured in the horses across the postorbital processes to establish the cephalic index. This measurement is not practicable in titanotheres. 5. Zygomatic breadth across the widest part of the zygomatic arches. 6. Premolar-molar length, or superior grinding series (p' to m^ if the first premolar is retained, otherwise p^ to m^). 7. Molar length, anteroposterior measurement along middle of crowns (m' to m') . EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 255 8. Molar length and width, anteroposterior measurement of first superior molar (m') and transverse measurement of the same (least width, near center of crown). Indices of skull (expressed as per cent) - „ , . , ,. ■ J breadth across zygomata 9. Zygomatic-cephalio mdex=r — n , .. ^ — —nr ■^ ° basilar or cephalic length length of six superior r, 1 1 1, 1- • J grinding teeth, p^-m' Premolar-molar cephahc index=^ iT't — j — <£ iiiujc^i ^^ij^ia.j^ iiiiic^v cephalic length I, ,, , u 1- ■ J length of upper true grinders, m'-3 II. Molar-cephalic index =■ — ^, -,.-, -,-, '^ cephalic length , „ - , , . , breadth of a molar, mi or m^ 12. Molar index= -. rr — c i length of a molar lensth of face ^' 13. Faciocephalic index= ^r^. — , — -r;— '^ cephalic length length of cranium -' cephalic length 14. Craniocephalic index =- Flexures of skull 15. Palatooranial flexure = angle that the palate, from the posterior to the incisive border, makes with the basal line of the cranium. (Not used in the study of titanotheres.) 16. Faciocranial flexure = angle that the preorbital part of the skuU, determined from the optic foramen (see figs. 213, 214) to the incisive alveolus, makes with the line from the optic foramen to the middle of the occipital condyle. SUMMARY AS TO CRANIOMETRY 1. Direct measurement. — Since the fossil skulls and dental series are rarely complete or perfect, the paleon- tologist requires an additional series of direct detailed measurements of parts of the skull and teeth that are not needed by the zoologist. 2. Significance. — Every one of these direct measure- ments, indices, and angles is significant, because all skulls are in a continuous process of movement, or evolution. The indices are even more significant than the direct measurements, because every genus and probably every species has its distinctive indices in adult specimens, and the direct measurements vary greatly with the age, sex, and individual variation of the specimen. 3. Imperfection. — In fossil skulls the indices are often difficult to determine; a slight crushing or dis- tortion seriously disturbs the index, for a skull that is crushed on its side is narrowed and lengthened at the same time. Nevertheless, the indices and ratios should be used wherever obtainable. 4. Age. — The proportions between the several parts are largely altered with the age of the animal; this statement is especially true of progressive allometrons, such as the proportion between the face and the cranium. Thus the faciocephalic or craniocephaHc index may alter rapidly as the titanothere advances from youth to maturity; similarly the flexure (cyp- tocephaly) becomes extreme only in mature skulls. The age of the animal measured is thus to be con- sidered in all the indices and ratios of the skull, teeth, and skeleton. '* As projected on basilar line. 5. Sexual correlations. — Certain proportions and indi- ces are correlated sexual characters — that is, in brachy- cephalic phyla the males have relatively broader heads than the females. For example, we observe in the genus Brontops the following proportions : Males of Brontops validus (braohycephalio), indices 73-87. Females of Brontops validus (mesaticephalic), indices 60-70. FiGUBE 211. — Standard measurements of Eocene titanothere skulls Palatal view of a skull of a middle Eocene titanothere, Mesatirhinus petersoni, showing (by arrows) how the basilar or cephalic length, the zygomatic width, and the length and width of the upper pre- molars and molars are measured. 6. Effects of crushing. — The indices of fossil skulls are profoundly modified by vertical or lateral crush- ing: vertical crushing tends to give brachycephalic indices; lateral crushing tends to give dolichocephalic indices. To these facts are due in part the wide variations in the tables of indices, especially in the chapter on the Oligocene titanotheres. 256 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA CHANGING PROPORTIONS OF THE CRANIUM AND FACE The back of the eye socket, or orbit, is the dividing line between the face, which lies in front of it, and the cranium, which lies behind it, as indicated by the shaded and unshaded parts of the skulls in Figure 212, which shows that in the oldest true titanothere (Eotitanops) of lower Eocene time, as in the oldest true horse (Eohippus) of the same period, the face (shaded) and the cranium (outline) are equally long — in fact, in Eotitanops the face is a little longer than the cranium. The imiversal allometric character of titanotheres is abbreviation of the face (proopic region) and elonga- tion of the cranium (postopic region) — that is, brachy- opy and dolichocrany. Thus in all middle and upper Eocene titanotheres the cranium is longer than the face. This disparity keeps increasing until in the Oligocene titanotheres, such as Brontotherium (fig. 212), the face is greatly abbreviated and the cranium greatly elongated. In the horses (Equus) this allome- try is just reversed: the face becomes very long (fig. E7otita7iop5 Figure 212. — Unequal elongation of face and cranium in titanotheres and horses Eoliianops and Eohippus, primitive perissodactyl type; face and cranium sub- equal in length. BTontoiheTium, titanothere type; face abbreviated, cranium elongated. B^uus, Equidae type; face elongated, cranium abbreviated. 212), but the cranium remains very short (dolichopy and brachycrany). These differ- ences are expressed in the so-called facio- cephalic index, which is obtained as follows: length of face including orbits X 100 basilar length of skull The relative faciocephalic indices in titano- theres and horses are as follows: author (Osborn, 1912.382); the term "cyptocephaly" is correct. In primitive ungulate skulls and in the fetal skull the anteroposterior planes of the face and palate and of the basicranial axis are more nearly in parallel lines — that is, the skulls are "orthocephalic." In certain specialized ungulates there is either an upward or a downward deflection of the face on the Figure 213. — Faciocranial flexure, or cypto- cephaly In the reindeer {RoTigifeT) the face is much less bent upon the cranium than in the hartebeest (Bubalis). A similar but less pronounced contrast is seen in the Eocene titanotheres Palaco- syops and Solichorhinus. cranium, which appear respectively to be adapted to different forms of feeding, as follows: (1) Horizontal and upward flexure of the face is characteristic of certain browsing types, such as Alces and Rangifer; (2) downward flexure of the face and palate on the basicranial axis is characteristic of certain grazing types, such as the hartebeest (Bubalis) and other grazing antelopes (see fig. 213); (3) in the young of certain species of Equidae, Bovidae, and Cervidae the palatal line makes an angle of 19° to 25° with the Titanotheres : Brontotherium, 33. Eotitanops, 56. Horses: Equus, 70. Eohippus, 53. In the titanotheres the imiversal tendency of facial abbreviation and cranial elongation distinguishes all the branches alike, but since the allometric movement takes place at un- equal rates each genus or phylum has its YiGvnn 214.— Faciocranial flexure in Palaeosyops (A), orthocephalic, and Dolichorhinus (B), cyptocephalic distinctive faciocephalic index CYPTOCEPHALY, OR FACIOCRANIAL FLEXURE The upward or downward flexure of the facial and palatal parts of the skull upon the basicranial axis was first erroneously termed "cytocephaly" by the basicranial line, which may increase to 53° in the adults of extremely deflected types. Among the many authors who have more or less directly contributed to this subject are Riitimeyer EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 257 (Cervidae, 1882.1), Flower (1885.1, pp. 185-201), Lan- kester (Giraffidae, 1902.1), Ewart (Equidae, 1907.1). Riitimeyer pointed out that the Cervidae, as brows- ing animals, have an extremely horizontal axis of the skull, in contrast with that of the Bovinae, grazing animals, in which the face is strongly deflected. Lankester pointed out that in the hornless Olcapia, a forest animal that browses on the leaves of trees, the facial and cranial regions are orthocephalic, or nearly in the same horizontal plane; in the related Giraffa, however, which also browses on high trees but possesses horns, the face is deflected on the cranium almost as much as in the grazing sheep (Ovis), which feeds upon the ground. Lankester consequently attributed the Osborn's examination of the horses (1912.382, p. 96) shows that in the adult domesticated horse the palatocranial angle ranges from 10° to 17°; in the Burchell zebra the palatocranial angle increases with age from 19° at three years to 25° at six years. In the domesticated horses it varies from 20° to 23°. In the Burchell zebras it increases from 15° at birth to 25° at the fourth year. Since the asses are more given to browsing than the horses or zebras the slight difference in flexure may be attributed to the prevailing browsing habit. (See p. 259.) In titanotheres the faciocranial angle is measured as shown in Figure 214. In a comparison of the brachy- cephalic Palaeosyops, presumably a browsing type with Figure 215. — Cranial proportions of Eocene titanotheres — Palaeosyops, Manteoceras, and Dolichorhinus A, Brachycephalic (Palaeosyops major), zygomatic-cephalio index 77. B, Mesaticephalic (J/orjieocfrasTnaHfeoccras), zygomatio-cephalio index 63-68. C, Dolichocephalic (DoUchorMnus Jiyognathus) , zygomatic-cephalic index 43^6. h, Eudiments (rectigradations) of the horns. deflection of the face to the possession of horns. Ewart applied cyptocephaly as a means of distinguish- ing the various phyla of horses and, like Riitimeyer, explained the flexures as adaptations to a prevailing browsing or grazing habit, respectively. He pointed out that Alces and Ovis illustrate the two extreme types of skull: (1) the elk (Alces) is a short-necked, forest form adapted to feeding on shrubs and trees — that is, to holding the head in a nearly horizontal position, (2) whereas the sheep {Ovis) grazes or browses on the ground and is adapted to holding the head when feeding in a nearly vertical position. There are excellent reasons for believing that a bent skull facilitates grazing on short herbage. short-crowned teeth, the lines of the face and the cranium are more nearly parallel, the angle being 154°. In the extremely dolichocephalic DolicJiorTiinus , which has more hypsodont teeth and presumably subgrazing habits, the face is sharply bent down on the cranium, forming an angle of 135°. (See fig. 214.) Cyptocephaly, whatever its adaptive significance, is certainly one of the important progressive characters in the transformation of the ungulate skull and is decidedly marked in certain titanotheres. DOIICHGCEPHAIY, BRACHYCEPHAIY, AND CORRELATION In 1902, when the measurements of Oligocene titanotheres were brought together, the conclusion was again reached that dolichocephaly and brachy- 258 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA cephaly are among the dominating tendencies affecting the skull and grinding teeth of titanotheres, but that they are not invariably correlated with similar abbreviation or elongation iu the trunk and limbs. The principle of correlation, however, was found to explain a vast number of dry detailed facts concern- ing all parts of the skulls of titanotheres, including the teeth, which had been recorded by Cope, Marsh, Earle, Osborn, and others without any appreciation of their morphologic significance. In brief, the progressive doUchocephaly or brachy- cephaly of the skull is found to dominate the shape of every bone in the skull but more particularly that of the nasals, horns, zygomatic arches, and palate, as well as the confluence or separation of the foramina Manteoceras J)oiic?>orfiinus Figure 216. — Cranial proportions in man (A) and in the titanotheres (B) Viewed from above. Used in reference to man the words brachyce- phalic, mesaticephalic, and dolichocephalic denote, respectively, brachycranial, mesaticranial, and dolichocranial. In other words, they describe the proportions of the cranial cavity. Used in refer- ence to the titanotheres the same words describe the relative length and breadth of the entire skull. in the base and sides of the skull, the form of the occiput and of the mastoid, and the relations of other bones around the auditory meatus, the shape of the premaxillary and mandibular symphyses, the dias- temata between and behind the teeth, the number and shape of the teeth, the shape, number, and rela- tions of the cusps of the teeth, and even, it would appear, the cingulum around the grinding teeth. The three skulls of middle and upper Eocene titanotheres shown in Figure 216 also illustrate admirably the extremes of brachycephaly and doUcho- cephaly and the intermediate condition of mesa- ticephaly. The skulls are those of the species Palaeosyops major, with its extremely broad head, of the mod- erately broad-headed Manteoceras manteoceras, and of the extremely long and narrow-headed DoKcJiorMnus Jiyognathus. The cranium of these titanotheres varies in width like the skulls of man (fig. 216) and the rhinoceros, and the excessive width is contributed chiefly by the great expansion of the zygomata. The skull of Doli- chorhinus as a whole, however, is far more elongate than that of Palaeosyops. We find also very pronounced differences of proportion in every bone and every Aa Figure 217. — -Natural and artificial brachycephaly and doUcho- cephaly. After Osborn, Science, 1908, pp. 750, 751 A, Palaeosyops, extreme brachycephalic type, superior view; Aa, Palaeosyops, arti- ficial dolichocephaly produced by stretching A to length of C; B, Mesatirhinus, primitive dolichocephaly, palatal view; Ba, Mesatirhinus, artificial dolicho- cephaly, palatal view, produced by stretching B to length of C ; C, Dolkhorhinus, progressive dolichocephaly, superior view; C, BolichoThinus, progressive dolicho- cephaly, palatal view. tooth when we compare Palaeosyops and DolichorMnus minutely. The table on page 259 presents some of the extremes of structure observed especially in the titanotheres, in which the most careful comparison of dolichocephalic and brachycephalic skulls has been made. When we compare a long-skulled with a short- skulled titanothere the skull at first appears to be EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 259 compressed laterally or stretched out longitudinally as if composed of india rubber, all parts being elongated alike. (See fig. 217.) This appearance is entirely deceptive, because every bony element of the skull has a different rate of elongation. As above noted, although the face of the titanothere is abbreviated (brachyopic), the cranium is elongated (dolichocranic), and it is chiefly the midregion of the cranium that is notably elongated — that is, the region between the orbits and the postglenoid processes. There is no predetermined innate or invariable correlation of brachycephaly or dolichocephaly in all parts of the skull, nor is there any fixed correlation between elonga- tion or abbreviation of the skull and of the limbs respectively. (See law of correlation, Chap. XI.) ZYGOMATIC-CEPHAHC INDICES IN THE TITANOTHERES AND OTHER PERISSODACTYLS RELATIVE VALUE OF INDICES The proportions and indices applied to different groups of mammals are largely relative. The terms applied to the human cranium and the indices are as follows : Brachycephalic = brachycranial, 80.1-100 Mesaticephalic == mesaticranial, 75. 1-80 Dolichocephalic = dolichocranial 60-75 Among the perissodactyl ungulates that have ordinal relationships to the titanotheres some of the indices of total length and width of skull are shown in the accompanying table. Indices oj length of sTcuU of perissodactyl ungulates related to the titanotheres Sumatran rhinoceros (Rhinoceros (Dicerorhinus) sumatrensis) . Black African rhinoceros (R. (Opsiceros) bicornis) South American tapir (Tapirus terreStris) White African rhinoceros (R. (Ceratotherium) simum) Indian tapir (Tapirus indicus) Domestic horse (Equus caballus) Domestic horse (Equus caballus) Domestic ass (Equus asinus) Basilar length, premaxillaries to condyles (millimeters) 580 568 355 387 513 Transverse width across zygomatic arches (millimeters) 345 320 178 183 202 59 56 50 43-50 47 39 40. 4-44 1 46. 9-49. 9 Most of the skulls of the above-named species are dolichocephalic in comparison with the skulls of titanotheres. A standard of skuU proportions among the perissodactyl ungulates, including the rhinoceroses, tapirs, horses, and titanotheres, may be established as shown in the table below. Summary of the zygomacephalic indices of the perissodactyls Form of skull Index Perissodactyl ungulates Dolichocephalic : H yperdolichocephalic Dolichocephalic Subdolichocephalic Mesaticephalic - - 39. 0-50 50. 1-55 55. 1-60 60. 1-70 70. 1-75 75. 1-85 85. 1-91 + Horse, tapir (Indian), white rhinoceros, Dolichorhinus, tapir (South American), Eotitanops. Mesatirhinus petersoni, Rhadinorhinus. Black rhinoceros, Sumatran rhinoceros, Mesatirhinus megarhinus. Telmatherium ultimum, Metarhinus earlei, Manteoceras, Menodus sp., Allops marshi, Menodus giganteus. Limnohyops laticeps, AUops serotinus, Brontotherium curtum. Brachycephalic : Subbrachy cephalic . _ Palaeosyops leidyi, Brontotherium platyceras. Brontops robustus, Diploclonus amplus. Hyperbrachycephalic INDICES OF SKULLS OF EOCENE AND TITANOTHERES OLIGOCENE The study of these proportions and indices of the titanotheres demonstrates that the skull in this family presents an ascending scale from primitive dolichocephalic ancestors like Lambdotherium and Eotitanops, which, on the one hand, evolved into extremely broad-headed forms like Palaeosyops and Megacerops, and, on the other, into extremely long- headed forms like Mesatirhinus and Dolichorhinus, as shown in the following table: 260 TIT.-USrOTHERES OP ANCIENT "WYOMING, DAKOTA, AND NEBRASKA Zygomatic-cephalic indices in the tiianotheres [Arranged in chronologic and taxonomic order] Lower Eocene: Eotitanops borealis (dolichocephalic) Lambdotherium popoagicum (dolichocephalic) Middle Eocene: Limnohy ops monoconus (brachy cephalic) Limnohyops laticeps (brachycephalic) Palaeosyops leidyi (brachycephalic) Upper Eocene: Telmatherium ultimum (crushed) (mesaticephalic)- Sthenodectes incisivus (mesaticephalic) Manteoceras manteoceras (mesaticephalic) Mesatirhinus megarhinus (dolichocephalic) Mesatirhinus petersoni (dolichocephalic) Doliohorhinus superior (dolichocephalic) Dolichorhinus hyognathus (dolichocephalic) Metarhinus fluviatilis (dolichocephalic) Metarhinus earlei (dolichocephalic) Rhadinorhinus abbotti (dolichocephalic) Lower Oligocene: Brontops brachycephalus, 9 (mesaticephalic) Brontops brachycephalus, & (brachycephalic) Brontops validus (brachycephalic) Brontops robustus (brachycephalic) Allops marshi (mesaticephalic) Allops serotinus (brachycephalic) Diploclonus amplus (brachycephalic) Diploclonus tyleri (brachycephalic) Menodus giganteus (mesaticephalic) Menodus varians (brachycephalic) Menodus heloceras (brachycephalic) Megacerops coloradensis (brachycephalic) Megacerops acer (brachycephalic) Brontotherium leidyi (brachycephalic) Brontotherium gigas (brachycephalic) Brontotherium sp. div. (brachycephalic) Index 50 (?) 72 75 74-75 61 63-65 63-68 56-59 49-52 49 46 58 59-63 51-52 64 72 85 76-83 64^69 72-78 91 85 62-70 73 79(?) 76 84 66 84 74-80 Generic tendencies. — Certain generic ascending series are progressively brachycephalic — for example, Bron- tops and Megacerops; others are progressively dolichoce- phalic — for example, Menodus. Zygomatic-cephalic indices of the titanotheres and other perissodactyls [Arranged in ascending numerical order] Dolichocephalic : Hyperdolichocephalic — Index Domestic horse (Equus caballus) 39, 40. 4^44. 1 Dolichorhinus hyognathus (Eocene) 43-49 White African rhinoceros (Ceratotherium simum) 43-50 Domestic ass (Equus asinus) 46. 9-49. 9 Indian tapir (Tapirus indicus) 47 Mesatirhinus petersoni (Eocene) 49 Dolichorhinus superior (Eocene) 49 Eotitanops princeps (Eocene) 50 Dolichocephalic — Mesatirhinus petersoni (Eocene) 51, 52 Rhadinorhinus abbotti (Eocene).. 51-52, 53, 54, 54. 9 Subdolichocephalic — Black African rhinoceros (Opsiceros bicornis) _ _ 56 Mesatirhinus megacrhinus (Eocene) 56-57 Metarhinus fluviatilis (Eocene) 58 Metarhinus earlei (Eocene) 59 Sumatran rhinoceros (Rhinoceros sumatrensis).. 59 Mesaticephalic : index Telmatherium ultimum (Eocene) 61 Metarhinus earlei (Eocene) 60. 1-61, 62-63 Menodus giganteus (Oligocene) 62-70 Sthenodectes incisivus (Eocene) 63-65 Manteoceras manteoceras (Eocene) 63-68 Allops marshi (Oligocene) 64-69 Brontops brachycephalus, $ (Oligocene) 64 Brontotherium leidyi (Oligocene) 66 Menodus giganteus (Oligocene) 69. 9, 70 Brachycephalic : Subbrachycephalic — Allops serotinus (Oligocene) 72 Brontops brachycephalus, c? (Oligocene) 72 Menops varians (Oligocene) 73 Brontotherium curtum (Oligocene) 74 Palaeosyops leidyi (Eocene) 74-77 Limnohyops laticeps (Eocene) 75 Brachycephalic — Megacerops? coloradensis (Oligocene) 76 Brontotherium curtum (Oligocene) 78 Allops crassicornis (Oligocene) 76 Palaeosyops major (Eocene) 77 Allops serotinus (Oligocene) 78 Brontops dispar (Oligocene) 78-79 Brontotherium platyceras (Oligocene) 80 Brontotherium peltoceras (Oligocene) 80 Brontotherium curtum, 9 (Oligocene) - 80 Brontotherium gigas (Oligocene) 82, 84 Brontops validus (Oligocene) 83 Megacerops acer (Oligocene) 84 Diploclonus tyleri (Oligocene) 85 Hyperbrachycephalic — Brontotherium gigas (Oligocene) 87? Brontops dispar (Oligocene) 87 Brontops robustus (Oligocene) 87? Diploclonus amplus (Oligocene) 91? DIFFERENCES IN TERMINOLOGY OF SKULL PROPORTIONS IN TITANOTHERES AND MAN In the Oligocene genus Menodus the zygomatic- cephalic index rises above 60, and the skuU is there- fore "mesaticephalic," as defined above, rather than "dolichocephalic," as described in the author's earlier papers. The term "stenocephalic," meaning narrow headed, may therefore be used to describe the narrow cranium and dentition of Menodus, in contrast with the wide and truly brachycephalic cranium and denti- tion of Brontops, Megacerops, and Brontotherium. According to the indices adopted for all perisso- dactyls, no Oligocene titanotheres are truly dolicho- cephalic; all are mesaticephalic or brachycephalic as compared with many Eocene titanotheres. The term dolichocephalic, as used in the section on the Oligocene, may be considered equivalent to "stenocephalic" (see Chap. V) as applied to Oligocene titanotheres having relatively narrow face and teeth but a zygomatic index of 64-70. The terms given above are not used in the same sense as in anthropology. The anthropologists for cranial form should have introduced the terms "dolicho- cranial" and "brachycranial," but as a matter of fact they used "brachycephalic" and "dolichocephalic." There is no other word left for craniometry, because EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHEKES 261 there is no other Greek word descriptive of the head as a whole. We can not use "dolichocranial," because our indices measure the whole skull, not the calrarium alone. The standard zygomatic-cephalic indices adopted in this monograph are as follows: Dolichocephalic 39-60 Mesaticephalic 60. 1-70 Brachycephalic 70. 1-91 + It is true that in the top view of Brontoiherium platyceras the skull top itself is long, but the indices show that the head as a whole is extremely brachy- cephalic. In uncrushed skulls of Bro7itotherium gigas the index is 82-S7, which is doubtfully exceeded only by B. rohustus and Diploclonus amplus. CONTRAST IN FEATURES OF BRACHYCEPHALIC AND DOLICHOCEPHALIC SKULLS AND TEETH The later brontotheres are excessively brachyopic, not only in measurements but in all the characters of the teeth, including molars and premolars. This brachyopy, no doubt, supervened upon an earlier stage in which the middle portion of the cranium was elongate, and it is the elongation of the middle part of the cranium that gives a dolichocephalic tendency. Far from being elongate as viewed from below, the palatal and basicranial regions of brontotheres are excessively wide and short, as well as the zygomata and the face, and thus contrast very strongly with the narrow face and unexpanded zygomata of Menodus. Hence there is no available substitute for the term brachycephaly for the brontotheres. Contrasts in features oj hracJiycepJialic and dolichocepTialic teeth and slculls Brachycephalic type DolichocephaUc type Teeth: Grinding series Diastema between cutting and grinding Anterior premolars, p \ Intermediate tubercles or conules of molars. Opposite cutting and grinding series Incisor series Canine teeth Grinding teeth, or molars Cingulum between molar teeth Skull: Entire skull Most of the constituent bones Palate Nasals Abbreviated. Closed Suppressed, or one fang suppressed. Persistent Bridge over infraorbital foramen. Infraorbital foramen Lacrimal bone Lacrimal foramen Zygomatic arches Areas of insertion of masseteric and temporal muscles. Mastoid portion of periotic Exoccipital, postglenoid and post-tympanic processes. Postglenoid and post-tympanic processes. . Tympanic bulla Foramen ovale and foramen lacerum medius. Alisphenoid canal Presphenoid Vomer Premaxillary symphysis Frontonasal horns Converging or arched. . Placed transversely Rounded or broadened. Shortened and widened. Suppressed Shortened and broadened . do Broadened and flattened.. Shortened and spreading. Narrowed Not seen on side of face Crowded toward orbit Crowded into orbit Broadened, especially in the buccal plates; in section broad rather than deep. Increased in thickness Exposure abbreviated or covered. Broadened Jaw: Ramus of jaw. Area of insertion of temporal muscle. Coronoid process Mandibular symphysis Approximated, especially below, in- closing the external auditory meatus inferiorly. Thrust inward Approximated Abbreviated do Thrust backward Abbreviated and massive Transversely expanded Shortened, thickened, deepened Reduced do Abbreviated and massive Elongated. Open. Persistent and spaced. Reduced or aborted. Parallel or elongated. Convergent anteriorly. Elongated or compressed. Lengthened and narrowed. Persistent. Lengthened and narrowed. Do. Narrowed and transversely arched. Elongated with curved and straight borders. Broadened. Conspicuous on side of face. Exposed on side of face. Seen on edge of orbit. Elongated and vertically deepened; in section deep rather than broad. Elongated horizontally. Expanded and exposed.- Deepened and narrowed. External auditory meatus not closed inferiorly. Exposed laterally. Separated by a bridge o i bone. Elongated. Do. Not thrust backward. Elongated. Less expanded transversely. Elongated with straight lower borders and backward produced angle. Balance maintained. Lengthened anteroposteriorly. Elongated. 262 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Comparison of the auditory region in the skull of these seven divergent Eocene species shows that the auditory meatus tends to close in the brachycephalic types, is moderately open in the mesaticephahc types, and is widely open in the dolichocephalic types of titanotheres, and thus parallels the auditory region of some modern perissodactyls, as shown in Figure 379. The chief aUometric characters that distinguish the Eocene titanotheres are the following: 1. Abbreviation of the face (brachyopy) and elongation of the cranium (dolichocrany), combined with general brachycephaly or with general doliohooephaly. 2. Flexure of the face upon the cranium ( = cyptocephaly, upward or downward bending). 3. Broadening and elongating of the nasals or narrowing and recession of the anterior narial openings. 4. Pneumaticity, development of great pneumatic cavities in the upper part of the face and cranium. We observe a long series of modifications of all the great adaptive functions in the evolution of the mammalian skull — namely, (1) prehension of food, (2) mastication of food, (3) passage of food to the fauces, (4) channels of respiration, (5) lodgment of sense organs, (6) lodgment of the brain, (7) offensive use of the canine teeth, (8) offensive use of the horns. It would appear that the two functions last indicated (7, 8) exert little influence in the middle Eocene titanotheres. In wide contrast are the latest Eocene and the Oligocene titanotheres, in which the horns predominate. The canines vary greatly in size and in the telmatheres become effective offensive weapons. The prehensile functions of the lips and anterior teeth vary with the development of a broad muzzle, the firmly united premaxUlae, the greater or less recession of the nasal bones ; but there is little evidence of strong development of prehensile powers in the upper lip such as is seen in the skull of the tapirs. nST OF ABBREVIATIONS USED IN ILLUSTRATIONS OF SKULLS The following abbreviations are used in this mono- graph in the illustrations of skulls. Names of com- plete bones are begun with capital letters; names of parts of bones with small letters. As. Alisphenoideum. Bo. Basioccipitale. Bs. Basisphenoideum. c. As (car. ex. mx.). canalis alisphenoideus, carotis externa, ramus maxiUaris. c. i. o. (V2 car. ex.). canalis infraorbitalis, nervus maxillaris, carotis externa, cond. ac. condylus occipitalis accessorius. cond. condylus occipitalis, cr. lamb, crista lambdoidea. cr. sag. crista sagittalis. or. tem. crista temporalis, dct. la. ductus nasolacrymalis. em. ar. eminentia articularis. em. Fr. eminentia frontalis. Ex. o. Os exoccipitale. f. 0. (XII). foramen condylare, nervus XII. f. la. foramen lacrimale. f. 1. a. (Ill, IV, V, VI). foramen lacerum anterius, nervi III, IV, V, VI. f. 1. m. (car. in.), foramen lacerum medium, carotis interna, f. 1. p. (IX, X, XI). foramen lacerum posterius, nervi IX, X, XI. f. mg. foramen magnum, f. mn. (V3). foramen mentale, nervus V3. f. ms. (jug.), foramen mastoideum. f. op. (II). foramen opticum, nervus II. f. ov. (V3). foramen ovale, nervus V3. f. pi. a. (N. nas. pi.), foramen palatinum anterius, nervus nasopalatinus. f. p. gl. foramen postglenoideum. f. pi. p. (N. pi. post.), foramen palatinum posterius, nervus palatinus posterior, f. r. (V2). foramen rotundum, nervus Vj. f. sph. pi. (V2). foramen sphenopalatinum. f. stm. (VII). foramen stylomastoideum, nervus VII. f. su. or. foramen supraorbitale. f. ven. foramen venosum. fis. nar. ant. fissura narialis anterior, fos. gl. fossa glenoidea. fos. interpa. fossa interparietale. fos. men. in. fossa menisci interna, fos. nar. ant. fossa narialis anterior, fos. nar. post, fossa narialis posterior, fos. st. hy. fossa ossis stylohyoidei. Fr. Os frontale. HI Umbo cornu ("horn"). I. P. Os interparietale. La. Os lacrimale. lig. nu. ligamentum nuchae. m. a. e. meatus acusticus externus. m. obi. cap. sup. musculus obliquus capitis' superior. m. rect. cap. lat. musculus rectus capitis later- alis. m. rect. cap. post, musculus rectus capitis posticus. Ms. mastoideum. ms. Per. pars mastoidea, ossis periotici. Mx. maxiUa. Mx. (alv.) maxilla (processus alveolaris). Mx. tb. maxilloturbinale. Na. Os nasalis. nar. post, naris posterior. obi. cap. sup. musculus obliquus capitis superior. Orb. Orbis. Pa. Os parietale. petr. pars petrosa ossis periotici. p. gl. Sq. Processus postglenoideus ossis squamosi. PL Os palatinum. Pmx. Premaxilla. p. o. Ex. Processus paroecipitalis ossis exoccipitalis. p. o. Fr. Processus postorbitalis ossis frontalis. p. o. Mai. Processus postorbitalis ossis malaris. pr. cor. processus coronoideus. pr. i. o. Mai. processus infraorbitalis ossis malaris. pr. p. o. processus postorbitalis. Psph. Os presphenoideum. Pt. Os pterygoideum. pt. As. Processus pterygoideum ossis alisphenoidei. p. ty. Sq. processus post-tympanicus ossis squamosi. rect. cap. musculus rectus capitis (antice) . rect. cap. post, musculus rectus capitis posticus. sin. lat. eth. sinus lateralis ethmoidalis. So. Os supraoccipitale. Sq. Os squamosum. (attachments). EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 263 t. la. tuberculum ossis lacrimalis. XII. foramen condylare (nervus XII). z. Sq. processus zygomaticus ossis squamosi. 7i. Mx. processus zygomaticus maxillae. TERMINOLOGY OF THE UPPER MOIAR TEETH The accompanying table shows the terms used to designate the upper molar teeth of ungulates: Comparative terminology of the superior molar teeth All ungulates: Terminology based upon evolution from a tritubercular, bunodont ancestral molar type. Os- born (1888, 1892) Primary molar cones: Protocone Paracone-. Metacone. Hypocone. Intermediate molar cones: Protoconule Metaconule Premolar cones " : Protocone Deuterocone Tritocone Tetartocone Secondary pillars or styles: Parastyle Mesostyle. Metastyle.. Hypostyle. Secondary crests: Ectoloph Protoloph- Metaloph_ Valleys : Medisinus_. Postsinus Secondary folds: Crochet Antecrochet- Crista Secondary pits: Pre-, medi-, and postfossettes. Gingulum Titanotberes: Leidy (1873), Cope (1883), Marsb (1877) Antero-external lobe. Postero-external lobe. Postero-internal lobe. ■' Tubercles "- Antero-internal be. Buttress Median fold Basal ridge. Horses: Huxley (1876) and Lydekker (1886, p. 67) Anterior pillar Anterior crescent. Posterior crescent Posterior pillar Anterior ridge. Middle ridge Posterior ridge.. Posterior promi- nence. Rhinoceroses: Cuvier (1836), De Blainville (1846), Gaudry (1878), Pavlow (1892) Dentioule interne du pre- mier lobe. Dentticule externe du pre- mier lobe. Denticule externe du second lobe. Denticule interne du second lobe. Crete externe. Colline seconde = Crete ou lobe antSrieur. La troisieme colline = crete ou lobe post^rieur. Vallon oblique Fossette post^rieure. Crochet. Crochet ant^rieur. Antecrochet Fossette post&ieure=fos- settes. Bourrelet Rhinoceroses, English au- thors: Boyd-Dawkins (1867), Busk (1877) and Lydekker (1882), Joote (1874) Second costa. Costae (in part) First costa (buttress) . Posterior collis (in part) . External lamina (dor- sum). Anterior collis Median coUis. Anterior valley.. Posterior valley. Posterior combing plate (uncus, cro- chet) . Antecrochet Anterior combing plate = crista. Posterior collis (in part) = cingulum, guard. Rhinoceroses and ungulates, Ger- man and Russian authors: Riiti- meyer (1863) and Kowalevsky (1873) Innenpfeiler des Vorjochs. Innenpfeiler des Nach- jochs. Pericones, Randgipfeln. Aussenwand. Vorjoch. Nachjoch. Wulst. ■ Premolar cusp; term proposed by W. B. Scott (1891). 264 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA SECTION 2. INTRODUCTION TO THE ANATOMY OF THE SKULL AND TEETH OF THE EOCENE TITANO- THERES TYPES OF SKULL OF EOCENE TITANOTHERES For reasons that are fully set forth in Chapter VIII, on the origm and descent of the titanotheres, we regard the skull of Eotitanops horealis (figs. 250, 251), from the lower Eocene, as the ancestral type from which all the highly modified Eocene skulls were derived. The structure of the middle Eocene skulls is corre- lated with certain feeding habits and exhibits a marked contrast to that of the Ohgocene skulls. In middle Eocene time the horns had not yet become weapons of. offense and defense. The forms of the skulls of the following Eocene titanotheres are noteworthy: 1. LimnoJiyops priscus, a primitive hornless titano- there, had a moderately brachycephalic skull and primitive low-crowned grinding teeth. Figure 218. — Contrasting forms of upper teeth in Eocene titanotheres Brachycephalic (A) and dolichocephalic (B) types of upper premolar-molar series. One-half natural size. A, Pa!(icos!;op5Zei(iv!,Am.Mus.l544(t5'pe); B, DoUcliorhinusliyogmthus, Am. Mas. ISbl. 2. Palaeosyops leidyi (figs. 275-278) was entirely hornless and represents the extreme brachycephaUc and brachyodont type. 3. The skull of TelmatJierium ultimum (figs. 294-296) may be regarded as an elongated or mesaticephalic modification of the primitive Limnohyops type. The horn rudiments are retarded, and the crowns of the teeth are more elongated than in Palaeosyops. In many respects this skull resembles that of the suc- ceeding type, Manteoceras. 4. The skull of Manteoceras manteoceras (figs. SOS- SOS) differs from that of TelmatJierium ultimum in the vigorous development of the very precocious horn rudi- ments, which are seen in profile above and in front of the eyes. It is also mesaticephaUc, and the molars are more brachyodont than those of TelmatJierium. 5. The skull of MesatirJiinus petersoni has passed from mesaticephaly into dolichocephaly. It resembles an elongated skull of Manteoceras in having similar rudiments of horns above the eyes, and the conforma- tion of the zygomatic arch is similar to that of Manteo- ceras and very distinct from that of Palaeosyops and TelmatJierium. 6. The skuU of DolicJiorJiinus JiyognatJius (figs. 347- 349) is a decidedly long-headed derivative of Mesati- rJiinus petersoni (figs. 327-329). The horn rudiments are much more prominent and show some progressive characters, such as the flattening of the top of the cranium, which is analogous even to the cranium of the Oligocene titanotheres. It is also decidedly cyptocephalic, the face being strongly bent down on the cranium. This is perhaps a river-loving type. 7. The skull of MetarJiinus earlei (fig. 361) presents a striking contrast to that of MesatirJiinus petersoni. It is less dolichocephalic and shows a marked recession of the narial openings and very prominent orbits, indicative, perhaps, of semiaquatic habits. FEEDING HABITS OF BROAD-HEADED AND LONG-HEADED TITANOTHERES Peculiar forms of tJie teetJi. — Tlue mode of feeding and the food of the titanotheres can not be inferred with certainty, because their dentition differs considerably from that of any modern mammal. In middle Eocene titanotheres the grinding teeth were per- fectly adapted to a combination of cutting and crushing the food, as noted below. This adaptation implies a choice of succu- lent food consisting of relatively coarse leaves, grasses, buds, twigs, roots, and tubers such as would be found in forest and stream habitats. In later Eocene and Ohgocene titanotheres the shearing action of the teeth was more perfect and the food may have included smaller objects of tougher fiber. Although the structure of the grinding teeth of the titanotheres is very different from that of the grinding teeth of members of related families — the tapirs, rhinoceroses, and horses — the titanotheres neverthe- less present certain analogies in the form of the head, from which we may infer that analogies existed also in the feeding habits. Again, a survey of the feeding habits of the existing Perissodactyla reveals a certain family likeness running throughout the families of this order, which was prob- ably manifest also among the extinct Perissodactyla. Primitive types. — The primitive form of head and tooth of Eotitanops is analogous to that of the primitive paleotheres and horses, in which the proportions of the cranium and face and the structure of the grinding teeth are again similar. We may infer that all these animals had a marked similarity of diet, from which the Eocene titanotheres diverged in two directions, developing into the extremely brachycephalic Palaeo- syops and into the extremely dolichocephalic Doli- cJiorJiinus. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 265 Brachycephalic types. — In the short-skuUed Palaeo- syops we observe heavy canine tusks, large canini- form outer incisor teeth, deep and heavy zygomatic arches and lower jaw, high and relatively thin sagittal for plucking and tearing up succulent bulbs, tubers, and roots from the ground as well as for browsing on twigs and leaves, a diet much in favor with the Amer- ican tapir. But it differed from the tapir in that the £o tita n ops ' A^anteocercLS Figure 219. — Skulls of Eocene titanotheres of the principal genera Side views. One-eighth natural size. A, Eotitanops borealis, lower Eocene, Wind River formation; B, Limnoliyops prisms, middle Eocene, Bridger formation, horizon Bridger B; C, Palaeosyops leidyi, middle Eocene, Bridger formation, horizon Bridger D; D, Telmatherium uHimum, upper Eocene, Uinta formation (Uinta C); E, Manteoceras manieoceras, middle Eocene, Bridger formation, horizon Bridger D; F, Mesatirhinus petersoni, middle Eocene, Bridger formation, horizon Bridger D; G, Meiarhinus earlei, upper Eocene of Uinta Basin, level Uinta B 1; H, DolichorUnus hyognathus, upper Eocene of Uinta Basin, level Uinta B 2. H, Horn. crest, and large areas of attachment for the temporal and masseter muscles. This indicates a notably ver- tical movement of the jaw and great power in crush- ing the food. Such an animal would seem well fitted 101959— 29— VOL 1 20 nasals are not retracted, and there is no evidence that the upper lip had exceptional prehensile power. (See fig. 220.) This titanothere presented the extreme of the browsing type. It had a lumbering gait and 266 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA spreading feet, and to judge from the associated fauna in comparison with that of the Uving tapirs it inhab- ited semitropical forests, especially those near streams. Dolichocephalic types. — The other extreme of struc- ture among Eocene titanotheres is the long-skulled Dolichorhinus, which succeeds Palaeosyops in geologic time, belonging more to the upper Eocene. The muzzle of this animal (fig. 219) was rather expanded The cheek teeth were relatively long-crowned with pointed cusps and constituted a relatively elaborate cutting and triturating apparatus, as compared with the very short-crowned grinders of Palaeosyops. The excursion of the more slender mandible was partly vertical, partly oblique. The oblique position of the grinding teeth produced an oblique shearing action. Conditioning these changes the length and proportions of the masticating muscles and their angles of action were also changed. (See Chaps. V, VIII for de- tails. These features of the head of Dolicho- rhinus indicate that the food of this animal required finer cutting and better trituration than that of Palaeosyops. Although in no sense a grazing animal as compared with the grazing Equidae and Bovinae, Dolichorhinus was better adapted to grazing than Palaeosyops. Its remains are very frequently found in coarse sandstones laid down by rapid streams, and it may well have hved partly in the rivers and along their banks. Intermediate types. — The other Eocene titanotheres, such as Manteoceras (fig. 220, C) and Telmatherium (fig. 220, B) are more or less intermediate between these extremes in the form of the head. Thus Manteoceras has very heavy, almost boarlike tusks and large, blunt incisor teeth, together with cheek teeth that are more elongate than those of Palaeosyops. Telmatherium had much more trenchant canine tusks, pointed incisor teeth, and somewhat elongated grinding teeth. ORIGIN AND STRUCTURE OF THE IN TITANOTHERES 'HORNS" -Heads of Eocene titanotheres of four phyla Modeled by Charles R. Knight. A, Palaeosyops, brachycephalic; B, Telmatherium, mesaticephalic; C' Manteoceras, mesaticephalic; D, Dolichorhinus, dolichocephalic. The nostrils were actually more nearly terminal than those shown in the models, and the upper lip may have been more markedly pointed or prehensile. H, Horn rudiments. or truncate. The incisors were arranged in a semi- circle and made some approach to the cropping in- cisors of a ruminant, being also partly cupped as in Oligocene species of the horse. The space behind the canine tusk was longer, as in typical herbivorous forms. The canines were recurved, compressed, or sharp-edged and may have been used in fighting, as in the camels. The offensive power of the front teeth was, however, much less than in Palaeosyops. The so-called horns of titanotheres arise as rectigradations; they consist of osseous protuberances of the skull above the eyes, where the frontals overlap the nasal bones. In life they were prob- ably covered with tough sldn, rather than with horn. In the earliest titanotheres, of lower Eocene age {Lamb- dotherium, Eotitanops), the frontonasal junction shows no beginning of the horns. In the genera Palaeosyops and Limnohyops (middle Eocene) most of the skulls were equally hornless, but some very old males of Palaeosyops show an incipient nasofrontal protuber- ance and roughening of the outer tabula of the bone. (See PI. XVI.) In the middle Eocene contemporary D. S. GEOLOGICAL SURVEY MONOGUAPH 55 PLATE XVI r ^ ■^ B \ THE REGION OF THE HORN SWELLING IN PALAEOSYOPS, MANTEOCERAS, AND TELMATHERIUM A', Palaeos^iops robuiiui (Am. Mus. 1554), superior view; A', the same, vertical longitudinal secftion. B, Manteoceroj manteoceras (Am. Mus. 1569). C, Telmatherium ultimum (Am. Mus. 2004). fr, Frontals; h, rudimentary horn; mx, maxillary; na, nasal. All natural sise U. S. GEOLOGICAL StrKVET MONOGRAPH 55 PLATE XVII THE REGION OF THE HORN SWELLING IN MANTEOCERAS, MESATIRHINUS, AND DOLICHORHINUS Manteoceras manteoceras (Am. Mus. 23S3), lateral view, left side (right side reversed). B, DoUchorhinus hyognathus (Am. Mus. 1851), lateral view, left side (right side reversed). C, Mesatirhinus petersoni (Am. Mus. 1556), lateral view, left side. D, DoUchorhinus hyognathus (Am. Mus. 1851), superior view. c. i. o. /., c. i. o., Infraorbital foramen; /r, frontal; h, rudi- mentary horn; la, lacrimal; mo, molar; mx, maxillary; na, nasal; or, orbit; s. nar., external narial aperture EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 267 Telmatherium cultridens there appears to have been a distinct nasofrontal protuberance, but in the geologi- cally later Telmatherium ultimum only the faintest indication of its presence is found; it is possible that the horn retrogressed in this phylum. In Manteo- ceras manfeoceras of the upper part of the Bridger formation (middle Eocene) the protuberance, although small, is perfectly distinct and fully characteristic in form. In ProtitanotJierium of the upper Eocene the horns (figs. 317-319) consist of oval protuberances about 20 millimeters high and 90 millimeters long. In nearly all the lower Oligocene titanotheres the horns are of large size and finally become the domi- nant feature of the whole skull, affording generic and specific characters. The horns are believed to have evolved concomi- tantly with the fighting habits of these animals and with the general increase in size and body. The conditions of the horns in the titanotheres may be summarized as follows: Summary of character or condition of the horns in Eocene and Oligocene titanotheres Subfamily or genus Character or condition of horns Lambdotheriinae _ Eotitanopinae Palaeosyopinae, . . Lambdotherium zone (Wind River B) . do Telmatheriinae _ Sthenodectes Manteoceratinae Dolichorhininae : Mesatirhinus-Doliohorhinus_ Metarhinus Rhadinorhininae : Rhadinorhinus Manteoceratinae : Protitanotherium _ Eotitanotherium. _ Brontopinae Lower horizons of Bridger Basin to lower horizons of Washakie Basin, inclusive. Upper horizon of Bridger Basin to Uinta C, inclusive. Uinta B 1 only Upper horizons of Bridger Basin to Uinta C. Upper horizons of Bridger Basin to Uinta B 2. Lower horizons of Washakie Basin to Uinta B 1, inclusive. Uinta B 1 only_ Uinta C onlv_- Uinta B 2 Chadron A to C, inclusive. Menodontinae. . Megaceropinae. .do. Brontotheriinae. .do_ _do_ Hornless. Do. Hornless, or nasofrontal horn swelling barely perceptible. Do. Horn swelling small. Horn swelling small but distinct. Horn swelling more pronounced; on nasals only. Horn swelling small; chiefly on nasals. Horn swelling small. Nasofrontal horn swelling pronounced and progressive. Do. Nasofrontal horn swelling at first small, slowly Isecoming progressively larger. Do. Nasofrontal horn swelling of medium to large size. Nasofrontal horn swelling at first small, rapidly becoming progressively larger. PROPORTION AND RECTIGRADATION IN THE GRINDING TEETH OF EOCENE TITANOTHERES The chief characters of the grinding teeth in the Eocene titanotheres were evolved from the bunose- lenodont pattern (see fig. 221) and were modified by changes of proportion and rectigradation, under eight principles, as follows : 1. The primitive grinders seen in Lambdotherium and Eotitanops are extremely low crowned, or brachyo- dont. The numerous phyla can be distinguished chiefly by the different degrees and rates of elonga- tion of the crown, which shows progressive hypso- dontism. 2. The sLx main grinding teeth in the upper and lower jaws, p^-m^, p^-nis, are closely crowded together, and this crowding causes the crowns of the grinders to be closely proportioned to the brachycephaly or dolichocephaly of the skull. In brachycephalic titano- theres the transverse diameters of the grinding teeth generally exceed the anteroposterior diameters, whereas in dolichocephalic skulls the reverse is true. Thus we shall speak of the grinders as of the "brachy- cephalic" or of the "dolichocephalic" type. 3. The general tendency of the grinders in titano- theres is to become macrodont, because the pattern of the grinding teeth is mechanically imperfect, and the grinders compensate in size, in some degree, for what they lack in mechanical perfection, 4. The transformation of the "cone and crescent" or bunoselenodont pattern of the upper and lower grinding teeth in the titanotheres can be best under- stood by comparing that pattern with that seen in the other bunoselenodonts — the primitive chalicotheres, horses, and paleotheres — a pattern similarly derived from the same primitive type of upper and lower grinding teeth (fig. 222), which presents four main cones above and four main cones below, known as the "primary molar cones." 268 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 5. The secondary accessory folds, known as "styles," or pillars, and "lophs," or crests, as well as the "fossettes," or pits, may also be homologized by comparing the superior and inferior molars of the ECTOLOPH MESOSTYLE'' PARASTYLE PROTOCONULE- -METACONULE -HYPOCONE PROTOCONW PROTOLOPHID PARACONID- -HYPOCONID METALOPHW ENTOCONID METACONID \ METACRJSTID METASTYUD FiGUEE 221. — Upper (A) and lower (B) molars of bunosele- nodont pattern Molars of Lambdotherium, a lower Eocene titanothere. Enlarged. The worn enamel surface is cross hatched; the exposed dentine is shown in dense black. titanothere (TelmatJierium) and of equine forms {AncMtherium) with those of the rhinoceros. 6. Arrested or retro- gressive development is the chief characteristic of the titanothere molar evolution- — that is, parts are arrested or vestigial in titanotheres that evolve rapidly and strongly in paleotheres, chalicotheres, and equine s. Thus the titanothere molar be- gins its evolution in the form of the Lamhdothe- rium molar (figs. 221, 235) or of the Eofitanops molar (figs. 229, 249), in which 32 primary and secondary elements may be more or less clearly dis- tinguished in the typical upper and lower grinders. In the course of evolution these grinders, through arrested development, lose six or more of these ele- ments. Thus the grinding teeth are impoverished as compared with those of the other bimoselenodonts (fig. 223). The parts that gradually become vestigial or dis- appear in titanotheres are the following: Protoconule, anterior intermediate cusp of superior molars; degenerates. Metaconule, posterior intermediate cusp of superior molars; degenerates. Protoloph, anterior transverse crest of superior molars, formed of protocone, protoconule, paracone (inner base) ; disappears. Metaloph, posterior transverse crest of superior molars, formed of hypocone, metaconule, metaoone (inner base) ; dis- appears. Paraconid, antero-internal cusp of infe- rior molars, reduced or vestigial in all Peris- sodactyla; disappears. Hypoconulid, posteromedian cusp of in- ferior molars; abortive except in third inferior molar. 7. All stages in the reduction and disappearance of these six or more elements in the upper and lower grinding teeth are observed among the Eocene titanotheres (Pis. LIV-LXV), whereas the lower Oligocene titano- theres exhibit grinding teeth (fig. 381) in which all these parts have totally disappeared and certain new secondary rectigradations, such as the "fossette" and crochet, have appeared. 8. The appearance or disappearance of these single elements is generally gradual or continuous; yet it is much more rapid in certain phyla than in others. The variation in the rate of degeneration distinguishes the phyla from one another and thus becomes a char- B Figure 2 2 2. — Upper (A) and lower (B) molar patterns of Hyracotherium, a primitive Eocene equine perissodactyl (ancestor o f the horse) Enlarged. a Figure 223. — Bunoselenodont patterns of upper and lower molars in Tertiary perissodactyls Telmatherium vltimum, an upper Eocene titanothere, upper molar; a, Telmatherium cuUridens, a middle Eocene titanothere, lower molar; B, b, Moropus sp. Miocene hippoid. a Miocene chalicothere; C, c, Palaeotheriuvi sp., an Eocene paleothere; D, d, Atichitherium sp., a acter of generic value. The numerical gain or loss of one of these elements is of specific value and marks off the subspecific stages or mutations. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 269 MECHANISM OF THE TITANOTHERE GRINDING TEETH The pattern of the upper and lower grinding teeth of the titanotheres is one that has entirely disappeared among the existing mammals. It has no counterpart among any living ungulates, but it is closely analogous PARAGON e: FOSSA METACONE FOSSA Jvyd 772-2 METACONID ENTOCONID FOSSA FOSSA Figure 224. — Relations of upper and lower molars in an Eocene titanothere, Telmatherium cultridens Princeton Mus. 10027 (type). Two-thirds natural size. A, Second upper molar showing the crushing parts and the cutting parts, also the pits or fossae for the reception of the projections on the lower teeth; B, second lower molar, showing the fossae for the parts of the upper teeth; C, diagram showing how each lower molar articulates with two upper molars, the trigonid of ma wedging between mi and m^, and the talonid of ma receiving the protocone of m^. to that of many Eocene and Oligocene mam- mals, both artiodactyls and perissodactyls. There is no perfected grinding function, such as that between the upper and lower molars of the horse, nor could such a grinding function evolve out of the titanothere molar tooth. The dental mechanism is a combination of two functions which may be described as crushing and cutting, the crushing being effected by a double pestle and mortar or peg in socket mechanism, and the cutting by a mechanism of double shears or reversed double- bladed crescents. In the accompanying diagram (fig. 224) the manner in which this double function is sub- served in the upper and lower grinders is clearly shown. (1) The superior molars consist of a double- cutting shear W, composed of the paracone and metacone crescents, which oppose the reversed double shear M, composed of the crescents developed from the protoconid and hypoconid. (2) The chief crushing action is performed by the superior cones, the protocone and hypocone, which fit into the protocone and hypocone fossae of the inferior molars. (3) The apex (O) of each of the four primary superior cones (pro- tocone, paracone, metacone, hypocone) has its contact or abrasion point (oblique shading in figure), or fossa, in the inferior molars. (4) Similarly the apex of each of the four inferior .primary cones (protoconid, meta- conid, entoconid, hypoconid) has its contact or ab- rasion point (oblique shading), or fossa, in the superior molars. This double cutting and crushing function is me- chanically imperfect in the short-crowned molars of Lambdotherium and Eotitanops (figs. 229, 235, 242, 253). It becomes more efficient as the crowns become higher and the cones and crescents are vertically elongated in Telmatherium (fig. 225). The evolution of the titanothere grinders is directed to overcome the deficiency of this cone and crescent mechanism, which proves to be inherently defective in design. The crushing function of the grinders is best ob- served in the internal view (fig. 226) of the upper and lower grinders of a telmathere, in which the pestles (p-h) are sinldng into the mortars (m-e) in exactly the same manner as in the primitive insecti- vores. This closely correlated mechanism of the upper and lower grinding teeth, which was first studied by Cope (1889.3) and more fully by Gregory (1916.1), indicates that every new character (recti- gradation, allometron) added to the upper grinders must be correlated with a new and mechanically adaptive character (rectigradation, allometron) in the lower grinders. The cutting function performed by the W of the upper ectoloph and the M of the lower ectoloph is illustrated (figs. 224, 225) and dis- FlGTTRB 225 -Dental mechanism of titanotheres Interlocking relations of upper and lower premolar-molar series. One-half natural size. A, Telma- therium cultridens, Princeton Mus. 10027 (type); upper teeth (light line), with pattern of lower teeth (heavy line) projected upon them. Crown view. B, Internal view of the same teeth, showing the crushing action of the cones and conids. plays the close mechanical relation of the alternating crests as well as the simultaneous development of the new cusps (rectigradations) of the premolar teeth. 270 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The partial transformation in the titanotheres of a more vertical chopping, crushing, and cutting motion of the jaw into a more oblique sweeping or true grinding action of the molars finally results in the entire loss of the conules and transverse crests (protoloph, metaloph), in the partial molarization of the premolars, and in the development of two gigantic internal crushing cones (protocone, hypocone) and of a very powerful external cutting crest (ectoloph). 2. The excursion of the mandible was made from the outer side upward and inward, as in rhinoceroses and horses, in contrast with the opposite motion in ruminants. 3. In Eocene titanotheres the lack of a tetartocone in the upper premolars and of an entoconid in the lower premolars leaves an open space when the jaws are shut. This open space is filled in the Oligocene titanotheres by the opposing tetartocone and entoconid. MOLARIZATION OF THE PREMOLARS The titanotheres resemble all the other fami- lies of Perissodactyla in the gradual molariza- tion of the premolar teeth — that is, in the Figure 226. — Dental mechanism: Grinding teeth of a titanothere (A) and an insectivore (B) After W. K. Gregory. Internal view of the opposed upper and lower grinding teeth of Telmatherium cuUridens (A), natural size, and Erinaccus (B), much enlarged. The pro- tocones (p) fit into the talonid basins (fossae) between the metaconids (m) and entoconids (e). The hypocones (ft) fit into the trigonid basins (fossae) between the entoconids (e) and metaconids (m) . Similar relations are found in all primitive mammals. Three other interesting features in the evolution of the dental mechanism of the titanotheres are the following: 1. The marked protrusion of the roots on the outer sides of the upper molars in old Oligocene titanotheres is a result HYPOCONID M ESOSTYLE PROTOCONE Figure 227. — Contrast of molars of a brachyodont Eocene titanothere (A) and a semihypsodont Oligocene titanothere (B, B') A, Third left upper molar of Palaeosj/ops leiiyi, seen from the rear. The internal and external cones are subequal in height. B, Third left upper molar (unworn) of Menodus giganteus, seen from the rear. The internal cones are low; the external cones have greatly increased in height and have grown inward at the tip. B', The same seen from the outer side, showing the much deepened ectoloph. of the bunoselenodont pattern of the molars and of the vertical-oblique pressure of the lower teeth. As the outer side of the molar crowns becomes more hypsodont, in passing from lower Eocene to Oligocene titanotheres, so the external roots protrude more prominently. Figure 228. — Cross sections through second upper and lower molars of Lambdotherium and Menodus A, A brachyodont lower Eocene titanothere, Lambdotherium popoagkum, three-halves natural size; B, a semihypsodont lower Oligocene titanothere, Menodus giganteus, one-half nat- ural size. In A the excursion of the mandible was more trans- verse in direction than it was in B, where, in correlation with the deepening of the ectoloph, the movement of the mandible was more vertical. transformation of the premolars into the molar pattern. The mechanical inferiority of the teeth of the titanotheres lies in the fact that this trans- formation is never perfected ; it is very slow or retarded, and the premolars never completely acquire the molar pattern, as they do in the Equidae, for example, in which the premolars become actually superior to the molars both in pattern and in mechanical perfection. The arrested transfor- mation of the premolars in the titanotheres is undoubtedly a defect that is correlated with the abbreviation of the facial region and with the great increase in the relative size of the molars. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERE3 271 The primitive superior premolars of Eotitanops (fig. 229, A) when contrasted with those of Bron- totherium (fig. 229, B) are seen to be triangular in outline and surmounted with three more or less perfectly developed cusps. The transition between these primitive and specialized extremes, by the addi- tion of neomorphs or rectigradations, furnishes a whole series of specific characters and mutations in the ascending phyla of titanotheres. All the phyla exhibit a similar orthogenic tendency to molarization, but in some phyla this tendency is rapid and in others it is slow. Thus the different rates of molarization are of taxonomic value: they furnish distinctions between different generic or phyletic series. (See Velocity of movement. Chap. XI, pp. 810-812.) A careful review of the premolars in lower and middle Eocene titanotheres shows that the order of appearance of the premolar cusps is as follows: Lower premolars Upper premolars L Protoconid. 1. Protocone. 2. Hypoconid. 2. Deuterocone. 3. Paraoonid. 3. Tritocone. 4. Metacristid. 4. Parastyle. 5. Metaoonid. j 5. Tetartocone. 6. Entooonid. I 6. Mesostyle. In the above terminology of the upper premolar cusps the names are those proposed by Scott in 1892 in his paper on the evolution of the premolar teeth in the mammals (1892.1), but they are used through- out this monograph simply as names, without re- ference to the phylogenetic order of evolution, which Figure 229. — Upper premolar-molar teeth of the earliest (A) and latest (B) known titanotheres Both tooth rows drawn to the same length. A, Eotitanops lorealis, Am. Mus. 14887; lower Eocene. Premolars very primitive, a wide diastema behind pm'. B, Brontotlicrium gigas. Am. Mus. 492; lower Oligocene. The later type, showing the molarized pattern of the premolars and the great size of the molars. Pre- molars very advanced in pattern, diastema closed. is given above. The names of the lower premolar cusps are adapted from the terminology used by' Osborn for the lower true molars, but again without reference to the phylogenetic order of appearance. Summary of premolar evolution of the titanothe res Wind River Basin, Wyo. (Wind River B, "Lost Cabin") Bridger Basin, Wyo. Washakie Basin, Wyo. (Washakie A, B; B = Uinta A, B) Uinta Basin, Utah Titanotherium Brldger A, B Bridger C, D ( = Washakie A) Uinta A Uinta B Uinta C zone (Chadron A, B, C) pi with one cusp, except in later forms; p^ p', p' with three cusps: Primitive: evolving slowly. larly. More advanced - A little more advanced. Well advanced. But little changed. Of high type do Evolving slowly.. . Of high type Of high t do... Rhadinorhininae — Intermediate conditions: Diplacodontinae— Advanced pi-p' with four cusps; p' with three or four cusps (fourth cusp (te) absent or connected with third cusp (de)): Advanced, but changing very little. Do. Menodontinae— More advanced. but changing very little. Well advanced and P'-p* with four cusps (p^ with fourth cusp very large and not connected with third cusp) : progressing mod- erately. Very advanced, but changing little. Do. ' 272 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA CORRELATION OF DIMENSIONS OF UPPER AND LOWER TEETH In considering the generic and specific relations of isolated upper or lower jaws, it is frequently desirable to estimate some of the dimensions of unknown upper teeth from corresponding dimensions of the lower teeth and vice versa. The following table of equa- tions of measurements, prepared by W. K. Gregory, was based primarily upon the type of Telmatherium cultridens but has been verified as to other titano there s. Dimensions in the upper row of teeth (anteroposterior measure- ment) approximately equal to dimensions in the lower row M '-111^ = tip paraconid mi to tip entoconid 1113. P'-p* = hinder border pi to tip protoconid mi. P2-p'' = tip protoconid p2 to tip protoconid mi. P* = tip protoconid p4 to tip protoconid mi. M' = tip protoconid mi to tip protoconid m2. M2 = tip protoconid mj to tip protoconid ms. M3 = tip protoconid ms to tip hypoconulid mi. Tip metacone (tritooone) p'' to tip metastyle m^ = anterior border mi to tip hypoconulid ms. Tip paracone m' to tip paracone m^ = tip metaconid mi to tip metaconid ms. Tip parastyle m' to tip parastyle m3 = tip protoconid mi to tip protoconid mj. Some of these correlations have been frequently used in the identification of Eocene and Oligocene titanotheres, but they are equally applicable to many other groups of mammals, because they are based upon constant interlocking relations of the cusps in the upper and lower molars. GEOLOGIC SUCCESSION AND GEOGRAPHIC DISTRIBUTION OF THE EOCENE TITANOTHERES The following table shows the geologic succession and geographic distribution of the Eocene titanotheres as known in 1914: Geologic succession and distribution of the species of Eocene titanotheres [See flg. 41, p. 59) s .9 ffl a 1 m is 1 1 a i3 S B .1 ^\ X •so X 4(> X 48 X 47 X 4fi X 45 X X X X X X X X 44 43 .. __ - 4? X 41 40 3P X SS S7 X Sfi X X X X X X X X X X X S^i S4 •^s 3? M. earlei X 31 30 W 98 X ?7 X X X X ?6 25a X ''^ X X 94 X X X X X X X X X 9S ?,•>, "^1 ?0 X X IP IS P. leidyi 17 16 15 L. monoconus X EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHBRES Geologic succession and distrihution of the species of Eocene titanotheres — Continued 273 < s o a a 3 1 3 1 P 14 X X X X X n T> 11 in X X Pi q X X X X X X X X X X 8 7 R 5 4 S ?, In, 1 1 _ . SECTION 3. THE LOWER EOCENE TITANOTHERES ANCESTRAL TITANOTHERES OF THE lAMBDOTHERIUM ZONE OF WYOMING AT THE END OF LOWER EOCENE TIME The sudden appearance of two forms of titanotheres in the region now linown as Wyoming and Colorado at the end of lower Eocene time in the Lambdothe- rium zone is a very striking fact. The two forms are LamidotJierium, relatively small, swift, with slender limbs, very abundant (upland type); and Eotitanops, larger, with medium-sized limbs, less abundant (low- land type). No trace of the ancestors of either of these animals has thus far been found in the immediately underlying Heptodon zone and earlier beds, although the remains of horses (Eohippus) occur there in abundance. The evidence favors the theory that the titanotheres mi- grated into the ancient mountain region of North America near the end of early Eocene time. The beds in which they first appear belong to what is called the Lamhdotherium zone, because of the great abun- dance of the remains of this delicately formed titano- there, which is found there in numbers exceeded only by the remains of horses. In the typical Wind River formation Granger, in his collections made from the Lamhdotherium zone in 1905 and 1909, assembled out of a total of 727 specimens remains of 191 Eohippus, 111 Lamhdotherium, and 14 Eotitanops. The localities of the Lamhdotherium zone known up to the year 1912 were as follows: Thickness in feet Wyoming, Wind River Basin, Lost Cabin section 400 Wyoming, Big Horn Basin, Tatman Mountain section 325 Wyoming, Beaver Divide, Green Cove section 265 Colorado, Huerfano Basin, Garcia Canyon 400 The three sections in Wyoming are of nearly uni- form thickness throughout. The geology of the Lamh- dotherium zone in Colorado is described in Chapter II. There is also considerable uniformity in the size and character of the remains of Lamhdotherium. Most of the remains are referred to a single species, L. popoagi- cum, which, however, appears to split up into several subspecies. We Icnow only one phase in the evolu- tion of this animal. Other phases await discovery. PHYSIOGRAPHIC ENVIRONMENT AT THE END OF LOWER EOCENE TIME It is especially interesting to picture the geographic, climatic, and biotic conditions surrounding these early titanotheres. The picture may be drawn partly from the study of the rocks in which their remains occur and partly from the remains of the numerous mammals that are found with them in these Wind River deposits. The complete geologic relations of the deposits of the Lamhdotherium zone have been presented in Chapter II. We comment here chiefly on the geographic and climatic features of the period. WIND RIVER BASIN, WYOMING In lower Eocene time {Lamhdotherium zone) the Wind River Basin was a broad flood-plain valley with mountain barriers to the north, west, and southwest and an easterly drainage. (Sinclair and Granger, 1911.1, pp. 87-103, 105.) The materials of which the beds are composed came from these surrounding mountains. The fine material consists of highly colored clay, in places banded alternately red and blue, interstratified with pale greenish-buff and yel- low-brown sandstone in more or less continuous 274 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA lenses. The sandstone, which is composed largely of quartz sand in which fossils are rare, appears to have been laid down by swift-flowing streams. The de- posits of blue clay contain layers of lignite, ranging from mere dark bands to rather thick beds, indicating still water and a humid climate when vegetation was accumulating rapidly. Skeletons of mammals found in this blue clay were evidently swept into still-water areas and covered with river sediment; but fossils are rare in this stratum also. In many of the bands of red clay, on the contrary, or at the contact of the red and blue strata, great numbers of fragments of jaws and scattered teeth are found. Such levels probably represent parts of the basin floor as it was when these creatures died. The beds of red clay, according to Loomis, Granger, and Sinclair, were formed during the drier cycles, when the carbonaceous matter of decaying plants was completely oxidized, when iron compounds were concentrated and oxidized, and when the bones of animals exposed at the surface were weathered and broken before they were entombed. These signs of the alternation of moist and dry climate, indicated respec- tively by blue and red clays, are not accompanied by signs of excessive aridity, the mammals in the red and blue clay bands being the same. Similar alternations of red and blue clays are now found in the desert basins of Asia. BIG HORN BASIN, WYOMING The discovery of Lamhdotlierium by the Amherst College expedition of 1904 under Loomis and its localization by the American Museum expedition of 1911 (Sinclair and Granger, 1911.1) in the uppermost levels of the red-banded clay beneath the lignitic beds of Tatman Mountain demonstrated the deposi- tion of sediments of Wind River age in the Big Horn Basin. The true Lambdoiherium zone is exposed on all sides of Tatman Mountain and consists chiefly of red-banded beds. Granger and Sinclair observe (1912.1, p. 66) that the lower Eocene sediments of the Big Horn Basin, like those of the Wind River Basin, represent the filling in of a great trough surrounded by mountains. No volcanic ash occurs. The moun- tain streams have borne down gravel, sand, and clay and deposited them in stream channels or spread them over flood plains. No evidence of wind trans- portation has been observed. The red and blue banding of the clays occurs in more or less regular alternation. BEAVER DIVIDE, WYOMING The discovery of a typical Wind River fauna on Beaver Divide by Olsen, of the American Museum party of 1910, was a most important one, because it extends the range of this fauna many miles to the southwest. The entire fauna was obtained at or near a certain stratum of bluish-green shale resting on a band of red shale, the fossiliferous zone not exceeding 10 feet in thickness. (See Chap. II.) Remains of the animals listed below were obtained: Equidae: Eohippus craspedotus. Eohippus? venticolus. Lophiodontidae : Heptodon calciculus. Heptodon ventorum. Heptodon n. sp. Titanotheriidae : Lambdotherium popoagi- cum. Amblypoda: Coryphodon sp. Reptilia: Glyptosaurus (scutes). Crocodile (scutes, verte- brae, and teeth). Turtles (numerous frag- ments) . Insectivora: Hyopsodus n. sp. Hyopsodus sp. Creodonta: Didymictis? altidens. Primates: Microsyops sp. The fish and aquatic reptiles in this fauna indicate plainly that the deposit on Beaver Divide was fluvia- tile, and, as Granger and Sinclair observe, go far toward establishing the theory that the Wind River shales were flood-plain deposits, a theory that is further supported by the presence of numerous channel fillings of coarse sandstone. All the fossils from the shales are fragmentary and consist mostly of teeth whose roots are worn off, indicative of water trans- portation and abrasion. HUERFANO BASIN, COLORADO The Lambdotherium zone was discovered in Colo- rado by Dr. J. L. Wortman while he accompanied the writer in 1897 (Osborn, 1897.126) on a survey of the Huerfano Eocene deposits, which were first announced by Hills in 1888 (Hills, 1888.1). The zoogeographic significance of this discovery is evident from the fact that it carries the Lambdoiherium fauna eastward to the foothills of the Rocky Moun- tains, between the famous extinct volcanoes known as the Spanish Peaks. Wortman described these beds as follows: These beds of the lower division [Lambdoiherium zone] are indistinguishable, so far as their general appearance and litho- logical characters are concerned, from those of the upper level [Palaeosyops fonlinalis zone]. The fossils occur apparently in a single stratum not exceeding 10 or 15 feet in thickness and not more than 30 or 40 feet from the base of the formation. They underlie the beds of the upper division with perfect conformity, and there is at present no means of determining exactly where the one ends and the other begins. That sedimentation was continuous and iminterrupted from the beginning to the close of the whole [Huerfano] deposit, I do not think there can be the slightest question. The exact locality from which the greater number of the fossils of the lower beds were obtained is Garcias Caiion, about IJ-^ miles south of Talpa or the mouth of Turkey Creek. [Osborn, 1897.126, pp. 253-254.] The animals associated with Lambdotherium in this zone are provisionally identified by Osborn and Matthew as follows: Titanotheriidae Lambdotherium popoagi- cum. Creodonta Didymictis altidens. Didymictis leptomylus. Oxyaena lupina. Insectivora Hyopsodus sp. Amblypoda Coryphodon ventanus. Artiod actyla Trigonolestes secans. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 275 SUMMARY A summary of the life conditions in the Wind River and Big Horn Basins during lower Eocene time shows that there is no evidence of climatic change throughout lower Eocene time; that the conditions through the Systemodon zone and the succeeding Heptodon zone, into the overlying Lambdoiherium zone, remain sub- stantially similar. This fact accords with the sub- stantial similarity in the general character of the mam- malian fauna throughout the lower Eocene. The fauna evolves during this very long period; old forms give way to new; but it does not change in its general adaptation to conditions. Thus in the great mountain valleys sediments were being continually derived by erosion from the older rocks of the mountains and deposited in these great basins. Throughout Wind River time fluviatile dep- osition is indicated by numerous channels filled with coarse sandstones which irregularly traverse the finer clays or interstratify with them in the form of lenses. In the clays are found fish, crocodiles, and turtles, and occasionally beds of JJnio. Local swamps are indi- cated by the presence of lignitic areas in the blue clays and in the sandstones, but never among the red clays. The feldspars that wash down from the surrounding granitic mountains are fresh and angular, a fact that suggests rapid transportation of the fragments for short distances and burial beyond the reach of car- bonated waters. These conditions do not favor the idea of luxuriant Eocene tropical forests or of a warm, humid climate with the formation of a deeply decay- ing humus, but suggest rather a dry although not nec- essarily arid climate, with rapid changes of tempera- ture favorable to splintering the ledges of the granite cliffs. There were no frosts, but the climate may have been stimulating to a vigorous and actively competing fauna. In the Big Horn Basin fluviatile deposition is indicated throughout the entire Eocene epoch. The lignitic shales that cap the Lambdoiherium zone, as indicated by the fresh-water moUusks and plant contents, are both fluviatile and palustrine. The above picture of the physiography and the cli- mate of these Rocky Mountain basins of Wyoming in early Eocene time accords thoroughly with the analysis of the chief adaptive types of mammals whose re- naains are found in the lower Eocene rocks. These mammals are broadly divided into three types — flu- viatile, or river-living; river border, or palustrine; upland, or plains-living. The upland type sought and found hard ground, to which their narrow feet and compressed hoofs were adapted. Conspicuous among these dry-ground forms is Lambdoiherium itself, a swift-moving, or cursorial animal broadly analogous in structure to the horses of that time (Eohippus) and lophiodonts (Heptodon), as weU as to the archaic condylarths (Phenacodus) . The Wind River fauna of the Lambdotherium zone represents the closing chapter of lower Eocene mam- malian life. It is closely affiliated with the fauna of the typical lower Eocene or Systemodon zone, because the two contain 25 genera and 1 1 species of mammals in common. It is doubtful whether a single family of mammals of the Systemodon zone becomes extinct in the Lambdotherium zone, yet some of the archaic mammals begin to show a numerical reduction. On the other hand, the Wind River fauna is pro- gressive; the first appearance of these two genera of titanotheres, Lambdotherium and Eotitanops, and of seven other new genera of mammals is prophetic of the oncoming middle Eocene or Bridger life. During this transition of the Wind River mammals from their Wasatch forerunners to their Bridger suc- cessors the physiography and the climate apparently remained the same as in earlier Wasatch time, a fact attested not only by the geologic and physiographic evidence just considered but by the similar relative abundance of the adaptive types of mammals found in these two formations. The extent of the collections in the American j Museum, the total number of specimens collected, and the field records of Granger show that fossil mammals are about four times as abundant in the Systemodon zone as in the Lambdotherium zone. The relative numbers of the mammals in the Systemodon zone of the Big Horn Basin of Wyoming, as indicated by the number of specimens collected, are shown below: Mammals in the Systemodon zone Specimens Perissodactyla (horses, 1,202; tapirs, 370) 1, 572 Insectivora (Hyopsodus, 254) 306 Condylarthra (mostly Phenacodus) 264 Amblypoda (corj'phodons only) 209 Creodonta (various carnivores) 203 Primates (Pel3'codus, Anaptomorphus, etc.) 151 Artiodactyla (mostly Trigonolestes) 120 TiUodontia (Esthonyx) 73 Rodentia (Paramys) 16 Taeniodonta (?edentates, Calamodon) 2 Edentata (new type) 1 2,917 This table naturally is only approximately repre- sentative. The rodents, for example, were probably far more abundant numerically than the horses. The great number of horses, tapirs, and hyopsodonts in the assemblage listed above is due in part to the abundance of these animals in the "red beds." The table is valuable chiefly in expressing the relative abundance of the adaptive types of ungulates. In the Lambdotherium zone the relative abundance of the remains of ungulates undergoes a marked change: both of the archaic types of mammals, the condylarths and the amblypods, are relatively less abundant than in the Systemodon zone. 276 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Relative frequency of mammals in the Lambdotherium zone of Wyoming as indicated by the number of specimens collected Specimens Perissodactyla (horses, 191; titanotheres, 124; heptodonts, 56; hyrachyids, 2) 373 Primates (pelycodonts, 42; anaptomorphs, 9; Microsyops, 30) 81 Insectivora (Hyopsodus, 71) 75 Condylarthra (Phenacodus, 47; Meniscotherium, 1) 48 Rodentia (Paramys, 39) 42 Creodonta (various carnivores) 35 Amblypoda (Coryphodon, 29; Bathyopsis, 2) 31 Artiodactyla (Trigonolestes) 20 Tillodontia (Esthonyx) 12 Edentata (Taeniodonta) (Stylinodon, Calamodon) 5 722 It will be observed that the Condylarthra {Phena- codus) here drop to the fourth place in relative fre- quency, and the Amblypoda {Coryphodon) drop from the fourth to the seventh place. This reduction is partly in accord with the reduction of the archaic types of mammals generally, as shown also in the following faunistic comparison; in the Systemodon zone both the genera and species of archaic mammals numerically exceed those of modern type, whereas in the Lambdo- therium zone the genera and species of archaic and of modern types are evenly balanced. The relative frequency of the various adaptations to cursorial, ambulatory, and arboreal life is still more significant. The comparison of the adaptive types of the Systemodon and Lambdotherium zones is as follows : System- Lambdo- odon therimn zone zone Ungulates; cursorial, small, light-limbed 1,692 373 Ungulates; mediportal, medium in size 264 63 Ungulates; graviportal, large, heavy-limbed 209 31 Primates; arboreal, climbing -types 194 81 Insectivores; rodents, etc., ambulatory, small terrestrial and fossorial mammals 395 139 Carnivores; larger and smaller creodonts 203 35 2,957 722 It will be observed that although the fossils collected from the Lambdotherium zone are only one-third as numerous as those of the Systemodon zone the relative abundance of the adaptive types is approximately the same, a fact that sustains the inferences as to geologic and physiographic continuity or the absence of any marked changes of environment during lower Eocene time. Also, in the Lambdotherium zone, as in the Systemodon zone, there is still a numerical predomi- nance among the ungulates of cursorial types, the horses {Eohippus), the lophiodonts {Eeptodon), the titanotheres {Lambdotherium). Singularly, no primi- tive tapirs {Systemodon) have been found. Among the mediportal types may be noted the remains of Phenacodus and the titanothere Eotitanops. The graviportal coryphodonts, which may have been am- phibious or partly aquatic in habit, are comparatively The first author to analyze the Wasatch fauna with reference to adaptive types in their bearing on physi- ography and climate was Loomis (1907.1), who divided the Wasatch fauna into percentages, substantially as follows : Terrestrial and arboreal types 75 Aerial 3 Amphibious 12 Aquatic, including crocodiles, turtles, and fishes 10 The present analysis of the adaptations of foot structure in Wasatch and Wind River time combined gives the following relative degrees of abundance among the hoofed mammals: Ungulates; small, cursorial, light-Umbed types 2, 065 Ungulates; medium, mediportal types, proportioned like the tapir 327 Ungulates; graviportal, heavy-hmbed types {Cory- phodon) , proportioned hke the hippopotamus 240 The analysis both of the geologic and paleontologic evidence appears to show that in Wind River time there was a warm but relatively dry and invigorating climate in the Rocky Mountain region; that there were streams, swamps, and river borders for the coryph- odonts, forests and meadow borders for the true titanotheres {Eotitanops), and open spaces with harder ground for the diminutive horses, lambdotheres, and heptodonts. In the forests there were numerous lemu- roid or monkey types, as well as arboreal rodents, and on the borders of the savannas there were terrestrial and partly fossorial edentate-like mammals. Periods of aridity and areas of drier ground favored the de- velopment of the light-limbed ungulates. CONTRASTS AND RESEMBLANCES BETWEEN LAMBDO- THERIUM AND EOTITANOPS Geologic and biologic evidence of the existence of areas of dry, hard ground in Wind River time is thus adduced to explain the surprising fact that the feet and limbs of the little Lambdotherium are more highly specialized for cursorial locomotion than the feet of any of the known middle Eocene titanotheres. An alternative interpretation is that the ancestral peris- sodactyls wei'e small cursorial forms with narrow feet like Heptodon and Systemodon and that the widening of the feet is a secondary adaptation to mediportal habits. (See p. 586.) The skull of Lambdotherium is elongate and relatively Eohippus-like. Lambdo- therium was probably an early specialized cursorial member of the great titanothere family, a member that died out without leaving descendants. Under the law of local adaptive radiation it may have lived in the drier uplands; at all events its remains are especially abundant in the "red beds," in which all together no less than 111 specimens have been found in comparison with 14 of the bulkier Eotitanops. (See fig. 230.) Judging by the 14 specimens of Eotitanops that have been found in the typical Wind River Lambdo~ EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHBRES 277 therium zone, they had already shown considerable specialization both in structure and in size. The smaller and more primitive forms, such as E. gregoryi, only slightly exceed Larabdotherium popoagicum in central digit — it still resembles Lambdotherium, as shown in Figure 220. Its feet are adapted to softer ground, and we may conjecture that it resembled the tapir in its habits. Figure 230. — Reconstructed skeletons and restorations of Lambdotherium 'popoagicum (Ai, A2) and Eotitanops borealis (Bi, B2) Drawn by E. S. Christman under the direction of W. K. Gregory. About one-tenth natural size. These provisional reconstructions of skeleton and body are based on material in the American Museum. size, but species like Eotitanops princeps and E. major are little inferior to the existing American tapir {Tapirus terrestris). The gradations in size between these five or six species and mutations of Eotitanops may be judged from the accompanying outlines (fig. 231) of the lower jaws. Eotitanops is a more typical titanothere than Lamb- dotherium. In its limb structure it approaches espe- cially Mesatirhinus, of the middle Eocene, although in its mesaxonic foot structure — that is, its enlarged The principal contrasts theres are the following: Lambdotheri um Cursorial, light limbed, small, like Eohippus. Face decidedly elongate. Muzzle attenuate, pointed. Cranium abbreviate. Three premolars. Cropping teeth procumbent. between these two titano- Eotitanops Cursorial to mediportal, small to large, almost equaling Tapirus. Face moderately elongate. Muzzle relatively abbreviate, broad. Cranium intermediate. Four premolars. Cropping teeth more erect. 278 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA Both animals possess a number of features in com- mon: (1) Tliey show a similar cone and crescent (or Figure 231. — Lower jaws of Lamhdotherium., Eotitanops, and Tapirus One-fourth natural size. A, Lambdoiherium popoagicum. Am. Mus. 14906; Alkali Creek, Buck Spring, Wind River Basin. B, Eotitanops grcgoryi, Am. Mus. 14889 (type); Alkali Creek, Buck .Spring, Wind River Basin, C, E. brownianus, Am. Mus. 4885 (type); Wind River Basin. D, E. borealis, Am. Mus. 14891; west bank of Wind River, 3 miles above canyon (top of banded beds). E, E. princeps. Am. Mus. 296 (type); Wind River Basin. F, Tapirus ierrestris, Am. Mus. 1135; immature rpecimen. bunoselenodont) pattern of the superior grinding teeth, with either reduced or vestigial intermediate cusps or conules; (2) in both the preorbital (or facial) part of the skull is longer than the cranial (see fig. 232), a primitive characteristic of perissodactyls which at once allies these animals to other primitive perisso- dactyls and distinguishes them from the middle Eocene titanotheres, in which the cranium is longer than the face; (3) they show enlargement of the median digit (D. Ill), or mesaxonic structure of the fore and hind feet. It is quite possible (see Chap. VIII) that this mesaxonic specialization is itself common among perissodactyls — that is, that all perissodactyls are descended from quadrupeds with narrow feet, as sug- gested by Gregory. We should interpret this swift- footed structure as an adaptation that enabled the small, defenseless perissodactyls, without horns or tusks, to escape their pursuers. CBp7ialic70O% FiGTJKE 232. — Restored contours of skulls of La?7ibdothe- rium and Eotitanops Showing estimated proportions of face (shaded) to cranium. One-fourth natural size. A, Lambiotheriam, face 65, cranium 35. B, Eotitanops, lace 56, cranium 44. The special similarities of foot structure between Lamhdotherium, and Eotitanops embrace the follow- ing characters: (1) Enlargement of the median digit (D. Ill), which is provided with a broad proximal phalanx; (2) reduction of digits II and IV, with re- lative narrowing of the proximal phalanges; (3) reduc- tion of digit V (correlated with this metapodial structure we find that the lunar immediately rests on the unciform, with a narrow {Lambdotherium) or oblique (Eotitanops) facet on the magnum) ; (4) small, relatively high and narrow magnum. These points are well illustrated in the accompanying figures of the manus of Lamhdotherium and Eotitanops. The interpretation of this somewhat specialized form of foot as one derived from a cursorial type is considered in the introduction to the study of the skeleton of the Eocene titanotheres (p. 586). EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 279 EXPLORATIONS AND DISCOVERIES The explorations in the Wind River Basin for re- mains of titanotheres and the types collected were as follows : 1880. J. L. Wortman, for E. D. Cope; EoHtanops borealis (Cope), E. brownianus (Cope), and Lambdotherium popoagicum (Cope). 1891. J. L. Wortman, for American Museum of Natural History; EoHtanops princeps Osborn. 1894. F. B. Loomis, for Amherst College Museum; Lamb- dolheriavi primaevum Loomis. 1909, 1910, 1911. Walter Granger, for American Museum of Natural History; L. priscum Osborn, L. progressum Osborn, and E. gregoryi Osborn. The American Museum expeditions of 1909, 1910, and 1911, under Granger and Sinclair, made a series of important discoveries — first, in locating all these titanotheres in the Lamhdotherium zone; second, in proving that EoHtanops and Lambdotherium were contemporaneous; third, in collecting the remains of more than eight specimens of EoHtanops (Am. Mus. 14887-14894); fourth, in collecting a complete skull and jaws of Eotitanops iorealis, affording proof that EoHtanops is more primitive than the middle Eocene or Bridger forms and belongs to a distinct generic stage. The Wind River Lamhdotherium zone is 400 feet thick and may be divided into four levels, each in- cluding 100 feet. It is a remarkable fact that nearly all the remains of the 124 specimens of titanotheres found by the American Museum parties were collected between the 250 and 400 foot levels, as shown in the section taken from Granger's field records (fig. 48). Thus Eotitanops and Lamhdotherium occur contem- poraneously. It appears that the mutations or spe- cies of Eotitanops do not exhibit a continuously graded evolution or succession in ascending levels, for it hap- pens that the smallest and most primitive form known, E. gregoryi, occurs on a high geologic level, showing that Eotitanops was already polyphyletic in early Eocene time. This is an example of the extreme importance of an exact record of levels. SYSTEMATIC DESCRIPTIONS OF THE LOWER EOCENE TITANOTHERES Superfamily Titanotheroidea Osborn Perissodactyls with bunoselenodont superior molars and selenodont inferior molars. Distinguished from the Chalicotherioidea by normal limbs and hoofs. Distinguished from the Hippoidea by a persistently tetradactyl manus and tridactyl pes. Family Brontotheriidae Marsh Related to the type of Brontotherium. Evolving between early Eocene and early Oligocene time. Primitively cursorial in gait but early evolving into mediportal and graviportal forms. Terminal pha- langes and hoofs progressively reduced. Forward 101959— 29— VOL 1 21 portion of skull originally elongate but early becoming abbreviate; cranial portion progressively elongate. Earlier genera hornless; paired nasofrontal horns developing in middle Eocene time and becoming the dominant character of the skull. Subfamily Lambdotlieriinae Osborn Lower Eocene titanotheres, small, light limbed, of slender, cursorial proportions. Skull mesaticephalic; facial region elongate; cranial region abbreviate. Superior molars brachyodont, incipiently bunoseleno- dont, with lophoid paraconules and metaconules; lower molars with pronounced metastylids; premolars \ absent; lower premolars p2_4, progressively molariform. Lambdotherium Cope Plate LIV; text figures 27, 33, 103, 114, 143, 146, 154, 221, 228, 230-242, 244, 483, 484, 486-492, 503, 504, 510, 512, 521, 522, 661, 685, 688, 692, 694, 700, 701, 723 [For original description and type reference see p. 168. For skeletal characters see p. 590] Generic characters. — Skull of decided proopic doli- chocephaly. Anterior premolars wanting. Superior molars broadened transversely, with prominent para- styles and mesostyles, oblique ectolophs, large, free protoconules, and low metalophs; m' with prominent hypocone; inferior molars with metastylids; first up per and lower premolars wanting; posterior lower pre- molars progressive; ra^ with large crescentic hypoco- nulid. Manus numerically tetradactyl, functionally aniso tridactyl; lunar resting chiefly on unciform an- teriorly; magnum small, high, and narrow. We know neither the ancestors nor the descendants of this animal. It is already more highly specialized in many respects than certain of the middle Eocene titanotheres. Lamhdotherium is readily distinguished by its fine, delicate construction for swift movement, in which it has analogies to the lower Eocene horses. It has a long, slender snout and delicately prehensile jaw. We note especially that (1) the face is much elongated, the faciocephalic index being 65 as com- pared with 56 in Eotitanops and 60 in Eohippus; (2) this elongation is correlated with a very slender snout-, but the first upper and lower premolars are wanting; (3) the third and fourth lower premolars are more complicated than in Eotitanops, p4 in some forms closely resembling a molar tooth; (4) its sharply piercing canine teeth and chisel-shaped incisors are other features of specialization. History of discovery of Lamhdotherium. — Wortman' s discovery in the Wind River valley (1880) and Cope's original description have already been cited. Cope at once recognized the ancestral relationship of this form to the titanotheres. In 1889 he made Lamhdo- therium the type of a distinct family — "Lambdo- theriidae"- — to embrace all the Eocene titanotheres, a family name that was adopted by Flower and Lydek- ker. In his "Tertiary Vertebrata" (1885.1, pp. 709, 280 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 711) Cope mistakenly referred to this genus the two species L. ( = Eotitanops) hrownianum. and L. procyo- niiMm. In the same memoir he placed all the Eocene titanotheres in the family "Chalicotheriidae." In his memoir of 1892 Earle correctly considered this animal a probable member of a side line of titanotheres. In 1893 Osborn recognized the division as a subfamily — "Lambdotheriinae." In 1897 Osborn mistakenly pro- posed to remove Lamhdotherium from the titanotheres and related it to the Equidae on the ground of its slen- der foot structure. The renewal of the demonstration of its relation to the titanotheres is due to W. K. Gregory. In 1907 Loomis discovered five specimens of this genus in the deposits of Buffalo Basin, one of which he selected as the type of the new species L. primaevum, believing it to represent a stage somewhat more primitive and perhaps geologically older than L. popoagicum. In 1905 and 1909 the American Museum party under Granger finally determined that these animals were geologically contemporaneous with Eoti- tanops but confined to a comparatively narrow geologic zone. The principal collectors and the areas in which they worked were as follows: 1880. E. D. Cope, J. L. Wortman, for American Museum of Natural History; Wind River Basin, Wyo. 1896. J. L. Wortman, for American Museum of Natural His- tory; Huerfano Park, Colo. 1905. Walter Granger, for American Museum of Natural His- tory; Wind River Basin, Wyo. F. B. Loomis, for Amherst Museum; Big Horn Basin, Wyo. Walter Granger, for American Museum of Natural His- tory; Wind River Basin, Wyo. 1907. 1909. 1916. Walter Granger, for American Museum of Natural His- tory; Huerfano Park, Colo. 1918. Walter Granger, for American Museum of Natural His- tory; Huerfano Park, Colo. Geologic horizons. — The animals found in four ex- posures of the Lamidotherium zone in the Wind River Basin, the Big Horn Basin, the Beaver Divide, and Huerfano Park, each 300 to 400 feet thick, are not separated by marked differentiation or evolution; in all these basins and on all the levels of each formation the lambdotheres, so far as known, are substantially similar in size but differ markedly in the degree of evolution of the third and fourth lower premolar teeth. The range in size is indicated on page 282 . A very striking fact is that the extremes of premolar structure (-fig. 234) were found in animals collected around the great Alkali Creek "red stratum," which is such a conspicuous level mark (fig. 47), many of the specimens being just below the "red stratum," or 50 to 100 feet above the base of the Wind River forma- tion. The greater part of the collections have been made within a vertical distance of 200 feet, which would represent time for considerable evolution; but as the stages of evolution do not occur successively in the ascending levels, it does not appear practicable to separate any but the extreme forms as species or mutations, and the systematic order therefore appears as follows: Lamhdotherium progressum Osborn (most progressive) . L. popoagicum Cope, L. primaevum Loomis (inter- mediate) . L. priscum Osborn (most primitive). Measurements of teeth of Lamhdotherium, in millimeters [The numbers are those of specimens in the American Museum of Natural History] L . priscum L. popoagicum L. progressum L. sp., 14922 L. mag- U916 14912 14914 12822 (type) 14908 4863 (type) 14899 14904 14907 14902 14917 (type) 14918 14919 17527 (type) 68.7 60.9 50.9 26.7 42 7. 5 9.3 9.4 11.4 12 17.5 64 57 47 25 39 7.6 8.2 9 10.3 11. 1 16.2 69 61 51 26.4 42.4 9 9.5 1L4 12.6 17.4 70.6 62.3 53.3 27.4 43. 1 8.2 9 9.3 12.8 13.2 17.3 9.3 10.8 12.5 74 58 59 64 51.8 26.4 54 P2-P4 25 36 28. 5 39 41 41 44 P2 (ap.) 7.6 9 9.5 11.4 12.5 8.5 P3 (ap.) 8.5 9 10.9 11.8 16 9 9 10. 8 12 9.5 9.3 11 12. 5 16. 5 8 9 12 8.2 10 10. 1 15 9.5 P4 (ap.) 10 Ml (ap.) 11 11. 4 17.7 12 13 18.5 P2 m' 58.6 51 24 34 7 7.8 8.5 10.4 11. 1 12 pS jqS 50.6 V- D* 1 M' m^ 1 36 34 P2 (an ) P2 (an ) 7 8 10.3 10.9 11.4 P* (au ) 1 M' Can 1 1 11.5 12 12 M2 faD ■) M« (an ) EVOLUTION HE SKULL AND TEETH OF EOCENE TITANOTHERES 281 Lambdotherium popoagicum Cope Plate LIV; text figures 27, 33, 103, 143, 228, 230, 231, 233-237, 244, 483, 484, 486-492, 503, 504, 512, 521, 522, 661, 694, 700 [For original description and type references see p. 168. For slieletal characters see p. 590] Type locality and geologic horizon. — Wind River Basin, Wyo.; Lambdotherium zone (Wind River B). Specific characters. — P2-m3 69-56 millimeters; ps with paraconid, metaconid, and hypoconid intermediate in development. In superior molars the protoconules more or less free and distinct, metaloph low but dis- tinct, cingula not surrounding the crown internally. Materials. — The type species (figs. 234, H; 236, C) of a series of mutations of specific character is repre- sented by over 70 specimens in the collections of the American Museum, chiefly from the typical Wind River formation, but also from contemporaneous deposits in the Big Horn Basin, Beaver Divide, and Huerfano Basin. These specimens consist mostly of scattered upper and lower teeth and fragments of jaws but include several nearly complete jaws. One specimen (Am. Mus. 4880) affords a limited but signi- ficant knowledge of the skeleton. SJciill.— The imperfectly known skull is analo- gous to that of the primitive horses rather than that of Eotitanops or any of the typical middle Eocene titanotheres. There are only two speci- mens (Am. Mus. 14903, 14907) in which fragments of the skull are associated with the teeth, from which the conjectural restoration (fig. 233) is as- sembled, the outlines of the anterior part, or pre- maxillaries and nasals, being inferred from the attenuate structure of the lower jaw. The prin- General features of the teeth. — The dental formula, so far as known, is If, C\, Pf , M|. The inferior incisors, as observed in Am. Mus. 14899, 14906, 14920, repre- sented in Figures 233, 236, 237, are semicircular in arrangement, semiprocumbent, with spatulate or chisel-shaped crowns; the median incisors especially, which are distinctly chisel-shaped, are quite different from those of Palaeosyops, which are bluntly pointed. The inferior and superior canines are rounded, slightly compressed laterally, and sharply pointed. Ty2)e preinolars. — No trace of p^ or pi is to be found in any of the specimens; this tooth is ordinarily very persistent in the Perissodactyla. In the L. popoagicum type premolar series p2 is an elevated, laterally com- pressed cone, with a rudimentary paraconid and low, narrow heel bearing a hypoconid; ps presents an anterior lobe composed of a low paraconid, an elevated protoconid, a postero-internal metaconid elevated but slightly developed, a somewhat broader posterior Figure 233. — Skull of Lambdotherium "popoagicum, reconstructed cipal characters are the following: (1) Dolicho- MadebyL. M. sterling under the direction otW. K.Gregory. About two-fifths natural size. ,. . iUi"! IJ '^'''^ reconstruction is made from three specimens in the American Ivluseum, collected in Cephaly OI prOOpiC type that is, long, slender ^^^ ^^^^ jji^^j. Basin-No. 14899,Alkali creek, Buck Spring, lower jaw; No. 14907, Alkali skuU, in which the facial greatly exceeds the cranial creek, Wolton, maxilla, malar, and skull top; No. 14903, Alkali Creek, Buck Spring, , ,1,1 <•• 11--1 1- nr- squamosal and condyle. Missing parts conjecturally restored by comparison with Systemo- length, the faciocephalic mdex being 65, as com- ionB.niEoMppm. pared with 56 in Eotitanops; (2) sagittal crest rather low and slender; (3) external auditory meatus open inferiorly; (4) infraorbital foramen placed above the second premolar — that is, decidedly anterior in posi- tion as compared with that in the typical titano- theres; (5) an attenuated rostrum associated with the elongated symphysis of the jaw, suggesting the conformation of the skull of a ruminant rather than of a perissodactyl. Comparison. — This skull is very close in its propor- tions to that of Eohippus, and if it were not for the differences in the teeth might be mistaken for it. The faciocranial indices are similar, namely: Eohippus Lamb- dotherium Eotitanops Faciocephalic _ __ - 68-60 42-40 41 65 35 (?) 56 44 (?) heel or hypoconid, with a rudiment ry internal crest representing the entoconid; p4 is a more progressive or submolariform tooth with an anterior transverse crest composed of protoconid and metaconid behind which is a low, incomplete posterior crest supporting an elevated hypoconid and a depressed internal ridge. Lower premolars, primitive and progressive muta- tions or specific forms. — The structure of the cusps in P2, Ps, P4 is very important. The accompanying diagram (fig. 234) shows the wide range of progressive evolution in the lower premolar teeth which are exhibited in the large number of specimens in the American Museum collection. They embrace stages ranging from far less primitive to stages far more primitive than the lower premolar teeth of the type of L. popoagicum. These stages are especially impor- tant and interesting because they are recorded as coming from similar geologic levels. These records of geologic levels may be confused, but accepting them 282 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBjRASKA as correct, it seems impracticable to divide Lamb- dotherium into a large series of species, although the development of the premolar cusps certainly warrants specific separa- tion. These extreme stages are therefore grouped together for the present as muta- tions between Lambdothe- rium priscum, represented by the simplest type (No. 12822), and Lambdotherium progressum, represented by the most advanced type (No. 14917). Halfway between the extremes is the type species L. popo- agicum. The cusp evolution is very interesting, including the following elements: Ante- rior lobe — protoconid (pr*), paraconid (pa*^), metaconid (me"*), metastylid (ms"^); posterior lobe — hypoconid (h**), entoconid (en''). The series represented in Figure 234, including L. priscum at the bottom and L. progressum at the top, presents a complete mor- phologic transition or epit- ome of premolar evolution, it being essential to note that we do not know Figure 234.— Lower premo- whether this corresponds lars of three "species" or .,•■ i f T<7,fh o T-ooi succession m mutations of Lambdothe- rium, illustrating progres- with time. real At the base p2 and sive evolution of the pre- p^ are excessively simple, NatoalsL^'selnd, third, and fourth but at the Summit Pa haS lower premolars of the left side, inner a large paraCOnid and mcta- sideview. A-E, i. priscum; F-H, i. . , , , popoagimm; I-L, i. progressum. AH COnid, and P4 haS a para- American Museum specimens from „ -J rnptflponid metfl- the Wind River Basin. A, No. 12822 COUIQ, metaCOniQ, meia- stylid, and rudiment of an entoconid, which is devel- oped as a distinct cusp in certain specimens (such as (type), 3 miles east of Lost Cabin; B, No. 14916, Alkali Creek, Buck Spring; C, No. 14900, Alkali Creek, Wolton; D, No. 14912, Dry Muddy Creek, 18 miles above mouth; E, No. 14914, Alkali Creek, Wolton; F, No. 14915, Alkali Creek, Wolton; G, No. 14924, Dry Muddy Creek, 18 mUes above Am. MuS. 14924). mouth; H, No. 4863 (type). Wind 7-7 River valley; I, No. 14913, Muddy PremoiaTS relatively prO- Creek, south side, 18 miles above • tj_ • mouth; J, No. 14919, Alkali Creek, gressive.—lt IS vcry impor- Wolton; K, No. 14918, lower Alkali font in nntA tViot n nnrl Creek; L, No. 14917 (type). Alkali ^^^^ ^^ ^^^^ ^'^^^ Ps ^^^ Creek, Buck Spring. p^ in the progressive forms are much more advanced in evolution than the corresponding teeth in the contemporary Eotitanops; in fact, p4 is submolariform and lacks only the prom- inence of the metaconid to be like a molar. P2 in Lambdotherium is almost as progressive as ps in Eotitanops. Even in the middle Eocene species Palaeosyops leidyi, ps, p4 are not so far advanced as in Lambdotherium. We observe also another dis- tinctive character: Whereas in Palaeosyops the meta- conid arises as a bud or reduplication of the proto- conid, in Lambdotherium it springs from the posterior side of the protoconid. These details are of impor- tance as demonstrating the accelerated rate of evolu- tion of the premolar cusps as a character of lamb- do theres. Superior premolars. — Comparatively few well-pre- served superior premolar series are Icnown, so it can not be determined whether there is a corresponding series of mutations in the evolution of the upper teeth. In the specimens Am. Mus. 14902, 14900, 14911, 14907 the following characters are observed: (1) p^ very simple, with single external protocone and rudi- ments of the deuterocone and sometimes of the tritocone; (2) p', p* with rudimentary parastyle, deuterocone, tritocone, rudimentary crests connecting deuterocone with protocone and tritocone, respec- tively, faint conules sometimes observed on these crests. A series of deciduous premolars (Am. Mus. 14934) exhibits dp^ somewhat more complex than p^, dp^ elongate, quadricuspidate, with prominent parastyle and mesostyle. Molars. — The inferior molars are highly character- istic teeth, distinguished especially by the elevation of their crescents; the protolophid consists of an elevated protoconid, metaconid, and metastylid, or double internal cusp, which is very distinct in unworn teeth. This reduplicate cusp, which develops in the upper Eocene species of horses and also in the true chali- cotheres, is not present in the titanotheres of the middle Eocene, such as Palaeosyops. A rudimentary hypoconuHd is usually observed in nii and m2 and develops into a strong crescentic third or posterior lobe in 1113. The superior molars, as observed in five specimens in the American Museum (Nos. 14900, 14902, 14904, 14907, 14911), are fairly uniform in character, with very prominent parastyles, mesostyles, variable pro- toconules, rudimentary or lophoid metaconules. (PI. LIV, A, B; fig. 235, A.) A very distinctive feature is the large hypocone on m^ The following characters should also be noted: (1) The transverse diameter always exceeds the anteroposterior; (2) m' is a rela- tively small tooth; m^ is usually the largest tooth of the series; m' is usually intermediate in size but some- times is the largest tooth of the series. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 283 Measurements of superior molars of Lambdotherium popoagicum, in millimeters Ml M! M> Ap. Tr. Ap. Tr. Ap. Tr. Am. Mus. 4664 .._ 10.5 1L5 16 Am. Mus. 4880 . 11.5 11.5 15 14.2 Am. Mus. 14902, J. 10.5 13 12 14.5 Other distinctive characters of the superior teeth are the very obUque ectolophs, the prominent para- styles and mesostyles, the sculptured form of the cusps, especially apparent in the unworn specimens. The protoconule is distinct and usually of subtriangular form; it is connected with the protocone by a low crest. The metaconule proper is rather sessile, indis- tinct, or wanting; there is, in fact, a depressed metaloph or rudimentary posterior crest. The third superior molar (Am. Mus. 4664) exhibits an especially oblique ectoloph, also a prominent hypocone and low but distinct metaloph. The jaw.— The type jaw (Am. Mus. 4863, fig. 236) consists of two separate and incomplete rami figured by Cope (Tertiary Vertebrata, PL LVIII, B). The distinctive feature of the type species is the inter- mediate condition of the third premolar, which dis- tinguishes this animal from primitive and more pro- gressive specimens. There are five more complete jaws, namely, Am. Mus. 14899 (figs. 231, 236), 14905, 14906 (figs. 231, 236), 14909, which together afford a full knowledge of the characters of the jaw except the angular and condylar region ; the elongate and laterally compressed chin (fig. 236), even more extreme than that of the Eocene horses; the wide diastema between the canine and the second premolar; the incisive border extending somewhat to support the slender, recurved, prehensile canines and the row of chisel-shaped, semiprocumbent teeth; the coronoid process high, vertically placed, sharply defined, with flat anterior face. The extremes of measurement are shown below. Measurements of jaw of Lambdotherium, in millimeters L. popoagicum, Am. Mus. 4863 (type jaw) L.progressum, Am. Mus. 14919 Oargest L. priscum, Am. Mus. 14908 (smaJlest jaw) P2-m3, anteroposterior Mi-m3, anteroposterior M], anteroposterior M2, anteroposterior M3 , anteroposterior Depth of jaw below ms 68 41 11 12 16 32 70.6 43. 1 66 37 The premolars are 63 per cent of the length of the molars. Lambdotherium primaevum Loomis Plate LIV, C, D; text figure 114 [For original description and type reference see p. 178] Type locality and geologic horizon. — ^Big Horn Basin, Wyo. ; Lamhdotherium-Eotitanops-Coryphodon zone (Big Horn D). Specific characters. — Superior molars with crescentic protoconules; cingula completely surrounding the crowns. Measurements as in L. popoagicum. This type is significant as coming from the Big Horn Basin. The type superior first and second molar teeth (fig. 114, p. 178) may be readily dis- tinguished by the greater development of the internal cingulum, which completely surrounds the crown. Another feature is that the protoconules are large and subcrescentic, and the metaconules are lost in the metaloph. These measurements^^ are: D Figure 235. — Upper and lower grinding teeth of Lambdotherium Natural size. A, L. popoagicum, Am. Mus. 14902, Alliali Creek, Wolton; left upper premolar-molar series. B, L. priscum. Am. Mus. 14908, Dry Muddy Creek, 18 miles above mouth; light piemolar-molar series. C, L. progressum, Am. Mus. 14917 (type). Alkali Creek, Buck Spring; rightlower premolar- molar series. D, L. progTcssum, Am. JNIus. 14918, lower Alkali Creek; first lower molar of the left side, crown view. All from Wind River Basin. Measurements of molar teeth of Lainhdolhcrium primaevum Millimeters M' and m^, combined, anteroposterior 23. 5 M', anteroposterior 11 M>, transverse 13 M^, anteroposterior : 12 M^, transverse 15 M^, transverse, maximum along anterior border 18 Ml to M3, combined, anteroposterior 41 Ml, anteroposterior 11 M2, anteroposterior 12. 5 M3, anteroposterior _17 22 The measurements of mi and m* were accidentally transposed in Loomis' original description. 284 TITANOTHBRES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA ^4x^ kid^ Figure 236. — Lower jaws and teeth of Lambdotherium popoagicum Natural size. A, Am. Mus. 14899, Alkali Creek, Buck Spring; lower jaw, inferioi surface. Bi, Am. IMus. 14906, Alkali Creek, Buck Spring; lower jaw, infeiior surface; an older individual. Bi, The same, showing upper surface of symphyseal region. Ci, Am. Mus. 4863, Wind River valley; type jaw, outer side view. Cs, The same, lower piemolar-molar series, crown view. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 285 Figure 237. — Lower jaws and teeth of Lambdotherium popoagicum Side view. Natural size. A, Am. Mus. 4863 (type); Wind River valley; front part restored from Am. Mus. 14899. B, Am. Mus. 14906; Alkali Creek, Buck Spiing; an old individual. C, Am. Mus. 14899; Alkali Creek, Buck Spring. D, Am. Mus. 2989; Wind River valley. 286 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The three inferior molar teeth, mi-ma, measure longitudinally 41 millimeters, as compared with 42 in the type of L. popoagicum. The external crescents (protoconid, hypoconid), the internal cones (meta- conid, distinct metastylid, entoconid), and the cres- centic third lobe of ms (hypoconulid) are characteristic. Lambdotherium priscum Osborn Text figures 146, 234, 235, 238-240 [ For original description and type references see p. 194] Type locality and geologic horizon. — Wind River Basin, 3 miles east of Lost Cabin, Wyo.; Lambdoihe- FiGURE 238. — Front part of type lower jaw of Lambdoiherium priscum Natural size. Am. Mus. 12822, reversed, showing the long postcanine diastema, the three lower premolars, and the first lower molar. Locality, 3 miles east of Lost Cabin; Wind River formation. rium-Eotitanops-CorypJiodon zone (Wind River B); Granger, collector, American Museum expedition, 1905. Specific characters. — P2-P4, 25 millimeters; nii-ms (referred specimen), 37 millimeters; second and third lower premolars extremely simple, with rudimentary paraconid; ps, metaconid rudimentary, placed very low upon slope of protoconid, talonid narrow, de- pressed, with cingular rudiment of entoconid. (See fig. 238.) The extremely simple or primitive structure of the second lower premolar clearly distinguishes this stage. A referred specimen (Am. Mus. 14908) collected by Granger (American Museum expedition, 1909), is slightly more advanced in the structure of the second lower premolar (fig. 239) but is still much more primitive than the type of L. popoagicum. The measurements of these two specimens are shown below. Measurements of Lambdoiherium -priscum, in millimeters 12822 (type) 14908 (referred) P2-P4— . -. _. 25 7 8 5 9 6.5 12 7.5 P2, anteroposterior. 8 P3, transverse 5 P4, anteroposterior . 8. 5 P4, transverse Ml, anteroposterior 10 Mi, transverse. _. . 7 Mi-ms- 37 This Wind River species is identified in Huerfano A by a fine pair of jaws from Garcia Canyon (Am. Mus. 17526). The specific character of pa, without trace of metaconid, is clearly shown in Figure 240. This species is represented by another jaw, with teeth of the same size (Am. Mus. 17528) in which ps, also without metaconid, is in a slightly more advanced stage of evolution, the talonid being broader. This species is also doubtfully represented by the imperfect specimen of upper teeth referred to L. popoagicum by Wortman (Am. Mus. 2688), as well as by a newly found specimen (Am. Mus. 17529) of approximately the same size. In this new specimen, found 3 miles east of Gardner Butte, the isolated upper teeth of two sides, including p'-m^, show the following characters: (1) Molars sUghtly smaller than in the referred specimen of L. progressum, (2) conules and cingulum not so well developed, (3) measurements slightly inferior to those of the type of L. popoagicum. (See p. 283.) The types of L. popoagicum and L. priscum are both lower jaws from the Wind River, and as there are two lower jaws from the Huerfano positively referable to L. priscum and none referable to L. popoagicum it seems best to assign these two sets of upper teeth to L. priscum also. Lambdotherium progressum Osborn Text figures 147, 234, 235, 241, 242 [For original description and type references see p. 194] Type locality and geologic Tiorizon. — Wind River Basin, Alkali Creek, Buck Spring; Lamhdotherium- Trh2_ P2 Figure 239. — Incomplete lower jaw of Lambdoiherium priscum Natural size. Am. Mus. 14908; Dry Muddy Creek, 18 miles above mouth. A referred specimen. Oblique view of dentition. Eotitanops-Coryphodon zone (Wind River B); Granger, collector, American Museum expedition, 1909. Specific characters. — P2-P4, 16.5 millimeters. Sec- ond, third, and fourth lower premolars progressive: rudiment of metaconid on P2; Ps with elevated meta- conid subequal with protoconid, broad talonid with rudimentary entoconid; p4 with bifid metaconid and distinct entoconid. This is readily distinguished from both L. priscum and L. popoagicum by the advanced condition of ps, which may be described as submolariform. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 287 Measurements of type of Lambdoiherium progressum {Am. Mus. 14917) Millimeters P2-P1 26 P2, anteroposterior 8 P2, transverse (trigonid) 4. 8 P3, anteroposterior 9 This Wind River type is distinguished by ps, which has a strong metaconid — that is, it is submolariform. A series of molar teeth, p^-m^ (fig. 242), from the highest level of the lower Huerfano, is referred to L. progressum on the following grounds: (1) The upper teeth fit pretty well those of the type of L. progressum L. macjnum type Figure 240. — Jaws and teeth of Lambdoiherium Am. Mus. 17526, lower jaw of L. priscum, referred specimen from Huerfano A, outside and crown views. Am. Mus. 17527, outer view of type jaw of L. magnum, Huerfano A. Am. Mus. 15000, first and second upper molars of L. magnum, referred specimen from the Wind River horizon of the Big Horn Basin, Wyo. Natural size. After Osborn, 1919. P3, transverse 6 P4, anteroposterior 9. 3 P4, transverse 7. 3 Ml, anteroposterior 11. 5 Ml, transverse . 8. 5 M2, anteroposterior 12. 5 Mj, transverse 9. 5 from the Wind River; (2) the parastyle is especially prominent at the antero-external angle of m^, m'; (3) m^ has prominent hypocone and cingular hypostyle; (4) the outer cusps of the premolars are approximated, conules prominent; (5) the cingulum is strong on p^ and m'. 288 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA With these specimens (association doubtful) were found the calcaneum, portion of a tibia, and a proximal phalanx. Lambdotherium magnum Osborn Text figures 154, 240 [For original description and type references see p. 199] Type locality and geologic horizon. — Huerfano Park, Colo.; lower horizon of the Huerfano formation, Lamidotherium-Eotitanops-Coryphodon zone (Huerfano A). Specific characters. — Osborn writes: Exceeding in size any other known lambdothere is the type jaw (Am. Mus. 17527) from the Garcia Canyon, lower Huer- FiGUKE 241. — Lower jaw and teeth of Lambdotherium progressum Natural size. Am. Mus. 14917, type, reversed; Alkali Creek, Buck Spring; Wind River formation. fano, containing a complete inferior series, P2-m3 of both sides, represented in Figure 240. These teeth exceed in length over all (74 mm.) those of the type of L. popoagicum, in which the same teeth measure 69. P3 has a rudimentary metaconid and paraconid, in the same stage of evolution as in L. popoagicum. Of similar large size is a referred specimen, Am. Mus. 15600 (fig. 240), from the Big Horn, west end of Tatman Mountain. These referred grinders, m', m^, coincide closely in size with the type of L. magnum and may be regarded as a paratype. Below are given the measurements of teeth of species of lambdotheres. The numbers following the specific names are those assigned to the specimens in the American Museum of Natural History. Measurements of teeth of lambdotheres INIillimeters P2-m3; Huerfano A, L. priscum 17526 (referred) 67 Wind River B, L. popoagicum 4863 (type) 69 Wind River B, L. progressum 14917 (type) (esti- mated) 71 Huerfano A, L. magnum 17527 (type) 74 M'-m^ :Huerfano A, L. priscum 17529 (referred) 21.5 Huerfano A, L. priscMTO 2688 (referred) 22.5 Huerfano A, L. progressum 17530 (referred) 23.5 Wind River B, L. popoagicum 14902 (referred) _. 25 Wind River B, L. magnum 15600 (referred) 27.5 These measurements show that there is not a great range in size between the smaller and the larger animals referred to this genus (Osborn, 1919.494). Subfamily Eotitanopinae Osborn Lower Eocene titanotheres of intermediate size. Body proportions slender, submediportal rather than cursorial. Skull dolichocephalic; facial region longer than cranial region. Superior molars brachyodont; molar tooth proportions much as in the Palaeosyopinae, with reduced paraconules and metaconules. Inferior molars without metastylids. Premolars j- present; molarization of premolars retarded. Discovery. — The details of the discovery of Eoti- tanops {Palaeosyops horealis) in 1850 and the early history of opinion are in part related above. In Cope's "Tertiary Vertebrata" (1885.1, p. 703, pi. 58a, fig. 3) a full description is given of the type molar teeth and the imperfect radii of the animal that Cope called Palaeosyops horealis (Am. Mus. 4892). Materials. — In 1891 Dr. J. L. Wortman, who had discovered the type, enlarged our knowledge of this genus by the discovery of another specimen (Am. Mus. 296), including a complete lower jaw, two cervical, three dorsal, and one caudal vertebra, the femur, humerus, and the greater part of the fore foot. These bones were described by Osborn and Wortman in 1892 (1892.67) and were referred to the type species, Palaeosyops horealis; they are now known as E. princeps. Soon afterward Earle's memoir (1892.1) appeared, in which he treated Palaeosyops horealis as probably ancestral to the Bridger Telmatherium cultridens. In 1908 Osborn (1908. 318) revised" the Eocene titano- theres and placed P. horealis in the new genus Eotitanops. Granger's explorations in 1909 to 1911 resulted in the discovery of the type of E. gregoryi and have enabled us to make a systematic revision of these animals based upon materials in the American Museum collections, which are arranged below accord- ing to size and morphologic succession; their geologic succession is sho\vn in Figure 48. The numbers are AM.I7S30 Figure 242. — Upper teeth of Lambdotherium progressum Am. Mus. 17530, refened specimen from Huerfano A. Natural size. After Osborn, 1919. those assigned to the specimens in the American Museum of Natural History. E. major Osborn, 14894 (type), a third metatarsal of the left side (figs. 145, 506). E. princeps Osborn, 296 (type), jaw, manus, humerus, femur, etc. (figs. 144, 231, 246, 252, 484, 490, 494, 496, 498-600, 512, 686, 692, 700, 724 (Pis. XXVI, LIV). E. princeps Osborn, 4902 (referred?), fragments of pes. E. borealis (Cope), 4892 (type), superior molars p<-m^ radius, etc. (figs. 102, 497, 498; PI. LIV). E. borealis (Cope), 14887 (neotype), skull, jaw, atlas, pelvis, etc. (figs. 229, 232, 244, 250, 251, 494, 495, 501, 515, 721; PI. LIV). EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 289 E. borealis (Cope), 14890, portions of right and left jaws. E. borealis (Cope), 14891, complete jaws (figs. 231, 248, 249). E. borealis (Cope), 4886, anterior portion of jaw (figs. 246, 249). E. borealis (Cope), 14895, caleaneum, astragalus (reference doubtful) (figs. 503, 505, 522). E. borealis (Cope), 14888, jaws and fragments of skeleton and feet, right pes (figs. 246, 249, 494, 501-503, 521, 701). E. brownianus (Cope), 4885 (type), jaw fragment (figs. 104, 231, 246, 247). E. gregoryi Osborn, 14889 (type), jaws, also m'-s (figs. 142, 231, 245-247; PI. LIV). E. gregoryi Osborn, 14933 (referred) , portions of pes and tibia (fig. 503). The specimens listed above are arranged not in the ascending geologic order but according to size, E. gregoryi being the smallest and E. major the largest Manus numerically tetradactyl but functionally tri- dactyl, with a tendency to mesaxonic structure. This animal is separated generically from Larnbdo- therium by the possession of full eutherian dentition, including p|. The fact that the face is longer than the cranium constitutes its principal generic distinction from the middle Eocene titanotheres (fig. 256). As has been shown above, the genus is represented by five specific stages or mutations, which are distin- guished partly by size but more clearly, at least in four species, by the development of cuspules on the inferior premolar teeth, as follows : E. major Osborn, distinguished only by its large size. . E. princeps Osborn, distinguished by size and premolar com- plication. Figure 243. — Restoration of Eotitanops borealis, of the Wind River formation (Eocene) About one-twelfth natural size. Made by E. S. Christman in 1917 under the direction of W. K. Gregory. of the animals represented by these types and other specimens. The specific reference of the separate foot bones (Nos. 14893, 4902, 14895, 14933) is doubtful. Eotitanops Osborn Plates XXVI, LIV; text figures 10, 21, 25, 27, 28, 29, 33 142- 145, 155, 210, 212, 219, 229-232,243-253,405-408,482-485, 490, 492-503, 505-507, 512, 515, 521-523, 646, 648, 649, 661, 686, 688, 690, 692, 694, 695, 700, 701, 704, 709, 711, 717, 721-727, 733, 740, 742, 745 [For original description and type references see p. 179; for skeletal characters see p. 591] Generic characters. — Skull of proopic dolichoceph- aly. Incisor series obliquely anteroposterior. Pj with small, compressed single fang; p^-p* with single internal cusps; p'-p* with rudimentary lophoid proto- conules; p4 rather progressive. Superior molars sub- quadrate and rounded in form; protoconules small; metaconules wanting or rudimentary; inferior molars without metastylids; hypoconulid of ms subconic. E. borealis (Cope), distinguished by intermediate size and premolar simplicity. E. brownianus (Cope), distinguished by smaller size and premolar simplicity. E. gregoryi Osborn, distinguished by the smallest size and extreme premolar simplicity. The range of measurement in the species and muta- tions is shown in the following tables : Comparative measurements of Eotitanops, in millimeters [All specimens in American Museum] Pj-ma Mtc III Mts III, E. major, 14894 (type) . 103 E. princeps, 296 (type) E. borealis, 14891 (referred), E. borealis, 14890 (referred ). E. borealis, 14888 (referred) _ E. brownianus, 4885 (type). E. gregoryi, 14889 (type) — - E. minimus, 17439 (type) . . - 105 98 96 94 90 78.4 » 72 87 290 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Standard measurements of teeth of the species of Eotiianops, in millimeters [The numbers are those of specimens in the American Museum of Natural History] 'S.D, II f4 E. borealis 1 i 1 i i 1 a-s- is Pi-m' 108 92 P2-m3 1 P2-p^ 36 M'-m'- 35.5 54 17 16 18 20 19 P\ ap 11 14.6 18 18.3 P«, tr .- M', ap MS tr .-. M2, ap 14 15 5 W, tr _ MS, ap- . 13 3 18 M3, tr 17 22 Pz-ms '78 90 35 55 — - 94 36 58 98 38 59 96 36 60 11. 5 6 12 6.5 13 7.5 -105 P2-P4 29.4 49 8.8 6.2 9.5 5.2 39 66 P2, ap. - 12 6 12 7 12.5 8 - — 13 P2, tr . 63 P3, ap 125 P3, tr_ 63 P4, ap- -. _ 12 8 15.5 10 18 11.7 22 11.5 16.3 11 19 13 23.2 P,, tr Ml, ap_ _ 14.5 8.5 15.6 10.5 19.5 10. 7 183 Mi, tr . 12 Mj, ap 21 M2, tr 14 23 — - 25 M3, tr 14 Figure 244. — Skulls of the oldest known titanotheres Reconstructions by L. M. Sterling under the direction of W. K. Gregory. One- fourth natural size. A, Lamhdotherium popoagicum, Am. Mus. 14907, Alkali Creek, Wolton; 14899 and 14903, Alkali Creek, Buck Spring. B, Eotiianops bo- realis, Am. Mus. 14887, Dry Muddy Creek 12 miles above mouth. All specimens from the Wind River Basin, Wind River formation. Range of evolution. — As Eotitanops gregoryi, the smallest and simplest form, occurs on a high level, having been found 100 feet above the "red stra- tum" on Alkali Creek (see figs. 47, 48), and as specimens referred to E. horealis and E. princeps range from the "red stratum" on Alkali Creek, on the 250-foot level, to the 400-foot level, these species and mutations can not be arranged in monophyletic succession, but they afford evidence that even at this time the titanotheres were polyphyletic. Range in size. — The smallest of the Wind River titanotheres, E. gregoryi, measures about 18J^ inches, or 45.6 centimeters, at the shoulders. A larger form, E. princeps, measures about 26 inches, or 66 centi- meters, at the shoulders. The intermediate form, E. borealis, is more slenderly proportioned than the American tapir (T. terrestris); it is between 75 and 78 per cent of the height of the tapir, and thus about 75 per cent of the height of Mesatirhinus of the upper levels of the Bridger Eocene. E. major, judged only by the size of the pes, more nearly approaches T. terrestris in size, the median metatarsal of E. major measuring 103 millimeters and that of T. terrestris 108. It should be noted that Eotitanops includes the only known large lower Eocene perissodactyl. Even Eotitanops major, the largest Wind River species, appears to be considerably smaller than Palaeosyops jontinalis, the smallest Bridger species. Measurements of upper teeth of Eotitanops horealis and Palaeo- syops fontinalis, in millimeters M', anteroposterior M', transverse M', ectoloph, maxiUa M', transverse maxilla M^, transverse maxilla (pr.-pas.) Length of left zygoma (anterior bor- der malar to posterior border post- glenoid process) E. borealis from Wind River B, Am. Mus. 148S7 17.5 "18.5 19.5 23 25.5 » 129 P. fontinalis from Bridger A (type). Am. Mus. 5107 137 The measurements given show that in its dentition P. Jontinalis of Bridger A was much larger than E. horealis of the Wind River formation. The rela- tively small size of the zygoma in the type of P. fonti- nalis is consistent with the fact that the animal was very young, its milk dentition being still functional. A comparison of E. horealis (summit of known lower Eocene) with P. fontinalis (lower middle Eocene or lower Bridger) indicates a long period of titanothere evolution between these two species. P. fontinalis, although the oldest known Bridger titanothere, differs in two points — the superior dental series is 25 per cent larger than that of E. horealis; the cranium is elongate and the face abbreviate. EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 291 Eotitanops gregoryi Osborn Plate LIV; text figures 25, 27, 33, 142, 143, 231, 245-247, 253, 483, 492, 493, 503, 661, 726, 727, 742 [For original description and type references see p. 192] Type locality and geologic Jiorizon.- — Alkali Creek, Buck Spring, Wind River Basin, Wyo.; Wind River This very sharply defined species is named in honor of Dr. William K. Gregory. Its especial interest lies in the fact that it is the most primitive titanothere known. It represents, however, a persistent primitive stage, because its geologic level, 100 feet above the alkali "red stratum," is higher than that of the Figure 245. — Model of skuU of Eotitanops gregoryi Based on type specimen (Am. Mus. 14889) and on Eotiiamps iorealis. One-half natural size. formation, Lambdotherium zone, horizon Wind River B ("Lost Cabin"), 100 feet above heavy "red stratum." Specific characters. — -Very primitive and of inferior size, p2-m3, 78.4 millimeters; mi_3, 49; P2-3 with the I (See fig. 246.) typical and relatively progressive E. borealis. Its primitive condition is apparent in the comparison of P3 with the same tooth in E. borealis and E. princeps. FiGTJBE 246. — Lower premolars and molars of Eotitanops Natural size. American iVEuseum specimens from the Wind River formation. Wind River Basin. A, B, C, Inner side view of the third left lower premolar: A, E. gregoryi, No. 14889 (type), Alkali Creek, Buck Spring, upper level of "Big Red Pocket," 100 feet above heavy red stratum; B, E. iorealis, No. 14888, Alkali Creek Davis's ranch; C, E. princeps, No. 296 (type). D, E. gregoryi, No. 14889 (type), left lower premolars (p2, ps), inner side view. E, E. brownianus. No. 4885 (type), second left lower premolar, inner side view. F, E. borealis, No. 14891, west bank of Wind River, 3 miles above canyon (top of banded beds); left lower molars (mi, ma) , inner side view. internal cusps, paraconid and metaconid, consisting of rectigradations in a most rudimentary stage; hypo- conulid of m3 very small; m^ with a single internal cone, no hypocone. This third inferior premolar, ps, is much less pro- gressive than in E. princeps or even in Lambdotherium; the other premolars are correspondingly primitive, p2 short, compressed, with a very rudimentary hypo- 292 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA conid, Pa laterally compressed, hypoconid distinct, paraconid, metaconid, and entoconid extremely rudi- mentary rectigradations. In the molar teeth, mi_3, the metastylid and entostylid are also in an extremely rudimentary or rectigradational stage. In ma the hypoconulid is small, subconic, external in position. (See fig. 235.) Eotitanops brownianus (Cope) Text figures 104, 143, 231, 246, 247, 253 [For original description and type references see p. 169] Type locality and geologic horizon. — Wind River Basin, Wyo.; Wind Eiver formation, Lambdotherium- c-fSS^' Figure 247. — Lower jaws of Eotitanops gregoryi and E. brownianus One-half natural size. A, E. gregoryi, Am. Mus. 14889 (type), reversed; Alkali Creek, Buck Spring, upper level ol "Big Red Pocket"; Wind River formation, upper part (Wind River B, "Lost Cabin"). Contours partly restored from E. borealis. B, E. brownianus, Am. Mus. 4885 (type), reversed; Wind River Basin. Contours partly restored from E. borealis and E. princeps. Eotitanops- Cory phodon zone (Wind River B), exact level not recorded. Specific characters. — Size greater than E. gregoryi; P2-m3, 90 millimeters; mi_3, 55; fang of pi placed in close proximity to the canine; p2 compressed, hypo- conid distinct, elevated, entoconid invisible, paraconid and rudimentary rectigradations placed very low on the crown, metaconid extremely rudimentary if pres- ent; metastylid rudimentary. P2 (see fig. 246) is in a less advanced stage of evolution than ps in E. gregoryi. As shown in the comparative series of the jaws (fig. 231), in the table of measurements (p. 290; see also fig. 483), and in the accompanying figures, the type of this species belonged to an animal in size midway between E. gregoryi and E. borealis. The ramus of the jaw rather resembles that of E. borealis but with a pronounced swelling below ms; its vertical depth below the anterior face of ms is 40 millimeters; the symphysis is decidedly broad and massive. Eotitanops borealis (Cope) Cf. Palaeosyops borealis Cope Plate LIV; text figures 10, 28, 29, 102, 143, 219, 229-231, 243, 244, 246, 248-251, 405, 406, 482, 493-495, 497, 498, 501-503, 507, 515, 521-523, 646, 648, 649, 690, 694, 700, / 701, 717, 721, 724, 725, 745 (For original description and typa references see p. 168] Type locality and geologic Twrizon. — Wind River Basin, Wyo.; Wind River formation, Lambdotherium- Eotitanops- Coryphodon zone (Wind River B, "Lost Cabin"). Specific characters. — Of larger size; p2-m3, 94-98 millimeters; premolar teeth more complicated, as shown in neotype and associated specimens; p2 with very, rudi- mentary paraconid and metastylid; p^~* with progressively developing tritocones and single internal deuterocones back- wardly inclined, crowns sub triangular; m'~^ with distinct protoconules. Materials. — The fragmentary type speci- men is the historical Palaeosyops borealis (Am. Mus. 4892) of Cope, figured in the "Tertiary Vertebrata," Plate LVIII, A, Figures. It is marked No. 16 in the Wind River valley collection of J. L. Wortman, July, 1880. The very fine specimen se- lected as a neotype (Am. Mus. 14887, figs. 250, 251) consists of the skull and jaws found by Granger in 1909 on Dry Muddy Creek, 100 feet above the alkali "red stratum," and represents a slightly larger and somewhat more progressive mutation. Incisors of neotype. — The incisors show the char- acteristic titanothere feature of increase in size from i' to i^, the transverse measurement of the crowns being respectively i' 6 millimeters, i^ 6, i^ 8 (estimated). The crowns of i' and i^ are bluntly spatulate or chisel- shaped. P has a faint antero-internal cingulum; i^ is rounded and subcaniniform. The general arrange- ment of the series is obliquely anteroposterior rather than transverse. The canine is prominent, laterally compressed, the alveolus measuring, transverse, 13 millimeters (estimated); anteroposterior, 17. EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 293 Premolars of neotype. — P' is placed midway between the canine and p^ consistently with the relatively elongate preorbital region. The chief features of p^^* are the simple, backwardly directed deuterocones with low crests connecting them with the protocones and tritocones; the tritocones (see figs. 229, 250) in- crease progressively in p'"^; external cingula faintly indicated in p^ p*. The three premolars taken to- gether are subordinate in measurement (36 mm.) to the molars (54 mm.). Superior molars oj neotype. — The superior molars exhibit the characteristic bicrescentic ectoloph with prominent parastyles and mesostyles, and median ridges opposite the paracones and metacones (PL LIV, fig. 229); the protoconules are fairly prominent and faintly cres- centic in m'~', forming a ves- tigial protoloph; the internal cingula festoon but do not sur- round the inner sides of the crown; m^ entirely lacks the hypocone; protolophs distinct but sessile are observed on m'"^, also faint rudiments of metalophs on m\ m^ Lower molars of referred speci- mens. — The valuable series of jaws (Am. Mus. 14887, 14890, 14891, 14888, and 4886) com- plete our knowledge of the inferior dentition except the incisors, which are unknown (figs. 248-250). The premolar- molar series, p2-m3, exhibit progressive gradations of length from 94 to 98 milli- meters. (See table on p. 290.) They are thus superior to E. irownianus and inferior to E. princeps in measurement. The premolars afford the distinctive specific characters or muta- tions in the progressive stages of the internal cuspules or rectigradations. Besides the somewhat arbitrary association of the type and neotype, we also refer to this species the materials listed above, including a number of jaws and portions of the skeleton. (See figs. 231, 246, 248, 249, 494, 501-503, 505, 521, 522, 701.) Characters of the teeth. — The fourth superior pre- molar (Am. Mus. 4892; PI. LIV, H) measures antero- posteriorly 12 millimeters, transversely 14; it exhibits a faint external, distinct anterior and posterior, but no internal cingula, conical deuterocone, small proto- conule and larger convex protocone, a somewhat smaller and more plane tritocone and small meta- conule ridge. The superior molars exhibit faint external, more prominent anterior and posterior, and incomplete internal cingula; the ectoloph consists of sharply defined parastyle, paracones and meta- cones with median external ridges, and a prominent mesostyle; the most distinctive feature of the inner half of the crown in m' is the sublophoid character of the protolophs and hypolophs, which unite respec- FiGUEE 248. — Lower jaw of Eoiitanops borealis One-half natural size. Am. Mus. 14891; west bank of Wind River, 3 miles above canyon (top of banded beds); Wind River formation. Ai, Outer side view; A2, inferior view. tively with the distinct protoconule and a much less distinct metaconule to form a low or sessile crest. This rudimentary or vestigial lophoid character is even less evident in the middle Eocene species of titanotheres. M' measures 18 by 17 millimeters (ap. by tr.); it is a nearly quadrate tooth, in wide contrast to the transversely expanded tooth of L. popoagicum. In m^ the less worn paracone measures 7 millimeters in height ; the ectoloph is thus somewhat elevated in these molars, but its crescents are not 294 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA strongly concave and incurved as in the Bridger titanotheres; the protoconule is distinct, the meta- FiGURE 249. — Lower teeth of Eotitanops borealis Natural size. A, Crown view of left lower premolars and molars (P2--m3); Am. Mus. 1488S; Alkali Creek, Davis's ranch, Wind Hiver. B, Crown view of molars (mj-ms); Am. Mus. 14891; west bank of Wind River, 3 miles above canyon (top of banded beds). C, Crown view of premolars Cp!-P4); Am. Mus. 4886; Wind River valley, Wind River formation. Figure 250. — Skull of Eotitanops borealis One-fourth natural size. Am. Mus. 14887, Dry Muddy Creek 12 miles above mouth; Wind River formation. Ai, Palatal view of crushed specimen; Az, As, recon- struction of the palatal and side views of the same skull made by L. M. Sterling under the direction of W. K. Gregory. conule is faint. In m' similar characters are observed on the anterior half of the crown; the posterior half is broken away. The lower molars exhibit low brachyodont crowns, the crescentic external cusps alternating with the subconic internal cusps; extremely rudi- mentary hypoconulids, metastylids, and vestigial paraconids are observed; external cingula rudimentary, internal cingula en- tirely wanting, as in all titanotheres; hypo- conulid of m^ central, small, sublophoid. Slcull.—The discovery of the skull of E. borealis (Am. Mus. 14887, neotype) was an important event in the work of determining the morphology of the titanotheres because it connected these mammals closely with other early Eocene perissodactyls and sepa- rated them from the middle Eocene forms. The chief feature of the skull is that the proopic or facial region is longer than the opisthopic or cranial region, whereas in all the middle Eocene titanothere skulls yet known the face is shorter than the cranium and becoming progres- sively shorter throughout Eocene and lower Oligo- cene time. The skull is also relatively long and narrow, and the true molar series is relatively short as compared with the total length of the skull. These characters are well shown in the reconstruction of the skull (figs. 250, 251) and in the model of the head (figs. 646, 648, 649); they are expressed in the following indices, which are estimates only, because the skull is considerably crushed : Cephalic index 50 (vs^idth across zygomata -=- basal length = 160 millimeters h- Figure 251. — Skull of Eotitan- 313 [estimated]). Faciocephalio index 56 (length of face -;- basal length = 185-^313). Molar index 17 (length m"- m^ -^ basal length of skull = 54 -H 313). These fundamental proportions give the skull of Eotitanops (fig. 250) a striking resemblance to that of other lower Eocene perissodactyls. The type is technically known as proopic dolichocephaly. Attention may be called to the following details: (1) Premaxillaries slender, symphyseal union very slight, indicating feeble use of superior incisors, pre- maxillaries joining nasals superiorly, a primitive fea- ture; (2) infraorbital foramen placed above p'-p*, ops borealis A I, Top view; A2, occipital view. One- fourth natural size. Am. Mus. 14887, Dry Muddy Creek 12 miles above mouth; Wind River formation. Re- construction made by L. M. Sterling under the direction of W. K. Gregory. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 295 widely separate from orbit (closer to orbits in Eocene forms); (3) malars gently rounded, and zygomata moderately projecting; (4) superior profile believed to be plane or gently convex, slightly convex above the orbits; (5) greatest width of skull opposite glenoid fossae; (6) temporal fossae deep, brain case small, sur- mounted by high, thin parietal crest; (7) occipital crest overhanging condyle superiorly, deeply indented in median line; (8) postglenoid and post-tympanic open below auditory meatus; (9) small exposure of the mastoid between the post-tympanic and paroccipital process; (10) in occipital view (fig. 251) the parietal crest is narrow and flaring superiorly; (11) in palatal view basioccipital and basisphenoid keeled or com- pressed; (12) typical perissodactyl foramina separate^ namely, condylar, lacerum medium and posterius, ovale, and alisphenoid; (13) posterior nares deeply inclosed by pterygoids and pterygoid wings of alisphe- noids; (14) posterior borders of palatines not preserved; (15) palate relatively elon- gate, narrow and arched from side to side; (16) post- glenoid process narrow, internal in position ; (17) occipital condyles sharply convex, prominent, sepa- rated in median line. ing behind ms; symphysis moderately elongate, gently convex, incisive alveoli, indicating progressive increase of size from ii to is and semiprocumbent position of the incisors. Eotitanops prlnceps Osborn Plates XXVI, LIV; text figures 27, 33, 143, 144, 231, 246, 252, 407, 408, 483, 484, 490, 492-494, 496, 498-500, 512, 661, 686, 692, 700, 704, 709, 724 [For original description and type references see p. 193. For sljeletal characters see p. 690] Type locality and geologic horizon. — Wind River Basin, Wyo. ; Wind River formation, Lambdotherium-E otitanops- CorypJiodon zone (Wind River B, "Lost Cabin"). Measurements of Eotitanops borealis and E. princeps, in milli- meters Basilar length, premaxillaries to con- dyles (estimated) Zygomatic or transverse width (esti- mated) Width across occipital condyles Cranial length, postorbital process to occipital condyles Facial length, postorbital process to maxillary symphysis Length of lower jaw, symphysis to condyles (estimated) Height of jaw, condyle to bottom of angle Lower jaw, depth behind ma E. borealis. Am. Mus. 14887 (neo- type) E. princeps, Am. Mus. 296 (type) The jaws are well displayed in the neotype (Am. Mus. 14887) and in the referred specimens, especiaUy in the well-preserved jaw shown in Figure 248 (Am. Mus. 14891). The chief characters are the following: Ramus elongate, gently convex in vertical section, expanding toward symphysis; lower border suddenly compressed and descending below angle, thin posterior border; delicately retroverted coronoid, ramus slowly ascend- 101959— 29— VOL 1 22 riGTTHB 252. — Lower jaw of Eotitanops princeps One-halt natural size. Am. Mus. 296 (type), reversed. Wind River Basin; Wind River formation. Specific cJiaracters. — Of still larger size; Pa-ms, 105 millimeters (estimated). Inferior premolar teeth some- what more complicated, as shown in the type specimen; P2 with elevated, distinct, but very rudimentary para- conid and metastylid, entoconid very rudimentary if present, talonid narrow; ps, paraconid quite distinct, elevated, metastylid small, distinct, entoconid rudi- mentary, talonid broad; p4 submolariform, talonid broad, entoconid shelf distinct. Hypoconulid of ma rounded, more robust. Ramus larger and more robust. The more advanced development of the premolar rectigradations, the increased size of the teeth and of the jaw, the larger size of the hind feet in the referred specimen (Am. Mus. 4902) combine to distinguish this specimen as a mutation or subspecific stage between E. borealis and E. major. Lower jaw of type. — The weU-preserved jaw (fig. 252) of the type specimen (Am. Mus. 296) measures 253 millimeters from the back of the condyle to the symphysis, 99 from the condyle to the bottom of the angle, and 53 vertical depth of the ramus just behind ms. Its distinguishing features are (1) the elevation of the condyle above the grinders; (2) the rather slender, recurved coronoid with sharply angulated and flattened anterior border, which reminds us of the coronoid of the middle Eocene Mesatirhinus and DolichorJiinus rather than of that of Palaeosyops; (3) the well-defined superior fossa between the angle and the coronoid; (4) the depressed or delicate incurved 513 162 52 128 245 97 296 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA posterior border of the angle; (5) the elongate (70 mm., estimated) symphysis laterally compressed l)ehind the canines; (6) the slope of the anterior border of the coronoid directly into the fang of ms; (7) the moderately thick rami (18 mm.). The lower profile or contour of the jaw is convex below the molars, concave below the coronoid, extending backward into the angle. Inferior teeth oj E. princeps {type; PI. LIV). — There were apparently three inferior incisors, the crowns of which are not preserved. The alveoli of the inferior canines are slightly compressed laterally; the esti- mated measurements are 15 millimeters (antero- posterior) by 13 (transverse). The crown of pi is not preserved; its fang is single; the fang is separated from that of the canine by a very narrow diastema Figure 253. — Lower grinding teeth of three species of Eotitanops from the upper Huerfano formation (Huerfano B) Natural size. After Osborn, 1919. A, A', E. minimus (type), lower level of the upper horizon of the Huerfano formation; B, Bi, E. gregoriji (referred specimen), from the upper Huerfano; C, E. hrownianus (referred specimen), from near the base of the lower Huerfano. (3 mm.); behind it is a continuation of the diastema, 16 millimeters in width. This diastema points to a somewhat elongate character of skuU and jaw, since the total length between the canine and p2 is 31 millimeters. P2, measuring 13 by 6 millimeters, is an elongate, laterally compressed, bifanged tooth with an elevated paraconid and depressed metastylid or posterior cusp, noncingulate and with the faintest indication of valleys on the inner surface. P3, meas- uring 12 by 7 millimeters, is slightly more progressive, with its metaconid externally placed and a more clearly indicated posterior valley. P4, measuring 13 by 8 millimeters, exhibits a broader talonid and is thus submolariform. The molars are perfectly pi'eserved. aggregating 65 millimeters in length, 14 in maximum width of crown. The individual total measurements (ap. by tr.) are as follows: Mj, 17 by 12 millimeters; m2, 21 by 13; ma, 26 by 14. This progressive increase posteriorly accords with a similar increase of the upper molars posteriorly, as observed also in Larnhdotherium. The inferior molars exhibit faint external and no internal cingula; rather low but well-defined crescents; a progressive increase in size; paraconids partly defined on ms. The most distinctive primitive feature in ma is the small size, subconic form, and mesial position of the hypoconulid as compared with its backward extension and cres- centic form in some of the middle Eocene types. Eotitanops major Osborn Text figures 145, 483, 506 [For original description and type references see p. 193. For skeletal characters see p. 697.] Type locality and geologic Tiorizon. — Alkali Creek, Wind River Basin, Wyo.; Wind Eiver formation, Lamidotherium- Eotitanops - Cory- ;., phodon zone (Wind River B, "Lost Cabin"). 7 -•iMNoi744i Specific characters. — The type and only known specimen (Am. Mus. 14894) consists of a left me- dian metatarsal associated with the distal end of a tibia. It is distinguished from E. princeps by its notably larger size (length of Mts III, 104 mm., ' greatest width, 16). The skull and dentition are not known. Eotitanops minimus Osborn Text figures 155, 253 [For original description and type references see p. 199.] Type locality and geologic horizon. — Huerfano Park, Colo.; Huerfano formation, Eometarhinus- j_ Palaeosyops fontinalis zone (Huerfano B; lower 1 level). Specific characters. — As this is the smallest true titanothere known, Osborn (1919.494, p. 564) as- signed to the type lower molar teeth P4-m3 (Am. Mus. 17439) the specific name minimus. The measurement of p4-m3 (53 mm.) is much less than that (58 mm.) of the corresponding teeth in E. gregoryi, yet the other characters are so similar to those of E. gregoryi as to suggest that this is a related form. Figure 253 exhibits the form and size of three species, minimus, gregoryi, and hrownianus. A large number of measurements of Eocene titanotheres show that no single species exhibits so great a range in size. The discovery of this dwarf titanothere, together with the presence of titanotheres of the same size as E. gregoryi and E. hrownianus in Huerfano B and Wind River B, reveals the existence of what is probably a distinct phylum of diminutive titanotheres separable from the Eotitanopinae. We must, however, await the discovery of the skeletons before this supposition can be confirmed. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 297 SECTION 4. THE MIDDLE AND UPPER EOCENE TITANOTHERES PHYIA DISTINGUISHED Some of the middle Eocene titanotheres represented in the lower Bridger beds may have been evolved from forms related to the Eotitanopinae of the Wind River formation. A geologic interval covering a long period (including Huerfano B= Bridger A) separates the titanotheres of the Wind River B from those of Bridger B, and during this period there was a marked transformation in the proportions of the head, for in the titanotheres of Wind River B the face is longer than the cranium (dolichopic), whereas in those of Bridger B and succeeding subdivisions the cranium is longer than the face (brachyopic). In the titanotheres now to be described this change in faciocranial proportions probably occurred during the deposition of Bridger A and Huerfano B. In the 10 or 12 genera of titanotheres of the middle and upper Eocene the cranium is longer than the face. These animals fall broadly into two large groups, which are more or less theoretically subdivided (1917) into two groups and into six chief phyla or lines of descent as shown in the accompanying table. Characteristic features of groups of titanotheres [Compare flg. 219, p. 265] Palaeosyopine group: Palaeosyops, Limnohyops, Telmatherium, Sthenodectes Manteoceras-Dolichorhinus group: Manteoceras, MesatirUnus, Dolichorhinus, Metarhinus, Ehadinorlimus, Diplacodon Skull brachycephalic to mesaticephalic. Horn rudiments retarded in evolution. Occiput rounded or high. Zygomata deepened vertically. Canines more pointed, erect. Third superior incisor oaniniform. 1. Subfamily Palaeosyopinae "(Limnohyops, Palaeosyops), ex- tremely brachycephalic. 2. Subfamily Telmatheriinae (Telmatherium), mesaticephalic to brachycephalic. Skull mesaticephalic to dolichocephalic. Horn rudiments precocious in evolution. Occiput primitively low and broad. Zygomata shallow vertically. Canines more obtuse, recurved. Third superior incisor incisiform. 3. SubfamilyManteoceratinae = Brontopinae (Manteoceras, Pro- titanotherium, Brontops), progressively brachycephalic. 4. Subfamily Dolichorhininae (Mesatirhinus, Metarhinus, Doli- chorhinus) , mesaticephalic to dolichocephalic, facial region downturned. ?5. Subfamily Megaceropinae=?Rhadinorhininae (?Rhadino- rhinus, Megacerops), mesaticephalic, facial region up- turned. 6. Subfamily Brontotheriinae=?Diplacodontinae (Diplaco- don, Brontotherium) , horns precociously developed. The extreme forms of the two groups — namely, Palaeosyops and Dolichorhinus — also contrast widely in the detailed characters of the skull, as shown in longitudinal and cross sections in Figure 254. The subfamilies 1-6, according to the Osborn system (see Chap. I), correspond with the phyla, or vertical lines of descent, which have been established among the Eocene titanotheres, also between the Eocene and Oligocene titanotheres. Thus it is now Icnown that Manteoceras and Protitanotherium are related to Brontops of the Oligocene. Diplacodon of the upper Eocene is of uncertain affinities with the lower Oligo- cene genera. It is possible but by no means demon- strated that Rhadinorhinus is related to the Oligocene brontotheres and Megacerops, as suggested by Gregory. SPECIES OF PALAEOSyOPINAE AND DOLICHORHININAE FROM THE UPPER HUERFANO (TROGOSUS ZONE) The discovery of two very distinct phyla of true titanotheres in the lower Eocene confirms the theo- retic separation of the titanotheres into subfamilies as occurring in lower Eocene time. In the Huerfano formation we have evidence of two subfamilies, as follows : Palaeosyopinae (perhaps derived from the Eotitanopinae) Hornless. Slender nasals. Subbrachycephalic. More robust proportions. Dolichorhininae (Manteoceras-Me tarhi nuS'MesaUrkinus-VolichOThinus grouTp) Osseous horn rudiments at nasofrontal junction. Nasals very stout, laterally decurved. Mesaticephalic. Smaller proportions. The first subfamily is represented by numerous specimens of Palaeosyops fontinalis Cope; the second group is represented by the single type specimen of the new genus Eometarhinus {E. huerfanensis) . SYSTEMATIC DESCRIPTIONS OF THE MIDDLE AND UPPER EOCENE TITANOTHERES THE PALAEOSYOPINE GROUP (PALAEOSYOPS, LIMNO- HYOPS, TELMATHERIUM, STHENODECTES) DISTINCTIVE FEATURES AND GEOLOGIC HORIZONS Osborn finally included the genus Telmatherium within the palaeosyopine group, although there are some grounds for placing it closer to the Manteo- ceratinae. The telmatheres appear to have had a long and independent evolution of their own (see fig. 697) and thus constitute the distinct subfamily Tel- matheriinae. 298 TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA The resemblances and contrasts between the three I proportional and numerical evolution explained on chief genera included in these two subfamilies are in- j pages 251-262. dicated in the following manner, on the principles of i Proportional and numerical characters oj titanotheres of the palaeosyopine group Telmatherium Skull and skeleton very robust; skuU de- cidedly broad and massive; feet short. Fore feet short, more paraxonic, the fifth digit larger. Skull rounded, occiput stout, sagittal crest of medium length, forehead pro- tuberant or convex. Jaws robust, chin prominent, angulate; mandibular rami massive below grind- ing teeth; coronoid at base very broad and concave anteriorly. Premaxillary symphysis short and rounded. Maxillary splint on side of malars; malar section rounded. Incisors more transverse; canines sub- lanceolate to round; premolars com- pressed anteroposteriorly; molars broad or quadrate, with strong, rounded para- styles. Conules on superior molars more or less persistent, rounded. Grinders persistently brachyodont. Last superior molar usually without hypo- cone or second postero-internal cusp; crown subtriangular, rounded. Ectolophs of superior premolars in some specimens resembling those of molars (that is, with mesostyles). Skull and skeleton more slender; skull broad, brachycephalic, less massive; feet narrow. Fore feet short, more mesaxonic, with the fifth digit reduced. A more elevated occiput, higher and thinner sagittal crest; forehead con- cave, without protuberance. Jaws somewhat more slender, chin slop- ing, rami less massive below grinders, anterior face of coronoid less broadened. Premaxillary symphysis rounded. Maxillary splint extending from side to beneath malars; malar section de- pressed. Incisor series obliquely placed; canines slightly more compressed and ridged; molars with ridged parastyles and ridged conules. Conules on superior molars persistent, ridged or lophoid. Grinders persistently brachyodont. Last superior molar usually with a dis- tinct hypocone; crown more quadrate. Skull and skeleton rather slender; skull decidedly elongate, dolichocephalic. Fore feet long, more mesaxonic, the fifth digit elongate. Occiput very high; sagittal crest elongate; forehead plane, no protuberance. Jaws more slender; chin deep; symph3'sis elongate. Premaxillary symphysis elongate. Maxillary splint elongate, extending be- neath malars; malar section rectangular. Incisor series more parallel; canines high, sublanceelate; premolars elongate; mo- lars narrow, more sharply crested or hyp- sodont, with feeble parastyles or none. Conules on superior molars disappearing early. Grinders progressively hypsodont. Last superior molar without hypocone; crown quadrate. SUBFAMILY PAIAEOSYOPINAE (STEINMANN AND DODEELEIN) The Palaeosyopinae consist of the Limnohyops and Palaeosyops generic phyla. They were abundant chiefly in lower and middle Bridger time, beginning to decline in upper Bridger time. Limnohyops is sub- brachycephalic to brachycephalic, mediportal; Palaeo- syops is brachycephalic to hyperbrachycephalic, graviportal. They were larger than tapirs, propor- tions stout, becoming graviportal; feet of brachypodal type; skuU broad, progressively brachycephalic, facial region abbreviate, nasals tapering distally, nasofrontal horns retarded in development; cranial region and zygomata broadening; grinding teeth persistently brachyodont; canines stout, subconical. They make their appearance at the base of the Bridger or in Bridger A, in the species Palaeosyops fontinalis, and the last member known is the species Palaeosyops copei, of Bridger D or Washakie A. The Palaeosyopinae thus formed the first titano there sub- family to appear in the middle Eocene and also the first, so far as known, to disappear geologically. The subfamily name Palaeosyopinae is taken from the name of the classic genus Palaeosyops leidyi, the first Eocene titanothere discovered. These titano- theres are broad-headed, chiefly of lower and middle Bridger age, reaching a climax and beginning to decline in upper Bridger time. The cranial region of the skull is longer than the facial region; the head is short and broad (brachycephalic); the horns are relatively late or retarded in development; the feet are short and broad (Palaeosyops), or less broad (Limnohyops). The two phyla, Palaeosyops and Limnohyops, were contemporaneous, their remains being found in the same deposits. SEPARATION OF PAIAEOSYOPS AND IIMNOHYOPS GENEKIC PHYIA In the middle Eocene of the Bridger region in west- ern Wyoming the animals known as Palaeosyops and Limnohyops are the earliest to occur geologically — namely, in Bridger A, B, and C. They were browsing animals, with short-crowned teeth and broad heads, which increase in breadth in the successive descendants EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHEEES 299 of the original forms. They exhibit many features in common, yet they seem to represent two distinct contemporaneous phyla. Of these two phyla Lim- nohyops is the more primitive; it is in many features more central or intermediate than the second phylum, ferent lines of descent, the ancestral members of each line (Bridger A and B) are not easily distin- guished. The abundance of remains of Palaeosyops is wel- comed by the student of evolution because so many Figure 254. — Sections of skull of brachycephalic (A, B) and dolichocephalic (C, D) Eocene titanotheres One-fourth natural size. Ai, Palaeosyops Icidyi, Am. Mus. 1544 (type). Median section of cranial region. Note the back part of the olfactory chamber (with remnants of the ethmoturbinals), the cribriform plate, the frontal sinus, and the cellular character of the expanded cranial vertex above the brain chamber. (Section line shown in A2.) As, The same, rear view. The left side shows the widened vertex oi the occiput; the right side shows in section the frontal and ethmoidal sinuses. B, Limnohyops prisms, Princeton Mus. 10044. Cross section through the basioccipital, periotic, squamosal, and paiietal, seen from front. In this primitive form the sagittal crest has not expanded into a flattened cranial vertex, and hence this region is without any large cavities. C, Dolichorhinus Jiyognathus, Am. Mus. 1851. Median section of the whole skull slightly to the right of the median plane, showing the enormous olfactory chamber, the elongate fronto-occipital sinus, and the small brain chamber. The much enlarged maxilloturbinal (mx. tb.) is produced backward and downward, appearing as a prominent swelling in the roof of the narial channel; the primary border of the posterior nares is at pn', the secondary at pn'. D, Dolichorhinus longicepsf (Jiyognathus f) , Am. Mus. 1852. Cross section through the middle part of the brain chamber (near line A-A of figure C) looking forward. Note the fossae for the anterior lobes of the brain, the lateral ethmoid sinus, the mesethmoid septum, the remains of the ethmoturbinal scrolls, and the large fronto-occipital sinus. consisting of the very massive, broad-headeu Palaeo- syops. The genus Palaeosyops of Leidy was the first known, and the LimnoJiyops of Marsh may be regarded as a subgenus. Although the animals belong to two dif- mtergradations or mutations are found. But this very abundance renders more difficult the definition of species because the sharp lines of specific separation and distinction breah down ; the forms merge into one another. 300 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 255. — Cross sections of the skull in middle Eocene titanotheres One-fourth natural size. Ai-Ei, Sections across the face just behind the lacrimal and through the malar and m'; A2-E!, sections across parietals, alisphenoids, and zygomatic process ol squamosal. Ai, Aj, Palaeosyops leidyi, Am. Mus. 1516; Bi, Ba, Telmathermm ultimum, Am. Mus. 2060 (type; crushed laterally); Ci, Cj, Manteoceras manteoceras, Am. Mus. 12678; Di, Ds, Mesatirhinus pclersoni, formerly Am. Mus. 1566, now in British Museum; Ei, Ej, Bolkhorhinus hyognathus, Am. Mus. 1851. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 301 Probably the pbysiographic conditions in this region during the early stages of the Bridger deposition were peculiarly favorable to these animals. Whatever the cause in Bridger B and C their remains are as plentiful as those of other titanotheres are rare. In Bridger D, however, remains of Palaeosyops become mingled with those of titanotheres of other kinds, which are Z. i'Ti n ohyops Figure 256. — Three skulls typical of the palaeo- syopine group One-eighth natural size. A, Zimnohyops prisms. Am. Mus. 11687 (type), middle Eocene, lower Bridger; B, Palaeosyops leidyi, Am. Mus. 1544 (type), middle Eocene, upper Bridger; C, Telmatherium ultimum, Am. Mus. 2060 (type), upper Eocene, TJinta C (true Uinta formation). equally or even more abundant and include forms that apparently had undergone their antecedent evolution in another part of the mountain region of the continent. (See fig. 257.) COMMON CHARACTERS OF THE PALAEOSYOPS AND IIMNOHYOPS GENERIC PHYLA The three most distinctive features of Palaeosyops and Liranohyops, as stated above, are brachyodonty, or persistently short-crowned grinding teeth; brachy- cephaly, or progressively increasing head width; brachypody, or broad and abbreviated foot structure (less defined in Limnohyops). The members of all the species known in both phyla are thus short-toothed, short-skulled, and more or less short-footed. In the accompanying outline of the Figure 257. — Distribution of the species of Palaeosyops and associated fauna in the Bridger formation, Bridger Basin, Wyo. parallel geologic distribution of the species belonging to these two phyla in the Bridger formation we observe that in about every 200 feet of sediment there is accu- 302 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA mulated a change in proportions and in a number of details of cranial and dental structure which we may collectively dignify by the term species, in the neo- Linnaean sense. The transitional or intermediate stages, distraguished by the infinitesimal advance in certain new characters, are mutations in the sense of that term as used by Waagen. The orthogenetic or direct and adaptive origins of single new characters are rectigradations in the sense of that term as used by Osborn. (See p. 812.) The progressive changes in certain characters — for example, in the rectigradations of the premolar teeth and in the rise of the horn rudiments on the frontals — occur nearly contemporane- ously in members of the two phyla. In some other characters the progression is dissimilar, or at different rates. Geologic horizons of Limnohyops and Palaeosyops Part of lormation Horizon Limnohyops Palaeosyops D3 . P. copei. Upper Bridger _. D2 D 1 D L. laticeps.. L. laticeps.. C4 C3 L. laticeps. - P. leidyi. P. leidyi. P. leidyi. P. grangeri. C 2. - C 1 B 4 P. major. P. major. P. major. P. paliidosus. P. longirostris. P. paludosus. B 3 Lower Bridger B 2 B l?-5? [L. matthewi L. monoco- nus. L. priscus- . [L. laevidens. B 1 A? PROGRESSIVE DISTINCTIONS BETWEEN PALAEOSYOPS AND IIMNOHYOPS It is extremely difficult — indeed, it may be impossi- ble — to distinguish parts of individuals belonging to Palaeosyops from parts of those belonging to Limno- hyops. Means of recognizing the differences and resemblances have been afforded by the cumulative work of Marsh, Earle, Osborn, and Gregory. The supposed distinctive generic character (Marsh) of LimnoTiyops, namely, the presence of a hypocone on m^, is possibly a primitive character, because of its presence in Lamidotherium. It prevails but does not appear to be constant in aU species of Limnohyops. It is typically absent but exceptionally present, by reversion perhaps, in Palaeosyops. In the proportions of the skull Palaeosyops is more robust and Limno- hyops is more slender, and this quantitative or pro- portional contrast prevails throughout all the cranial, dental, and skeletal parts, although it is often difficult to measure or define the finer shades of difference. When we compare the ancestral members of the two phyla in Bridger B, some of them are difficult to separate. As the successive specific stages of Limno- hyops are contemporaneous geologically with those of Palaeosyops it is well to enumerate the chief known distinctions which gradually develop and become fully apparent only after the two lines of descent have diverged from each other, as observed in the higher geologic levels — for example, in comparing P. rolustus and L. laticeps of Bridger D. These distinctions are as follows: 1. The upper and lower molar teeth of Palaeosyops are relatively larger, more rounded, and more robust than those of Limnohyops. 2. The vertical striations on the cones of the upper and especially of the lower molars of Palaeosyops are more distinctly marked. 3. On the upper molars (m'~^) of PaJaeosyops the conules are more variable, more rounded, and sepa- rate; in Limnohyops they are more constant, lophoid, ridged, or conjoined with the protocone and hypo- cone; this distinction, however, is not invariably reliable. 4. In m' of Limnohyops the hypocone is typically though not invariably present (L. laticeps), and the metaconule is extremely reduced, owing to the large size of the adjacent hypocone. In m^ of Palaeosyops the hypocone is typicaUy absent but sometimes pre- sent, as in the type of P. diaconus. In m^ of Palaeosyops the metaconule is generally present and in some specimens is so close to the raised posterior cingulum as to appear like a hypocone; thus the m' of Palaeo- syops is generally more triangular, whereas that of Limnohyops is more quadrate and sometimes actually bUobed internally. 5. The parastyle in Palaeosyops is rounded and obliquely placed across the outer angle of the crown, whereas in Limnohyops it is sharp and extends out- ward as a ridge, analogous to the parastyle of the Telmatherium type (Pis. LX, LXIII). 6. The nasals taper toward the extremities and are more pointed in Palaeosyops, whereas in Limnohyops the sides of the nasals are more parallel and they are more truncate at the extremities. 7. The suborbital bar in the two genera becomes quite different; in Palaeosyops the bar is roimded and the overlying maxillary process extends back on its outer side as a broad splint, whereas in Limnohyops the bar becomes more depressed and slightly rectan- gular in section and the maxillary process extends back as a long, slender splint on the lower side; in Telmatherium the suborbital bar is distinctly rectan- giilar and the maxillary process extends back as a long, narrow splint beneath the malar projection. 8. In LimMohyops the top of the cranium is slightly concave; in Palaeosyops there is a strong median con- vexity near the frontoparietal junction some distance behind the orbits. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 303 9. The sagittal crest of Palaeosyops is lower, broader, and passes more rapidly into the temporal ridges, whereas in Limnohyops as in Telmatherium the crest is higher and thinner and extends well forward before spreading into the temporal ridges. 10. The male jaws of Palaeosyops are at once recog- nized by (a) the more prominent, massive chin and, as seen from below, the short, depressed area for the digastric muscle on the posterior symphyseal line, features that contrast with the longer, more slop- ing chin of Limnohyops and its elongate median fossa for the digastric; (6) the massive breadth of the Palaeosyops rami, as seen from below, in contrast with the somewhat more slender inferior borders of the jaw in Limnohyops; (c) the extremely distinctive base of the anterior border of the coronoid process which in Palaeosyops is very broad and in progressive stages deeply hollowed out in front, whereas in progressive stages of Limnohyops it is somewhat narrower, less deeply excavated, and lies more to the outer side of the line of the molar teeth. Additional means of distinction are set forth in the descriptions of the genera and species. W. K. Gregory has observed that the above and other differences are in part quantitative; they are differences in the proportion of one and the same character, as in the form of the nasals, of the sub- orbital bar, and of the sagittal crest. The divergence is far less than that seen in the modern genus Cervus, for example. It may be noted also that certain of the numerical differences are variable; for example, the hypocone on m^. The known forms of Limnohyops are rather slender; thus a male jaw of this animal would resemble a female jaw of Palaeosyops. Limnohyops Marsh Plates LVI, LVII, LX, LXII; text figures 29, 87, 96, 115-117, 219, 254, 256, 258-266, 274, 484, 485, 510-514, 516, 518-523, 525, 527-532, 538, 672, 685, 686, 690, 701, 714, 717, 722, 723, 745, 760 [For original description and type reference see p. 170; for skeletal characters see p. 605] Generic characters. — Brachy cephalic; grinding teeth persistently brachyodont; conules on the molars per- sistent, usually lophoid; third superior molar sub- quadrate and usually with distinct hypocone. Pro- portions of skull and skeleton moderately robust. Manus slender. Five sacral vertebrae (type). Geographic and geologic distribution. — Limnohyops has thus far been found only in the geologic levels B, C, and D of the western or Bridger Basin (see geographic map on p. 8). As compared with Palaeo- syops the materials representing this genus or sub- genus are limited; we can not therefore trace at present the successive stages of its evolution. It is subdi- vided into five species — L. Inevidens, L. priscus, Z. laticeps, i. matthewi, L. monoconus. Resemblances to Palaeosyops. — From our present knowledge the geologically early species L. laevidens and L. priscus are so close to the type of Palaeosyops (P. paludosus) that they might readily be embraced within one and the same genus. In fact, material at first referred by the present author to L. priscus now appears to belong to P. paludosus, which is itself so primitive that it may almost be regarded as the an- cestor of Limnohyops. However, the sum total of the distinctions between these animals — in external form, color, and habits — was probably very consid- erable, and as we progress into geologically higher stages the cranial and dental differences become more apparent, as summarized below. Materials. — Besides the admirable type specimen from Bridger C 4 in the Yale Museum, on which Marsh founded the genus and species, American Museum parties have found five specimens of L. laticeps in levels Bridger C 4 and D 2. In the lower level of Bridger B 2 occurs the type of L. laevidens Cope, and here we have also found two specimens of the somewhat more progressive stage L. priscus. At present the species L. matthewi and L. monoconus are represented only by a single specimen each, and it is noteworthy that these also are of lower Bridger age. Thus our knowledge of Limnohyops at present rests on portions of about 16 individuals. Chief progressive distinctions from Palaeosyops. — (Compare pp. 302, 618, vertebrae; p. 612, limbs and feet.) So far as we know at present Limnohyops is distin- guished by somewhat more slender proportions. The skull in the larger species is equally broad but less massive ; the j aws are decidedly less massive. The long bones of the limbs referred to L. laticeps are practically of the same length as those of the contemporary Palaeosyops leidyi, but the foot bones of the manus of Limnohyops appear to be shorter (figs. 512, 520). Limnohyops may be described briefly as a relatively light-limbed, broad-skulled, short-footed type. As we have already given many of the details by which Limnohyops in its advanced stages may be distinguished from Palaeosyops, it is only necessary to summarize its chief diagnostic characters. Cranium: (1) The skull of Limnohyops has a concave instead of a convex forehead (fig. 256); (2) it has a high, thin sagittal crest; (3) the nasals are relatively broad anteriorly; (4) the suborbital bridge of the malars is shallow, narrow, and more or less quad- rangular in section or broadly depressed, with a rounder outer border, and the fiange for masseteric insertion is not extended so far forward as in Palaeo- syops; (5) the splint of the maxilla extends backward under the side of the malar; (6) only slight promi- nences indicate the osseous horn areas, and no horn rugosities have been observed; (7) the jaws have more slender rami, the chin is sloping, the digastric fossa is elongate, deep, and sharply defined, the lower border is less thickened below the grinders, the coronoid base is less broad and flaring anteriorly, when seen from 304 TITANOTHERES OF ANCIENT "WYOMING, DAEIOTA, AND NEBRASKA the front, and is set on the outer side of the line of the grinders. Dentition: In the superior teeth we note especially that (1) m^ is small; (2) the crested metaconules are confluent internally with the hypocone; (3) the parastyle is elongated on the outer side of the ectoloph and somewhat more sharply ridged; (4) the ectolophs of premolars have more sharply defined convex ridges opposite the paraconules and metaconules; (5) the hypoconulid of ms is rather sharp and prominent, laterally depressed, and very slightly crescentic. Measurements of species of Limnohyops, in millimeters Level in Bridger formation Species Basilar length of slsull pi-m3 P2-m» C-D B 2 B 1-2 B 1-2 L. laticeps Marsh L. monoeonus Osborn L. priscus Osborn L. laevidens (Cope) _. - "410 "439 375 153 156 149 141 139 142 133 129 • Estimated. The distinctions in measurement and proportion noted above are based upon our present loiowledge and are by no means so full and definite as we should wish. Comparison of the species. — The known species of Limnohyops do not form a progressive phyletic series in the ascending geologic scale, whereas the known species of Palaeosyops form a finely progressive phyletic series. The type of L. (Palaeosyops) laevidens Cope, from Bridger B 1-2, is a relatively small and primitive form, an undoubted Limnohyops in skull structure, distinguished by a small p^ of rounded form. All the teeth in the type are greatly worn. Considered in ascending geologic order from Bridger B 1 to D the species may be distinguished as follows: The type of L. priscus Osborn is recorded as from the same geologic horizon — namely, Bridger B 2 — but it is an animal of greater size and has a p^ of elongate, triangular form. The large and heavy type of L. monoeonus Osborn also occurs in Bridger B at Grizzly Buttes. It is named the "single-coned species" in reference to the absence of the hypocone on m'. Except in this character it is a typical Limnohyops. The type of L. matthewi also occurs in Bridger B 2, a surprising fact because of its extremely short and broad skull proportions. It exhibits the extreme of brachycrany. L. laticeps, the type species of the genus, occurs much higher up — namely, in Bridger C and D. It is less specialized in skull structure than L. matthewi and has the generic character of the presence of the hypocone on m^ very marked. QXTANTITATIVE EVOIUTION OF IIMNOHYOPS The accompanying table gives the measurements of 29 characters of proportion in 11 specimens, repre- senting 5 species, collected at ascending geologic levels in the Bridger formation. Evolution of proportions of Limnohyops [Measurements in millimeters] L. laevidens, Am. Mus. 5104 (type); Bridger B L. priscus L. matthewi, Am. Mus. 11684 (type); Bridger B 2 L. monoeonus L. laticeps Am. Mus. 11688 (cotype); Bridger B 2 Am. Mus. 11687 Bridger B 2 Am. Mus. 11679 (type); Bridger B 2 Am. Mus. 6102; Bridger B (3-5?) Yale Mus. 11000 (type) Am. Mus. 11710; Bridger D 2 Am. Mus. 12201; Bridger C 4 Am. Mus. 12198; Bridger D 2 Skull: "375 270 -439 "■320 °410 »310 452 118 -395 Height of occiput above for. 118 160 169 122 Width across postglenoid proc- pi-m3 141 129 57 85 15 21 23 27 31 34 149 137 64 87 18 23 24 27 32 35 "156 142 172 159 153 139 64 90 18 24 24 27 33 38 "75 p2-m' P'-p« M'-m^ » 30 '■93 19 23 26 29 33 36 103 20 26 27 33 36 40 32 38 » Estimated. EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES Evolution of proportions of LimnoTiyops — Continued 305 L. laevidens, Am. Mus. 6104 (type); Bridget B L. priscus L. matthewi, Am. Mus. 11684 (type); Bridger B 2 L. monoconus L. laticeps Am. Mus. 11688 (cotype) ; Bridger B 2 Am. Mus. 11687 (type); Bridger B 2 Am. Mus. 11679 (type); Bridger B 2 Am. Mus. 5102; Bridger B (3-6?) Yale Mus. 11000 (type) Am. Mus. 11710; Bridger B 2 Am. IVjus. 12201; Bridger C 4 Am. Mus 12198: Bridger D 2 Lower jaw: »278 -75 71 123 160 146 91 19 13 24 16 41 20 368 93 93 -175 191 172 111 22 14 27 17 48 24 357 "95 94 » 153 lower teeth: Pi-ms 196 173 116 20 14 28 19 52 25 " Estimated. The foregoing table brings out the following facts: 1. The cranial increases in length and width are not accompanied by proportional dental increases. 2. Relatively large-skulled and extremely brachy- cephalic animals (i. monoconus) occur in the lower Bridger levels. 3. There is no evidence of progressive monophyletic change such as we see in Palaeosyops. (See p. 313.) Limnohyops laevidens (Cope) Plate LVII; text figures 96, 258, 259 [For original description and type references see p. 163] Type locality and geologic horizon. — The type speci- men represents the smallest, most primitive, and geologically earliest LimnoTiyops at present known. It is somewhat doubtfully recorded from Bridger B 2(?), as represented in the deposits of Cottonwood Creek, Bridger Basin, Wyo. If from this level, it is slightly more recent than the geologically early and most primi- tive discovered stage of Palaesoyops, known as P. paludosus, referred specimens of which have been found in Bridger B 1 . Specific characters. — Inferior in all dimensions to type of L. laticeps; p'-m', 141 millimeters; p^-m^ 129; p'-p*, 57; p^ rounded or transversely oval, with tritocone rudimentary or absent. A large hypocone on m'. Materials. — The type specimen of L. laevidens (Am. Mus. 5104; see revision of the nomenclature, Chap. Ill) is a part of an aged skull containing a well-worn dental series, from which it is difficult to determine positively the characters of this animal. The specific name laevidens (from laevis, (Zens = imperfect tooth) was assigned to this specimen by Cope in recognition of the supposed absence of the tritocone on p^. It is probable that in the unworn condition this cusp was present but very rudimentary. Cope's type of Palaeosyops laevidens Teeth. — The teeth of the type of P. laevidens (Am. Mus. 5104), belonging to an aged animal, are especially Figure 258. — Anterior part of skull of Lim7iohyops laevidens One-fourtli natural size. Am. Mus. 5104 (type), reversed; Cottonwood Creeli, Bridger Basin; level Bridger B 3. Ai, Side view; A2, front view. interesting because they exhibit the influences of age in rounding off the angles, wearing away the cusps, and smoothing down the cingula. Professor Cope was probably misled when he described this type as lacking a tritocone on p'; we now recognize that the 306 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA apparent absence of this tritocone may be due to age; it probably has been almost completely worn off. The superior incisors arch gently forward, the total transverse series when in place measuring 56 milli- meters; there is a regular increase from i^ to i^; the anterior faces of the crowns are slightly crenulate, and a convex swelling or low ridge surrounds the base; the posterior faces are marked off by lateral ridges and by a posterior cingulum, which is irregularly pitted above; the transverse measurements of the anterior portions of the crown are: i' 10 millimeters, i^ 11 (ap. 12), i' 12. A narrow diastema (7 mm.), separates i^ from the canine. The canine has a stout fang and a crown laterally compressed at the base, the measurements being, anteroposterior, 21 millimeters; transverse, 18; height, 36 (estimated). The crown is defined by faint anterior and posterior ridges; it is slightly retro verted and inverted. In the premolar-molar series a very narrow diastema (5 mm.) separates the canine from p', a tooth which is continuous with the remaining grinders, the total length of the whole grinding series being exactly 141 millimeters, less than in the type of L. prisons (149 mm.). In MesatirMnus megarhinus the premolar-molar series ranges from 140 to 147 millimeters. P' is a simple, bifanged cone (ap. 11 mm., tr. 8), with faint anterior and posterior concavities on the inner side. P^ is an obliquely placed oval, measuring (ap. by tr.) 12 (ectoloph 15) by 15 millimeters, whereas in Mesati- rMnus megarhinus the anteroposterior diameter greatly exceeds the transverse. As Cope pointed out, this tooth is distinguished specifically by the simple rounded protocone, with a more sessile and internally placed tritocone, and a relatively small deuterocone on its lingual side. P^, measuring 15 (ectoloph 18) by 19 millimeters, is broader than long and exhibits relatively more prominent tritocones and deutero- cones. P*, 15 (ectoloph 18) by 21 millimeters, is also broader than long, the tritocones and deutero- cones are still larger, and the external cingulum begins to be defined, as well as the rudimentary anterior and posterior cingula; very rudimentary cingula are also observed in p^, p^, except on the lingual side of the deuterocones, which cusps are absolutely smooth and rounded in all these teeth, presenting in this respect a sharp contrast to the condition in M. mega- rhinus, or even (in less degree) to that in the type of L. priscus. The entire length of the premolar series is 58 millimeters, as compared with 64 in a small individual of M. megarhinus. The molar series measures 84 millimeters. The imperfectly preserved m' (ap. 2.3 mm., tr. 27) exhibits rudimentary external and antero-internal cingula and a subquadrate crown; in m^, also badly damaged, we observe evidence of sessile conules and a low anterior cingulum; in the better preserved m' (ap. 30 mm., tr. 35), also a sub- quadrate tooth, there are rudimentary external. anterior, and postero-internal cingula, the last giving rise to a low cingulate hypocone; there is some evi- dence of small, well-worn protoconules and metaco- nules; the crown, as in the other molars, is singularly smooth. SlcuU. — Although only the anterior portion of this cranium is present (see fig. 258), it affords conclusive evidence of ancestral relationship to L. laticeps in the rounded shape of the nasals. Its general or palaeosyopine characters are especially seen in (1) the downward V-shaped extension of the nasals on the sides of the face; (2) the prominent antorbital Icnob of the lacrimals; (3) the backward extension of the infraorbital portion of the maxillaries beneath the malar arch; (4) the narrow median symphysis between the premaxiUaries; (5) the comparatively slight lateral decurvature of the nasals; (6) the extreme upward arching of the zygoma as a whole, the mid-depth being 51 millimeters, and the "depression and angula- tion" of the malar 19 millimeters behind the orbits. This cranium not only differs in its smaller size but in a number of other proportional characters from that of L. laticeps. The nasals are relatively more elongate, narrower posteriorly, and relatively broader an- teriorly — that is, the sides of the nasal in front of the narial notch are more nearly parallel, the narrowest midportion measuring 44 millimeters, the broadest terminal portion measuring also 44. From the an- terior border of the orbit to the narial notch the measurement is 61 millimeters. The zygomatic bar immediately below the orbit is more angulate and less rounded than in P. leidyi, the inferior face of the malars being more flattened and the sharp masseteric ridge defining the malars inferiorly being less extended fore and aft. In palatal view we observe the trans- verse extension of the glenoid facets for the condyles of the jaw, the opening of the nares behind m^, the abbreviation of the palatines, and the relative flatness of the palate. Limnohyops prlscus Osborn Plates LVI, LX, LXII; text figures 29, 115, 219, 254, 256, 259, 260, 266, 274, 690, 717, 745 [For original description and type references see p. 180] Type locality and geologic horizon. — Grizzly Buttes, west Bridger Basin, Wyo. ; Bridger formation, Palaeo- syops paludosus-Orohippus zone, level Bridger B 2. Specific characters. — P^-m^ 148 (type) to ?161 milli- meters. Second superior premolar obliquely elongate with a very rudimentary tritocone. Large hypocone on m^. This is apparently a larger and relatively more advanced animal than L. laevidens, but, as the specific designation priscus indicates, it is still very primitive. Materials. — L. priscus is represented by the type skull (Am. Mus. 11687; see fig. 259), from the Bridger formation at Grizzly Buttes, level B 2, and by the type or cotype jaw (Am. Mus. 11688), found EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 307 near the same place. An occiput in the Princeton Museum (No. 10044), also from Cottonwood Creek, which was described and figured by Earle (1892.1, p. 353) as L. laticeps, may also be provisionally referred to L. priscus. The generic or LimnoJiyops character is shown in the presence of a hypocone on m^. Divergence from the type of L. laevidens is indicated by the elongate form of p^ as distinguished from the rounded or transversely oval form which this tooth presents in L. laevidens. The progressive stage is indicated by the rudimentary condition of the tritocone on p' in the type specimen of L. priscus. The type sTcull. — The skull of the type specimen (fig. 259) is somewhat larger than that of the American tapir. It is extremely interesting to note that it more closely resembles that of the common ancestral form of the titanotheres than does the skull of the contem- porary species Palaeosyops paludosus. The skull and m^ are therefore more primitive than those of Palaeosyops. The specimen in hand is much crushed laterally, and the restoration represented in Figure 259 is an approximation to the complete form. The estimated total length from condyles to symphysis is 375 millimeters; from the crest of the occiput to the tip of the nasals 395. The cranium shows the typical LimnoJiyops characters enumerated above — namely, short supraoccipital exposure on top of the skull; moderately high, thin sagittal crest, which diverges into the supratemporal crests about 95 millimeters in front of the occiput; occiput moderately high (proportions not to be exactly ascertained owing to its crushed condition). The nasals measure 143 milli- meters (ap.) and taper gradually toward the extremities. In the base of the skull the paroccipital processes are separated by shal- low grooves from the post-tympanic processes, and the external auditory meatus is apparently open below. Dentition. — The superior teeth are well represented in the type cranium. The lateral incisor is enlarged as in Palaeosyops; the canine is slender, slightly recurved, and followed by a very narrow diastema; the grinding series (Pis. LVI, LX, LXII) is con- tinuous, p'-m^ measuring 149 millimeters; pm^ pre- sents a very large convex protocone and rudimentary tritocone. The inferior teeth are well shown in the cotype jaw (Am. Mus. 11688), found not far from the skull and probably belonging to the same individual. The two incisors preserved are cingulate posteriorly; the canine is rather slender, laterally compressed, and slightly recurved; behind this is a small, simple pi, followed by a narrow diastema; p2 and ps are com- paratively narrow and simple teeth, exhibiting ex- tremely rudimentary paraconids and slightly cres- centic hypoconids, and a faint rudiment of the meta- conid in ps; p4, on the contrary, is submolariform, exhibiting a well-developed metalophid and a rudi- mentary hypolophid or posterior crescent which lacks only the entoconid. The three true grinders are simple, with rudiments of vertical striations, with distinct paraconids, and with a sharply pointed sub- crescentic hypoconulid on la^. The characters of the jaw are well shown in Figure 259. Back of the cranium. — The back of a cranium in the Princeton collection (Princeton Mus. 10044) may be provisionally referred to this species. It was found on Cottonwood Creek, Bridger Basin, Wyo., in the Bridger formation, level B 3(?) by Francis Speir, of the Princeton expedition of 1877, and was rightly referred to Limnohyops by Earle. The deep (45 mm.) and nar- row (9 mm.) sagittal crest is not quite so elongate as that of the Yale Museum type of L. laticeps, the supra- FiGUKB 259. — Skull of Limnohyops priscus One-fourth natural size. Skull Am. Mus. 11687 (type); Grizzly Buttes (west), Bridger Basin, Wyo.; Bridger formation, level B 2. Partial reconstruction of crushed skull made by L. M. Sterling under the direction of W. K. Gregory. Lower jaw. Am. Mus. 11688, possibly belonging to same individual. Details of zygoma restored from Am. Mus. 5104 (type of L. laevidens); details of occiput from Princeton Mus. 10044, L. priscus. temporal ridges beginning to diverge rapidly 77 milli- meters in front of the crest of the occiput. In addition to the points noted in the type skull above described, we observe in this occiput (1) two large mastoid foramina, (2) two prominences just above the foramen magnum, (3) the broadly extended (48 mm.) post-tympanic processes closely conjoined at the base with the relatively narrow (12 mm.) paroccipital proc- esses; (4) the relatively broad (37 mm.) postglenoid processes. In palatal view the zygomata are seen to have a transverse measurement of 268 millimeters, the post-tympanic and postglenoid processes are slightly (3 mm.) separated; the articular facets for the condyles of the jaw are nearly transverse; a bridge of bone, 19 milluneters in width, separates the foramen ovale and foramen lacerum medium. These features are well illustrated in Figure 260 as compared with similar views of L. laticeps and L. matthewi. 308 TIT.\KOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Limnohyops matthewi Osborn Text figures 116, 261, 262 [For original description and type references see p. 180] Type locality and geologic horizon. — -Grizzly Buttes (west), Bridger Basin, Wyo.; Bridger formation, Palaeosyops palvdosus-OroMppus zone, level B 2. Specific characters. — Type of intermediate size as compared with L. laevidens and L. monoconus. M} small, with large hypocone and quadrate inner half. Occiput very high and narrow. Cranial portion of skull greatly abbreviated, bringing post-tympanic and post- glenoid processes into broad union. Temporal open- ings subcircular as defined by zygomatic arches. (See figs. 261, 262.) Figure 260. — Back part of skull of Limnohyops priscus One-fourth natural size. Princeton Mus. 10044; Cottonwood Creels:, Bridger Basin, Wyo.; lower part of Bridger formation. Ai, Side view; Az, basal view; A3, top view; A4, occipital view. This species is named in honor of Dr. W. D. Matthew, of the American Museum, who has done so much to advance our knowledge of the Bridger fauna and geology. As compared with the primitive type skulls of L. laevidens and L. priscus, above described, the cranial region of this animal is surprisingly specialized in its abbreviation, or extreme brachycephaly. This pro- gressive character is difficult to reconcile with the fact that it is geologically recorded in the same low level as that of L. laevidens. Materials. — The type (Am. Mus. 11684), consisting of the posterior portion of a skull which includes the orbits, is the only specimen at present referable to this species. It was found in 1903 by the American Museum expedition. Distinctive characters. — The brachycranial type of this species is readily distinguished from that of L. laevidens by the very specialized condition of the posterior portion of the skull, including the elevated occiput and the firm inclosure of the auditory meatus below. It differs from L. laticeps when seen from above in the greater height of the occiput and in the transversely oval form of the temporal openings as defined by the zygomatic arches. It differs from L. monoconus in the presence of a large hypocone on m' and in the quadrate form of the inner side of this tooth, also in the rounded temporal openings. In the lateral view of the skull we observe that the zygoma descends rapidly anteriorly and thins out as it passes into the anterior portion of the malar, which constitutes the suborbital bridge; this bridge is de- pressed in section but ex- hibits a rounded rather than angulate outer bor- der. This species differs from Palaeosyops in that the masseteric insertion ridge is not carried for- ward any great distance, and that there is a slender splint of the maxillary ex- tending back below the malar bridge. The sagit- tal crest extends 50 milli- meters above the brain case proper; it is even higher and thinner than in L. laticeps. As seen from below the temporal open- ings are transversely oval, the anteroposterior meas- urement being 82 milli- meters, and the transverse measurement from the malars to the alisphenoids 95. The sagittal crest divides these great insertion cavities of the temporal muscles and is thin at the summit, measuring 8 millimeters at the thinnest point; it is also elongate, extending 94 millimeters from the tip of the occiput to the point where it bifurcates into the supratemporal ridges. The chief feature of the occiput is the foreshortening and compression of its lower portion against the back portion of the squamosals and zygomatic arch, causing a broad junction of the postglenoid and post-tympanic processes and a very characteristic flattening of this region. Seen from behind (fig. 262) the occiput measures 116 millimeters above the foramen magnum, and 157 above the bottom of the occipital condyles. Conforming to the smaller size of the skull as a whole, the condyles measure 86 milli- EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 309 meters transversely as compared with 99 in L. mono- conus. The width of the occiput across the top is 108 millimeters. At the sides of the exoccipital por- tion are indistinctly seen the mastoid foramina. Just Figure 261. — Skulls of three species of Limnohyops Occipital view. One-fourth natural size. A, L. monocomis, Am. Mus. 11679 (type); Grizzly Buttcs, west Bridger Basin, Wyo.; Bridger formation, level B 2. B, L. laticeps, Yale jMus. 11000 (type, vertically crushed); Bridger Basin; upper? Bridger. C, L. matlhewi, Am. Mus. 11684 (type); Grizzly Buttes, west Bridger Basin; Bridger formation, level B 2. above the foramen magnum are very faintly indi- cated the pair of facets characteristic of this genus, more distinctly marked in Telmatherium. Limnohyops monoconus Osborn Plate LXII; te.xt figures 117, 261-263, 484, 485, 510-514, 516, 519-523, 525, 527-530, 685, 686, 701, 723 [For original description and type reference see p. 130. For skeletal characters see pp. 604, 612] Type locality and geologic Tiorizon. — Grizzly Buttes (west), Bridger Basin, Wyo.; Bridger formation, Pal- aeosyops paludosus-Orohippus zone, level B 2. This specimen was discovered by the American Museum expedition of 1903. It was on the level of L. laevidens and L. matthewi and thus belongs to a much lower horizon than L. laticeps. This specimen may rep- resent, however, the geologic intrusion of a fauna from a higher level, and it is therefore quite possible that it was not contemporaneous with L. laevidens. The presence of these specialized forms, L. mattheivi and Figure 262. — Skull of Limnohyops matthewi One-fourth natural size. Am. Mus. 11684 (type) ; Grizzly Buttes (west) , Bridger Basin, Wyo.; Bridger formation, level B 2. Ai, Side view; As, basal view. L. monoconus, on the same geologic level as the primitive forms L. laevidens and L. priscus is contrary to the general law of succession observed among other forms in the Bridger Basin. It may indicate either some source of error in the geologic records or some deviation from the generally horizontal dis- tribution of the Bridger titanotheres. Another ex- planation is possible: that Limnohyops was evolving more rapidly in other geographic centers, from which these progressive forms may have migrated. Specific characters. — M^ without hypocone, roundly triangular in form, with broadly extended ectoloph and parastyle. P*-m', 142 millimeters; p'-m^, 156. Condyle to incisive border 439 millimeters (estimated); 310 TITAKOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA occiput very high; cranium relatively elongated, with space (4 mm.) between post-tympanic and post- glenoid processes; temporal openings as defined by zygomatic arches elongated. This type (Am. Mus. 11679) is an exception to all the other species referred to Limnohyops in the ab- sence of the hypocone on m^. The specific name, monoconus, refers to the existence of but a single internal cusp (protocone) on this tooth. The animal is provisionally referred to the genus Limnohyops on strong evidence in five other points of cranial structure which are cited below. As compared with the type of L. matthewi, the animal on which this species is founded is of very FiGUBE 263. — Skull of Limnohyops monoconus Palatal view. One-fourth natural size. Am. Mus. 11679 (type); Grizzly Buttes, west Bridger Basin, Wyo.; Bridger formation, level B 2. robust size; its size is especially robust for the low geologic level from which it is recorded — namely, B 2. Its proportions agree fairly well with those of L. lati- ceps, which is referred to level D 2. Materials. — The species is certainly Icnown only from the type (Am. Mus. 11679), which consists of the crushed cranium of a robust male specimen, as indicated by the large, recurved canines. Other doubtfully referred specimens (Am. Mus.' 5102, 12680) are recorded from B 5. As the hypocone on m^ is the alleged "generic" character of LimnoTiyops and is present in the other species L. laevidens, L. matthewi, and L. laticeps, its absence in this form is very exceptional and causes some doubt as to the propriety of the generic refer- ence of this species to Limnohyops. The other grounds for referring this animal to Limnohyops rather than to Palaeosyops are strong, however — namely, (1) the doubly ridged ectolophs of p"-p*; (2) the very elevated occiput; (3) the more or less angulate form of the suborbital bridge in the malars; (4) the splint of the maxillary extending on the lower side of the malar bridge; (5) nasals not perceptibly narrowing anteriorly. Specifically this animal is readily distinguished from all other species of Limnohyops by the absence of the hypocone on m^. As compared with L. laevidens it is an exceptionally large form, the measurements (esti- mated) being, from the condyle to the incisive border 439 millimeters, width across the zygomata 320. It is also distinguished by the very robust and recurved canines. From L. laticeps it is distinguished by the higher occiput, by the absence of a hypocone on m^, and by its supposed lower geologic level. It is dis- tinguished from L. matthewi by the more elongate cranium, correlated with which are the oval openings circumscribed by the zygomatic arches, and by the separation between the postglenoid and post-tympanic processes. The skull is of massive proportions, with widely arched zygomata (320 mm., estimated) as compared with the total length (439 mm., estimated). (See fig. 263.) The anterior portion is too much crushed for recognition, except that the nasals have the form char- acteristic of Limnohyops, with more parallel sides than in Palaeosyops. Seen from behind the occiput is more elevated than in L. laticeps and resembles that of L. matthewi. (See fig. 262.) The extreme height of the sagittal crest above the foramen magnum is 133 millimeters, and above the bottom of the condyles 179; the condyles measure 99 millimeters in width. Seen from above the nasals are narrow and long, measuring 175 to 186 millimeters as compared with 168 in L. laticeps. In the region of the frontonasal horn swelling there is a rugose area which may have exhibited a rudimentary frontonasal horn. Dentition. — The canine is exceptional in its robust size and recurved form, the height being estimated at 41 millimeters and the diameters at the base of the enamel being, anteroposterior, 25; transverse, 25. The crown approaches that of Manteoceras in the swelling of the base. The premolars are primitive in lack of complication: (1) there is a small trito- cone on p^, (2) p^ and p^ lack the internal cingula entirely, a primitive condition, (3) the double ridging of the ectolophs is a Limnohyops character. The mo- lars, m'-m^, measuring 93 millimeters, are very distinc- tive in form, with oblique ectolophs and prominent parastyles. The triangidar m^ especially is of quite different form from that of L. matthewi, the tooth narrowing toward the inner side and having a long oblique outer border. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 311 Limnohyops laticeps Marsh Plates LVII, LXII; text figures 87, 92, 261, 264-266, 511, 531, 532, 714, 760 [For original description and type references see p. 160. For skeletal character see p. 6181 Type locality and geologic horizon. — Bridger Basin, Wyo. Marsh's Fork, the level of which is not cer- tainly known, is the type locality. The American Museum specimens closely resembling the type are from Bridger C 4, D 1, and D 2. Specific characters. — Of intermediate size; p'-m^, 153 millimeters; p^-m', 139; second and third in- ferior premolars of more advanced type than in L. FiGiTRB 264. — Skull of Limnohyops laticeps Top view. One-fotirth natural size. Yale Mus. 11000 (type). Bridger Basin, Wye; upper (?) part of Bridger formation. priscus; p^ with well-developed tritocone; m' with a large hypocone (fig. 265). Condyles to incisive border 410 millimeters (estimated); breadth across zygomata 320; smooth and extremely rudimentary horn swell- ings on nasofrontal sutures. Cephalic index 75 (esti- mated). This was one of the earliest of the Palaeosyopinae to be described, and for a long time it was not clearly separated from the genus Palaeosyops. The most dis- tinctive character assigned by Marsh was the hypocone oa the last superior molar (fig. 265). Many of the distinctive cranial characters were clearly pointed out 101959— 29— VOL 1 23 by Earle. Additional materials in the American Mu- seum collections enable us to fully define this species, especially from the full characters of the skull, the carpus, and the manus. Materials. — L. laticeps is represented by two speci- mens — by the type cranium and parts of the skeleton (Yale Mus. 11000), belonging to an individual not fully grown, and by Am. Mus. 11710, a fragment of the maxilla containing two molars, from level D 2, Bridger. Specific characters of the type. — In addition to the specific characters enumerated above the occiput is moderately high (144 mm. above the condyles, 118 above the foramen magnum) ; the condyles are moder- ately broad (95 mm.); the mesostyle on p*, which is seen as a shadow rudiment or rectigradation in L. priscus, is here quite distinct; in p^ the tritocone is much larger and more distinct than in L. priscus; m' as in L. priscus is small (ap. 24 mm., tr. 27); m^ is much larger than in that species; the metaconules are larger than in L. priscus and confluent internally with the hyipocones; there is little or no diastema be- hind the canine. The type slcull. — Our knowledge of the skull is based mainly on the crushed but very complete type cranium of L. laticeps (Yale Mus. 11000), which gives us the principal characters. (See fig. 264.) (1) The pro- portions of the skull are approximately the same as in P. leidyi — namely, 410 millimeters in length and 310 across the zygomata. (2) The skull of L. laticeps is distinguished from that of L. priscus and approaches that of P. leidyi in the slight narrowing of the nasals anteriorly: posteriorly they measure 53 millimeters in width ; anteriorly they diniinish to 43 millimeters, being still much broader anteriorly than in P. leidyi. Other distinctions from P. major and P. leidyi are found in the following principal characters : (3) The sagittal crest is very high, extending 65 millimeters above the brain case, and thin at the summit (9 mm.), extending for- ward a considerable distance (103 mm.) before the crest begins to spread into the supratemporal ridges, whereas in the least progressive specimen of P. leidyi described below the thinnest portion of the crest measures 13 millimeters and begins to expand rapidly into the plane of the vertex; (4) the occiput as seen from behind is well defined by a sharp crest and is rounded superiorly, extending 118 millimeters above the foramen magnum and 125 millimeters trans- versely; (5) the postglenoid and post- tympanic proc- esses are slightly separated; (6) the zygomata arch widely, the malars being compressed inferiorly and Figure 265. — Third right up- per molar of Limnohyops laticeps Natural size. Am. Mus. 11710, re- versed. Henrys Fork, Lone Tree, Bridger Basin, Wyo.; Bridger for- mation, level D 2. 312 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA forming a sharply convex ridge below the orbits, with a narrow V-shaped union with the maxillaries anteri- orly ; (7) in lateral view the nasals are slightly deciu-ved anteroposteriorly; (8) the infraorbital foramen is above the third and fourth premolars, or about 34 milli- meters below and anterior to the orbit. Dentition of the type. — The premolars of the type of L. laticeps are much more progressive than those of the types of L. laevidens and L. priscus. The range of progression is parallel with that of P. leidyi in the following respects: (1) A larger tritocone is developed on p^; (2) a distinct mesostyle is observed in the ectoloph of p*; (3) the protoconule of p'* is larger. Comparison of these teeth with Leidy's superior pre- molar and molar types of P. paludosus shows that the ectoloph of p^ is identical in measurement in the two specimens and that the measurements and characters of m^ are very similar except that in the type of L. laticeps the anterior cingulum is somewhat more accented. In the type of L. laticeps p^-m^ measures a few millimeters less than in Am. Mus. 2361. The inner side of the crown of p^ (ap. 1 1 mm.^^) is preserved, show- ing a postero-internal cingulum; p^ is a transversely oval tooth like that of P. laevidens; p^ (ap. 14 mm., tr. 19) is more progressive than in L. priscus in its slightly better developed tritocone, more sharply ribbed protocone, stronger internal cingula; in p* (ap. 17 mm., tr. 24) we observe the slightly more distinct development of the conule above mentioned and the presence of a mesostyle on the tooth of the right side, which, however, is wanting in that of the left. The molar series measures 90 millimeters in length, and the anteroposterior by transverse dimensions of the teeth are, m' 24 by 26 millimeters, m^ 31 by 35, m^ 33 by 38, the teeth thus being broader than long; the slopes of the cusps are vertically striated but less strongly so than in Palaeosyops; both protoconules and metaconules are present, small, of transversely lophoid shape; in m^, however, the protoconule is more distinct, the metaconule is vestigial, and the cingulum rises into a distinct cingular hypocone (see fig. 265); the vertical ridges of the ectoloph are slightly fainter than in Leidy's cotype of P. jmludosus, and the external crescents are slightly more open. Jaw oj the type. — The posterior portion of the jaw as preserved shows that the angle descends almost vertically below the condyle and does not extend backward so far as in Palaeosyops. The lower border of the angle is 157 millimeters below the condyle. The coronoid is stout, relatively low and broad. 23 The anteroposterior measurements given here and elsewhere are talien across the middle of the crown. Palaeosyops Leidy Plates XVI, XXVII, XXVIII, XLIV, L, LIII, LV, LVI LVIII-LXII; text figures 27, 28, 33, 88, 88, 89, 97, 108, 118- 120, 210, 214-220, 227, 254r-257, 266-288, 305, 405, 407, 482- 485, 508-516, 519-523, 528, 533-537, 539-550, 552, 559, 571, 645, 661, 685, 686, 703, 711, 713, 714, 716, 718, 721, 724, 727, 732, 733, 737-742, 745, 760 [For original description and type reference see p. 157. For skeletal character see p. 619] Type locality and geologic horizon. — Bridger Basin, Wyo., Bridger formation, levels B, C, and D; Wash- akie Basin, Wyo., Uintatherium-Manteoceras-Mesati- rhinus zone (Washakie A). Specific characters. — Brachycephalic. Grinding teeth persistently brachyodont; metaconules on the molars persistent or absent; third superior molar with- out hypocone. Skull and skeleton robust; feet broad; manus with well-developed fifth digit; lunar resting subequally on magnum and unciform. Four sacral vertebrae. Geographic and geologic distribution. — This type genus of the family Palaeosyopinae embraces a re- markable series of stages of evolution of animals, which are subdivided into eight species, found in ascending geologic succession — namely, P. fontinalis, P. longiros- trisf, P. paludosus, P. major, P. grangeri, P. leidyi, P. roiustus, and P. copei. The members of the phylum belong chiefly to the Bridger formation and increase steadily in size from its base to its summit, or from level B to D. P. grangeri and P. copei are not in- cluded in the direct line. The specimens listed below were collected from the Bridger formation at the levels indicated. 13032, B 1. 10276, B 1 or B 2. 12182, B 3. 13116, B 2. 12185, C 3. 10009, C or D. 12196, C 2. 1544, C 4. 11678, D 4. 5106, D (?). 10282, D (?). 11683, D 3. 12189, C 1. 11708, D 3. 11692, B 2. 11680, B 1. 12181, B 3. 12165, B 2. 12183, B 3. Type of P. r, 2(?). 12200, C 4. 12188, C 5. 12205 a, D 1. Comparatively few specimens of Palaeosyops have been found in the more easterly Washakie Basin, only 80 kilometers (50 miles) distant. This fact implies a difference of living conditions, because the deposits of the Washakie Basin are much less rich in fossils and because the greater part of the fossiliferous "Washa- kie" exposures are of more recent age than the Bridger. Only the lower "Washakie" (horizon A) was syn- chronous with the summit of the fossiliferous Bridger (horizon D). The evidence afforded by our present knowledge in- dicates that the quadrupeds now known as Palaeosyops disappeared from this region or died out during or soon after the deposition of Bridger D. Palaeosyops EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 313 was by far the most abundant of the Bridger Basin quadrupeds. The identified materials in the American Museum from the Bridger Basin embrace parts of more than 60 individuals, including 12 more or less com- plete skulls. Materials. — Altogether, more than 70 specimens have been examined for this monograph, including those in the collections in the American, Yale, Na- tional, and Philadelphia Museums. Most parts of the skeletons are dissociated from skulls; even the jaws are rarely found with skulls; in only three of the speci- mens of Palaeosyops studied are the jaws associated with the skull. The following table shows the length of the lower grinding series, the length of the upper grinding series, the length along the bottom line of the skull, and the breadth across the zygomatic arches : Comparative measurements oj species of Palaeosyops, in millimeters Bridger D 3 P. copei Osborn Bridger D ' P. robustus (Marsh) . Bridger C 2-4 _ Bridger C 1 _ _ . Bridger B 2-4. Bridger B l-2_ Bridger B Bridger A Huerfano B P. leidyi Osborn P. grangeri Osborn.. P. major Leidy P. paludosus Leidy _. P. longirostris Earle. P. fontinalis Cope P. fontinalis Cope__. (?) 164 151 143 170 180 174 180 155 137? 153 170 158 165 147 124? (?) (?) (?) (?) (?) (?) '•440 415 (?) 389-435 (?) (?) (?) 146 (?) '•340 310 (?) ' 290-335 (?) (?) (?) (?) There are steady increases in every dimension measured until we reach the single specimen known of the ultimate species, P. copei, from Bridger D 3, in which a sudden falling off in size is observed. (See also detailed table on p. 316.) Palaeosyops in general steadily evolves from a smaller although very robust animal into a larger, ex- tremely broad-skulled, or brachycephalic animal. Certain characters are persistent in all the species. There are added through orthogenesis other new char- acters, or "rectigradations," which distinguish the more advanced stages from those found in the lower levels. The chief rectigradations and allometrons are the following: (1) Increase in size and brachycephaly of the skull; (2) steady increase in all dimensions of the grinding teeth; (3) certain cusps in the lower pre- molars (p2, Ps) analogous to the paraconid, metaconid, and hypoconid of the true molars ; (4) the second cusps (or tritocones) on the outer wall of the second upper premolars (p^), cusps foreshadowed in the ancestral species; (5) the median ridges (or mesostyles) added on the outer walls of the two posterior upper premolars (p^, p*), which happen to be quite distinctive because these ridges do not arise in all titanotheres as they do in all horses; (6) the second postero-internal cusp, or tetartocone, on the superior premolar teeth is late in evolution in all titanotheres and only appears in very rudimentary form in P. copei; (7) the horns, or naso- frontal bosses, are rectigradations, which appear to be more retarded in evolution in Palaeosyops than in several other phyla. Summarized, these seven principal cranial and den- tal changes are shown below. I. Allometrons (proportional, or metatrophic) : 1. Increase of the skull in size and in brachycephaly. 2. Steady increase in all dimensions of the grinding teeth (see table on p. 316). 3. Cusps on the lower premolars P2, Pa analogous to the paraconid, metaconid, and hypoconid of the true molars; foreshadowed in P. paludosus and in- creasing progressively in the higher stages. II. Rectigradations (numerical, or the addition of new elements): 4. Tritocone on p^ first occurring in P. major and in- creasing in all higher stages. 5. Mesostyles added to p', p*, first occurring in P. leidyi and characterizing all higher stages. 6. Tetartooones first appearing on superior premolars in P. copei (no higher stages at present known). 7. Osseous frontonasal horns first appearing in P. leidyi and increasing in all higher stages. Incisors. — The superior incisors exhibit low crowns with irregularly folded posterior cingula; the opposite series are usually separated by a median diastema and from the canines by lateral diastemata. The incisors increase regularly in size from i' to i'; the crowns are convex anteriorly, with a basal subcingulate expan- sion; the posterior faces slope gradually into an irregularly folded basal cingulum; the lateral incisors i' are much the largest and decidedly subcaniniform, with obliquely sloping internal cingulum; i' is more symmetrical, with short, compressed root; i^ is less symmetrical, with longer root; i' is very asymmetrical, with a very long, large root. The inferior incisors are readily distinguished by their narrow crown, sharply convex in front, and sloping posterior face divided by a median ridge. As shown in P. leidyi the crowns increase slightly but regularly in height from ii to is. In the lower incisors the opposite ij have very long roots nearly meeting in the midline 314 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA below; is have short roots and are being crowded out byia. Canines. — The superior canines are rounded at the base and when unworn are very sHghtly recurved and sharply pointed, differing from those of Telmatherium Figure 266. — Lower jaws of Limnohyops and Palaeosyops One-fourth natural size. A, L. prisms. Am. Mus. 11688 (cotype), reversed; Grizzly Buttcs (east), Bridger Basin, Wyo.; Bridger formation, level B 2; coronoid and condylar region reconstructed. Bi, P. copeiT, Am. Mus. 12205a; Lone Tree, Henrys Fork, Bridger Basin; level D 1, 15 feet above white stratum. Bj, The same, section through mj. C, L. laticeps. Am. Mus. 12201,$, reversed; Henrys Fork Hill, Bridger Basin, level C 4; angle restored from P. paludosus, Am. Mus. 11690 in their circular section and strongly convex inner sides. In females the canines are long, pointed, and less robust, whUe in males they are much more rounded and robust. The inferior canines are of very similar form, nearly erect, with crowns rounded at the base, differing from those of Manteoceras in the absence ol the marked posterior expansion of the base, and from those of Telmatherium in the rounded, nonlanceolate form. A peculiar feature of the lower canines is the directly internal or lingual position of the posterior ridge, the two ridges being thus brought very close together. Premolars. — The law of cusp addition in the pre- molars, which constitutes a most important means of distinguishing the specific stages, is clearly set forth above. In the general brachycephaly the premolars acquire a rounded form rather than the elongate or angulate form seen in Telmatherium. P' is either slightly separated from the canine or placed im- mediately behind it, according to the degree of brachycephaly; in form it is either an extremely small, rounded, bifanged tooth or considerably larger, with a posterior internal cingulum in the higher stages. The Figure 267. — Lower jaws of , male and female Inferior view. One-fourth natural size. A, P. paludosus, Am. Mus. 11680, S (neotype); Little Dry Creek, Bridger Basin, Wyo.; Bridger formation, middle beds, level B 1. B, P. copeif, Am. Mus. 12206a, V; Henrys Fork, Lone Tree, Bridger Basin; Bridger D 1 , 15 feet above white stratum. chief point to note in p^ is the strengthening and in- creasing convexity of the tritocone or posterior outer cusp (P. major). A very sharp distinction is seen in the progressive transformation of p' and p* in Palaeo- syops — namely, in the gradual development of a mesostyle or median ridge separating the protocone from the tritocone (P. leidyi), which appears to in- dicate that in Palaeosyops the ectolophs of the premo- lars are tending to become somewhat like those of the molars, whereas in Manteoceras the ectolophs never assume the molar form. The outer cusps of the premolars of titanotheres never become crescentic externally, however, and are thus essentially different from those of the molars. In general, the grinding teeth are distinguished by persistent brachyodont or short crowns, and the persistence of the conules is undoubtedly correlated with the persistent breadth of the grinders. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 315 Molars. — The superior molars are progressively distinguished (1) by the increased development of the cingula; (2) by the intensified striations of the sides of the cones; (3) by the robust and rounded form of the parastyle and increasing prominence of this style; (4) by the relative persistence of the conic or sublophoid protoconules; (5) by the more variable, rounded, or lophoid metaconules^*; (6) by the subtrian- gular or subselenoid form of the hypocone in m' and m^, the protocone remaining rounded or bunoid; (7) by the fact that in no specimen of Palaeosyops has a distinct hypocone on m^ been observed, although the metaconule is often enlarged and might easily be mistaken for a hypocone, and the posterior cingulum is often elevated. The inferior molars are readily distinguished progressively (1) by the rounded, bulb- ous character of the sides of the main cusps; (2) by the vertical grooving or striation of the sides of the cusps; (3) by the festooning of the external cingula; (4) by the prominence of the paraconids, the increased development of the metastylid, entostylid, and parastylid; (5) by the increasingly central position and rounded form of the hypoconulid in m.3, a cusp which is subcrescentic when entirely unworn but, as pointed out by Earle, wears into a circle in old age; (6) by the median ridges strengthening the internal slopes of the protoconid and hypoconid crescents, which are evidently dynamically correlated with the presence of the analogous median external ridges on the outer slopes of the paracone and metacone of the upper molars. The internal ridges also serve to comminute the food by pressing against the pro- tocones and hypocones. Slcull and jaws. — The detailed primitive and pro- gressive characters of the jaw and of the male skull may best be indicated under the descriptions of the various species. In general, the skull (fig. 276) is distinguished by (1) nasals tapering anteriorly; (2) a prominent convexity above the orbits covering a large frontal sinus; (3) rudimentary osseous horns which appear on the sides of the face; (4) a relatively short, free sagittal crest; (5) a moderately elevated occiput; (6) deep and outwardly arched zygomatic arches; (7) in earlier stages post-tympanic and post- glenoid processes slightly separate; (8) a very large mastoid foramen; (9) occipital condyles moderately expanded; (10) constantly increasing expansion of the masseteric insertion beneath the orbits; (11) premaxUlary symphysis short and rounded. In the jaw marked differences are seen between the sexes (figs. 266, 267). In the males the chin is very deep and prominent, correlated with the insertion of the long fangs of the extremely robust canines. In the females the chin is more shallow and more sloping and rather resembles that of Limnohyops. The dis- tinctive characters of the male jaw (fig. 268) are (1) the marked curvature of the lower border; (2) the great thickening of the lower border beneath the 2* These cusps appear to be entirely wanting in the primitive species Palaeosyops fontinalis, from Bridger A. I alveoli of the grinding teeth, which first appears beneath the premolars and then extends progressively backward beneath the molars; (3) the comparatively short digastric fossa for the insertion of the digastric muscle below the posterior portion of the symphysis; (4) the greater distance between the posterior molar and the back of the angle as compared with Limno- hyops; (5) the breadth of the anterior border of the Figure 268. — Lower jaws of three species of Palaeosyops One-fourth natural size. A, P. longirosMs, Princeton Mus. 10275 (type), Bridger Basin, Wyo.; B, P. paludosus. Am. Mus. 11680 (neotype), Millersville, Little Dry Creek, Bridger Basin, level B 1; C, P. major, Am. Mus. 12181 (neotype jaw), Cottonwood Creek, Bridger Basin, level B 3. coronoid, which becomes progressively hollowed out in front; (6) the reduction of the upper portion of the coronoid process, which in some higher stages assumes a more simple form with an abbreviated posterior hook. The characters of the skeleton are fully described on page 619. QUAHTITATITE EVOLUTION OF PALAEOSYOPS The following table gives the measurements of 28 characters of proportion in 16 specimens, belonging to 6 species, collected at ascending geologic levels in the Bridger formation: 316 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Evolution of proportions of Palaeosyops [Measurements in millimeters. Level, if known, given after specimen number] 3 g fi a" 1 P. major P. leidyi P. robustus 3 1 on .p si < a a D 1 a < a < 1 3 ■- a"" < 3q a < if || 3 g ■3 >< S a -< a"- if so It F Sliull and upper teeth Condyles to incisive border -389 '■290 435 335 414 •276 405 416 310 397 112 196 174 158 120 73 100 19 25 "341 I Widtli across postglenoid proc- 188 169 156 119 Pi-ms 161 144 79 71 91 18 22 25 26 32 34 1 162 150 115 - «170 174 163 127 180 165 125 71 107 21 31 30 35 38 43 P2-m'... ! 147 160 'im . •170 169 131 P!-m'..- _ - 116 P>-p< . 70 102 20 27 ■ 29 35 37 42 M>-m' 17 25 32 37 94 19 24 27 31 33 40 74 98 18 24 97 19 25 29 »31 35 39 96 18 26 29 32 32 41 102 18 25 29 32 34 39 94 17 24 -102 19 26 100 20 27 •103 20 26 30 36 37 42 96 31 34 39 36 38 74 33 37 36 40 42 Cephalic index... 1 P. paludosus P. major P. leidyi ■o a.- li M-' pi 1 4 1^ a o P a 1" a a i a 2 s a 3 SO 3 ." a*^ 1 s a < 1 3 a a 1 3 a a 1 a r p^ Lower Jaw and lower teeth -340 -85 85 165 -169 152 98 17 12 26 18 43 23 370 100 97 170 182 168 112 20 14 26 19 49 358 93 98 174 193 166 107 19 14 29 19 46 Length of symphysis Depth of ramus behind ms. 86 167 164 106 19 13 26 17 45 24 95 87 182 91 192 171 113 20 15 -107 -170 190 173 109 21 15 •90 184 91 Pi-ms. . -. 180 169 109 19 14 27 19 47 27 189 192 178 115 21 15 •126 170 113 20 14 98 19 12 24 15 41 22 118 22 15 29 19 60 26 «115 105 112 108 20 14 27 18 46 115 21 14 29 21 52 112 20 13 29 19 62 114 26 19 45 25 29 19 48 25 -50 48 26 47 25 60 60 31 52 28 'Specific reference doubtful. The accompanying table brings out the following facts: 1. The law of regular progressive increase in size as we pass from P. paludosus (of Bridger B 1) through P. major (of levels B 2-3), P. leidyi (C 2 to C 4?), to P. robustus. 2. The exceptionally large P. major skull, Am. Mus. 13116, from B 2 interrupts this regular increase, but its tooth row is not larger than in other P. major skulls (that is, the skull and the teeth are differential) . 3. So far as the measurements are evidence, the larger speci- mens of P. major overlap the smaller specimens of P. leidyi, and the larger P. leidyi overlap the smaller P. robustus. 4. P. grangeri, although it is not so advanced in its stage of premolar evolution as P. leidyi and although it comes from a lower level, yet exceeds P. leidyi in size. It 5. P. leidyi seems to be very closely allied to P. robustus. exhibits considerable fluctuation in size. 6. P. copei, although more advanced in premolar evolution than P. robustus, has a somewhat smaller dentition (differential evolution) . 7. While the premolar series remains relatively stationary in length the molar series increases from 91 millimeters in P. paludosus to about 102 in P. robustus (differential evolution). 8. The fourth premolar and first molar exhibit a marked increase in size, especially in width (differential evolution). Other conclusions from the measurements are con- sidered under the heading "Differential allometrons," on page 825. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 317 Palaeosyops fontinalis (Cope) Test figures 97, 269-271 [For original description ^nd type references see p. 165] Type locality and geologic horizon. — Bridger Basin, Wyo.; Bridger formation, level not clearly recorded but probably EometarTiinus-Palaeosyops fontinalis zone (Bridger A), as the type skull is recorded from Green River. Also recorded from Huerfano Park, Colo., in Bridger formation, horizon Huerfano B ( = Bridger A). The six specimens from Huerfano B (see below) range from 250 feet to 500 feet below the top of the Huerfano formation. Specific characters .—01 relatively small size. Superior molars with sharply defined crescents and cusps; m' extremely small (ap. 22 mm., tr. 25), with protoconule and no metaconule. This little-known animal resembles Palaeosyops in its cranial structure but differs quite widely from any known species in the form of the cusps of its grinding teeth. It is the smallest, probably the most primitive, and certainly the oldest Bridger titanothere known, and these facts in connection with the very low geologic level give it great importance. Materials. — This species is repre- sented in Bridger (A?) merely by the very immature skull (Am. Mus. 5107) collected on Green River near Big Sandy Creek. In Huerfano B it is represented by six specimens in the American Museum of Natural History, as follows : 17411. Superior dentition and portion of palate (fig. 271, A), Huerfano-Muddy divide, 2 miles west of Gardner, Colo., 414 feet below the top of tlie Huerfano formation. 17413. Two upper molars and incisor (fig. 271, C), 3 miles north of Gardner, 400 to 500 feet below the top of the Huerfano formation. 17414. Three superior molars, fragmentary, 3 miles north of Gardner, 400 to 500 feet below the top of the Huerfano formation. 17417. Ml, p', and milk teeth (fig. 271, B), 2 miles north of Gardner, 400 to 500 feet below the top of the Huerfano formation. 17425. Series of right upper grinders, p'-m^ (fig. 271, D), 2 miles north of Gardner, 400 to 500 feet below the top of the Huerfano formation. 17450. Lower canine and fragment of ma, Huerfano-Muddy divide, 2 miles west of Gardner, about 250 feet below the top of the Huerfano formation. The immaturity of the type specimen is determined by the fact that only one true upper molar (m') has come into use, the second molar (m^) being still deeply embedded in the jaw. In superior view (fig. 269, A3) the cranium is valuable as exhibiting the suture between the supraoccipital and the parietals, a suture which closes very early in Palaeosyops. Similarly in the lateral view (fig. 269, Ai) the maxilla sends out a broad, spurlike process directly on the outer side of the malar as in Palaeosyops, instead of on the under side as in Limnohyops. The infraorbital bridge is rounded as in Palaeosyops, rather than FiGiTEE 269. — Young skull of Palaeosyops fontinalis One-third natural size. Am. Mus. 5107 (type). Basin, Wyo.; Bridger formation, level A?. Green River near mouth of Big Sandy Creek, Bridger Li, Side view reversed: A2, palatal view; A3, top view. The geologic horizons of these specimens range rom 250 to 500 feet below the top of the Huerfano formation. angulate as in Limnohyops. The depressed or sessile character of the supratemporal crest is probably due to immaturity. Behind the post-tympanic process the mastoid is exposed as a narrow strip. The specimen thus probably pertains to the genus Palaeosyops, though its grinding teeth are not entirely of the typical Palaeosyops form ; its specific distinction from the larger and more robust forms is readily determined from the teeth. Dentition. — The part of the crown of dp* that is preserved exhibits the protocone more internally 318 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Comparative measurements of teeth of P. fontinalis, in millimeters placed than the hypocone; this tooth is therefore fully quadrate though not precisely molariform. The well preserved first superior molar (m') exhibits trenchant or pointed main crescents and cones; the ectoloph is divided by small, sharply ridged parastyles and Figure 270. — Upper molars of Palaeosyops fontinalis Natural size. Am. Mus. 5107 (type) . Green River near mouth of Big Sandy Creek, Bridger Basin, Wyo.; level Bridger A? mesostyles; a protoconule but no metaconule is pres- ent; in size this tooth is diminutive, measuring (ap. by tr.) 22 by 25 milHmeters. The a.m. /7413 i anterior half of the second molar (m^) is preserved; it measures 35 millimeters from the outer side of the parastyle to the inner side of the protocone; the protocone and paracone are prominent; the protoconule is reduced; the meta- conule is not preserved. As compared with the molars of P. leidyi, we note the following differ- ences: (1) Ectolophs and cusps more trenchant and flatter; (2) outer border of the metacone ectoloph more in- clined; (3) styles more sharply ridged; (4) hypocone of dp* projecting more internally than in corresponding tooth of P. leidyi. This species is certainly not a typical Palaeosyops in its denti- tion, as stated above. Measurements may be taken from the natural-size figures of the teeth in Figure 270. Of the six specimens from Huerfano B (see above) a finely preserved palate (Am. Mus. 17411; fig. 271, A) of an aged individual and the unworn upper teeth (Am. Mus. 17425, fig. 271, D) of a young individual afford a close comparison with the two permanent teeth of the type of Palaeosyops jontinalis from Bridger A and are very similar both in characters and in measurement. Am. Mus. 6107 (type); Bridger Am. Mus. 1742S; Huer- fano B Am. Mus. 17411; Huer- fano B Am. Mus. 17414; Huer- fano B Am. Mus. 17413; Huer- fano B P. palu- dosus Am. Mus. 13032; Bridger B 1 M'-m3. _ 83 16. 5 21. 5 23 26 29 34 "146 ■^63 77 16.3 21. 5 22. 5 26 27 31.5 141 63 91 18 22 M', anteroposterior M', transverse M^, anteroposterior M^, transverse 22 26 30 34 23.5 °26 28 32. 5 25 26 32 34 P'-m' . . _ 160 P'-p^ 71 General specific characters of P. fontinalis. — The dis- tinctive specific characters of P. fontinalis are (1) parastyle very prominent; (2) protoconules reduced; (3) meta- and hypocones closely compressed; (4) no Figure 271. — Teeth of Palaeosyops fontinalis One-half natural size. Referred specimens from Huerfano B. (See p. 317.) trace of metaconules; (5) no hypocone on m^; (6) pre- molars very simple; (7) restored palate of considerable breadth; and (8) the maxillaries send back a spHnt on the outer side of the malars, as in typical Palaeosyops. EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 319 Since all these six individuals from Huerfano B agree closely in size with each other and with the type of P.fontinalis, they establish Huerfano B and Bridger A as the Palaeosyops fontinalis zone, distinguished by true ancestors of Palaeosyops inferior in size to any known members of this genus in Bridger B. Palaeosyops longirostris Earle Plate LVI; text figures 108, 268 [For original description and type references see p. 172] Type locality and geologic horizon. — Cottonwood Creek, Bridger Basin, Wyo; Bridger formation, Palaeosyops paludosus- Orohippus zone (Bridger B). Specific characters. — Pi-ma, 156 milli- meters (estimated); p2-m3, 143; mi_3, 90; P4 rather simple. Wide posterior extension of the ramus of the jaw behind ms. Canine large, semiprocumbent. Mandibular sym- physis elongate. This little-known form appears to be closely related to but somewhat smaller than the typical P. paludosus. The type specimen, the left ramus of a lower jaw (fig. 268, A), was well described by Earle in his memoir of 1892. No other known material has been referred with certainty to this species. Palaeosyops paludosus Leidy Plates LII, LVI, LVIII, LIX, LXII; text figures 86, 88, 267 A, 268 B, 405, 407, 732 [For original description and type references see p. 157] Type locality and geologic horizon. — Bridger Basin, Wyo.; Palaeosyops paludo- sus-Orohippus zone (Bridger B). Bridger B 1, as represented by the base of the sec- tion of Church Buttes, is apparently the type geologic level. The Bridger B 2 specimen is from the lower portion of the Grizzly Buttes deposition. Specific characters of type and of referred specimens. — Of small size; pa-nis, 152 milli- meters; p^-m^, 144; m2 (ap. by tr.), 33 by 20.5; second and third superior and inferior premolars more primitive than in succeed- ing stages ; p^ with a single external cone — that is, no tritocone ; p^ with a rudimentary tritocone; superior molars subquadrate and premolars without mesostyles. No rudi- ments of osseous horns. Joseph Leidy, over 50 years ago, aptly characterized this animal specifically as "swamp or marsh loving" ijpaludosus) because its teeth are manifestly adapted to the softer kinds of herbage. By Leidy himself and by subsequent authors the term "paludosus" was erroneously applied to the more progressive species which are classified in this monograph under P. major, P. leidyi, and P. roiustus. Materials. — Many of the specimens other than the type which were referred to this species in the volumi- nous literature belong on higher geologic levels, such as those from upper Cottonwood Creek (Bridger B 4 and 5) and from Henrys Fork (Bridger C and D), and consequently to higher stages of evolution. P. paludosus in the restricted sense is represented by Leidy's isolated type teeth in the National Museum FiGUEE 272. — Skull of Palaeosyops major One-fourth natural size. Am. Mus. 12182 (neotype skull). Cottonwood Creek, Bridger Basin, Wyo.; Bridger formation, level B 3. Crushed downward but slightly reconstructed from Am. Mus. 1516 (P. leidyi), especially in the infraorbital region. Ai, Side view; As, top view. (No. 762 in part), which are carefully distinguished in Chapter III of this monograph. An upper dental series probably preserved in the Philadelphia Acad- emy collection was also referred by Leidy to P. palu- dosus (Leidy, 1873.1, pi. 4, fig. 3) and may possibly be conspecific with the type. In the same stage of evolution apparently is the fine lower jaw (Am. Mus. 11680) from MillersvUle, Bridger Basin, level B, which 320 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA may be regarded as a neotype. Agreeing well with this neotype lower jaw is the finely preserved upper dentition Am. Mus. 13032, also from Bridger B 1. Three other jaws (Am. Mus. 11692, 11711, from Grizzly Buttes, Bridger B 2, and 13118, from Moun- tain View, Bridger B 2, Grizzly Buttes) may also be referred to this very primitive species. The inferior molar, m^, of Leidy's original type, now taken as the lectotype, measuring (ap. by tr.) 33 by 20.5 millimeters, exhibits crescents with striated sides, very low antero-internal cusp, very rudimentary metastylid, prominent entoconid, with a distinctly de- veloped entostylid, also anterior and posterior cingula wanting, even between the valleys. The superior premolar type (Nat. Mus. 762, in part) consists of the ectoloph of p^ of the left side and of the inner half of the crown of p* of the right side, two teeth conjoined by wax. The accurate transverse measurement of the crown, therefore, can not be given; the ectoloph measures 19 millimeters anteroposteri- orly and 13 millimeters (estimated) from the base of the crown to the tip of the partially worn protocone; it consists of a prominent anterior style, a protocone, sharply convex externally, spreading into an external cingulum which surrounds the less convex tritocone, a cone which is slightly smaller than its fellow the pro- tocone. The inner half of the crown of p^ of the oppo- site side consists of a deuterocone, a faint ridge ex- tending anteriorly toward the protocone, measuring 16 millimeters anteroposteriorly; on either side are anterior and posterior cingula which rise gently to- ward the apex but do not tend to surround the smooth inner side of the protocone; this condition is exactly intermediate between that observed in L. laevidens and P. major. As remarked above, the association of these upper teeth with the lower is doubtful; the upper teeth may belong to LimnoTiyops. The second superior molar (Nat. Mus. 758) consists only of the anterior half of the tooth, estimated at 35 millimeters transversely; it exhibits a stout parastyle, a median rib opposite the paracone, a distinct and somewhat transversely elongated protoconule, a low, conical protocone, a low and slightly worn cingulum which rises at the antero- internal edge of the protocone. The anterior cingu- lum is incomplete at a point anterior to the tip of the paracone (cf. P. major, Am. Mus. 12182). The slopes of the cusps, like those of the molars of P. major, are vertically striated. The anterior crescent is consid- erably smaller, whereas in higher stages the two are subequal. Specific characters oj the types. — It is difficult to define this species clearly from the lectotype specimens which are described in detail above. Reference should be made to the very carefully prepared natural-size drawings of these teeth on Plates LII and LIX. The following is a provisional definition: Second inferior molar (lectotype) with distinct but depressed entoconid, and metastylid and entostylid folds; median ridges within the crescents; cingula not prominent; dimensions, anteroposterior, 33 millimeters; transverse, 20.5; superior premolars without trace of internal cingulum at base of deuterocone; subequal protocone and tritocone on p*, no mesostyle; superior molars with moderately developed cingula and proto- conule, moderately open external crescents. The superior teeth referred by Leidy to this species are the principal ones among those described by Leidy as P. paludosus which possibly belong to this primitive species. The figure copied herewith is taken from Leidy's memoir of 1873 (pi. 4, fig. 3). The specimen is recorded from Grizzly Buttes, Bridger Basin, Wyo., probably Bridger B 2. It is seen at once to belong to a small animal in a very simple stage of evolution. The premolar teeth are readily distinguished specifically, as shown in the figure, by the extremely simple charac- ter of the ectoloph of p^; it exhibits not even a rudiment of the tritocone, a cusp which is strongly developed in the geologically successive P. major. The measure- ments of the teeth as figured on Leidy's plate, said to be of natural size, are suspiciously small — namely, p'-m' 137 millimeters, p^-m^ 133 — and may indicate, if the drawing is correct, that the specimen belongs to a smaller form, such as P. longirostris. Oshorn's neotype and other referred material. — The neotype jaw (Am. Mus. 11680, fig. 268, B) is referred to the same species as the lectotype (see above) on account of the absolute similarity in form and size of the second inferior molar (m2) in the two specimens. Although small it belongs to a male animal, as indi- cated by the very deep and prominent chin and robust canines, which measure (ap. by tr.) 24 by 22 milli- meters at the base of the crown. The lower borders of the rami are especially thickened beneath the pre- molar series, the vertical depth of ramus in front of p2 being 60 millimeters and behind ms 81 or 84. While broad, the anterior face of the lower portion of the coronoid process lacks the concavity which distin- guishes P. major. The grinding series, P2-ni3, measures 152 millimeters as compared with 163 to 169 in P. major and 174 in P. rohustus. P2 is in a simple stage, not showing the distinct rudiments either of the paraconid or of the crescentic conformation of the hypoconid. In ps faint rudiments of the paraconid, of the metastylid, fold, and well-developed hypoconid crescents are seen. In p4 all these characters are strongly accented, but this tooth can not be described as molariform, as it still lacks the entoconid, the eleva- tion of the hypoconid, and the equalization of the two lobes. The outline form of this jaw is represented in Figures 267, 268, B, and the detailed characters of the teeth are shown in Plates LVI, LXII, Figure 268. Another jaw (Am. Mus. 11711) from the bluff above Mountain View, Bridger Basin, Wyo., Bridger forma- EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 321 tion, level B, has the same general characters but is of smaller size and unfortunately lacks the teeth. In a third, rather young jaw (Am. Mus. 11692) from Grizzly Buttes, level B 2, the molars are only a shade smaller than those of the type, with which they agree in most details; but this younger jaw is slightly more progressive in the structure of p4, of the paraconids, and of the metastylid folds in mi-ma. In a fourth jaw (Am. Mus. 13118), from B 2, the characters and measurements agree perfectly with those of the type and neotype. In a fifth jaw (Am. Mus. 12679), from B 3, the dental measurements are slightly smaller than in the neotype. The finely preserved upper teeth (Am. Mus. 13032) fit exactly with the neotype jaw. These upper teeth represent one of the oldest (level B 1) and certainly most primitive Bridger titanotheres known. They differ from the type of L. priscus in the following re- spects: 1. They are from a lower level, very low in B 1 (Granger). 2. They are larger. 3. The premolars are far more primitive — the most primitive known, in fact, among Bridger titanotheres. 4. The hypocone-cingule on m^ is imperfect. They agree well with the neotype lower jaw of P. paludosus in the following respects : 1 . They are from the same general level (B 1 ) . 2. They correspond in general size. 3. They show exact fitting of upper teeth (Am. Mus. 13032) with lower teeth (neotype of P. paludosus, Am. Mus. 11680) — that is, certain measurements be- tween cusps in the upper jaw agree with corresponding measurements between interspaces and valleys of the lower teeth. The accuracy of this correspondence is highly significant. 4. They show correspondingly backward develop- ment in the upper premolars of No. 13032 and of the lower premolars in the neotype of P. paludosus. 5. The canines are large and rounded and are charac- teristic of the genus Palaeosyops. In short, this specimen. No. 13032, appears to fill the great want of an upper dentition of P. paludosus. The presence of a cingule-hypocone on m^ does not neces- sarily excluded it from Palaeosyops, because this cusp is more or less variable, a fact shown by its absence in L. monoconus and its presence in P. diaconus ( = rolustus). Specific cTiaracters of the neotype and other referred specimens. — Although the first of the Eocene titano- theres to be discovered and constituting the classic type of the genus Palaeosyops, and also the oldest in point of evolution, this primitive species is still im- perfectly known because of the rarity of the speci- mens on this low geologic level. A vast amount of confusion has attended the previous description of P. paludosus. We are now for the first time enabled to characterize it sharply as a stage in which the second and third superior and inferior premolars are ex- tremely simple in point of cusp evolution. Referring the reader to the previous systematic discussion in Chapter III, we may here summarize our knowledge of the neotype and the referred specimens. The lower jaw (Am. Mus. 11680) is taken as the neotype, in which m2 agrees exactly with that of the type. This jaw exhibits the following specific characters: P2-m3, 151 millimeters; p2 extremely simple, with faint trace of paraconid fold and noncrescentic hypoconid; ps with rudimentary paraconid, metastylid fold, and crescentic hypoconid ; p4 with very decided paraconid, elevated metaconid, distinct metastylid fold, broadly crescentic hypoconid, extremely rudimentary ento- conid; m2 closely agreeing in form and measurement with that of the type; ms with a narrow, subcrescentic hypoconulid, median in position. The following measurements of two specimens re- ferred to Palaeosyops paludosus should be compared with the table of measurements on page 316: Measurements of Palaeosyops paludosus Upper teeth, Am. Mus. 13032, Bridger B 1: Millimeters Pi-m3 102 P2-m3 144 P>-p< 71 M>-m3 : 91 PS ap. by tr 18X22 Ml, ap. by tr 25X26 MS, ap. by tr 32X34 Neotype lower jaw, Am. Mus. 11680, Bridger B 1: Condyle to symphysis (estimated) 340 Length of symphysis (estimated) 85 Depth of ramus behind ms 85 Condyle to angle 165 Pi-m3 (estimated) 169 Mi-ms 98 PS ap. by tr 17X12 Ml, ap. by tr 26X18 MS ap. by tr 43X23 Palaeosyops major Leidy Plates LVIII, LXII; text figures 89, 215, 268 C, 272, 279, 515, 516, 533-535, 546, 550, 686, 721, 741 [For original description and type reference see p. 158. For skeletal characters see p. 620] Type locality and geologic horizon. — Bridger Basin, Wyo. ; Bridger formation, levels B 2 to 4, Palaeosyops paludosus-Orohippus zone Leidy's type jaw is simply recorded from Grizzly Buttes, equivalent to Bridger B 2. The geologic range of this species, as exposed in the upper portions of Grizzly Buttes and the lower portions of the Cottonwood Creek section, covers Bridger B 2, B 3, and probably B 4. Specific characters. — Of intermediate to large size; total length of skull (estimated), 389 to 436 millime- ters; p2-m3, 164; p^-m', 147; lower premolars somewhat more progressive; superior premolars without meso- styles; p^ with two external cones — -that is, both pro- tocone and tritocone. No rudiments of osseous horns. Cephalic index 74 to 77. 322 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA In 1873 Leidy named this quadruped P. major, in reference to its larger size as compared with P. palu- dosus, but he was unable to characterize it fuUy. At least six rudimentary new characters, or recti- gradations, may now be observed in the grinding teeth alone of this mutation, or subspecific stage — so much for numerical change. The quantitative or propor- tional changes (allometrons) are equally significant of progressive evolution. Type and neotype. — Finding it impossible to define this species from Leidy's type specimen, which con- sists of an incomplete and abnormal fragment of a ramus only, a neotype jaw and skull were selected by Osborn from the same geologic level — namely, Am. Mus. 12181 and 12182, from middle Cottonwood Creek, level B 3 in the Bridger Basin. From these specimens discovered by the American Museum expe- ditions the species is defined as above. In continua- tion of the systematic description above, P. major may be clearly distinguished as an important early stage of evolution, successive to P. paludosus. Materials. — This species is represented by Leidy's pathologic and fragmentary type ramus (Philadelphia Acad. Nat. Sci. collection, fig. 89) and by six referred specimens in the American Museum from levels B 2 and B 3 of the Bridger Basin, as follows: A crushed but finely preserved skull (Am. Mus. 12182), probably belonging with the mandibular ramus (Am. Mus. 12181), both from Cottonwood Creek, Bridger Basin, level B 3; a stiU finer specimen (Am. Mus. 13116), from middle Cottonwood Creek, level Bridger B 3; a skuU (toothless) and excellent limb bones (Princeton Mus. 10276); an imperfect palate from Smiths Fork, probably from level B 3 of the Bridger Basin; lower jaws (Am. Mus. 12183) from middle Cottonwood Creek, level Bridger B 3; a fine set of lower teeth (Am. Mus. 12165) from the same locality, level Bridger B 2; fine jaws (Am. Mus. 5101), locality and level unknown. The measurements of the jaws indicated above agree approximately with the few measurements that may be taken from Leidy's very imperfect type, and there is consequently little doubt about the final identification of this species, which, like P. paludosus, has been confused with species belonging to higher stages and higher geologic levels. Specific cJiaracfers of type. — The only specific char- acters that can be drawn from the fragmentary type specimen (fig. 89) are the measurements of the lower jaw (97 mm. below ms) and the length of the inferior molar series (mi_3, 115 mm.). P. major can not, however, be distinguished specifi- cally by characters taken from the type, such as the length of the true molar series or the depth of the ramus behind ms, because specimens of Palaeosyops leidyi having the same measurements are found at higher levels. It must therefore be distinguished by the characters of the neotype specimens found at the same geologic level as the type. Characters of neotype jaw. — The jaw (Am. Mus. 12181, fig. 268, C), from Bridger level B 3, middle of Cottonwood Creek, is taken as a neotype, although it belongs to a young adult or a female and is somewhat less robust than the type. This jaw was found near the skull (Am. Mus. 12182) and may belong to it. It affords, however, the following distinctive characters as compared with the referred jaws of P. paludosus: Ps-ms 164 millimeters; p2 with rudimentary para- conid, metastylid fold, and subcrescentic hypoconid (characters all of which are wanting in P. paludosus) ; Pa with distinct paraconid, very rudimentary meta- conid, with metastylid fold subcrescentic and elevated hypoconid; p4 with broadly prominent meta^'.onid, metastylid fold, hypoconid low, broad, and relatively elevated. More in detail, in p2 we see a tooth slightly more progressive than that of P. paludosus in the following respects, as shown in Plate LXII: It exhibits a very rudimentary paraconid and rudimentary metastylid fold and the beginning of a hypoconid crescent, yet these rectigradations are in their very inception. In pa the paraconid, metastylid fold, and hypoconid crescents are accompanied by the ento- conid in its most rudimentary form. In p^ in the unworn condition (Am. Mus. 12165) we see a distinct entoconid and a relatively more elevated hypoconid than in P. paludosus. P. major is therefore dis- tinguished as a mutation or higher stage than P. paludosus by a number of rudimentary cusplets on P2_4 and by the general progress of these teeth toward the molar form. Similarly, in the molar teeth, paraconids, striations on the sides, ridges within the crescents, and festoonings of the external cingulum seem more clearly defined, as well as the entostylids. A very distinctive character also is the hollowing out or concavity of the base of the coronoid process behind m.3, not observed in P. paludosus; the free portion of the coronoid process is still quite high and recurved, not having assumed the triangular form seen in P. leidyi; the thickening of the lower borders of the rami now extends back below the first molar. The characters of the jaw of P. major are also exhibited in Am. Mus. 12183 and 5101. They are distinguished by the following principal features: (1) The decided curvature of the lower border; (2) the posterior thickening of the symphysis (ap. 89 to 103 mm.); (3) the depth of the ramus behuid ma (86 mm., cotype, female; 96, type, male; 97, Am. Mus. 12183); (4) the slight reduction of the free portion of the coronoid process; (5) the deep excavation of the ante- rior border at the base of the coronoid process. Characters of the neotype sTcuU. — The fine skull. Am. Mus. 12182 (figs. 272, 279), from level B 3 of the Bridger Basin, top of Grizzly Buttes, may also be taken as a neotype and possibly belongs with the neo- type jaw above described. It exhibits the following characters, which are well displayed in Plates LVIII, LXII: Superior teeth, p^-m', 147 millimeters; no meso- styles thus far observed on p^ very faint shadow rudiment of a mesostyle on p*, cingulum not completely EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 323 embracing inner sides of superior premolars, tritocone on p^ small, no external ciagulum ; ectolophs of molars with "wide-angle" or open crescents; parastyle but- tresses not very prominent; rudimentary external cingula opposite valleys; the protoconules very large, angulate; the metaconules small. The skull is that of a young adult; in an old adult the masseteric ridge below the zygoma would be more strongly developed. It exhibits clearly the convexity of the forehead, the abbreviated sagittal crest, the broad, low occiput, the separation of postglenoid and post-tympanic processes, the absence of even a rudi- ment of the froiitonasal horn; and these characters in connection with its inferior size indicate a decidedly lower stage of development than that of the skull of P. leidyi or P. rohustus. (See table on p. 316.) The superior cutting teeth are unknown. Superior premolar-molar series. — This series is well represented in the neotype skull (Am. Mus. 12182), possibly also in Am. Mus. 5105, Cope's paratype of P. laevidens, and in Princeton Mus. 10276. The molars are readily distinguished from those of P. leidyi and P. rohustus by their smaller dimensions. (See table, p. 316.) As pointed out above, the pre- molars are distinguished by the absence of well developed mesostyles in p^~^ and by the smaller size of the tritocone of p". Measurements of another specimen. — A bea-utifully preserved superior premolar-molar series (Am. Mus. 2361, Pis. LVIII, LXII), probably from level A of the Washakie Basin, is in a more advanced stage of pre- molar evolution than P. major and differs from the neotype of that species in having a larger tritocone on p^, a larger deuterocone and stronger "protoconule ridges" onp^~*, more prominent protoconule ridges on m'~^ and a well-marked metaconule ridge on m'. The specimen also differs in details from those referred to P. leidyi, P. rohustus, and P. copei, and it may repre- sent a new species or subspecies characteristic of Washakie A. Comparative measurements of this specimen are as follows: Comparative measurements of species of Palaeosyops, in milli- meters Palaeo- syops sp., Am, Mus. 2361 p. major P. copei?, Am. Mus. 13177 P. copei, Am. Mus. 11708 (type) Am. Mus. 6105 Am. Mus. 12182 pi-m^ 155 145 112 94 17 24 26 30 33 35 " 170 P2-m3 - --- -- -- ''US 112 95 17 24 26 "31 38 147 116 94 19 24 27 31 33 40 17 26 26 29 " 153 P2-m2 .- . - 118 M'-mS P^, anteroposterior P^, transverse M', anteroposterior 96 19 26 24 27 M^, anteroposterior M', transverse 34 40 Palaeosyops leidyi Osborn Plates XVI, XXVII, XLIV, L, LIII, LVI, LIX-LXII; te.xt figures 27, 28, 33, 118, 217-220, 227, 254-256, 273-283, 305, 482, 483, 485, 511-513, 520, 522, 523, 536, 537, 539, 540, 643, 546, 552, 559, 645, 661, 703, 713, 716, 727, 737, 741, 742, 745 (For original description and type references see p. 181. For skeletal characters see p. 620] Type locality and geologic horizon. — Henrys Fork, Bridger Basin, Wyo.; Bridger formation, Uinta- therium- Manteoceras- MesatirTiinus zone. Bridger levels C 2, C 3, C 4, and C 5?, as exposed on Henrys Fork in the Bridger Basin, are the geologic levels of this species, which is well above that of Palaeosyops major — ■ approximately 200 feet. Specific characters. — Of larger size; total length of skull 415 millimeters; p^-m', 158; P2-m3, 168. Diaste- mata behind canines. Posterior superior premolars with mesostyles. Barely defined swellings repre- senting the rudiments of osseous frontonasal horns. At least four new numerical characters, or rectigrada- tions, in the grinding teeth. Cephalic index, 74. This species is named in honor of Joseph Leidy, the founder of American vertebrate paleontology and first contributor to our knowledge of the titanotheres. P. leidyi is noteworthy as the earliest form to have the visible beginnings of horns (PL XVI, fig. 281). It is the only species of which the bony structure is known in nearly every part; this knowledge is obtained chiefly from materials collected by the American Museum expeditions under Dr. J. L. Wortman. The form of the occiput in the type skull (figs. 277, 279, Bi) is very exceptional and may be an individual variation. The premolars of P. leidyi are more advanced than those of P. major, but this and the incipient horn bosses are about the only decisive characters separating the two. The average size of the skull in P. leidyi was larger than in P. major, but exceptionally large individuals of P. major are actually larger than small individuals of P. leidyi (see table on p. 316); in fact, P. major and P. leidyi might by some systematists erroneously be regarded as successive mutations (in the sense used by Waagen) from P. paludosus rather than as species in the Linnaean sense, and this conception might be conveniently expressed by trinominal names, such as P. paludosus paludosus, P. paludosus major, and P. paludosus leidyi. In the opinion of the present author we should hold a heredity conception — that of germ evolution through the independent advance of a very considerable num- ber of single characters, including new rectigradations, such as cusplets on the teeth and horn bosses on the skull; and new proportions or quantitative characters (allometrons). Expressed in another way, P. leidyi succeeded P. paludosus after a vast interval of time, as indicated by the intervening 400 to 600 feet of sediment. In the long .series of generations that separated these stages new tendencies of character 324 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA and proportion, such as brachycephaly, were one by one added in heredity, so that the offspring of P. leidyi were born on a more specialized heredity plane than those of P. paludosus. P. leidyi was by no means the same animal; it was an enriched stock; it possessed in its germ substance a number of characters not found in its ancestor, and probably it lost some other germ characters. Materials. — This species is by far the most richly and abundantly represented in remains of skull, type of P. leidyi, a broad-topped skull with lower jaw from Henrys Fork, with a large part of the skeleton associated, level probably upper C. Also the following superior teeth: No. 12208, m'-m\ level C 4; No. 1552, c-m^ left, p^-m^ right, a small female, very progressive, level probably upper C; No. 12196, p^-m' of right side, level C 2; also No. 1565, milk premolars and m'-m^, from Henrys Fork, levelprobably C; in the Princeton Museum, skull and jaws. No. 10009, level probably upper C Figure 273. — Skull and head of Palaeosyops leidyi Restoration by Erwin S. Christman made under the direction of W. K. Gregory. tooth, and skeleton. The American Museum collec- tions contain the following principal specimens: No. 1516, a perfectly preserved female skull, level not definitely ascertained, probably C 4; No. 12185, a male skull transitional between P. leidyi and P. major, level C 3; No. 1581, a laterally crushed skull with a convex forehead and faint rudiments of the horns, associated with portions of the skeleton (possibly P. rohustus), level probably Bridger C; No. 1544, the The best specimen of a lower jaw is that associated with the type skull (No. 1544), certainly a male. There are also Am. Mus. 1585, 1522, 1564, 12200, all probably males; 12197, which is in the milk stage, corresponding closely in size with the upper milk teeth (1565); 5103, possibly a female; and 1549, a female; also Leidy's cotype of P. major (Acad. Nat. Sci. Philadelphia). None of the foregoing lower jaws are positively recorded as to level. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 325 Specific and age characters. — The materials enum- erated above exhibit a considerable range of measure- ment (see table on p. 316), as well as progressive development of all the mutational characters. The skull form in Am. Mus. 12185 is the most primi- tive of the series and in many respects takes a position intermediate between that of P. major and that of P. leidyi; on the other hand, the type skull (Am. Mus. 1544) has an extremely broad cranial roof and differs from all known specimens of Palaeosyops in this respect. In addition to these differences, which are due to actual progressive stages of development, there are apparent differences due to age. For ex- ample, as explained above, the rudimentary cusplets (such as the paraconid and metastylid), seen especially on the unworn premolar teeth of young individuals, disappear on the worn premolar teeth of old indi- viduals. Another very important age character is the faint frontal rugosity prophetic of a horn seen in old male skulls such as Am. Mus. 1581 but absent in all the younger male and female skulls. With these ex- ceptions P. leidyi may be provisionally characterized as follows: Specific cliaracters of P. leidyi hased on the type. — Premolars, especially in specimens from the upper levels, slightly more complex than in P. major; para- conid distinct and metaconid rudimentary on p2; metaconid distinct on ps; entoconid distinct on p4; tritocone larger on p^ becoming convex or ridged; p^ with mesostyle rudimentary or variable ; p^ with meso- style distinct, sometimes large. Superior molars with parastyle somewhat more prominent, ectoloph conse- quently more oblique, external cingula more distinct, all cingula heavier in specimens from the upper levels, valleys of the external crescents somewhat narrower; conules variable, often reduced, sometimes very large; metaconules lophoid on m' and m^, often much re- duced, sometimes very large on m'. In the skull, sagittal crest variable, sometimes narrow, in the type broadened into a fiat crest (figs. 276-279), occiput confluent with cranial roof superiorly (in type), post- tympanic and postglenoid approximated, almost touching, coronoid process of jaw broadly concave inferiorly. Incisors. — The superior series measures 70 milli- meters from side to side (Am. Mus. 1544); the incisor teeth increase in size from i' to i^, transversely measur- ing i' 11 millimeters, i^ 12, i^ 16. Similarly the inferior incisors increase slightly but regularly in height and breadth, the lateral teeth being either continuous with or slightly separated from the canine. There are su- perior diastemata between the grinding series and the canines, also between the lateral incisors and the canines. Canines. — The canines are more slender in females, measuring vertically 32 millimeters (Princeton Mus. 10009); the superior canines in this specimen measure 34 millimeters. In the males the canines are more robust, the fangs at the base measuring (ap. by tr.) 21 by 20 millimeters, and when unworn are very slightly recurved, sharply pointed, differing from those of Telmatherium in their circular section and strongly convex inner sides. In one specimen (Am. Mus. 1549) the posterior base of the crown is horizontally grooved, apparently as a re- sult of use of this tooth in uprooting plants or pidling down twigs. Superior grind- ing teeth in the type and other speci- mens. — P' exhibits diastemata both tn front and behind (seePls.LX,LXII), whereas in the more progressive speci- mens of P. rolusfus the diastema be- hind p^ is closed; p' is occasionally large (Am. Mus. 1552, 5102). The succeeding premo- lars are distin- guished by sharply convex protocones, flattened or very slightly convex tritocones, internal cingula variable, mesostyle wanting on p', variable, often very distinct on p^ ; external cin- gula of the pre- molar and molar ectolophs are vari- able but especially strong in progres- sive specimens; in figure 274.— Incisors and canines of general, more Limnohyops and Palaeosyops sharply marked One-half natural size. A, L. priscus, Am. Mus. than in P maior ^'"^^ (type); Orizzly Buttes, west Brldger Basin, . '. ^ ' Wyo.; Bridger formation, lower beds. B, P. It IS a Strikmg fact Iddyi, Am. Mus. 1616; Sage Creek, Bridger Basin, that in all the tvpi- '^' ^' '"'''''' ^™- ^^''^- IS*^ (type); Henrys Fork, , . Bridger Bafin; probably Bridger upper C or D. cai specimens (Am. Mus. 1544, 1516; Princeton Mus. 10009) refer- red to this species the metaconules are wanting, while the protoconules are quite distinct. M^ is a large tooth but still inferior in measurement and especially in de- velopment of the parastyle to that of P. rohustus. The series p^-m^ measures from 150 to 159 millimeters, as compared with 145 to 147 in P. major and 163 to 170 in P. rohustus. 326 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Injerior grinding teeth. — The most distinctive char- acters of the lower grinding teeth (PI. LVI) are the prominence of the paraconid, the distinctness of the metaconid, and the crescentic form of the hypoconid on P2 as compared with that of P. major; in more progressive specimens of P. leidyi p2 is almost as pro- gressive as p3 in P. major. Similarly, ps and p^ are gressive than those of P. major, and several stages are represented in the five skulls described below. Some of these stages belong to animals related to the type of P. leidyi; others are intermediate between P. leidyi, P. major, and P. rolustus. Of para- mount interest is the origin of the osseous horns. First stage: Transitional skull (Am. Mus. 12185) from level Bridger C 3, found at the mouth of Summers Dry Creek, appears to be the most primitive in its dentition, ps lacldng the mesostyle, p^ comparatively primitive, and the premolar-molar series measuring 162 millimeters, yet the sagittal crest is much broader (17 mm. at narrowest part) than in the P. major hypo type skull; the zygomata are more massive and widely expanded, and the measurements through- out are more robust. In this specimen the protoconules are greatly reduced and the metaconules are small and lophoid. It is apparently a male, the canines meas- uring 36 millimeters vertically and 20 across the base of the crown. The molar crescents are of the "wide-angle" type seen in the hypotype of P. major. Second stage: A higher stage is repre- sented by the female skull Princeton Mus. 10009, in which as a progressive feature a very distinct mesostyle is observed on p* and a rudimentary mesostyle on p^; p^-m' estimated at 150 millimeters. The supe- rior molars exhibit very distinct protoco- nules but no metaconules. The lower jaw exhibits the more oblique chin of the female type, and the mandibular ramus meas- ures 87 millimeters behind nis, in which the hypoconulid is progressively conic in form. In this specimen, however, m2 is very simple. Third stage : The third stage exhibits the horn swellings without rugosity. It is rep- resented by the finely preserved, apparently female skull Am. Mus. 1516 (fig. 275), in which the zygomata are moderately ex- panded and the sagittal crest has a mini- mum breadth of 13 millimeters. More in detail, the superior aspect represents FiGUEE 275. — Skull of Palaeosyops leidyi an adult but not aged animal. Although One-fourth natural size. Am. Mus. 1516; Sage Creek, Bridger Basin, Wyo.; level probably Bridger practicaUy of the Same size in itS length (415 mm.) it is somewhat narrower (275 mm. as against 310) than the type (Am. Mus. 1544) and ap- pears to be in a stage only slightly advanced beyond that of skull Princeton Mus. 10009, because the sagittal crest is just beginning to broaden out into the plane of the vertex, the vertex of the crest measuring 13 millimeters transversely. The supratemporal crests are very prominent, sharply overhanging the tempo- ral fossae and terminating anteriorly in prominent C or D. Ai, Side view, reversed (canines from Am. Mus. 12185J; Aj, top view. much more advanced than the corresponding teeth of P. major. In the true molars a marked feature is the more conic form of the hypoconulid of nis. All these specific characters, however, exhibit fluctuations either toward a more primitive or a more progressive type. Stages of evolution represented hy the sJculls. — All the characters of the cranium of P. leidyi are more pro- EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 327 \ postorbital processes. Between the orbits the skull is broadly convex. The nasals extend 175 millimeters anteroposteriorly; posterolaterally they send down a broad flange beneath the adjoining parts of the max- illaries, lacrimals, and frontals. This flange is not (as in Manteoceras, etc.) largely covered by the forward extension of the frontals but appears in side view as a V-shaped area lying between the frontals and the maxillaries, a feature which is very characteristic of Palaeosyops and Limnohyops. The maxillaries rise in front of this point «<^'5*?>. and arch over the maxillary notch. Another character- istic feature is that the nasals diminish toward the tips both in width and in the depth of the lateral decurvature. Still another feature is the broad en- trance of the maxillaries below the anterior portion of the zygomatic arch. Horn swellings: By far the most important feature of this skull is the lateral horn swelling (fig. 275) on each of the frontals behind the nasofrontal sutures, which are entirely smooth. They would certainly never have been observed if atten- tion had not been directed to this particular region of the skull by the distinct and rugose horn rudiments seen in a subsequent stage (P. rohustus). The type stage (muta- tion): A more progressive stage is the type cranium. Am. Mus. 1544. In this male skull the frontonasal horn swellings are so slight that they can barely be distinguished. This is a very important point because in the female skull just described the horn swellings are quite apparent. This fact, in connection with corroborative evidence in other phyla, tends to prove that in their inception the horns are not sexual characters. The supratemporal crests at the narrowest point are separated 36 milli- meters by the broadly plane vertex of the skull, which passes uninterruptedly into the occiput by a gentle curve, there being no definite supraoccipital border. Immediately behind the orbits the vertex measures 136 millimeters transversely; between the orbits, 119 101959— 29— VOL 1 24 transversely. The frontoparietal, intei'frontal, and frontonasal sutures are all closed by age. The nasals narrow from 76 millimeters at the broadest point posteriorly to 47 at the tips. In the palatal aspect we observe that the palate is relatively short and broad and not decidedly arched, the horizontal plates of the palatines being abbreviated. The postnarial space is relatively short and deep but less excavated than in Telmatherium ultimum. A relatively narrow bridge of bone (14 mm.) separates the foramen ovale from .pr-% r\ yf Figure 276. — Type skull of Palaeosyops leidyi Am. Mus. 1544; upper part of Bridger formation, level C or D. Drawings by R. Weber. One-sixth natural size. the foramen lacerum medium. The postglenoid proc- esses are transversely extended but less decidedly so than in T. ultimum. A very characteristic feature is the broad triangular plate formed by the basioccipital and basisphenoid, slightly keeled in the median line, with a prominent rugosity (insertion of rectus capitus and constrictor muscles) at the junction of the basi- occipital and basisphenoid and a very narrow bridge between the condylar foramen and foramen lacerum posterius. This is very different from the more elongate and laterally compressed region in Tel- 328 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA matJierium uitimum. The post-tympanic processes are broadly oval, and the paroccipital processes are more slender and acute than in T. uitimum. A dis- tinctive feature is the relatively sharp ridge con- stituting the inferior border of the malars from the point where the arch leaves the skull to their junction with the squamosals. The occipital view of the skull fu o (J^ ^car.ex.) Figure 277. — Type skull of Palaeosyo'ps leidyi Top view. One-fourth natural size. Am. Mus. 1544 (type) . Henrys Fork Bridger Basin, Wyo.; Bridger formation, level probably upper C or D. reveals a large mastoid foramen. This is also observed in LimnoJiyops laticeps and appears to be a distinctive feature of this series of crania. Unlike that of L. laticeps (figs. 261, 264) the summit of the occiput is not defined by a crest but passes directly into the vertex. In the lateral view, in addition to the features already mentioned, attention should be called (1) to the prominent antorbital knob or process on the lacrimals, to the abbreviation of the face, the space between the orbit and narial notch measuring but 70 millimeters; (2) to the absence of any horn rudi- ment; (3) to the convexity of the maxillaries as seen in side view; (4) to the abbreviation of the premaxil- laries; (5) to the semicircular section of the malars below the orbits, which passes into the deep and later- ally compressed section, with the inferior masseteric ridge beneath and behind the postorbital processes. The anterior and superior views of the cranium (figs. 277, 279) illustrate the characteristic rounded pre- maxillary symphysis and the abbreviation of these elements and the correlated rounding and depression of the maxillaries below the narial notch. Type lower j aw : The perfectly preserved j aw of this specimen (fig. 280, C) exhibits the massive chin of the male form. On the lower surface we see the short (24 mm.) digastric fossa, the massive thickening of the mandibular rami (35 mm.), now extending back below m2, the curvature of the lower border of the ramus, the increased depth (98 mm.) behind m^, the triangular form of the free portion of the coronoid process, the broadly transverse expansion of the base of the coronoid process behind nis, the transverse extension (74 mm.) of the condyles, the great elevation (175 mm.) of the condyles above the bottom of the angle, the extension of the angle backward to a less degree than in Manteoceras, also on the outer surface the subdivision of the masseteric fossa by a horizontal xictr \ -ms.per Figure 278. — Type skull of Palaeosyops leidyi Palatal view. One-fourth natural size. Am. Mus. 1544 (type). Henrys Fork, Bridger Basin, Wyo.; Bridger formation, level probably upper C or D. ridge extending forward and downward from the condyle. Dentition of the type: Diastemata appear between the opposite incisor series and behind both the incisors and the canines. P* is too much worn to show the mesostyle. On the molars the conules are almost completely worn ofi'. External cingula are well marked, especially on m'. In the lower jaw the EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 329 incisors exhibit a compact transverse series measuring 53 millimeters. In p, the paraconid is well marked. The worn grinding teeth, although perfectly preserved, in such a specimen as the type do not present the distinctive characters of the series; but they demon- strate most conclusively that every element in the crown finally comes into some degree of use in the comminution of food and therefore has an adaptive significance; the styles, the cingula, the cones, the crescents, the conules, all are distinctly worn and blunted either by opposing elements in the lower teeth or by attrition of the food. The crown in this stage of wear presents an ineffective grinding and cutting apparatus and serves little more than a crushing function, because the low crenulated cres- cents and cones exhibit none of the hypsodont tendency so characteristic of the Telmatherium series. Fifth stage: Mutation transitional to P. robustus: The male skull. Am. Mus. 1581 (represented in fig. 281), exhibits as its most interesting and important feature a rugosity and a very slight elevation of the frontals just behind their junction with the nasals, which represents the horn rudiment in this species in the incipient rugose stage. Viewed from above (fig. 28 1 , A2) this rugosity is seen to be very slightly convex — that is, it rises above the surrounding surface as an elevation involving the posterior border of the nasals and a portion of the nasofrontal suture. This horn is thus slightly posterior to the position which it occu- pies in the skull of the allied genus Telmatherium. In this skull the facial convexity is very prominent. Although m3 is well worn, the conules are persistent and very slightly affected by the wear. The meso- style on p*, originally present, has been worn away. Comparison with dentition of other forms: Tho measurements of several other maxillary series are given in the table on page 316. Among the large num- ber of specimens examined some (such as Princeton Mus. 10009) are smaller and are more primitive in structure, approaching P. major, whereas others, such as Am. Mus. 5102 (P. leidyi advanced, but no meso- style on p*) are larger and more progressive. In the former the ectoloph and styles of the premolars are less strongly developed; in the latter the ectoloph, styles, and cingula are very strongly developed. In Am. Mus. 12208, from C 4, the protoconules are pres- ent in the second and third molar teeth. In Am. Mus. 5102, an aberrant form, the third and fourth superior premolars are without mesostyles but at the same time exhibit a very marked tendency toward molari- zation and have strongly pinched ridges opposite the protocones and tritocones. This specimen is pro- gressive in the great prominence of the parastyle. Fluctuations and progressive characters in the in- ferior teeth: In the several fine jaws referred to this species (Am. Mus. 1585, 1564, 1546, 1549) we observe fluctuations or individual variations of mutative and specific characters. In P2 the paraconid is always dis- tinctly marked; in ps the protolophid and hypolophid crests are very well defined; in p4 the metaconid ap- pears in a rudimentary cuspule; in ms the hypocon- ulid varies from a rounded (progressive) to a more crescentric (primitive) form; No. 1549 exhibits the triangular form similar to that seen in the type jaw; in No. 1585 we observe the more recurved form seen in P. major. Figure 279. — Skulls of Palaeosyops major and P. leidyi One-fourth natural siie. A, P. major, Am. Mus. 12182 (neotype skull); middle Cottonwood Creek, Bridger Basin, Wyo.; Bridger formation, level B 3; occipital view, crushed downward. Bi, P. leidyi, Am. Mus. 1544 (type); Henrys Fork, Bridger Basin; Bridger, level probably upper C or D; occipital view. B2, The same, front view. Juvenile dentition and crania: In many ungulates the deciduous premolars are more molariform than their permanent successors, and this law is well illustrated in Palaeosyops, as in the milk dentition referred to P. leidyi (see below) dp^, dp', and especially dp"* are more molariform than their successors p^, p', p*. It seems not impossible that p^ in the titanotheres represents a persistent milk tooth, or dp^ In the milk dentition under discussion (Am. Mus. 1565) the 330 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA first premolar (dp^) is about as large as in the adult Princeton Mus. 10009. The large alveolus of the canine appears to leave no room for the formation of Stage with milk teeth and one permanent molar in use: This stage is represented by a maxilla (Am. Mus. 1565, PL LIX, figs. 282, 283), probably referable to Palaeosyops leidyi, and by a lower jaw (No. 12197, level C 2) belonging to another indi- vidual of the same size. In these specimens the length of the ramus is estimated at 300 millimeters. In the upper jaw two upper milk incisors and the milk canines are in place; dp^ is a simple, conical tooth, smaller than its successor p^; dp^ exhibits two ex- ternal crescents, a parastyle and mesostyle, and two internal cones, but the crown is oblong and not molariform; dp' exhibits the same elements more fullj'' developed, the crown small and more quadrate in form, small conules, submolarif orm ; dp* is entirely molariform, with protoconules and meta- conules and fully quadrate contours; m' is also in use. The juvenile jaw, Am. Mus. 12197, belong- ing to an animal of exactly the same size as that to which the above-described maxilla pertained, exhibits the following characters: Dp2 is of small size but similar in form and not more progressive than its permanent successor p^; dps and dp4 are in every respect fully molariform. Leidy's type of Palaeosyops Tiumilis: The single tooth from the "Dry Creek beds," Bridger formation, figured in Leidy's memoir of 1873 (1873.1, pi. 24, fig. 8) is the third superior milk molar (dp') of the left side of the jaw, belonging to an animal slightly inferior in size to P. leidyi. Its agreement in all essentials with dp' of the milk dentition, Am. Mus. 1565, above described, makes it clearly referable to the genus Palaeosyops, but geologic evidence for this reference is lacking, as its exact level is unrecorded; it might therefore belong to P. major, P. leidyi, or P. rohustus. Hence it seems best to regard P. Tiumilis as an indeterminate species. Conclusions. — (1) There is evidence in Palaeosyops of the presence of at least two upper milk incisors, one milk canine, and Figure 280. — Lower jaws of Palaeosyops leidyi three milk premolars on each side above, or One-fourth natural size. Ai, Am. Mus. 1546 (reversed); Bridger Basin, Wyo.; side view; condyle 10 rlppirliinnci tpptli nhnvo nil +ncrot>iAr' (9^ restored from Am. Mus. 6103, angle from Am. Mus. 1622. A,, The same, anterior view of section -^^ aeCmUOUS tCetU aDOVe ail tOgCtUer, {Z) through line b. Ai, The same, anterior view of section through line c, with coronoid and condyle, the first Upper and loWCr premolars (Pt) are B, Am. Mus. 16C4; Henrys Fork, Bridger Basin; upper Bridger; coronoid and chin supplied from . . i i • i • i Am. Mus. 1585, incisors and canine from Am. Mus. 1544. C, Am. Mus. 1644 (type); Henrys Simple, COniCal tCCth, whlch are retained Fork, Bridger Basin; Bridger, level probably upper C or D. i. Abnormal, extra premolar. • .1 rl 1 + a permanent successor, or p'. The evidence is still insufficient, however. In the Oligocene titanotheres p' may also be a persistent milk tooth (dp'), as it is in recent horses. dentition — that is, they are probably without predecessors, or milk teeth; (3) the second lower deciduous premolar (dp2) resembles in form its successor p2 except that the posterior lobe is better developed; (4) in correlation EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 331 with the submolariform shape of the corresponding- upper teeth, the third and fourth lower deciduous pre- molars (dps and dp4) are more molariform than their successors ps, p4, especially in having large posterior lobes with high entoconids, which latter are lacking in the permanent teeth; (5) the second and third upper deciduous premolars are quadricuspidate, not fully quadrate, and may be described as sub- molariform; (6) the fourth upper decid- uous premolar (dp^) is fully molariform. Palaeosyops robustus (Marsh) Plates LV, LVI, LVIII, LXII; text figures 94, 284-288, 508-511, 521-523, 542, 544-546, 571, 685, 714, 718, 724, 737, 741, 760 [For original description and type references see p. 161. For skeletal characters see p. 626] Type locality and geologic Jiorizon. — Bridger Basin, Wyo., especially the upper exposures of Henrys Fork; Bridger forma- tion, Vintaiherium - Manteoceras -Mesati- rJiinus zone (Bridger D). Specific cliaracters. — Of massive breadth and proportions; slightly more brachy- cephalic than P. major and P. leidyi; meas- urements (all estimated), total length of skull, 440 millimeters; basilar length, 440; zygomatic breadth, 340; p^-m^, 163-170; mesostyles variable on p^, more constant on p*; molars with oblique ectolophs; m^ enlarged, with prominent parastyle, ectoloph oblique, molar conules strong; distinctly rugose frontonasal horn swellings. This stage is less perfectly known than P. leidyi, our knowledge being confined to the structure of the cranium, of the superior dentition, of a few of the ver- tebrae and of numerous fragmentary limb bones. Materials. — The type maxillary teeth are in the Yale University collection (No. 11122). In the American Museum collec- tion the following are the principal speci- mens: No. 11683, Bridger level D 3, ca- nines to m^, progressive, close to P. robustus, type; No. 1580, crushed skull with rudi- mentary horns and associated skeletal fragments, from Henrys Fork; No. 1554, skull with rudimentary horns, same stage as type; No. 11678, Bridger level D 4, a broad skull with smaller teeth; No. 5106, Cope's type of Palaeosyops diaconus, from Henrys Fork, progressive; Princeton Mus. 10282b, maxillary from Henrys Fork. Also the fol- lowing less progressive forms: Am. Mus. 1584, palate and teeth; Am. Mus. 1552, palate and teeth, from Twin Buttes, level Bridger C or D; Am. Mus. 1558, also Twin Buttes, level Bridger C or D, palate with p^-m^; Am. Mus. 1586, i'-m^ unprogressive; and Am. Mus. 1590, fragments of lower jaw, mi_3. We still lack the complete lower jaw. General specific cliaracters of P. robustus. — The horn swellings, as compared with those of three of the skulls Figure 281. — Skulls of Palaeosyops leidyi and P. copei robustus) One-fourth natural size. Ai, P. leidyi, Am. IVlus. 1581; Henrys Fork Hill, Bridger Basin, Wyo.; Bridger formation, level probably upper C or D; old male skull, side view, showing rugose horn swelling (purposely emphasized in the drawing); skull straightened from Am. Mus. 1544. As, The same; top view of the region of the horn swelling. B, P. copeil, Am Mus. 12205a; Lone Tree, Henrys Fork, Bridger Basin; level Bridger D 1; top view of skull. described under P. leidyi, are here more strongly developed. The upper grinding series averages 6 millimeters longer than that of P. leidyi. The depth of the ramus behind ma is estimated at 98 millimeters. A metatrophic character is the relatively large size of m^ measuring 42 millimeters transversely in the type, exhibiting large conules and more distinct external 332 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA cingula. There seems to be considerable range of variation in the size of m^, as indicated below, so that its large size in the type may not be truly specific. P. major, 32 by 38 to 33 by 40 millimeters. P. leidyi, 34 by 39 to 36 by 38 millimeters. P. robustus, 33 by 37 to 37 by 42 millimeters (type). P. granger!, 38 by 43 millimeters. P. copei, 34 by 40 millimeters. The size of the conules is also variable, as would be expected in a character which was losing its hold in Figure 282. — Jaws and deciduous teeth of Palaeosyops leidyi? One-fourth natural size. Am. Mus. 1565; Bridger Basin, Wyo.; upper jaw. Am. Mus. 12197; Henrys Fork Hill, Bridger Basin; Bridger formation, level C 2; lower jaw. most titanotheres. The forward extension of the an- terior part of the masseter and deepening of the malar is a progressive metatrophy, as are also the closing of the diastema (postcanine), the closure of the ex- ternal auditory meatus, and the large size of the skull. A very constant brachycephalic character is the closing up of the postcanine diastemata, which are either reduced or wanting The sagittal crest is powerful but is differently formed from that of the type of P. leidyi, although not dis- similar to that of other P leidyi skulls. In addition to the specimens which exactly or very closely resemble Marsh's type, there are others which appear to oc- cupy an intermediate position between P. robustus and the older form P. leidyi of level C. Fluctuations. — The mesostyle and other premolar characters (Pis. LVIII, LXII) in these intermediate forms also show considerable fluctuation, but on the whole there is a recognizable metatrophic advance over P. leidyi. It appears that at each actual period of geologic time Palaeosyops would show a consider- able range of variation, partly individual, partly vari- etal. For example, large size appears as an excep- tional variation in a B level P. major (Am. Mus. 1.3116) and in the P. grangeri of Bridger C 1, while small size of grinding series appears as an exceptional character in the high level (Bridger D 3) P. copei and in the species of Palaeosyops from the Washakie Basin. Mutations. — Tliere seems to be considerable evi- dence for the view that these "transitional" dentitions and skulls bridge over the structural gap between P. leidyi and P. robustus; indeed, it would appear that this is clear. This view contradicts the idea expressed elsewhere that P. leidyi "stands apart and does not appear to form a connecting link between P. major and P. robustus"; but that statement applies only to the broadened occiput of the type of P. leidyi, and since other skulls with narrow crests make up the bulk of the species P. leidyi, too much should not be made of the exceptional condition in the type. It may well be that in one or two trifling characters P. robustus may be shown ultimately to be descended not from the true race of P. leidyi which lived at Henrys Fork Hill during Bridger C 3 time but from some other race of P. leidyi living to the north and perhaps during Bridger C 1 to 3 time. However, by such hairsplitting we obscure the grand evolution lesson that P. major, leidyi, and robustus form suc- cessive mutations which are very nearly if not quite in a direct line, which might perhaps have been desig- nated by trinomial names such as P. paludosus paludosus, P. paludosus major, P. paludosus leidyi, and P. paludosus robustus. Less progressive mutations, transitional from the P. leidyi stage. — Some of the less progressive forms are so much more primitive than the typical P. Figure 283. — Deciduous cheek teeth of Palaeosyops leidyi? Am. Mus. 1565. Natural size. The identity of mi, m' is positively established by comparison with the adult dentition. The deciduous molars dp^, dp', dp*, are more molariform than the teeth which succeed them, p2, ps, p<. In this speci- men the alveolus for the permanent canine lies closely appressed to p^ while p> has been forced out into association with the milk molars dp^, dp', dp*. Thus in this middle Eocene titanothere the relations of the milk and permanent teeth are the same as in the Oligocene titanotheres. robustus that they might be placed with equal exactness in P. leidyi. They are especially interesting biologi- cally in demonstrating the gradual inception of such specific characters as are seen in Palaeosyops robustus rather than the sudden saltation of this species out of its predecessors. In comparing the following four mutations we note especially the very gradual evolu- tion of the rectigradations — namely, of the premolar mesostyles — also the gradual atrophy of the conules. First mutation: Of these the least progressive is Am. Mus. 1586, consisting of a palate with full denti- EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 333 tion. There is a short diastema behind the canine; p' is of small size. The following measurements are much inferior to those of the type of P. roiustus: P* 23 millimeters, transverse; width of last molar 39, -Fragments of jaws of Palaeosyops One-fourth natural size. A, P. granger!, Am. Mus' 12189 (type); Twin Buttes, Bridger Basin Bridger formation, level CI. B, P. roSus/MS, Am- Mus. 1590; Bridger Basin, Wyo.; inner view. as in P. leidyi; small conules on the molars; a faint "shadow" rudiment of the mesostyle on p^. Second mutation: Am. Mus. 1558 also exhibits a faint "shadow" mesostyle rudiment on p^; a very strong mesostyle on p*. The following measurements are similar to those of P. leidyi: Width of p* 26 mil- limeters; width of m' 39. Small conules on the molars. P' is unusually large. Third mutation: Am. Mus. 1552 is the palate of a small female individual. Premolars and molars ex- hibit strong external cingula; mesostyle is rudimentary but distinct on p^, very strong on, p*. The measure- ments are, however, the same as in P. leidyi, namely, p*, transverse, 23 millimeters; m^ transverse, 39. Fourth mutation: Am. Mus. 1584 exhibits an advanced mesostyle on p^ and p''. The diastemata are closed up, as in the type of P. roiustus, and p* shows a slightly increased width (27 mm.) transversely. The molars exhibit very small conules and faint cingula. Detailed characters of the type and other progressive forms. — The dentition of the type specimen is fully described above and figured on Plates LVI, LVIII. In this specimen p* attains a width of 26 millimeters, and m^ a width of 41; the measurement of m^ as seen in its oblique diameter, measured from the para- style to the hypocone, is 52 as compared with 48 in P. leidyi and 56 in the type of P. grangeri. This diagonal expansion of m^ is characteristic of the spe- cies. The mesostyles of p^ and p* are worn off or possibly were not present in the type. The cingulum nearly closes in around the inner sides of the premolars. Both pro to- and metaconules on the molars are large. Stages similar to the type : Very close to the stage represented by the type is the dental series Am. Mus. 11683 (level Bridger D 3), measuring, p^-m^ 167 millimeters; width of m', 42; width of p"*, 27. The cingula are progressive on the inner side of p*; as in the type the postcanine diastema is very narrow, and, as observed above, the parastyle expansion is very marked. The masseteric ridge of the malar is very deep below the orbit. In a similarly advanced stage is Princeton Mus. 10282b, with heavy cingula and a large mesostyle on p*. Cope's type of Palaeosyops diaconus. Am. Mus. 5106 (Henrys Fork, Bridger Basin, Wyo., level Bridger D?), is also very progressive, with "shadow" mesostyle on p' (nearly worn off) and p^. The internal premolar cingula are nearly in contact on the inner sides of the deuterocones; similarly the cingula nearly embrace the protocones of the molars internally. The transverse measurement of p* is 26 millimeters. FiGtiRE 285. — Skull of Palaeosyops rohustas Top view. One-fourthnatural size. Am. Mus. 1S64; Henrys Fork, Bridger Basin, Wyo.; Bridger formation, probably level D. A-A, Section tine across born swelling (cf . fig. 210) . A pecuhar feature is the expanded metaconule of m^ which is unfortunately broken away in the posterior half. Cope believed that this metaconule represented a second internal cusp or hypocone, as in Limnohyops; 334 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA but by comparison with other specimens this cuspule is seen to be certainly a metaconule and not a true hypocone. A similar " pseudo-hypocone " condition of the meta- conule on m^ is also observed in a skull from Henrys Figure 286. — Hyperbrachycephalic old male skull robustus One-fourth natural size. Am. Mus. 1580; Henrys Fork, Bridger Basin, Wyo.; Bridger formation. conules, prominent parastyles, little or no postcanine diastema. (See fig. 286.) The close concurrence of measurements and pro- gressive and retrogressive characters in the above- mentioned specimens with those of the type of P. rohustus fully establishes this species as a dis- tinct stage of evolution. Jaws. — The jaws of this species are not yet fully known. There are portions of the rami of the young adult (Am. Mus. 1590) contain- ing mi_3, which measure 119 millimeters (esti- mated), as compared with 107 in P. leidyi. In these molars the cingulum is progressive and the paraconids are large. These teeth appear to correspond with the P. rohustus stage. Sicull of P. rohustus. — Three skulls are at- tributed to this species in the American Mu- seum collection, namely, Nos. 11678, 1554, and 1580. Horns. — The matter of chief interest is the structure of the osseous horn knobs. Kudi- mentary frontal horns are seen on Am. Mus. 1554 (fig. 285), which are even more prominent than in the transitional skull Am. Mus. 1581 attributed to P. leidyi. The position of the protuberances is on the frontals behind the nasal sutures; they are a little more posterior in position than the rugosities observed in P. leidyi, the center of the protuberances being 18 millimeters behind the suture and 59 milli- meters from the median or internasal suture. The convexity of the horn is a complete oval, approximately 29 millimeters in diameter in transverse and longitudinal sections (PI. XVI). The frontal bones are thickened and more can- cellous beneath the horn. In a very aged and robust skull. Am. Mus. 1580 Palaeosyops probably level D. Skull crushed downward. Shows horn swelling (?i) and extreme rugosity, fr- 286) which is COVCrcd with CXOStOSCS the horn Fork (Am. Mus. 11678), level Bridger D 4; this tooth, like that in Cope's type of P. diaconus, entirely lacks the true hypocone. Although this skull is a male its molar-premolar series is relatively short, the three true molars measuring only 94 millimeters, as compared with 101 in the type of P. rohustus. Mesostyles on p'"^ if originally present are worn away. Progressive features are the marked external cingula of the molars. This important skull is illustrated in Figure 287. A skull that exhibits rudimentary horns (Am. Mus. 1554) shows in its detailed measurements (p^, trans- verse, 28 mm. (estimated); m^, transverse, 40; m', oblique, 52) close approximation to the type measure- ments. The mesostyle is absent on p", wanting or worn off on p^ (See fig. 285.) A more robust skull with horns (Am. Mus. 1580), Henrys Fork, level Bridger D?, exhibits measurements (p^ 28 mm.; m^ transverse, 41; m^ oblique, 52) which are very close to those of the type; also large knobs are more prominent and rugose and still more Figure 287. — Basicranial region of Palaeosyops rohustus One-fourth natural size. Am. Mus. 11678; Henrys Fork, Lone Tree, Bridger Basin, Wyo.; Bridger formation, level D 4. posterior in position than in the specimen above described, the center being 23 millimeters back of the frontonasal suture. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 335 Cranial evolution. — The breadth of the skull, even in the earlier stages (P. major), is the most conspicuous feature of the quantitative and differential evolution. The comparative measurements in the three succes- sive stages are as follows : Measurements of skull in species of Palaeosyops, in millimeters P. major (Am. Mus. 12182) P. major (Am. Mus. 13116) P. leidyi (Am. Mus. 1544) P. robustus (Am. Mus. 11678)__ Longitu- dinal Trans- verse 390 ''290 435 335 415 310 "440 °340 Assuming that these measurements are fairly repre- sentative, between P. major and P. rohustus we note little if any rise in the cranial index — that is, relative increase in breadth over length of skull. In the same period the grinding series (p--m') has increased in length from 10 to 15 per cent over that of P. major, or about as rapidly as the cranial length. Prominent features of the aged skull (Am. Mus. 1580) are (1) the width and power of the zygomatic arch, including the deepening of the forward extension of the masseteric insertion, which now has a depth of 62 millimeters below the orbits; (2) pterygoid wings of the alisphenoid are very heavy, for the insertion of the external pterygoid muscles opposing the temporals and masseters; (3) sagittal crest, while largely broken away, apparently broadened, as seen in the aged skull; in the younger skull (Am. Mus. 1554) still narrow (11 mm.); (4) occiput apparently broad and low, not confluent superiorly with the vertex of the cranium, as in the type of P. leidyi, resembling rather that of P. major, with the broadly flaring pillars above the condyles; (5) similarly paroccipital and post-tympanic processes suturally separate, as in P. major, and not closely conjoined, as in P. leidyi. Viewed from below the basioccipitals are sharply keeled, the keel bifurcating posteriorly into the occipital condyles, as in P. major, and dissimilar from the same region in the type of P. leidyi. While these advances upon P. leidyi are bridged over by several other skulls, yet they are all metatrophic and thus significant. These characters (assuming them to be specific and not merely individual) would seem at first to indicate that P. rohustus is to be regarded as a successor of P. major rather than of P. leidyi, and that in the intermediate levels (Bridger C) we should look for the species contemporaneous with P. leidyi but directly intermediate between P. major and P. robustus. In this connection, however, we should bear in mind the apparent variability in metatrophic characters which is displayed in the skulls referred to P. leidyi (see also remarks under "Mutations, " above). Nasals. — Skull Am. Mus. 1510 exhibits the nasals (fig. 288) robust and tapering anteriorly, strongly arching from side to side and anteriorly, extremely solid in section. Palaeosyops granger! Osborn Plates LIX, LXII; text figures 119, 284 B [For original description and type references see p. 181] Type locality and geologic horizon. — Bridger forma- tion, UinfatJierium- Manteoceras- MesatirJiinus zone, level Bridger C 1, is recorded as the geologic horizon of this species. The type is from Twin Buttes, Bridger Basin, Wyo., 200 feet below the "red stratum. " Specific characters. — Exceeding P. rohustus in certain dental proportions; p^-m^, 165 millimeters; fourth Figure 288. — Nasals of Palaeosyops rohustus One-fourth natural size. A, Yale Mus. 11122 (type). Bridger Basin, Wyo.; upper (?) part of Bridger formation. Bi, Am. Mus. 1510; Bridger Basin, top view. B2, The same, side view. superior premolar enlarged; molars with extremely prominent parastyles and oblique ectolophs. This species was named in honor of Walter Granger, associate curator of fossil mammals in the American Museum of Natural History, whose explorations have done so much to advance our knowledge of the Bridger titanotheres and of Bridger stratigraphy. The ani- mal seems to be a collateral rather than a main-line form, distinguished by several peculiarities of its grinding teeth. Materials. — The only specimen known is the type, consisting of a palate and grinding teeth, with por- tions of the jaw and skull (Am. Mus. 12189). General specific characters. — The species appears to be collateral to the stage represented by P. rohustus. It comes from Bridger C and was found 200 feet below the "red stratum"; there is no exposure of Bridger D at Twin Buttes. It is more progressive than P. rohustus, chiefly in its enormous size, for the premolars (p^~^) are quite backward in development, in both the ectoloph and internal border. 336 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA P. grangeri may be a descendant of some large strain of P. major, such as Am. Mus. 13116; in fact, very projecting parastyles are seen in Am. Mus. 12185, transitional between the P. leidyi and the P. major stage, and also in Am. Mus. 12680, from B 5, referred provisionally to P. paludosus. The species is especially distinguished by the extremely prominent parastyles of the niolar teeth, which result in the very oblique direction of the ectoloph. The principal measurements are, p'-m^ 180 millimeters, breadth of p* 31, of m^ 43, oblique or diagonal measurement of m' 57. The grinding teeth form a continuous series behind the large and laterally compressed canines. The single incisor preserved exhibits a subcaniform crown, 20 millimeters in height; the mesostyles are not observable on p^ or p'' but were possibly present (though small) in the unworn condition. The molars exhibit sharply defined median ridges in the valleys of the ectolophs. Strong development of the cingulum, which surrounds the entire crown excepting only the inner sides of the protocones, is a very characteristic feature. The conules are also well developed but relatively less than in typical specimens of P. roiustus. In p^ the internal cingulum is complete though faint. The fragment of the lower jaw which has been preserved (fig. 284) indicates that the thickening of the lower border which we have traced in successive stages from P. major is now carried back below m.3. The ramus measures 86 millimeters below m^; mi_3 estimated at 126 millimeters. The anterior lobe of nis measures 31 millimeters transversely. This tooth has strongly striated sides, festooned external cingula, and strong entoconules. Palaeosyops copei Osborn Plate LX; text figures 120, 266, 267, 281, 484, 511-514, 519, 543, 547-550, 724 For original description and type references see p. 181. For skeletal characters see p. 629] Type locality and geologic Tiorizon. — Henrys Fork, Lone Tree, Bridger Basin, Wyo.; Bridger formation, Vintatherium- Manteoceras- MesatirM.nus zone, level Bridger D 3. Probably also from level A of Washakie Basin, Wyo. Specific characters. — Tooth row of somewhat smaller size. The most progressive species of Palaeosyops known in superior premolar and molar evolution. Heavy cingula embracing the inner sides of the crowns. P*, p', p^ very advanced, with subquadrate contours and subequal protocones and tritocones, in- cipient tetartocones on p*, and distinct tetartocone constriction on p^ (rectigradations). This little-known animal represents a most ad- vanced stage. It is, so far as known, the terminal stage of Palaeosyops evolution. In view of its pro- gressive character this species is appropriately named in honor of Edward D. Cope, one of the founders of American vertebrate paleontology and the describer of Lambdotheriura, "Palaeosyops" horealis, and other species of Eocene titanotheres. Materials. — This species is positively known only from the American Museum series of superior grinding teeth (No. 11708) from Lone Tree, Henrys Fork, Bridger level D 3, including the premolars and molars of opposite sides (PL LX; fig. 120). Detailed meas- urements are given above. As shown in Plate LX this is by far the most specialized or advanced of the species of Palaeosyops in respect to the molarization of the premolars. It shows the following features: (1) The cingida are carried broadly around the inner sides of p'""*, a character approached but not so fully attained in any of the previous stages of the evolution of the premolar teeth of this genus; (2) a rudimentary tetartocone is present on p^, as indicated by a con- striction of the deuterocone to form this cusp, very apparent on the outer side of the deuterocone and less strongly marked on the inner side; (3) the decided convexity of the protocone and tritocone ridges of the ectoloph approaches that of some of the uppermost Eocene titanotheres and is quite different from that in P. rohustus or P. leidyi; (4) p^ is a very progressive elongate tooth (17 mm. as compared with 12 in P. leidyi), with rudimentary deuterocone; (5) the molar cingula are very broad and heavy, continuous around the inner side of the protocone in m^; (6) the inner side of all the premolars is more filled out, more sub- quadrate. P. copei is also very probably represented in Washa- kie A by Am. Mus. 13177, a very aged skull, in which the teeth, so far as preserved, closely resemble those of the type but are a little larger. Portions of the skull indicate an animal about the size of the P. leidyi type, resembling the leidyi-rolustus group in its very convex forehead, nasals, and basicranial re- gion. The nasofrontal horn swelling was if anything more pronounced than in P. rohustus. The nasal sinus beneath the horn, so prominently developed in Oligocene titanotheres, was present. A specimen doubtfully referred to P. copei? is the young jaw (Am. Mus. 12205a, level Bridger D 1) that belongs with the cranium and skeleton described on page 629. The associated top of the cranium (fig. 281) is almost certainly that of a Palaeosyops, but the specific reference is uncertain. In the jaw of this specimen (fig. 266, B) the measurement from the angle to the incisive border is 340 millimeters; p2-m3, 172. The second and third incisors are approximately equal in size. The canine is comparatively small and probably indicates that this animal is a female. Close behind it is pi, followed by a narrow diastema (5 mm.). P2 and Pa are very narrow, simple teeth, but slightly more progressive than in Limnohyops prisons, the metaconid being quite distinctly formed on the inner side of ps. P4 is decidedly more progressive than that EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 337 of L. prisons, the posterior crest being well defined and the entoconid ridge being somewhat more decided. A larger and more progressive jaw, also of doubtful specific reference, is Am. Mus. 12201 (fig. 266, C), from Bridger level C 4. In this the measurement from the condyle to the incisive border is 365 milli- meters; p2-m3, 171. The paraconids are distinctly de- fined on p2 and pa. The teeth are otherwise very simple, although the posterior crescent (hypolophid) is deepened and slightly broadened. In p4 a distinct entoconid is seen. The true molars measure 111 milli- meters; the ramus behind ma, 94 millimeters. A jaw (Am. Mus. 12198) from Henrys Fork (level D 2) is in a slightly more advanced stage of evolution on the evidence presented in its inferior premolar teeth. Palaeosyops copei is the last known species of this palaeosyopine race. Its specialized condition and its reduction in size may indicate that it was declining and on the point of extinction. On the other hand, it may have migrated from this region. SUBFAMILY TEIMATHEEHNAE OSBORN Middle and upper Eocene titanotheres of larger size. Skull with elongated cranial and abbreviated facial region, mesaticephalic {Telmatherium) or sub-brachy- cephalic {Sthenodectes) . Basicranium abbreviated. Nasofrontal horns retarded in development. Denti- tion of macrodont type; large cingula; incisors heavy, sublanceolate; premolars progressively molariform; molars large, progressively subhypsodont. Geologic liorizon and geographic distriiution. — The genera are Telmatherium, mesaticephalic, of levels Bridger C 3 to Uinta C 1, and Sthenodectes, mesati- cephalic to brachycephalic, of level Uinta B 2. These are the least known of the middle and upper Eocene titanotheres. Remains are infrequently found, and the parts preserved are incomplete. The Bridger region was probably beyond the center of their favorite habitat. The telmatheres appear to have been the most elegant and graceful as well as the most progres- sive and active of the middle Eocene titanotheres; in this respect they correspond with the Menodontinae of the lower Oligocene. As shown in Figure 257 they appear suddenly in the upper Bridger (levels C and D) and extend up into Uinta C 1, which probably rep- resents the end of upper Eocene time. No known telmathere is directly related to the Oligocene Menodus, yet certain telmatherines may have given rise to the Oligocene offshoots known as the Menodontinae. The resemblances and differences between Telmatherium and Menodus are set forth below. Resemllances to contemporary titanotheres. — The tel- matheres may be regarded as intermediate in anatomy between the Limnohyops-Palaeosyops type and the Manteoceras type. The earliest species known, T. cultridens, exhibits certain resemblances to Manteo- ceras, others to Limnohyops. The skull as a whole is long; the basicranial region is short; the zygomata are moderately arched. A distinctive feature of the face is that the frontonasal horn swelling is feeble or wanting, perhaps because the horns were compensated for by the large, powerful canine tusks. These relatively dolichocephalic, subhypsodont, supposedly subdolichopodal, and subcursorial charac- ters doubtless indicate that the telmatheres frequented firmer ground and made longer excursions for harder kinds of food than did members of the Palaeosyops phylum. They were also probably more intelligent and alert. Since the rise of Telmatherium, Manteo- ceras, and other phyla possessing relatively long- crowned molars occurred simultaneously with the decline of the earlier group of Palaeosyops and Limnohyops, the general replacement of the latter by the former may be attributed to the mechanical superiority of their grinding teeth as well as to physio- graphic changes from forests and lakes to more open flood-plain country. Compensation for small horns hy large tusTcs. — The pronounced development of the canine tusks in the telmatheres indicates that they were probably com- bative and vigorous fighters; another respect in which they resemble the Oligocene menodonts. This development of the tusks may have compensated for the absence or retarded development of the osseous nasofrontal horns. In the earliest known species, T. cultridens, this horn rudiment was evidently represented by a slight nasofrontal convexity. The condition of the horns in T. validum is unlcnown, but even in a male of T. ultvmum the horn rudiment is only slightly developed (PL XVI), while the canine tusks and lateral incisors are both enlarged and tusldike (PI. LV). Two suhphyla. — There is evidence of an early division of the Telmatherium phylum into Telmathe- rium (mesaticephalic to dolichocephalic, index 60, incisors moderately large) and Sthenodectes (mesati- cephalic, index 63-65, incisors greatly enlarged). Geologic succession. — The telmatheres appear in Bridger C 3 and extend into Uinta C 1, through a vertical thickness of over 1,500 feet. Their span of life thus covers a very long period of geologic time. History of discovery. — As stated above, the Tel- matherium phylum is comparatively little known, probably because the known areas of deposition did not present a habitat favorable to these animals; they are very rare in the upper Bridger deposits; they are as yet unknown in the deposits of Washakie Basin; and only a few specimens have been found in the deposits of the Uinta Basin. The first remains of a member of the group to be dis- covered were the maxilla and superior teeth that Marsh described as Telmatherium validum in 1872. His type description was brief and was published without illustration; the geologic entry is simply the 338 TITANOTHBRES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Bridger formation, but the type specimen may have been found in Bridger D. A maxilla discovered by the Princeton expedition of 1877 in Bridger C or D was considered by Scott and Osborn to represent a new genus of animals, to which they gave the name Leurocephalus, the type being the species L. cultridens ; but in 1891 Earle pointed out that Leurocephalus is generically identical with Telmatherium. All the exploration of many subsequent years in the Bridger Basin has not revealed anything certainly similar to these types. In the upper deposits of the Uinta Basin (Uinta C or true Uinta formation) Peterson, of the American Museum expedition of 1894, secured a fine skull of a female specimen to which the name Telmatherium ultimum has been given; and in the same deposits was found the anterior portion of a male skull of the same species (fig. 297). These skulls fortunately throw a flood of light on the cranial structure of these animals, which were previously Ivnown only by upper and lower jaws. These animals reappear (Riggs, 1912.1) in the up- permost levels of horizon B 1 of the Uinta Basin (in the " Metarhinus sandstones"), and in the middle of horizon Uinta B 2 Douglass discovered in 1908 the type of T. incisivum to which Gregory (1912.1) gave the name Sthenodectes, a telmathere with very large incisor teeth. Finally a large jaw was found by Peterson in Uinta C which apparently represents the latest known member of this series, to which the name T. altidens has been given by Osborn. Irregular geologic distribution. — It is important to note that these animals appear simultaneously with the first species of Mesatirhinus and Manteoceras in the upper Bridger levels; that they have not been recorded thus far in the Washakie Basin levels nor in the lower levels of the Uinta Basin; and, finally, that they are sparsely found in the upper or true Uinta levels. As above intimated, this rarity of geo- logic distribution appears to prove that they dwelt apart or in another food region and rarely invaded the region inhabited by Manteoceras and Mesatirhinus. General structure and habits. — Little can be said re- garding the general proportions of these animals until the skeleton has been discovered. The known indi- viduals of T. cultridens somewhat exceed in size the largest tapirs, T. validum is somewhat larger, and T. ultimum of Uinta C is still larger, having a skull about 20 inches long. In divergent adaptation the telmatheres were probably swifter and of more grace- ful build than Palaeosyops and Limnohyops. The incisor, canine, and grinding teeth are much more elevated, sharp, and trenchant (hence the specific name T. cultridens) than in Palaeosyops and were reinforced with distinctly defined cingula. Phyletic affinities of the telmatheres. — The first ques- tion that arises is, Are these animals more closely related to Limnohyops, to Palaeosyops, or to Manteo- ceras? The answer in brief is that although they are somewhat intermediate in position they are related by most of their ancestral or hereditary characters to Limnohyops and Palaeosyops. This real ancestral affinity was long obscured by the general mesatice- phalic character and correlations of the different parts of the skull, jaws, and teeth in Telmatherium, which are the dominant distinguishing features of this animal. Affinities to the Palaeosyopinae. — The ancestral affin- ities of the telmatheres to the Palaeosyopinae are indi- cated (1) in the transversely subconvex contour above and in front of the orbit, correlated with the very retarded development of the frontonasal horns {T. ulti- mum); (2) in the subrectangular, rounded rather than shelf -like section of the malars below the orbits; (3) in the deep, laterally compressed form of the zygo- matic arches and the progressive development of a vertical flange {T. ultimum); (4) in the tusklike en- largement of the third or outer superior incisors; (5) in the rounded rather than angular posterior borders of the temporal fossae {T. ultimum); (6) in the ele- vated and rounded superior contours of the occiput (T. ultimum); (7) in the absence of distinct lateral occipital pillars above and on either side of the fora- men magnum {T. ultimum); (8) in the presence of two facets for the atlas just above the foramen magnum (T. ultimum); (9) in the shape of the base of the skull, which is sub-brachycephalic or mesaticephalic, perhaps the strongest indication of affinity with the Palaeosyopinae. The concurrence of these resem- blances in so many different parts of the skull is strong evidence of a community of descent; moreover, the species Limnohyops monoconus exhibits several char- acters that are seen also in Telmatherium — (1) the very high, rounded occiput, with thin sagittal crest; (2) a deep superior flange on the squamosal portion of the zygoma; (3) a similar, though less quadrate infra- orbital portion of the malar. Comparative indices in telmatheres and related species Cephalic Faciocephalic Molar Telmatherium ultimum Sthenodectes incisivus Manteoceras manteoceras.- Palaeosyops leidyi Palaeos3'ops major 60 63-65 60-66 70 74-75 " 49 " 48 49 46 26 "27 20-23 24-28 Influence of dolichocephaly. — It appears that the Telmatherium craniurri is a partly elongate or drawn out Limnohyops type of cranium, and that correlated with this incipient dolichocephaly are the beginnings of numerous familiar dolichocephalic characters; but this incipient dolichocephaly affects chiefly the facial and midcranial regions, while the base of the cranium EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 339 proper remains relatively short. In the teeth the incipient dolichocephaly appears in the following characters: (1) Incisors somewhat compressed, oppo- site pairs ranged in convergent series; (2) canines laterally compressed, or lanceolate, rather than rounded; (3) premolars and molars generally with elevated crowns somewhat compressed transversely, and with decidedly compressed crescents and sharply pointed cones; (4) conules reduced or vestigial; (5) first inferior premolars laterally compressed, with dia- stemata on either side; (6) molars laterally compressed. In the skull we first observe the elongate, deep, and narrow premaxillary symphysis and the corresponding form of the "median suture." This is the generic character originally pointed out by Marsh and em- phasized by Earle, in contrast with the shallow, rounded symphysis and median suture of Palaeosyops. The anterior aspect (fig. 295) of the symphysis is very characteristic of the species of this genus as compared with Palaeosyops and Mesatirhinus but is not greatly different from the Manteoceras type. The zygomata bend outward widely but not so much as in Palaeosyops; they are deeply extended vertically into flanges. The external auditory meatus remains widely open below. Distinctions from Manteoceras. — The distinctions from- Manteoceras are seen in a number of prominent characters in the Telmatherium series: (1) The horn rudiments are less prominent and the facial concavities less pronounced (T. ultimum); (2) the malar section below the orbits in T. cultridens is roundly angulate and in T. ultimum it is more rounded, approaching that in Palaeosyops, whereas in Manteoceras it is sharply angulate externally, foreshadowing the shelf- like flattening and rudiment of the infraorbital shelf which is so prominent a feature in MesatirJiinus and Dolicliorliinus; (3) the canines are elongate, laterally compressed, and lanceolate, while in Manteoceras they are suboval and incurved rather than vertical; (4) the lateral superior incisors of TelmatJierium (Pis. LV^ LXIV) rapidly increase in size, progressively becoming caniniform (T. validum, T. ultimum), but in Manteo- ceras the lateral incisors are moderately large and increase in size progressively, though the disparity between i^ and i^ is less marked than in T. cultridens: in DolicTiorTiinus they progressively diminish in size, but the lateral incisor, while the largest of the three, is both relatively and absolutely smaller than in Telmatherium and Manteoceras; (5) in Telmatherium ultimum the ectolophs of the superior premolars (PI. LXV) exhibit a very pronounced development of the cingulum, which rises in a festoon upon the protocone, producing an asymmetry of the outer face (a highly progressive character), whereas in Manteoceras the cingula are less pronounced and the protocones and tritocones are less subequal on the ectoloph; (6) in Telmatherium the deuterocones of the premolars are longitudinally compressed (Pis. LXIII, LXIV, LXV, fig. 291), with a tendency to a ridged apex, which becomes more marked in T. validum and very decided in T. ultimum, whereas in Manteoceras the deuterocones of the premolars are more oval or conical; (7) in Tel- matherium the mesostyles of the superior molars are sharply compressed (Pis. LXIV, LXV, fig. 292), the buttress rising into a horizontal ridge, which becomes a very decided character in T. validum and T. ultimum, while in Manteoceras the mesostyles are more robust and rounded; (8) in the members of both genera the conules tend rapidly to disappear owing to the lateral compression of the crown and the elongation of the ectoloph, but the ectolophs in Telmatherium seem to be even more elongate, progressive, and trenchant than in Manteoceras. There are, however, some peculiar features which distinguish the incipient dolichocephaly of this phyhun from the more pronounced dolichocephaly seen in the genera Mesatirhinus and Dolichorhinus — namely, the free nasals are relatively short; the sagittal crest is elongate and relatively persistent; the basicranial region is relatively abbreviate. These differences are consistent with the general law that dolichocephaly is a process of differential growth of different parts of the cranium, not all parts being elongated equally. Affinity to Manteoceras. — There are important fea- tures in which T. cultridens, from the upper Bridger, the earliest known member of this series, resembles the contemporary representatives of Manteoceras, as shown in a comparison of Figures 290 and 308. There appears to be a similar development of the nasofrontal horn rudiment and a somewhat similar concavity in front of the orbits, though unfortunately this region of the type of T. cultridens is fragmentary (fig. 290). A de- cided resemblance to Manteoceras and Limnohyops and distinction from Palaeosyops are seen in the form of the nasal bones, which in T. ultimum are elongate but with a short free portion which is laterally decurved and truncate instead of pointed distally (contrast Palaeosyops). With these exceptions the progressive affinities of Telmatherium to Manteoceras and Mesatirhinus appear to be adaptive and convergent characters rather than ancestral or genetic characters. It thus appears that the distinctions from Manteo- ceras outweigh the resemblances and that the resem- blances to Manteoceras and Mesatirhinus are in part attributable to parallel or convergent adaptation, in part to similarity of origin. '° Progressive and conservative or stationary characters. — It appears at present that the horn rudiments are not progressive in the telmatheres; they are found to be even less prominent in the Uinta T. ultimum than in the Bridger T. cultridens — a feature possibly com- " W. K. Gregory regards the species T. cultridens as linked by intermediate stages (Am. Mus. 12193, 12194) with M. manteoceras and as very closely related in all char- acters, a resemblance not due to convergence. T. cultridens, according to this view, is intermediate between the manteoceratine and the palaeosyopine divisions. 340 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA pensated for, as above noted, by the development of the tusks. As a second conservative character it is important to note that the first and the second lower premolar teeth exhibit in T. culfridens a distinctively high, laterally compressed, and secant character, and that vestiges of this character are conserved in the species T. altidens of Uinta C. The free or projecting portion of the nasals remains relatively short (T. ultimum) . All the other distinctive incipient dolichocephalic characters of Telmatherium appear to be progressive: (1) the lateral superior incisors are decidedly progres- sive, becoming elongate and tusklike; (2) the hypso- donty in the superior grinding teeth becomes more marked; (3) the posterior superior premolars (p^"*) acquire similar internal ridges longitudinally placed, which in T. ultimum tend to develop tetartocones; (4) there is a decided elongation of the postcanine diastema, culminating in the very long diastema of T. altidens; (5) there is a marked elongation of the third inferior molar (ms); (6) while the canines of T. ultimum are not relatively larger than those of T. cultridens, the canines of T. altidens of Uinta C are exceptionally large and show progressive development of this character; (7) the ectolophs of the superior premolars as seen in T. ultimum of Uinta C tend to develop symmetrical convexities of protocones and tritocones such as are characteristic of all Oligocene titanotheres. Sex cliaraders. — Differences in sex are indicated very markedly in the male and female specimens of T. ultimum in the inferior size of the canines in the female, and apparently also in the absence or faint development of the horn rudiments. Contrary to an earlier opinion of the author it now appears that even in their first development the horn swellings are less prominent and rugose in the female than in the male Eocene titanotheres. Resemhlaiices to the Oligocene Menodus. — There are many resemblances in Telmatherium ultimum to the characters of the Oligocene genus Menodus, as follows: (1) Middle region of the skull between the orbits and the postglenoid processes lengthened; (2) molar series enlarged absolutely and proportionally both in length and in breadth; (3) canines of sublauceolate form; (4) grinding teeth sharp and hypsodont; (5) post- temporal and occipital regions similar in their rounded and elevated form, also in the absence of the separate condylar pillars at the back of the occiput and in the presence of accessory articular facets above the fora- men magnum; (6) zygoma deep in section in both Telmatherium and Menodus, with a vertical flange; (7) coronoid process high and slender. Contrast with Menodus. — On the other hand, Tel- matherium appears to be excluded from the ancestry of Menodus by the wholly different trend of develop- ment of certain parts: (1) the elongation of the post- canine diastema seems to be a progressive feature culminating in T. ultimum, whereas in Menodus this diastema is much reduced; (2) the progressive increase in size of the incisors contrasts with the extremely vestigial condition of the incisors in Menodus; (3) all the Uinta Basin species of Telmatherium and Dolicho- rhinus are characterized by the great prominence of the incisor series and by sharp constriction of the face back of the enlarged canines, probably indicating grazing habits, and no species seems to fulfill all the ancestral conditions of any of the Oligocene titanotheres. It therefore can not be said that we now know any species of Telmatherium that would fill the ancestral characters of the Oligocene Menodus. Telmatherium Marsh (Leurocephalus Osborn, Scott, and Speir) Plates XVI, XLVI, LI, LV, LXIII-LXV; text figures 126, 127, 210, 219, 220, 223-226, 255, 256, 289-300, 508, 516, 588, 592, 593, 647, 717, 733, 735, 745 Generic characters. — Skull as a whole long, basi- cranial region short, zygomata spreading, with deep malar flanges; frontonasal horn swellings rudimentary or wanting. Incisors large, with heavy posterior cingulum; i^ very large and pointed; canines large, compressed, pointed, with sharp anterior and posterior borders; upper premolars relatively advanced, with complete internal cingulum; upper molars sub- hypsodont, relatively large, with rudimentary conules and slender parastyles and mesostyles, m^ without hypocone. These animals are without difficulty dis- tinguished from the species of Mesatirhinus and Dolichorhinus, but the earlier stages show certain resemblances to Manteoceras manteoceras. The known specific stages are as follows : T. cultridens, from Bridger ?C or D. The premolar- molar series measures 180 millimeters. The premolars are somewhat simpler in structure than in T. validum. The animal is inferior in size to T. validum and is of the same size as the smaller members of M. manteoceras. T. validum, Bridger D. The premolar-molar series measures 195 millimeters. This animal is represented by part of a male skull, the only specimen Icnown. The lateral superior incisor is more distinctly canini- form than in T. cultridens, and the premolars and molars are somewhat more progressive. T. ultimum, from Uinta C. The premolar-molar series measures 217 millimeters. The lateral superior incisors are greatly enlarged and caniniform; the superior premolars are progressive and have more symmetrically convex protocones and tritocones and well-defined external cingula. The second superior premolar is simpler than the third and fourth. T. altidens, from Uinta C (? lower levels). The inferior premolar-molar series measures 330 milli- meters. The anterior premolars are primitive and laterally compressed. The canines are exceptionally prominent, hence the name T. altidens. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 341 Measurements of skull and teeth of species of telmatheres, in ■millimeters Basal length of skull Zygomatic breadth of skull Length of dental series Pi-m3 P'-p< M'-m3 I', ap. by tr__ P, ap. by tr_. C, ap. by tr__ P*, ap. by tr_. M', ap. by tr_ IVP, ap. by tr_ 255 180 80 103 9X 8 MX 13 20X ? 21X26 29X29 36X39 270 79 113 UX 9 22X30 30X30 39X38 330 295 207 82 128 22X21 25X22 27X24 22X36 38X38 38X43 S5. as 300+ 305 218 89 130 14X14 19X18 25X35 38X40' 46X52' 229 95 137 14X14 22X20 2.5X 22 27X35 40X37 49X48 The history of this species has already been given. (See pp. 167-168.) Materials. — As noted above, the type (Princeton Mus. 10027) is from the Bridger Basin, Henrys Fork Hill, level C or D. It represents a smaller and considerably more primitive animal than T. validum, especially in the more incisiform character of the superior lateral incisor. Another specimen from Bridger C 2 referable to this species (Am. Mus. 12209) consists of p"*, m', and m^ Another specimen that is certainly referable to this species is a young lower jaw (Am. Mus. 1560), with ms not yet entirely exposed, recorded from Twin Buttes, level Bridger C or D. Another well-preserved lower jaw (Am. Mus. 12193), from Henrys Fork, level Bridger C 3, agrees closely with the type in the dentition. Am. Mus. 12685, which includes m\ with a deciduous molar, and an unerupted p-, from Sage Creek Spring, level Bridger C 3, may represent a primitive phase of this species. A lower jaw (Am. Mus. 12687) from Henrys Fork Hill, level Bridger D 3, is somewhat more progressive than the type in p2 and ps. Another specimen from the Bridger Basin (Am. Mus. 1546a), consists of pMn^, B "=^ C Figure 289. — Progressive hypsodonty of the molars in Telmaiherium Natural size. Posterior view of third left upper molar. A, T. cuUridens, upper Bridger (C or D) ; B, T. ralidum, Bridger D; C, T. uUimum, Uinta C (true Uinta) . The table illustrates (1) the marked increase in the size of the skull and dentition as we pass from T. cultridens of the upper Bridger to T. ultimum of Uinta C; (2) the relatively larger size, in the later stages, of the true molars as compared with the pre- molars; (3) the increase in both the length and the breadth of the molars. Telmatherium cultridens (Osborn, Scott, and Speir) (? Leurbcephalus cultridens) Plates LV, LXIII, LXV; text figures 101, 223-226, 289-293, 735 [For original description and type references see p. 168] Type locality and geologicliorizon. — Henrys Fork Hill, Bridger Basin, Wyo.; Bridger formation, level C or D. Also recorded from Bridger C 2 and probably other horizons, as described below. Specific characters. — P'-m', 180 millimeters. In males superior canines elongate (46 rnm.), laterally compressed; premolars less progressive than in T. validum. right and left, and the lower border of the orbit. It differs from the type in the more complete exter- nal cingulum on p*, but the malar closely resembles that of the type. This specimen also approaches M. manteoceras in some respects. A young lower jaw from the Washakie Basin (Am. Mus. 2356), with the milk molai's in place, is more advanced than the type in the characters of the permanent p2. Type of T. cultridens. — So little is preserved of the cranium of this type (Princeton Mus. 10027) that it can only be partially characterized. As shown in Figure 290 there is a slight concavity at the side of the face and distinct evidence of the existence of a nasofrontal horn rudiment. The premaxUlary in side view approaches the Manteoceras type but is dis- tinguished by the greater depth and by the emphasis of the dorsal symphyseal keel. It is more elongate and more angulate superiorly than the short, rounded premaxillary of Palaeosyops and is vertically deeper than in Mesatirhinus, Metarliinus, and Dolichorhinus. Its dimensions are, depth from symphyseal crest to 342 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA internal alveolar border 38 millimeters, length from anterior border to the anterior edge of the canine 45, extreme length 96. Behind the canine convexity the sides of the maxillaries are somewhat flat. The infraorbital foramen is well exposed on the side of the face, the distance from the antorbital border of the malar being 36 millimeters. Only the anterior portion of the malar is preserved, but the relations of the malar and maxillary are shown by close examination and comparison to have been about as they were in Manteoceras and Limnohyops — that is, the maxillary contributed an antorbital process and a long internal inferior sliver; there is no infraorbital shelf; imme- diately below the orbits the malars are gently convex on the outer surface and broadly flattened on the inferior surface, the vertical extent of the outer face pas. pci- nts. FiGUEB 290. — Upper jaw of Telmatherium cultridens One-half natural size. Princeton Mus. 10027 (type), reversed, showing the region of the horn swelling (A) and the overlap of the maxilla on the nasal. The fragment of the nasofrontal region here figured is said to be associated with this type- Upper part of Bridger formation, Bridger Basin, Wyo. being 27 millimeters, and the transverse extent of the slightly concave inferior face opposite m^ 23. The teeth in general are distinguished by the sharply defined, finely sculptured character of all their elements. Incisors. — In the type (Princeton Mus. 10027) the superior incisors have the typical generic character of the opposite sets, forming acutely convergent or V-shaped rather than gently convergent series; the incisors increase rapidly in size from i' to i^, the fangs measuring S, 10, and 14 millimeters, respectively. The antero-external faces of the crowns are readily distin- guished from those of Mesatirhinus megarlnnus by a sharp anterior ridge, wliich divides the crown into a flattened external portion, feebly convex and cingulate, and a narrow anterior portion, feebly concave. Simi- larly the postero-internal face is flattened, with a median basal ridge and a very prominent postero-internal cingulum, especially upon i^ 1^, which is less per- fectly preserved, is a large subcaniniform tooth, with a sloping and less prominent internal cingulum. The single infei'ior incisor preserved (is. Am. Mus. 1560) is, in contrast to its mate above, typically incisiform, with a more uniformly convex antero-external face, feebly cingulate, and a nearly plane postero-internal face, ^vith a median ridge and sessile postero-internal cingulum; the long axis of this tooth is oblique, the diameters being 14 by 11 millimeters. Canines. — In the type the superior canines have not fully emerged, the crown measuring 49 millimeters verti- cal, 24 anteroposterior, 22 transverse (estimated); the tusk has the true generic lanceolate or lateraUy com- pressed character, the anterior and posterior ridges being sharply defined and terminating in the piercing apex; the outer face is broadly convex; the inner is more nearly plane, with a convex median swelling. The inferior canines belonging to an animal of the same size (fig. 293; Am. Mus. 1560) are somewhat smaller (vertical 30 millimeters, anteroposterior 17, trans- verse 15), distinctly lanceo- late, with a sharply defined anterior ridge, which be- comes especially prominent and inflected near the base of the crown; the posterior ridge is much less sharply defined. Premolars. — The superior molar-premolar teeth in the type specimen (see PI. LXIII, fig. 291) have the laterally compressed, sharp- cusped, secant, and pierc- ing form so characteristic of T. validuin, the type of this genus; they are distinctly smaller — ISO milli- meters, as against 195 millimeters in T. validuin. The superior premolars (81 mm.) exhibit nearly complete internal cingula, excepting p^ also an exter- nal cingula except directly opposite the protocone sweUing. P' is a narrow tooth (ap. 15 mm., tr. 9), with sharply compressed ridges extending forward and backward from the protocone, a rudimentarj^ trito- cone, and well-defined but low anterior style. P^ is a subtriangular tooth; the deuterocone in this tooth is double, extending backward to the postero-internal portion of the crown. As seen externally (fig. 292) the protocones and tritocones of p^~* are of equal height, but the convexity of the protocone broadens characteristically at the base into an anterior and posterior cingulum; the tritocones present narrow vertical external ridges, which enable us to distinguish these teeth from the typically more flattened tritocones of Mesatirhinus mega.rhinus and Manteoceras manteo- ceras. In p' the ectoloph exhibits the same characters EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 343 and the same length as in p^ but p^ is relatively much broader, the length of the ectoloph being 21 millimeters and the breadth across the crown 22; the deuterocone is more median in position, elongate, and flattened internally, as in T. ultimum; this tooth also exhibits a low antero-external style. In p'' this style is still more prominent, the protocone and tritocone convexities of the ectoloph are more symmetrical, and the crown seen from above is more quadrangular, the deuterocone being sharp and slightly flat- tened internally. Of the inferior premolars pi (preserved in Am. Mus. 1560) is spaced, lying 13 milli- meters behind the canine and 3 millimeters in front of P2; it is laterally compressed (12 by 7 mm.), sharply pointed, and simply plano-convex in section, with a notice- able paraconid, or anterior style. P2 is elongate, laterally compressed, 22 by 10 millimeters (in Am. Mus. 1560), with an elevated protocone, rudimentary antero- internal cusp (=metaconid) and well- developed postero-external cusp (=hypo- conid), slightly concave internally. P3 ex- hibits more symmetry; the protoconid still being more elevated than the postero-external cusp (=hypoconid), the proportions of the crown being a shade larger in the type than in Am. Mus. 1560 (ap. 22 mm., tr. 14); the analogous to the metaconid; the anterior lobe (=tri- gonid) is much higher, however (15 mm.), than the posterior (12 mm.). Molars. — The superior molars (103 mm. in type) exhibit prominent external cingula feebly continuous P3 ^ ^p^ FiGUHE 291. — Upper and lower teeth of Telmatherium cuUridens, showing their mechanical relations One-half natural size. Princeton Mus. 10027 (type). A, Crown view; upper teeth (light line), with pattern of lower teeth (heavy line) projected upon them. B, Internal view of the same, showing the crushing action of the cones and conids. around the styles in m\ m^, and internal cingula nearly continuous on the inner sides of m^, m'. The conules are represented merely by a vestige in m'. The internal cones (protocones and hypocones) Figure 292. — Upper and lower teeth of Telmatherium cuUridens, interlocked Outer side view. One-half natural size. Princeton Mus. 10027 (type), reversed; upper part of Bridger formation (level C or D), Bridger Basin, Wyo. This view shows the shearing or cutting action of the grinders by the interaction of the superior and inferior crescents form is thus feebly molariform. In Am. Mus. 1560 the posterior half of ps, p4 is widely expanded transversely. P4 is submolariform (ap. 23 mm., tr. 15 in the type), with cusps analogous to the paraconid, entoconid, and metastylid of the molars, and a very prominent cusp 101959— 29— VOL 1 25 are sharply pointed and slightly more flattened in- ternally than externally. In m^ the unworn proto- cone and ectoloph measure in height 18 and 31 milli- meters, respectively, exhibiting incipient hypsodonty. They are sharply pointed and closely approximated. 344 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA the apices being only 12 millimeters apart. M' ex- hibits a rudimentary hypocone, also a rudimentary swelling of the anterior cingulum corresponding with the protostyle of T. ultimum and Menodus. The in- ferior molars constitute a long, narrow series (122 mm.) in the type specimen and exhibit distinctly defined paraconids as well as rudimentary metastj^- lids and entostylids. In the elongate nis (54 by 22 mm.) the apex of the hypoconulid is placed nearly in line with the outer wall, and this cone is deeply cres- centic within. Mechanical correlation oj upper and lower teeth. — The significance of these accessory cuspules as well as the mechanical relations of the upper and lower cusps is well demonstrated in the accompanying drawings (figs. 291, 292), which show that the metastylid be- low serves to press the food against the protocones and their ascending internal cingula ebove. The adaptive significance of these minute features in the comminution of the harder food which was probably selected by this species is thus clearly brought out. Figure 293. — Lower jaw of Telmatherium cuUridens One-fourth natural size. Am. Mus. 1560. Twin Buttes, Bridger Basin, Wyo.; upper part of Bridger formation (Bridger C or D). It is seen also that the entoconid and paraconid below press the food against the hypocones above, that the tip of the hypocone fits squarely into the antero-internal part of the basin of the trigonid, that the single deuterocones of the superior premolars fit into the posterior internal valleys (=talonids) of the inferior premolars, and that the elongate p^ above is effectively correlated with the elongate and secant P2 below. Millc dentition of fT. validum. — A pair of young jaws from the Washakie Basin (Am. Mus. 2356) exhibit dp2, dps, dp^, m', nr in place, with the true premolars as well as nis still deeply buried in the jaw. Each of the deciduous premolars is fully molariform, with sharply defined double crescents; precocious molarizatiou is, in fact, characteristic of mUk pre- molars of titanotheres in general. Dps measures (ap. by tr.) 19 by 11 millimeters, dp4 24 by 14. The enamel is vertically crenulate on the outer surface. An important fact is that this jaw is in a more ad- vanced stage of evolution than the type of T. cul- tridens, since the second permanent premolar (P2) has the talonid V much better developed, and it may therefore belong to T. validum. Lower jaw of Telmatherium cultridens. — The par- tially preserved type jaw (fig. 292) exhibits (1) two mental foramina, the second indistinctly shown, the larger and more anterior being below p^; (2) a gradual increase in depth from 58 millimeters behind p2 to 65 behind m2 and 76 behind nia, with a thickness of 20 millimeters below mo. A more perfectly preserved young jaw (Am. Mus. 1560, fig. 293) in which ms is not fully emerged exhibits a long (91 mm.) and rather shallow (26 mm.) symphysis and laterally compressed chin (47 mm.); the ramus exhibits two mental foramina and gradually increases in depth from 51 millimeters behind pa to 58 behind m2 (inside), the thickness being 19 millimeters below m2. All these measurements would naturally increase with advancing age. The depth of the angle below the condyle is 134 millimeters; the coronoid attains a free height of 49 millimeters and is regularly hooked or recurved from base to tip. The angle is thin but extended downward and backward very decidedly, as in Manteoceras manteoceras. Compari- son with Am. Mus. 12193 (Bridger C 3), which prob- ably belongs with this species, brings out the differ- ences due to age. In the younger jaw (Am. Mus. 1560) the chin and ramus are shallower, the whole ascending ramus narrower, the angle less depressed, the coronoid shorter, more delicate and recurved, less truncate at top, and with the lateral flange much less pronounced. Measurements of Am. Mus. 12193 are as follows: Pi-ms, 194 millimeters; pi_4, 68; mi_3, 125. Telmatherium validum Marsh Plate LXIV; text figures 93, 289 [For original description and type references see p. 160] Type locality and geologic horizon. — Bridger Basin, Wyo.; Bridger formation, Uintatherium-Manteoceras- Mesatirhinus zone, level Bridger D. Specific characters. — P'-m^, 195 millimeters. In males, superior canines large, elongate (55 mm.), lateral superior incisors subcaniniform; ectolophs of superior premolars with sharply cingulate ridges; pre- molars more progressive than in T. cultridens; trans- verse measurements of p'-m' greater than in T. cultri- dens. Frontonasal region unknown. The only teeth definitely known are those of the type in the Yale Museum (No. 11120), a male indi- vidual first characterized by Marsh in 1872 and fully discussed later by Earle. The animal is young, since the crown of the last molar is entirely unworn, and all the distinctive characters of the surfaces of the teeth are still sharply defined. As noted above in the description of Palaeosyops these surface characters disappear rapidly by the wearing action of the food. The type specimen represents a comparatively large and powerful animal. The canines, relatively more EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 345 prominent than in T. ultimum, suggest a possible affinity to T. altidens of Uinta C The lateral superior incisors are almost as distinctly caniniform as in T. ultimum. As compared with T. cultridens, distinctive characters are the more progressive rectigradations, seen principally in the premolar teeth, as enumerated below, also the greater width of the premolars. From M. manteoceras this animal is readily distinguished by its very long and less curved canines. A comparison of the detailed measurements of the teeth is given in the following table : Measurements of teeth in Telmatherium validum and T. cultri- dens, in millimeters T. validum, Yale Mus. 11120 (type) T. cultridens, Am. Mus. 1560 Antero- posterior Transverse Antero- posterior Transverse 26 18 24 18 23 34 39 42 13 IS 17 23 11 20 26 32 34 41 40 12 15 16 24 15 21 18 21 30 37 36 13 16 pl 9 P2 19 PS .. --- 22 p<_ 26 Ml M^ - 29 38 M3 . - __..__ 40 D 9 V 12 13 15 In T. validum the opposite superior incisor series converge slightly. I' has an angulate antero-exter- nal face and sharply defined postero-internal cingulum; i^ is a larger tooth, with a broad and sharply defined cingulum; while in i' we have a subcaniniform crown of very large size, with compressed anterior and posterior edges and somewhat less prominent internal cingulum, sloping downward and backward. The inferior in- cisors are not Icnown. The superior canine (55 mm.) is a powerful lance- shaped tooth, larger but of the same form as in T. cultridens, with a very convex antero-external and more plane postero-internal face, bounded by the sharply defined anterior and posterior ridges. The superior grinding series, including the spaced p', extends 195 millimeters anteroposteriorly, as com- pared with 180 in T. cultridens (both males). In general, the teeth are similar to those of T. cultridens, but besides the larger size we note the following pro- gressive features: (1) On p' the internal cingulum is well defined, with the rudimentary posterior cusp larger; (2) p^ is slightly broader and shorter (ap. 24 mm., tr. 20); (3) there are protoconules on p^"*. The protoconules on the true molars are the only variable or reversional characters. P' is much larger than in T. cultridens. It is separated by narrow intervals both from the. canine (9 mm.) and from the second premolar. In p^ be- sides the greater breadth we note the somewhat more anterior position of the deuterocone and the more sub- equal convexities of the protocone and tritocone on the ectoloph, although the base of the protocone is still much broader than that of the tritocone; p^ has a rudimentary protoconule but no suggestion of a tetartocone. In p' we have a more quadrangular crown with a more elevated ectoloph than in T. cul- tridens, and a more symmetrical development of the deuterocone and tritocone, although the former is still widely expanded at the base. In both p^ and p* the internal cingulum is slightly less complete than in T. cultridens. On p** there is a very striking ele- vation of the ectoloph accompanied by greater promi- nence of the antero-external style and greater sym- metry of the deuterocone and tritocone convexities. The molars represent a progression upon those of T. cultridens, with sharp prominent styles, serrate external cingula, elevated anterior cingula, pointed protocones and hypocones, which are somewhat flattened and vertically striated on their inner faces; m^ has a prominent and serrate posterior cingulum but no trace of a hypocone. Telmatherium ultimum Osborn Plates XVI, XLVI, LI, LV, LXV; text figures 126, 219, 223, 255, 256, 289, 294-298, 300, 508, 516, 592, 593, 647, 717, 745 [For original description and type references, see pp. 177, 184. For slteletal characters see p. 653] Type locality and geologic horizon. — White River, Uinta Basin, Utah; Uinta formation {Diplacodon- Protitanotherium-Epihippus zone, Uinta C 1). Specific characters. — Skull very large (basal length 510 mm.), zygomata arching (zygomatic breadth 300 mm., estimated). Incisors and canines large, i' very large, subcaniniform. P'-m^ 218 millimeters (type); premolars progressive; p^ p* with the two outer cusps subequal and externally convex; well-developed internal and nearly complete external cingula; very rudimentary tetartocone swellings; molars large (m'-m^, 129 mm.) and progressive. The discovery in Uinta C, by 0. A. Peterson, of a female skull and jaw (Am. Mus. 2060) and the ante- rior half of a male skull with well-preserved dentition (Am. Mus. 2004) representing this species was a most important one. The animal was at first sup- posed by Osborn to be a terminal member of the Palaeosyops series. Subsequently it was compared point by point with Marsh's type of Telmatherium validum and was found to exhibit the most striking resemblances in the dentition and those parts of the skull in which comparison could be made in both. Highly distinctive is the premaxillary symphysis, more deep and elongate than in Metarhinus, deeper than in the type and paratype of Manteoceras manteoceras, more abbreviate than in Dolichorhinus. The region of the malars below the orbits is also characteristic and similar to that of T. cultridens but shows a more decided depression for the anterior 346 TITAJSrOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA portion of the masseter, and the posterior end of the malar has a deep vertical flange. In dentition, as enumerated above, T. ultimum is directly progressive from T. validum. Figure 294. — Type skull and lower jaw of Telmatherium ultimum One-fifth natural size. Am. Mus. 2060, from Uinta C (true Uinta); White River, Uinta Basin, Utah Measurements of Telmatherium ultimum, in millimeters SkuU, basal length Skull, breadth across zygomata. Face, length Cranium proper, length Free nasals Dental series, total length i'-m^. Pi-m3 Pi-p^ Mi-m3 P, ap. by tr P, ap. by tr P, ap. by tr C, ap. by tr C, vertical P', ap. by tr P^, ap. by tr P^, ap. by tr PS ap. by tr Ml, ap. by tr M\ ap. by tr MS ap. by tr Am. Mus. 2060 (type); Uinta C 510 "300 270 "250 85 303 218 90 129 14X14 15X15 19X18 19X12 20X19 21X28 26X35 38X40 46X49 46X51 Am. Mus 2004 (paratype); Uinta C 320 229 95 137 15X14 15X17 22X20 25X22 44 19X12 20X22 23X28 27X35 40X37 49X44 50X48 568 In general comparison with Manteoceras the skull of this species of Telmatherium presents very pro- nounced differences: (1) the occiput differs widely in its height and rounded summit and in the presence of two large facets above the foramen magnum; (2) in front of this, on the vertex of the skull, is a relatively long, delicate sagittal crest without the characteristic pit of Manteoceras and lacking the over- hanging supratemporal ridges; (3) the nasals are laterally recurved and distally truncate, as in Manteo- ceras, but the free portion is relatively shorter; (4) the zygoma resembles that of the Palaeosyops or LimnoJiyops type — that is, it is without the infraorbital shelf — and especially parallels that of Palaeosyops in the de- velopment of a deep flange on the lower surface of the malars, which is an ad- vance on the M. manteoceras condition. The above table shows rather marked differences in proportions of the teeth between the type and paratype; the cheek teeth in the paratype are all relatively longer and narrower. As a whole the skull is mesaticephalic. Comparison of the outline dorsal and palatal views of Telmatherium ultimum and Manteoceras (figs. 296, 303) brings out a large number of very distinctive characters. The horn rudiments are so inconspicu- ous in both the male and female skulls that they were not observed by the author for a long time. In the female they may be said hardly to exist, and in the male (PI. XVI) they can be seen only by very close scrutiny. As above noted, it is difficult to say whether they are in a retrogressive or stationary condition. They are certainly far less progressive than in Manteoceras. Slcull of T. ultimum. — The cranium of this species is represented by the type, a superbly preserved female skull (Am. Mus. 2060), and by the anterior portion of the paratype, a male skull (Am. Mus. 2004), in which the youthful age is such that many of the sutures can be made out. The skuU of the type is laterally crushed in the anterior half, but the width across the zygomata has probably not been greatly lessened. The general proportions are mesaticephalic, the cranium being very much longer than that of P. leidyi but much less elongate and deeper vertically than that of D. hyogna- thus. The measurements are, length 510 millimeters, breadth 300 (estimated). It is readUy distinguished from all other crania by the combination of the follow- ing principal characters: (1) Nasals relatively short (free length 78 mm.), the lateral downward extensions being wholly covered by the maxUlaries; (2) prominent narrow sagittal crest; (3) greatly elevated occiput; (4) deeply extended malar and squamosal flanges of the zygomatic arch; (5) premaxillary symphysis ver- tically extended; (6) frontals with horn swellings rudi- mentary — that is, consisting of convexities so slight (paratype) that they are with difficulty observable. In palatal aspect the brachycranial proportions decidedly predominate over the dolichocranial in the basicranial region of the skull as shown in the following characters: (1) The shortness of the anteroposterior measurements (as from glenoid facet to mastoid proc- EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 347 ess, from foramen ovale to condyle 100 mm.), as compared with the transverse measurements (across zygomata, 300 mm., estimated; across postglenoid processes, 195; across mastoid processes, 180); (2) the upward slant of the basisphenoid; (3) the shortness of the distance (140 mm.) between the pterygo-alisphe- noid wing and the condyle; (4) the postglenoid and paroccipital processes greatly flattened or extended transversely with very moderate anteroposterior diam- eter. More in detail: The posterior nares open immedi- ately between m^ and m^, whereas in P. leidyi they as compared with 38 in D. Tiyognathus. Between the foramen lacerum medium and foramen lacerum posterius the basioccipital forms a prominent, later- ally compressed keel. The occipital condyles are comparatively slender and widely separate below the foramen magnum. This aspect of the skull illus- trates admirably (1) the broadly transverse extension of the articular facets for the condyle of the jaw, (2) the broadening of the postglenoid processes, (3) the separation of the postglenoid and post-tympanic, which is much wider than in the brachycephalic P. major but very much narrower than in the dolichoct - •p.ty.sq. ^'pgl.sj. Figure 295. — Type skull of Telamatherium uliimum One-fourth natural size. Am. Mus. 2060. White River, Uinta Basin, Utah; base of Uinta C, true Uinta formation. Ai, Side view. The depth of the sltull in the middle region and immediately in front of the orbit has been increased by lateral crushing. The double lines mark the plane of the sections in Figiu-e 255, Bi, Bj. Aj, Front view. As, Occipital view. open opposite the posterior half of m^, and in Oligocene titanotheres they often open opposite the posterior half of m'. The postnarial space is relatively deep, or ver- tically extended, and short anteroposteriorly ; the line of junction between the pterygoid wings of the alisphenoids and the palatines can not be clearly made out. Unlike those of M. megarJiinus or P. major the pterygoids and lateral wings of the alisphenoids descend abruptly. The foramen ovale is separated from the foramen lacerum medium by a bridge, 24 millimeters phalic D. hyognathus; (4) also the sharply produced downward flange of the posterior portion of the malar. The superior aspect of the skull (fig. 296) fails to give the actual shape of the nasals owing to the marked crushing at this point. The entire length of the nasals is 219 millimeters, as compared with 520, the entire length of the vertex. Horn rudiments. — In the type female skull there is no evidence of the existence of a horn swelling at the junction of the frontals and nasals. In the paratype 348 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA male skull (Am. Mus. 2004), however, there is a low swelling (PI. XVI) at the junction of the nasals, maxillaries, and frontals, at a point above and some- what in front of the anterior rim of the orbit, which betokens the presence of a horn rudiment in an even more incipient stage than that of M. manteoceras or D. hyognathus . The uncrushed skull was evidently rather broad between the orbits. The supratemporal crests are moderately defined anteriorly, but as they enter the ing over of the downward lateral extension of the nasals; (3) the wide interval (109 mm.) between the antorbital border and the narial notch; (4) the anterior extension of the malars below the orbit; (5) the clear definition of the lacrimals, partly external to and partly within the orbit; (6) the prominence of the postorbital processes of the frontals and malars re- spectively; (7) the gently rounded conformation of the malar below the orbit, which most nearly resembles that in T. cultridens; (8) the sharp downward or in- ap/iij^ Figure 296. — Type skull of Telamatherium ullimum One-fourth natural size. Am. Mus. 2060. White River, Uinta Basin, Utah; base of Uinta C, true Uinta formation. Ai, Palatal view; A2, top view. Lateral crushing has narrowed the frontal region and distorted the zygomata. parietals they become more sharply defined, leaving a shallow groove between the summits of the short sagittal crest. The superior border of the lateral occipital crest is rather delicate ; in fact, the entire skull is slender rather than broad and massive. The lateral aspect of the skull is distinguished by the following characters: (1) The relatively short (85 mm.) free portion of the nasals; (2) the elevation of the maxillaries on the sides of the face, somewhat as in M. megarJiinus and D. hyognathus, and the cover- ferior flange of the malars beneath their junction with the squamosals; (9) the moderate upward extension of the zygomatic squamosal bar; (10) the presence of a cranial depression at the point of junction between the parietals and the frontals, or above the mid-cranial region, the skull being gently arched upward in front of this point. The anterior aspect of the skull (fig. 295) exhibits the relatively deep premaxdlary symphysis as compared with that of P. leidyi, and the absence of the extremely EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 349 long and deep maxillary union so characteristic of D. hyognathus. The nasals are much less thickened and decurved at the sides than in D. hyognathus. This aspect of the skull also exhibits the depth of the zygoma, including the malar and squamosal portion, compared with the extreme shallowness of this arch in D. hyognathus. The occipital view of the skull is still more charac- teristic, owing to its great height (194 mm.) as com- pared with its breadth (137 mm.), also to the pres- ence of a pair of oval prominences on either side of the superior portion of the foramen magnum, as in Menodus giganteus. This view also illustrates the breadth of the paroccipital and postglenoid processes. The dentition is finely represented in the complete type skull (Am. Mus. 2060) and in a somewhat more progressive stage in the paratype, consisting of the anterior portion of a skull (Am. Mus. 2004). The incisors are superbly shown in Am. Mus. 2004 and 2060 (PL LV; figs. 294-297). The superior in- cisors in the type are pointed, decidedly cingulate or cupped posteriorly, and sharply convex anteriorly, and increase in size rapidly from i' to i'; the lateral incisor (i^) is more caniniform than incisiform; the crown of i' measures 15 millimeters vertically, and a slight ridge extends down the posterior face to the apex of the basal cingulum which branches on either side to form lateral depressions; the crown of i^ (measuring 19 mm. vertically) is slightly larger and of exactly similar form; in i^ the caniniform crown (measuring 28 mm. vertically) is distinguished on its postero-internal sur- face by a lanceolate face with sharply defined antero- internal and postero-external ridges, which sweep at the base into the low, broad cingulum, exactly as in the canine. In Am. Mus. 2004 the superior incisors are even larger and the posterior cingulum is more strongly accented. All the cutting teeth, both incisors and canines, bear a striking similarity to those of T. cultri- dens and T. validum — in fact, they are almost directly progressive upon them, the only difference being that the posterior angles and cingula are a little less sharply accented. The inferior incisors are not known. The superior tusks, which are completely preserved only in the paratype (Am. Mus. 2004), are much more decidedly of the lanceolate, typical Telmatherium type than those of M. manteoceras, although the anterior and posterior ridges are not quite so promi- nent and sharply defined as in T. cultridens, T. validum, or D. hyognathus; the tusks are none the less long (43 mm.) and transversely narrower (22 mm.) at the base of the crown than those of M. manteoceras (25 mm.); the ridges pass inferiorly into a strong postero-internal cingulum, which also clearly dis- tinguishes these tusks from those of other species so far as observed. The superior molar-premolar series as a whole is not only larger (229 mm. in Am. Mus. 2004) but has a very marked individuality throughout, so that every tooth in the series can be distinguished by careful observation from those of either M. manteoceras or D. hyognathus. The type is distinctly telmatherioid, exhibiting peculiar pro- gressive modifications upon the dental type of T. cultridens and T. validum which partly anticipate those seen in Menodus. The series in the type (Am. Mus. 2060) is of somewhat smaller size and in an earlier or less progressive stage than that in Am. Mus. 2004. In both specimens the breadth of the molars equals or slightly exceeds the length, whereas in the more dolichocephalic D. hyognathus molars the length decidedly exceeds the breadth. The dental proportions are therefore mesaticephalic. Premolars. — The superior premolars of the type are so much worn as to obliterate certain of their rudi- mentary progressive characters. The following de- scription of these parts is accordingly based upon the unworn premolars of the paratype specimen, which appears to be in a somewhat more progressive stage of evolution. A narrow diastema, 12 to 16 millimeters. Figure 297. — Paratype skull of Telmatherium uUimum One-fourth natural size. Am. Mus. 2004, reversed; White River. Uinta Ba.sin, Utah; Uinta C, true Uinta formation. separates the canine from p^ The premolar series, measuring 90 (type) and 95 (paratype) miUimeters, as compared with 82 in M. manteoceras, exhibits not only increase in size but marked progression in pattern, as seen in the following characters: (1) Slightly in- creased compHcation of p^ (ap. 19 mm., tr. 12) in the rudimentary internal cingulum and more decided elevation of the tritocene; (2) in p^ to p* of the para- type (No. 2004) the deuterocones consist of antero- posteriorly elongate ridges, much more pronounced than in T. cultridens, convex on the lingual and flattened on the buccal surfaces, totally different from the smooth-sided deuterocones of M. manteoceras and from the apically compressed cones with faint lateral ridges in D. hyognathus; this feature, it should be added, is much more distinctly exhibited in the Httle worn series of Am. Mus. 2004 than in the much worn series of the type, Am. Mus. 2060; (3) this deu- terocone ridge is destined to give rise to the tetarto- cones by constriction, and in p^ p** a faint rudiment of the postero-internal cingulum can be observed in the unworn crown; (4) the internal cingulum is faintly defined around the entire lingual surface of the deutero- 350 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA cones; (5) the ectolophs are greatly elevated and con- sist of the two well-defined subequal protocone and tritocone convexities with a rudimentary external cingulum and pronounced antero-external style; (6) in p^ the progressive broadening tendency is illustrated by the fact that the breadth (22 mm.) equals the length on the ectoloph, a marlied advance upon what is observed in either M. manteoceras or D. hyognathus — in fact, this tooth now resembles p' in general pattern, although retaining a more elongate contour; (7) in p' the breadth considerably exceeds the length and the crown is broadened internally by the expansion of the deuterocone; (8) in p* we have a still more quadrangular and molariform tooth, the length being 27 millimeters and the breadth 35, but in this tooth the deuterocone is not quite so sharply defined. The superior premolars of the type (Am. Mus. 2060) are distinguished from the premolars of the paratype (Am. Mus. 2004) by the following characters: (1) The premolar series is somewhat shorter (90 mm. as compared with 95 in Am. Mus. 2004); (2) p^ in the type is less advanced, in that the deuterocone is smaller and placed farther back and the tritocone is less subequal with the protocone; (3) in p' also the deu- FiGURE 298. — Lower jaw of Telmatherium ulLimum One-fourth natural size. Am. Mus. 2060 (type). White River, Uinta Basin, Utah; base ot Uinta C, true Uinta formation. terocone and tritocone are somewhat less progres- sive and the tetartocone ridge is barely suggested; the cingulum also is slightly less developed; (4) in p^ the tetartocone is indicated by a low, obtuse swelling, whereas in Am. Mus. 2004 it forms the distal spur of a very prominent deuterocone ridge. These differ- ences in the tetartocones can hardly be due entirely to differences in degree of wear (the type being much the older of the two), because in the paratype the ridges in question are so strong that they would probably show even in the worn stage. These differences seem to indicate that the paratype is somewhat more ad- vanced than the type in its premolar evolution. The inferior premolars, as observed in the lower jaw of the type (Am. Mus. 2060), exhibit the following characters: The postcanine diastema is about 20 milli- meters in length; behind pi is a shorter diastema of 11 millimeters in length; pi and p2 are represented only by the alveoli; pa is much damaged but was incompletely molariform. P4 (ap. 27 mm., tr. 19) is submolariform, lacking only the prominence of the postero-internal cusp, which is analogous to the entoconid of the molars. The trigonid is higher than the talonid, and its V less sharply defined. A weak external cingulum appears opposite the outer midvalley and festoons the external slope of the hypoconid. Molars. — The superior molars are a powerful series of teeth measuring 129 (type) to 137 (paratype) milli- meters, with extremely elevated or subhypselodont cusps, the ectoloph of the least worn m^ reaching a height of 35 millimeters and the protocone of the same tooth 24. The external cingulum is more pronounced, especially in the type, Am. Mus. 2060, in which it prominently guards the outer valleys and begins to encircle the styles, reminding us of the cingulum development in Menodus giganteus; the internal cingulum is similarly prominent, embracing the entire inner side of the crown in m^ of the same specimen. A marked peculiarity which is an advance on both M. manteoceras and T. cultridens is the prominence of the anterior cingulum in m' to m', which swells into a large median cingule, comparable to the protostyle of most species of Menodus. The posterior cingulum is less prominent except in m', in which it is free and exceptionally high (type); in the paratype it is connected with an incipient hypocone swelling. The elevated ectoloph is accented by the sharp development of the parastyles, mesostyles, and metastyles. The hypo- cones of m^, m^ of both type and para- type are very large and prominent, an advance upon the conditions in T. cultridens and T. validum. A fur. ther peculiarity is that in the unworn paratype the buccal surfaces of the internal cones (protocone and hypocone) — that is, the surfaces facing the ectoloph — ■ are somewhat flattened and vertically striate, instead of rounded and smooth, as in M. manteoceras and D. hyognathus, which is faintly prophesied in T. validum. The only retrogressive elements are the conules, which have disappeared. The conules are largest in brachyodont titanotheres; with advancing hypso- donty the base of the paracones and metacones ex- tends linguad and either absorbs or crowds out the conules. The inferior molar series represented in the jaw associated with the type (Am. Mus. 2060) is of large size (155 mm.). A smaller jaw (Am. Mus. 2033), formerly referred to this species but now referred provisionally to Manteoceras uintensis, is somewhat shorter (147 mm.). Although this specimen probably represents another genus and species, the molar characters are somewhat similar to those of the type , EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 351 including as progressive features (1) the prominence and the backward extension of the paraconid into a parastylid; (2) the variable but distinct metastylid ridge; (3) the external position of the hypoconulid on 1X1} and the prominent internal ridge on it, which gives it a concave form internally; (4) the external cingulum slightly more progressive than in M. manteoceras and dipping somewhat into the valleys but not so deeply as in D. hyognathus. Lower jaw of T. ultimum. — The jaw of this specimen is represented by that of the type (Am. Mus. 2060). The type jaw retains the characters of T. cultridens in the rather slender recurved coronoid process but departs from them by its rapidly increasing depth posteriorly — in fact, the whole j aw is relatively deeper than in the ancestral species. The distance from the condyle to the incisive border is estimated at 435 millimeters in the type. The chin is strongly compressed laterally (54 mm.), and behind it the jaw gradually broadens and deepens, the lower border being more nearly straight than in M. manteoceras and terminating in the slightly depressed and back- wardly produced angle; the condyle exhibits two marked peculiarities: the outer half of the rotular facet extends broadly forward, whereas the inner half has a straight anterior border and unites posteriorly by a much broader union than in M. manteoceras with the broad facet for the postglenoid process. The coronoid process, perfect in the type, is rather narrow and uniformly recurved. The striking resem- blance to T. cultridens observed in the dentition of this species is therefore not seen in the jaws, which are relatively shorter, more massive, and deeper posteriorly (below ms) than in T. cultridens, all of which are pro- gressive characters. A second jaw (Am. Mus. 2033) was at first doubt- fully referred to the same species. In this jaw the second premolar is spaced as in the type. In other features, as in ps, in the coronoid process, and in the proportions of ma, this jaw resembles those of members of the Manteoceras phylum, to which this one is now provisionally referred. (See Manteoceras uintensis, below.) A skull in the Carnegie Museum (No. 2339) differs from the type and paratype in having a longer tooth row but shows generic agreemeiit with T. ultimum in the general form of the skull, especially of the zygomata, occiput, and nasals. Telmatherium altidens Osborn Plate LXV; text figures 127, 299, 300 [For original description and type references see p. 184] Type locality and geologic Tiorizon. — Uinta Basin, Utah; Uinta formation, Diplacodon-ProtitanotJierium- EpiMppus zone (Uinta C). Specific characters. — Pj-ms, 313 millimeters; a wide diastema (55 mm., estimated) behind the inferior canines; canines in males elevated (76 mm., estimated) and pointed; pi_2 laterally compressed, nonmolari- form; p3_4 submolariform. Subdolichocephalic, upper postcanine diastema elongate. Materials. — As described in Chapter III, this animal is known only from a single lower jaw (Am. Mus. 2025) with no parts of the skull or skeleton associated. Comparison. — The reference of this specimen to the genus Telmatherium depends chiefly upon (1) the large size and vertical elongation of the canines, as in T. validum; (2) the very large size of the lower incisors; (3) the exceptional elongation of the lower postcanine diastema, which is incipient in T. ultimum but was evidently carried to a much greater extreme in T. alti- dens; (4) p2 much less molariform than in Protitano- therium and hence more like the simple, laterally com- pressed p2 of T. ultimum; (5) the very large size of the lower molars (mi_3), the form of which indicates large, broad upper molars, as in T. ultimum. The skull when discovered may well prove that this animal represents a well-marked new generic stage. On the other hand, the very large incisors and lofty canines, the pronounced diastema, the characters of Pi, p2, and the large, broad molars, as noted above, appear to indicate generic Idnship with T. ultimum. Chief characters. — The exceptionally long mandibu- lar symphysis and wide postcanine diastema, as fore- shadowed in T. ultimum, distinguish this titanothere as possessing a relatively elongated facial region. This character, as well as the long, relatively shallow jaws, the elongation of ms, and the wide space behind ma, is evidence that the skull as a whole was sub- dolichocephalic, although far less so than that of Dolichorhinus. In common with T. ultimum, "T. inci- sivum," Protitanotherium, and all other upper Eocene and Oligocene forms, T. altidens had undergone a dif- ferential elongation of the middle part of the skull, which allowed the molars to become extremely large, both absolutely and as compared with the premolars. The elevated, piercing canines are also exceptional among titanotheres; they exceed those of the ances- tral species. Thus the animal is very readily distin- guished from any of the known species of the contem- porary Diplacodon and Protitanotherium. A fourth feature is the simple, nonprogressive, elevated, and somewhat laterally compressed form of p2, which is decidedly more primitive than the corresponding tooth in Protitanotherium. The cracked and much weathered teeth of this male individual (Am. Mus. 2025) fortunately include the median incisors (ij) of the opposite sides, the left canine, and the entire grinding series of the right side in sufficient preservation to define the species sharply. In detail the median incisors are much larger, with more pointed tips than those of Protitanotherium emarginatum, measuring 19 millimeters on the anterior face, 20 anteroposteriorly, and 15 transversely. These 352 TIT.USrOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA teeth distinctly suggest the upper median incisors of T. ultimum. The laterally compressed or convex anterior faces, the smoothly sloping posterior faces, the U-shaped posterior cingula also suggest the Pal- aeosyops type of tooth, although this dolichocephalic animal does not appear to present any affinity to that genus. It is difficult to determine the precise form and proportions of the canines, the fang measure- ments (vert. 76 mm., estimated; ap. 31; tr. 26) indicating a more laterally compressed or dolicho- cephalic type of canine than in Protitanotherium emar- ginatum. The height of this tall and slender canine exceeds 76 milHmeters (estimated), as compared with 53 in the male P. emarginatum and 56 (estimated) in the male P. superhum. The name T. altidens refers to this feature, as the tusk is the most elevated and Figure 299. — Hypothetical reconstruction of the skull of Telmatherium altidens One-sixth natural size. Designed to show especially the long postcanine diastema, generic resemblance to T. ultimum. The lower jaw is Am. Mus. 2025 (type of T. Am. Mus. 2060 (type of T. ultimum). piercing among all the known titanotheres, not except- ' ing the giant Menodus giganteus of the Ohgocene. Faint anterior and posterior ridges can be detected on the anterior and posterior faces of the crown, distinguishing this tooth readily from the canine of Palaeosrjops major, in which the posterior ridge is on the internal face of the crown. The very wide total diastema between the canine and p2 measures 70 milli- meters, as compared with 40 in Protitanotherium superium and 45 in Telmatherium ultimum, which is approached only by the wide diastema (51 mm.) in Dolichorhinus hyognathus. The grinding series as a whole measures 313 millimeters, as compared with 304 in P. emarginatum and 318 in P. superium, the lower grinding series being, therefore, slightly smaller than in P. superhum. Premolars. — Pi and p2 are not so much compressed as in Telmatherium cultridens but are somewhat swollen transversely. Pi (ap. 19 mm., tr. 12) is a simple, gently compressed cone, with a small posterobasal cusp rising from the posterior ridge. This cusp is less advanced than in Protitanotherium superhum or P. emarginatum. P2 (ap. 27 mm., tr. 15) is also less advanced than in those species, its posterior lobe being smaller, lower, and much less crescentic superiorly. It has a faint paraconid, no metaconid, and very faint anterior and posterior internal valleys. It is thus much like P2 of Manteoceras manteoceras, T. cultridens, and (so far as known) T. ultimum. In striking contrast with this is the progressive structure of pa (ap. 30 mm., tr. 18), especially its ex- tremely prominent median cusp ( = protoconid); the anterior and posterior crescents are correspondingly more defined than in p^; the rudiments of the metastylid and cusps appear, corresponding to the paraconid and ..-;: . entoconid in the molars. P4 is a decidedly larger tooth (ap. 35 mm., tr. 23) with prominent internal cusps ( = paraconid, metaconid, meta- stylid, entoconid). Molars. — The true molars measure 195 millimeters in length, as compared with 214 in Protitanotherium su- perhum. The measurements (ap. by tr.) are, nij, 45 by 29 millimeters; m2, 59 by 32; ma, 89 by 35. The very large size of the molars as in P. superhum and other Uinta C titanotheres is thus note- worthy. The much worn grinders give an imperfect picture of the distinctive characters of these teeth, but the elongated face, and the supposed altidens). The skull is restored from it would appear that the exter- nal cingulum and the meta- stylid are faintly indicated and that in ma the hypoconu- lid is placed more on the internal or lingual side of the crown, as in Palaeosyops paludosus. The grinding series, therefore, presents two resemblances to that of P. paludosus — namely, the prominent internal cusp on Pa and the more internal position of the hypoconulid on ma — yet neither of these characters is believed to indicate genetic affinity. The closest resemblances are to the inferior dentition of T. ultimum, from which this jaw differs, however, in its greater size, its rela- tively larger canines, and the more internal position of the hypoconulid. This last condition may be partly due to crushing. Jaw. — The jaw of T. altidens is readily distin- guished from the jaw of Protitanotherium emarginatum and that of P. superhum by its dolichopic characters, the wide diastema between the canine (55 mm., esti- mated) and Pi, and that between ma (54 mm.) and the anterior border of the coronoid process. The sym- EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 353 physisis extremely long (200 mm.); it is both, actually and relatively longer than in P. emarginatum (155) or P. superhum (158, estimated). The jaws are de- cidedly deep, measuring 107 millimeters below p2, 124 behind m2. The coronoid was probably elevated, ta- pering, and recurved toward the summit. The thick- ness of the rami in the type jaw has been reduced by crushing; below mi it is 40 millimeters. This jaw therefore represents a large but fairly slender and active animal, which in some respects is suggestive of relationship with species of the long-jawed genus Meno- ently short, broad proximally, and tapering distally; face concave in front of orbits; frontonasal "horn swellings" not evident; sagittal crest deep and nar- row; occiput low with thin crests; dentition extremely macrodont; incisors relatively larger than in any other known titanothere. Historical notes.^ln describing the type species {Sthenodectes incisivus) of this genus the author, Earl Douglass (1909.1, p. 305), said: "I think that this skull represents a different genus from Telmatherium, but I prefer to place it provisionally here rather than estab- FiGURE 300. — Lower jaws of Telmatherium ullimum and T. altidens One-fourth natural size. A, T. ultimum, Am. Mus. 2060 (type), reversed; White River, Uinta Basin, Utah; base of Uinta C, true Uinta formation. B, T. altidens, Am. Mus. 2025 (type); White River, Utah; Uinta C. dus of the Oligocene but in other respects is very unlike an ancestor of Menodus — namely, the excessively large size of the incisors, the retarded condition of pi and P2, and the very long postcanine diastema. Sthenodectes Gregory Plates LXV, LXVI; text figures 129, 130, 301 [For original description and type references see p. 190] Generic cTiaracters. — Skull mesaticephalic or sub- brachycephalic; cephalic index 62-65; malars with- out infraorbital shelf or protuberance; nasals appar- lish another genus." Through the courtesy of Doug- lass, Gregory (1912.1) was enabled to compare this type with the extensive material in the American and Yale Museums and reached the conclusion that T. incisivum represents a different genus or subgenus, to which he gave the name Sthenodectes, in allusion to the great power and development of the incisors and canines. The following characters were assigned by Gregory in the original description of the genus as compared with Telmatherium: (1) The incisors are much larger and more advanced in evolution; (2) the post- 354 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA canine diastema is reduced or absent; (3) the superior premolars p^ p\ p* are more progressive than in T. ultimum, having very heavy internal cingula and pro- nounced external cingula; (4) the basicranial region differs in many details. The type skull of the species (Carnegie Mus. 2398) is vertically crushed, a condition that led to some errors in the original description of the species which a second skull in the Field Museum (No. 12168) enabled Greg- ory to correct and to reach the following conclusion as to the affinities of this animal: Relation to Telmafherium. — StTienodedes is sharply separated from the Dolichorhininae and at the same time allied with Telmatherium by the following char- acters: (1) General contour of the skull in basal view, wholly unlike MetarMnus and resembling Manteo- ceras or Telmatherium; (2) complete absence of infra- orbital protuberance, the infraorbital portion of the malar more like that of either Manteoceras or Telma- tJierium; (3) midportion of malar with deep vertical flange as in Telmatherium (contrast Metarhinus) ; (4) incisors and canines readily derivable from the Telmatherium type (compare figures of side view, crown view; compare premaxillaries); (5) dentition extremely macrodont (microdont in Metarhinus, TQ.B.CYoAontm Telmatherium); (6) premolars more ad- vanced than in T. ultimum but derivable from the Telmatherium type (cf . T. validum) by enlargement of internal cingulum, filling out the internal contour of p^; (7) referred lower jaw (Field Mus. 12168) de- cidedly nearer to Telmatherium ultimum than to Metarhinus, macrodont, especially molars, ramus mas- sive and deep; (8) basis cranii with postglenoid, post- tympanic, meatus, and basioccipital nearer the subbrachy-mesaticephalic type of Telmatherium than to the subdolichocephalic type of Metarhinus. Through parallel evolution there are some marked resemblances to the Dolichorhininae, as follows: (1) Premolars (p^~*) with very heavy internal cingula and crowns well filled out on the inner side; (2) incisors cupped by upgrowths of heavy cingulum; (3) subhyp- sodonli or elongate character of the molars of the type specimen. Effects of crushing. — To the vertical crushing of the type skull is possibly due the wide displacement of the lacrimal bones on both sides of the face, result- ing in the false appearance of "lacrimal pits." To the crushing is also due the union of the postglenoid and post-tympanic processes, the depression of the occiput, and the abbreviation of the nasals. Sthenodectes incisivus (Douglass) Plates LXV, LXVI; text figures 129, 130, 301 [For original description and type references see p. 185) Type locality and geologic horizon. — About 3 miles northeast of well 2, Uinta Basin, Utah; upper levels of Eohasileus-Dolichorhinus zone (Uinta B 2). Specific characters. — Skull, length 488 millimeters, zygomatic breadth 305 millimeters, cephalic index 62. Dentition, p^-m^ 207 millimeters; m^-m^ 125; p^ large (ap. 19 mm., tr. 22), very progressive, with advanced tritocone and complete internal cingulum, deuterocones of p^~* relatively elevated, internal cingula heavy, complete, m' (ap. by tr.) 42 by 45 millimeters with reduced posterior metacone crescent. Materials. — Besides the type skull in the Carnegie Museum (No. 2398), on which the above specific characters are based, there is a well-preserved skull in the Field Museum (No. 12168), also a pair of lower jaws (Field Mus. 12166). According to Riggs (1912.1, p. 38) all three specimens are from the same locality and belong to the same species. The skull and lower jaws in the Field Museum were discovered by Abbott in the lenticular sandstones near well 2 at the foot of Coyote Basin. The skull, Field Mus. 12168 (PL LXVI), is shorter (460-300 mm., cephalic index 65) than in the type of S. incisivus but has the broad-crowned molars and massive incisors of that species. At approximately the same geologic level but half a mile distant was found the lower jaw (Field Mus. 12166) referred to this species (PL LXVI), which belongs to an older individual, as is evident from the worn molars and incisors. It is described below. This short-headed, massive-jawed titano there ex- hibits a remarkable combination of characters. It exceeds all other known titanotheres in the size of the incisor teeth, which are correlated with the massive jaws and the relative abbreviation of the skull, the general proportions of which suggest those of Manteo- ceras. The abbreviation of the facial region consti- tutes a differentiation directly the opposite of that which was occurring in the line which gave rise to Telmatherium altidens in Uinta C, in which the face, judging by the wide postcanine diastema, was elon- gated. The indices are significant. Indices of Sthenodectes incisivus Cephalic index Faoiocephalic index.. Molar-cephalic index Carnegie Mus. 2398 (type) Field Mus. 12168 (referred) 65 44 28 The grinding teeth are also proportionally very large. The skull is at once separable from that of Mesatirhinus and Dolichorhinus by the short basi- cranial region and the stout, wide, spreading zygo- mata, as well as by the heavy, short premaxillae and the absence of a rounded infraorbital protuberance, or shoulder. It also differs from any of these genera in the proportions of the molar teeth, m^ and m^ being wider, or more brachycephalic in type. It parallels the true" Dolichorhininae, however, in the advanced EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 355 condition of the premolars and in the cupping of the incisors. It resembles MetarTiinus, especially M. earlei, in the following characters: (1) Broad forehead; (2) concavity of the face in top view; (3) certain fea- tures of the premolars; (4) proportions of the occiput; (5) thin, high sagittal crest. From the contemporary species of Manteoceras, namely, M. uintensis, it is distinguished by (1) the characters of the incisors and canines; (2) the much more advanced condition of the premolars; (3) the less elongate m^; (4) the feebly constricted postcanine region. It parallels Manteoceras in the general pro- portions of the skull and in the form of the zygomatic arches, except that the malar portion of the arch has the deep flange characteristic of Telmatlierium. Sthenodectes suggests Tehnatherium ultimum in cer- tain features of the incisors, canines, and molars, in the detailed characters of the basicranial region and in the spreading zygomata; but it is distinguished from that form by (1) the much larger size and higher development of the incisors, (2) the more advanced condition of the premolars and premolar cingula, (3) the different form of m', (4) the lower occiput and sharper sagittal crest, (5) the wider forehead, (6) the more angulate section of the infraorbital portion of the malars, (7) the sharply tapering nasal bones. From the European genus Brachydiastematherium, which it resembles in having three large incisors, Sthenodectes is distinguished by the markedly lower evolution stage of the premolars (p2-p4). Side and top views. — The top of the type skull has been crushed downwai'd, especially above and in front of the orbits. The premaxillaries, though some- what flattened by pressure, are of very large size, in correlation with the exceptional dimensions of the incisors. The nasals are imperfectly preserved at the end but appear to be even shorter than in T. ultimum; they converge rapidly in front, about as in MetarTiinus, and proximally they spread rapidly and widely, measuring 125 millimeters transversely at the outer junction with the frontals. The latter were somewhat flattened but were very wide across the orbits (tr. 192 mm.). In front of the orbits there is a prominent vertical facial concavity suggesting the conditions in MetarMnus fluviatilis. The infraorbital foramen is large and prominent, apparently more so than in T. ultimum. Above this foramen and in front of the orbit is a triangular depression, in the position of the lacrimal bone, occurring on both sides of the skull but much larger in the right, which is referred to by Douglass as a vacuity. It now seems probable that these vacuities resulted from the down- ward crushing which has squeezed the lacrimals out of place; they lie immediately below the region where the horn swelling usually appears, but the presence of the latter is but vaguely if at all indicated. The fore- head, as already stated, is broad and flat, and the depth of the skull appears to be less than in T. ultimum The opposite postorbital temporal crests run backward into a long sagittal crest, which is quite high and thin. The occipital crests are thin, but the whole occiput is much lower than in T. ultimum. Palatal view. — In the inferior aspect of the skull we are struck by the great size of the dentition as a whole, the great size and spatulate outline of the incisor re- gion, the prominent pointed canine tusks, the long, straight tooth row, the virtual lack of a postcanine diastema, the wide, very progressive premolars, the relatively large, subhypsodont molars, the widely arching zygomata, and the short basicranial region — all these, with the exception of the prominence of the incisors and canines, being characteristic of Oligocene titanotheres. The infraorbital part of the malar is like that of Manteoceras in that it did not flare out- ward into an infraorbital protuberance; just behind the orbit the malar was very massive, and its broad inferior expansion shows an area for the attachment of the masseter; the postero-inferior portion of the malar is a deep vertical flange, as in Manteoceras and T. ultimum. The squamosal portion of the zygoma is very stout and broad anteroposteriorly; the post- glenoid process is rather small. The prominent ex- ternal auditory meatus of the type in side view appears to be closed below by the appression of the postglenoid and post-tympanic processes, but this is probably due to crushing, as the Field Museum specimen shows these processes widely separated. The palate is long, and the anterior border of the posterior nares is between m- and m". Incisors. — The anterior incisor (i^) is very large (ap. 22 mm., tr. 20), and closely appressed in the median line to its fellow of the opposite side; its large, blunt tip lies near the median line; back of this is a wide, oval basin, or pit, bounded by the very heavy posterior cingulum and by the external ridge; the front face is vertically deep (26 mm.). The median incisor (i^) has a low median tip and wide posterior basin. It remotely resembles that of DolicTiorhinus but is far larger (ap. 25 mm., tr. 26) even than that of T. ultimum. The very large canine (ap. 27 mm., tr. 27), as already observed, is long and piercing, with a verti- cal crown length of 57 millimeters, as compared with 42 in the paratype of T. ultimum. Its transverse diameter is 27 millimeters, as compared with 22 in T. ultimum. It has similar antero-internal and postero- external edges but is distinguished by its heavier poste- rior basal cingulum. Premolars. — The premolars are larger and wider than in T. ultimum. There is little if any postcanine diastema, p' being crowded in behind the base of the canine. Its crown is not preserved, but it appears probable that this was broader — that is, more ad- vanced — than in T. ultimum. In p^, p^, p* the trito- cones are nearly equal to the protocones, and both 356 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA external and internal cingula are extremely progres- sive, the external cingula being well defined across the base of the protocones and tritocones, and the inter- nal cingulum forming a wide basal shelf extending around the whole anterior as well as the posterior in- ternal border of the crown. P^ is thus almost like p' (except for its smaller size and relatively smaller by the more advanced condition of the deuterocone of p", of the tritocones of p^"*, and of the internal cingula. At the same time the premolars of S. in- cisivus simulate those of Dolichorhinus longiceps, espe- cially in their advanced tritocones, but are distin- guished from them by their greater breadth and far heavier internal cingula. A still nearer resemblance Figure 301. — Type skull of Sthenodectes incisivus Ona-fourth natural size. Carnegie Mus. 2398 (type). About 3 miles northeast of well 2, Uinta Basin, Utah; upper levels of EobasUevs-DolichorMnus zone (Uinta B 2). Ai, side view; Ai, top view; A3, palatal view. transverse diameter), whereas in the type of T. ulti- mum p^ is much simpler than p^ P'* (ap. 23 mm., tr. 32) and p* (ap. 23 mm., tr. 39) are correspondingly advanced but unlike T. ultimum show no trace of tetartocone ridges and swellings. The premolar series is thus readily distinguished from that of the contemporary Manteoceras uintensis is with the premolars of Metarhinus earlei, in which p^ is almost as progressive and p^ and p* have heavy internal cingula. Molars. — The molars of the type are distinguished from those of T. ultimum by the greater minimum transverse diameter of m\ m^, by the less prominent hypocone on m", by the weaker internal cingula on EVOLUTION or THE SKULL AND TEETH OF EOCENE TITANOTHERES 357 m^, m', and by the detailed form of m^ From those of the contemporary Manteoceras uintensis they are distinguished by their smaller size, by the greater rela- tive breadth of m^, more quadrate contour, especially of m^ relatively smaller parastyles and mesostyles, sharper external cingula. M^ is wide anteriorly (53 mm.) and narrow posteriorly; the posterior V is rela- tively small. The molars of MetarMnus earlei are relatively longer anteroposteriorly. Comparative measurements of Telmatherium and Sthenodectcs, in millimeters ■ Basal length of skull Zygomatic breadth of skull. . Cephalic index Length of dental series (i'-m^) . P'-m3 P'-p* M'-m3 I', ap. by tr P, ap. by tr P, ap. by tr C, ap. by tr C, vertical P*, ap. by tr Ml, ap. by tr M^, ap. by tr M^, ap. by tr T. ultimum, Am. Mus. 2060 (type) 500 300 + 60 305 218 89 130 14X13 15X14 19X17 * 25X23 * 42X? 25X34 39X36 45X46 44X51 Carnegie Mus. Field Mus. 2398 (type) 12168 490 "■310 63-65 295 207 84 125 22X21 25X26 25X27 27X27 67X? 23X39 40X44 45X49 41X46 211 132 " Estimated. ' Am. Mus. 2004. Lower jaw. — A lower jaw (Field Mus. 12166), found on the same geologic level as the skulls but at some distance, belongs to an aged individual. The crowns of the incisors are almost worn away. The following description and measurements are from Riggs (1912.1, pp. 38, 39): The mandible is 10 millimeters shorter than would be required to fit the skull, but the dentition matches closely. The molars have the strength necessary to oppose the massive upper series; the canines and incisors, though not so massive as those above, show such wear as would be expected in this form. The canines are worn away diagonally at the point of contact with the third upper incisor, but very little from contact with the upper canines. There is a short diastema between canines and premolars. The mandible as a whole is titanothere-like — deep through the ramus, broad at the angle, concave in the tooth line, and tapering toward the anterior extremity. The coronoid is short and recurved at the tip. Lower jaw of Field Mus. 12166 Millimeters Length, condyles to incisors 360 Height, condyles above angle 168 Length of molar-premolar series 215 Length of molar series 130 Length of crown of canine (estimated) 30 Diameter of crown of canine 19 Depth of ramus from base ofps 60 Depth of ramus from base ofms 84 THE MANTEOCERAS-DOLICHORHINUS GROUP (mANTE- OCERAS, MESATIRHINUS, DOLICHORHINUS, SPHENO- COELUS, METARHINUS, RHADINORHINUS) Stages, series, and subfamilies. — This second great group of middle and upper Eocene titanotheres is characterized by precocious horn swellings above the eyes and many other featui'es in common. The single specimen of this group (Eometarhinus) , discovered in the Huerfano B ( = Bridger A) horizon of the Huerfano formation of Colorado, is the sole known forerunner. With this exception, this group is of much later geo- logic appearance than the first group {Palaeosyops, Telmatherium), being found in the upper levels of the Bridger Basin, in the Washakie Basin, and in the lower and middle levels of the Uinta Basin. The group commenced to flourish in the Bridger and Washalde regions during the period of the decline of the Palaeo- syops phylum and survived it for a very long period, but it was contemporaneous with the Telmatherium, phylum. We find that this group radiates into four series, as follows : Series included in the Manteoceras-Dolichorhinus group Progressively large and mesati- cephalic to brachycephalio Progressively large and dolicho- cephalic Arrested in size, mesati ephalic to dolichocephalic Nasals wide Nasals pointed Later stage Earlier stage Protitanotherium ... Unknown Metarhinus Eometarhinus. . _ . .. (?)• Rhadinorhinus. Besides the rudimentary horns there are very numerous characters which tie the members of this second group together and distinguish them from the palaeosyopine group. These characters point indis- putably to a common ancestor. An underlying unity of descent is at once observed in the accompanying figures (fig. 302) of the four types of skulls included in this group, which are all reduced to the same scale. The four series are grouped into subfamilies and genera as shown below. 358 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Suhdiinsions of the Manteoceras-JDolicJiorMnus group Subfamilies Manteoceratinae (=Brontopinae) Dolichorhininae RhadinorMninae Horns on frontals. No infraorbital shelves. Mesaticephalic to brachycephalic. Nasals broad. Horns chiefly on nasals. Large infraorbital shelves. Dolichocephalic to hyperdoliohocephalic. Nasals broad. Horns retarded. Rudimentary infraorbital shelves. Dolichocephalic. Facial region upturned. Nasals pointed. Genera Protitanotherium (Eocene) . Manteoceras (Eocene) . Mesatirhinus (Eocene). Dolichorhinus (Eocene) . Metarhinus (Eocene) . ?Sphenocoelus (Eocene) . Eonietarhinus (Eocene) . Rhadinorhinus (Eocene) . The phyletic position of the recently discovered Eometarhinus, from Huerfano B ( = Bridger A), is ancestral either to Metarhinus or to Rhadinorhinus. Of these genera Manteoceras and Mesatirhinus rep- resent phyla which appear contemporaneously in the upper Bridger but which have already diverged from each other toward brachycephaly and dolichocephaly, respectively. As these subphyla diverge more and more the resemblances which are observed between the lower members of each series become fewer, and the differences become greater. Thus Manteoceras and Mesatirhinus are much nearer each other than the forms to which they respectively gave rise, namely, Protitanotherium and Dolichorhinus. The Rhadino- rhinus phylum may prove to be a distinct one, and in some characters it points toward the Oligocene Mega- cerops (Symhorodon) . SUBFAMILY MANTEOCERATINAE {=BRONTOPINAE) OSBOEN, EOCENE ANCESTOES A branch of the same stock as that of Mesatirhinus and Dolichorhinus. Precociously horned animals, known from the upper deposits of the Bridger Basin, from the Washakie Basin, and from the Uinta Basin. First referred to Telmatherium and subsequently de- scribed as Manteoceras, or "prophet horn." In all known characters more nearly central or ancestral to the Oligocene titanotheres of the genus Brontops than any of the Eocene forms thus far discovered. Manteoceras General structure and habits.- — The presence of the rudiment of a horn above and in front of the eyes is the most distinctive and interesting feature of the middle Eocene Manteoceras, which is the earliest known member of this subfamily. Many more characters both of the skull and the teeth make this a prophetic or ancestral form of great significance and interest, worthy of the most thorough, detailed study. Alto- gether more than fourteen such prophetic characters have been found in these animals. In point of size the known individuals are intermediate between the largest tapirs and the smaller rhinoceroses, such as Rhinoceros (Dicerorhinus) sumatrensis. The skull in these animals is moderately elongate, or mesaticephalic. The fluctuations are between mesaticephalic and brachycephalic types. Female skulls tend to be somewhat more long and narrow; aged male skulls tend to be broader and more robust. The parts of the limbs and feet which signify speed, especially the humerus, femur, and manus, indicate that the quadrupeds belonging to this genus were swifter than Palaeosyops but slower than Mesati- rhinus. They were brachypodal as compared with Mesatirhinus but considerably longer footed than Palaeosyops. The large tusks of the males and the earlier development of horn rudiments as compared with the palaeosyopine group indicate that these quadrupeds were vigorous fighters. In a large per- centage of the adult specimens the teeth are much worn, indicating that the food was somewhat harder and drier than that of Palaeosyops. As feeders these animals were better equipped than the members of the Palaeosyops and Limnohyops series, for their grinding teeth were decidedly more trenchant or cut- ting, but even in the later members of Manteoceras the grinding teeth are somewhat less efficient than those of the contemporary telmatheres, because the molar ectolophs are a little shorter and the premolars are less advanced in evolution. History of discovery. — The discovery of these animals was one of the turning points in the history of the evo- lution of the titanotheres. In 1894 the American Museum expedition was working under the direction of Dr. J. L. Wortman in a layer of brown sandstone 3 miles north of the base of Haystack Mounain, in what is now loaown as the Washakie A level. Here two skulls (Am. Mus. 1569, 1570) were found, and as partly exposed in the field they attracted the attention EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHEKES 359 of Doctor Wortman as seemingly different from any previously discovered. He described them in a letter written to Professor Osborn from the field as exhibit- ing rudimentary horns at the junction of the f rentals and nasals and suggested the generic name Manteo- ceras or "prophet horn." On the arrival of these skulls at the American Museum Professor Cope, the writer, and others who examined them expressed great doubt as to whether the tuberosities (PI. XVI; figs. 305, 307) above the orbits could really be re- garded as incipient horns. These doubts were soon removed by the discovery of similar horns in Dolicho- rhinus cornutus { = 'hyognathus) of the middle Uinta, an^ Doctor Wortman's observation was thus verified. As detailed in Chapter III (p. 151) the animal was first identified by Osborn with the imperfect upper CO type teeth of the species Palaeosyops (Telmatherium) vallidens Cope, previously found by Professor Cope in the Washakie Basin; but it was subsequently ascer- tained that this species, now provisionally referred to the genus DolichorTiinus, belongs in a higher level, Washakie B, whereas the types of Manteoceras man- teoceras were both found in Washakie A. These animals {M. manteoceras) were first supposed to be confined to the lower levels of the Washakie Basin, but subsequent exploration of the upper Bridger by the American Museum expeditions has proved that they were still more numerous in the Bridger Basin; altogether the remains of more than twenty animals of the type species (M. manteoceras) have been found by the American Museum parties, including seven skulls in Bridger D and four skulls in Washakie A. In the upper levels of horizon A of the Washakie Basin a more advanced stage has been found, M. washaJciensis. Thus far these animals have not been found in the Uinta Basin in beds of level B, deposited during a period when they undoubtedly lived; but in the lower part of Uinta C the genus reap- pears in the important species described by Douglass as Manteoceras uintensis. In the lower part of Uinta C an animal nearly related to Manteoceras, if not its direct successor, was discovered by the Princeton expedition in 1894 and was subsequently recognized by Hatcher as probably a successor of Manteoceras, and named by him ProtitanotTierium emarginatum. Geologic distribution. — The geologic levels at which the remains of these animals have been found are shown in Figure 334, and as the remains are numerous in the upper Bridger, levels C and D, and in the lower Washakie, level A, they indicate that these deposits are contemporaneous. As observed in the text on Telmatherium, the advent of Manteoceras appears to have been contemporaneous with the last stage in the development of the Palaeosyops-Limnohyops phylum and with the first appearance of the Mesatirhinus- Dolichorhinus phylum. The abundance of remains of these animals in the upper Bridger deposits is very 101959— 29— VOL 1 26 striking. It is possible that they are represented also by skeletal remains in the lower Bridger. Affinities to other Eocene titanotheres. — The resem- blances and contrasts between Manteoceras and Tel- matherium have been pointed out in some detail in the JDolicTzorh in us A^anteoceras Figure 302. — Skulls of titanotheres of the Manieoceras-Dolichorhinus group One-eighth natural size. A, Manteoceras manteoceras, middle Eocene of Bridger Basin. Wyo.; upper Bridger. B, Mesati- rhinus petersoni, middle Eocene of Bridger Basin, Wyo.; upper Bridger. C, Melarhinus earlei, middle Eocene of Washakie Basin, Wyo.; summit of Washakie A. D, Bolichorhinus hyo- Snaihus, middle Eocene of Uinta Basin, Utah; Uinta B 2. descriptions of Telmafherium. They may also be very clearly seen by comparing the crania of the types of these two general (figs. 210, 219). To summarize Manteoceras is distinguished from Telmatherium by (1) deeper facial concavities; (2) much more prominent 360 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA and rugose frontonasal horns; (3) progressive anterior flattening of the vertex of the cranium and recession of the sagittal crest; (4) presence of a supraparietal pit and strongly bifid sagittal crest; (5) broad and de- pressed occiput; (6) oblique shelf-like suborbital part of the malars; (7) widely arched zygomata with de- scending flange of malar relatively shallow; (8) pro- gressively more round-topped superior incisors, the outer relatively smaller than in Telmatherium; (9) shorter, heavier, and rounder superior canines, with very heavy roots; (10) less progressive tritocones, deu- terocones, and cingula on superior premolars; (11) somewhat less pronounced hypsodonty of the grinding teeth; (12) broader and more rounded mesostyles; (13) the less deep and finally more elongate premax- illary symphysis. Comparisons with MesatirTiinus and DolichorTiinus. — The general resemblances of these animals have been enumerated above. A number of resemblances in general conformation are seen by comparison of similar views of the crania of Manteoceras and of Mesatirhinus. These indicate a closer ancestral af- finity to MesatirTiinus than to Telmatherium. De- tailed points of resemblance between Manteoceras and Mesatirhinus are seen in (1) the tendency to form a suborbital shelf, which is more pronounced in Mesati- rhinus than in Manteoceras; (2) the depth of the facial concavities, giving prominence to the nasofrontal horn rudiments (a distinction must be noted here, however, that the horn rudiments in Mesatirhinus and Dolichorhinus are borne rather by the nasals than by the frontals, whereas in Manteoceras the reverse is the case); (3) pronounced affinities in the foot and limb structure. The statement may be made very emphatically, therefore, that Manteoceras and Mesatirhinus have risen from a common stock. The distinctive characters of Manteoceras lie prin- cipally in the proportions of the skull, dentition, and feet and in the divergent evolution of the premolar- molar series. Manteoceras is mesaticephalic in skull and tooth structure and subbrachypodal in foot structure, while Mesatirhinus is progressively both dolichocephalic and dolichopodal. Incipient horns. — As observed above, a notable characteristic of these animals is the precocious horn development. The horn swellings are borne directly over the frontonasal suture (Pis. XVI, XVII). They involve very slight convexity and are slightly rugose only in the more aged specimens. As they are exhibited in various degrees in all the skulls known, they were certainly present in both sexes, although less prominent in the females. These horn swellings have a different origin in Dolichorhinus, as well as in Mesati- rhinus, for in these genera (PI. XVII, figs. B, C\ C), although placed about as in Manteoceras, they are borne chiefly on the nasals and partly on the frontals — that is, in front of the frontonasal suture. Facial concavities. — The second distinctive character that is correlated with or lends itself to this precocious development of the horns is the concavity in the side of the face, in front of the orbit, beneath the nasal. This gives a greater prominence to the horn rudiments and in life would permit the warty epidermal swellings that covered these rudiments to be used more ef- fectively in butting. This overhanging frontonasal suture showB a wide contrast to the condition seen in Palaeosyops. The concavity of the face in front of the orbit, beneath the horn, is a very prominent feature also in the Oligocene titanotheres and in Sthenodectes incisivus of level B of the Uinta Basin, Utah. The vertex. — In lateral or profile view the skull is convex above the brain region, concave in the mid- cranial region, and convex again in the nasal region, as in Mesatirhinus. The horn rudiments, or hornlets, are thus thrown into considerable prominence both laterally and superiorly. The concave midportion of the skull is again a progression in the direction of the saddle-shaped top of the titanothere cranium. When viewed from above the cranium also exhibits a spread- ing of the space beneath the supratemporal ridges in such a manner that the sagittal crest proper is limited to the posterior region. In the V-shaped space on top of the skuU between these converging temporal ridges (or bifid sagittal crest) it is especially interesting to observe that a deep pit is developed in the more progressive and older forms, because we shall find a vestige or reversion to this pit on top of the large, flattened crania of some of the Oligocene titanotheres {Brontops, compare figs. 304, 307, 374). Dentition. — The superior incisors form a more A-shaped series than in Dolichorhinus, where they tend to form a fl, and the inferior incisors are more transverse in position. The grinders are less hypso- dont on the ectoloph, and the protocone tips are more blunt than in Mesatirhinus and Dolichorinus. The premolars are less advanced than in Mesatirhinus and Dolichorhinus, because the tritocones and deuterocones are relatively smaller, the ectolophs less flat, and the "ribs" on the external face of the outer cusps wider at the base. The relative degree of progression of the premolar ectolophs in Dolichorhinus and Manteo- ceras is a very complex matter, but after careful comparison it may be summarized as follows: EVOLUTION OF THE SKULL AND TEETH OF EOCENE TXTANOTHERES Characters oj 2?^ in Manteoceras, Mesatirhinus, and DolichorMnus 361 Manteoceras Mesatirhinus Dolichoihinus Very broad at base Strong Very broad at base, but "rib" appearing. Often pinched or riblike. Gently rounded. Still more anterior. Filling out. Tritocone nearly equal to tocone. Tritocone convexity. __ _ More anterior More rounded Tritocone relatively somewhat larger. Relative size of protocones and tritocones. Variable pro- In general p^ in DolichorMnus is in a much more advanced stage than in M. manteoceras. In p', p* these differences become more pronounced. The premolars were thus evolving along divergent lines in Manteoceras on the one hand and in Mesatirhinus and Dolichorhinus on the other. The general subfamily kinship of M. manteoceras with Mesatirhinus and Dolichorhinus is shown especially in the comparison of p^, p*, in M. washaJciensis and Mesatirhinus petersoni, but the generic differences are still evident. Jaw structure. — The jaws are prophetic of the Oligo- cene type, especially in the posterior region, with an elevated coronoid, and with the border sharply de- pressed below the angle (fig. 310); the chin, however, is weaker and the coronoid relatively much larger. Sex characters. — Differences in sex are indicated by the smaller size of the canines in the females, as observed in M. manteoceras. It is difficult to deter- mine positively whether the horns are also less promi- nent in the females than in the males. One well- preserved, very old Manteoceras skull (Am. Mus. 12678) from Bridger C 5 has small canines and appears to be a female. In it the horns are hardly less promi- nent than in the type male. The type of M. washa- Tciensis has very minute horn swellings and might be taken for a female, but its canines are of intermediate size. Mesaticephalic slcull proportions. — The skulls are in- termediate in proportion, or decidedly broader than those of Mesatirhinus and Dolichorhinus and much longer and narrower than those of Palaeosyops, the breadth being about three-fifths the length, and they may thus be described as mesaticephalic. In the ear- lier forms of M. manteoceras of the middle Eocene the zygomatic arches are rather stout and well arched. In the much later M. uintensis they are more slender than in Telmatherium ultimum but diverge widely, forming a decided angle with the glenoid region. There is only a rudiment of the infraorbital shelf that is so characteristic of most species of Mesatirhinus and Dolichorhinus. Detailed features. — Characteristic detailed features, some of which trend progressively toward the Oligocene titanotheres, clearly distinguish these animals from Palaeosyops and in a less degree from Mesatirhinus: (1) The premaxillary symphysis is long and firm as compared with that of Palaeosyops but shorter than in Dolichorhinus; (2) the nasals are very characteristic, being relatively short and stout, decidedly truncate, distally somewhat spreading and laterally much recurved; (3) in the sagittal line of the skull the suture between the frontals becomes obliterated in adults, as in many other ungulates with large diploe; (4) the occiput is low and broad (fig. 306), very distinct in form from that of Palaeosyops, and in the more advanced specimens {Manteoceras washaJciensis) it exhibits the lateral pillars which are so characteristic of the Oligocene titanotheres. Summary of progressive characters of Manteoceras toward Brontops and other Oligocene titanotheres. — Hatcher, at the time of the discovery and description of the animal now called Protitanotherium emarginatum of Uinta C, pointed out the fact that Manteoceras is in or near the main ancestral line of the Oligocene titano- theres rather than Dolichorhinus, which Osborn had supposed also in that ancestral line. The progressive characters of Manteoceras toward the Oligocene forms are naturally somewhat more marked in old than in young specimens. The follow- ing items relate chiefly to the species M. manteoceras, which is the most fully known and seems to lead espe- cially toward the Oligocene Brontops: (1) Middle part of the skuU elongate, face never very long, elon- gation becoming very pronounced in the Oligocene titanotheres; (2) rudimentary frontal-nasal horns ap- parently increasing in size with age and probably more pronounced and more rugose in the males, as in all the Oligocene forms; (3) concavities in front of the orbits, causing the rudimentary horns to overhang the sides of the face (very prominent in the later Oligocene gen- era); (4) nasals broad distally, shorter than in Doli- chorhinus and in M. uintensis, suggesting the Brontops 362 TITANOTHERES OP ANCIENT "WYOMING, DAKOTA, AND NEBRASKA and Menodus nasals; (5) premaxillary symphysis deepened and keeled (compare Oligocene genera); (6) middle or frontal portion of the cranium flattened, the flattening being associated with the progressive ob- literation of the suture between the frontals and with the abbreviation of the sagittal crest (compare Oligo- cene genera); (7) middle portion of the skull saddle- shaped in the region between the frontal-nasal horns and occipital crest, showing a tendency that becomes extreme in the Oligocene forms; (8) overhanging supratemporal crests or ridges characteristic of age, a tendency observed also in Oligocene titanotheres; (9) occiput broadened and lateral pillars above the con- dyles incipient, a feature observed in all Oligocene titanotheres; (10) incipient expansion of the zygo- matic portion of the squamosals and flattening out of the squamosal portion of the zygoma, as in later titanotheres; (11) deep backward angulation and depression of the angle of the jaw, a feature observed in certain Oligocene genera; (12) incisors tending to become round-topped {M. uintensis), a tendency that becomes very pronounced in the Oligocene genera; (13) crowns of the canine teeth abbreviated, with stumpy recurvature, foreshadowing the Oligocene Brontops and Brontotherium; (14) ectolophs of the premolar and molar grinding teeth elongated vertically, a character that becomes pronounced in all Oligocene titanotheres; (15) premolar ectolophs, showing incip- ient double convexities, a character that becomes well marked in all Oligocene genera; (16) fourth premolar showing a famt suggestion of the tetartocones {M. wasliakiensis) ; (17) premolars retarded in development. Despite the approaches of Manteoceras to the Oligo- cene Brontops in these 17 characters, there are reasons why none of the known species of Manteoceras, and especially the best known, M. manteoceras, can be considered the direct ancestor of any known Oligo- cene titanothere. This species differs from the Oli- gocene titanotheres notably in the sharp postcanine constriction of the face, the shallowness of the malar below the orbit, and the slenderness of the malar behind the orbit; and it is not yet known whether these are progressive tendencies leading away from the Oligocene type or are characters that were lost during the transformation into the early Oligocene types, such as Brontops iracTiycepJialus. Manteoceras Hatcher Plates XVI, XVII, XXIX, XLVI, LI, LIII, LV, LXIII, LXVII; text figures 27, 29, 33, 87, 113, 121, 131, 132, 210, 215, 219, 220, 255, 302-313, 323, 324, 380, 406, 408, 409, 483, 484, 508, 510, 512-517, 521, 551-557, 566, 641, 646-649, 661, 673, 674, 685, 686, 688, 690, 701, 709, 710, 712, 717, 720, 721, 723, 724, 733, 745 [For original description and type references see p. 177. For slceletai characters see p. 630] Localities and geologic horizons. — Bridger Basin, Wyo., levels C and, chiefly, D; Washakie Basin, Wyo., level A; Uinta Basin, Utah, lower part of level C {M. uintensis). Generic characters. — Facial concavities pronounced; horn rudiments borne chiefly on the frontals; suborbital portion of malars flattened, with a rudimentary shelf. Superior incisors moderately enlarged; canines robust, pointed, progressively more obtuse. Grinding series subhypsodont; molar conules vestigial or wanting; molars broader than in DolicliorJiinus or its allies; premolar evolution retarded as to tritocones, deutero- cones, and cingula; ectolophs with two convexities in tandem. As described in detail m the revision of the nomen- clature (pp. 177-178) the synonymy of the genus and type species has been confused and complicated, but it has now been definitely cleared up according to modern principles. The honor of discovering this important evolution stage of M. manteoceras and of first re- cognizing its prophetic character belongs to Wortman, who also invented the apt name Manteoceras (prophet horn). Osborn in 1895, the first to publish a descrip- tion of the skull of this animal, refrained from giving it a new name on account of the general resemblance in the teeth to the very imperfect type of Cope's "Palaeosyops vallidens." Hatcher, later in the same year, proposed the generic name and correctly de- fined the genus. Still later Osborn in manuscript referred to this form as Palaeosyops manteoceras; but this specific name is technically to be credited to Hay, who (1902.1) first fastened the specific name man- teoceras to the previous description and figures of the original "prophet horn" skulls, so that the name now stands as Manteoceras manteoceras Hay (Osborn MS.). It is very important to note that seven skulls and uppfr dentitions from the upper deposits of the Bridger Basin (mostly level D) equal or exceed in measurement and progressive characters three skuUs from level A of the Washakie Basin and thus afford corroborative evidence of the simultaneous deposition of those sediments. Materials. — A somewhat detailed enumeration of materials seems to be important in this case for purposes of geologic correlation. 1. Manteoceras manteoceras Bridger C 2: A fragmentary adult skull (Am. Mus. 12194), from Burnt Fork post office (Henrys Fork). The sagittal crest bifid with deep intermediate pit. This is in an early stage of development. Bridger D: A male skull (Am. Mus. 12683), from Sage Creek Spring, is important as supplementing the characters of the type. It probably belongs to a somewhat early stage and presents certain resemblances to the type of Telmatherium cultridens. The measurement of p'-m^ is 176 millimeters, as compared with 181 in the type of M. manteoceras. The most striking feature (see figs. 305, 307) is the depth of the preorbital concavities, which throws the frontonasal horn ridges into exceptional prominence. The horn surfaces are slightly pitted or rugose. Bridger C or D: A fuUy adult skull (Am. Mus. 1511) found on Henrys Fork. Basilar length, 447 millimeters; p>-m', 184; canines large. Probably a male specimen. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 363 Bridger C or D: Skull of a very old male (Am. Mus.) 1545 found on Henrys Fork. Basilar length, 523 millimeters; pi-m', 197; large fanged, recurved canines. Horn rudiments rugose. Bridger C or D: Skull, jaw, and parts of skeleton of a male adult, with open sutures (Am. Mus. 1587) from Henrys Fork. Affords characters of the feet. Bridger D 2: Skull, jaws, and parts of skeleton (Am. Mus. 12204). Basilar length estimated at 490 millimeters. Probably a male, aged. Grinding teeth relatively small. Affords knowledge of the femur and part of the feet. Bridger C 5: Very old female skull (Am. Mus. 12678). The first superior molar of both sides has dropped out. Canines short, recurved, cingulate posteriorly. Basilar length, 500 2. Manteoceras washakiensis Osborn Washakie A (upper levels) : Crushed but complete skull (Am. Mus. 13165) from the base of Haystack Mountain, summit of the brown sandstone, or upper part of Washakie A. A female with relatively small, obtuse, recurved and posteriorly cingulate canines. Horn rudiments slightly defined. Grinding series, pi-m^, 200 millimeters. 3. Manteoceras sp. Washakie B : Of the three specimens or cotypes described by Cope as Palaeosyops vallidens, the jaws (Am. Mus. 5098) from Mammoth Buttes appear to belong to Manteoceras sp. indet. The other cotype (upper teeth) is referred to Dolichorhinus (see below) . Figure 303. — Skulls of Manteoceras manteoceras One-fourth natural size. A, Top view; Am. Mus. 1587, Henrys Fork, Bridger Basin, Wye, upper (?) levels. B, Palatal view, chiefly from Am. IVlus. 2353, south of Haystack Mountain, Washakie Basin, Wyo., lower beds; partly restored from Am. Mus. 1570, La Clede, Washakie Basin; some details and sutures from a specimen now in the National Museum, formerly Am. Mus. 1545, Bridger Basin. millimeters; zygomatic breadth, 294 (estimated); p'-m^, 187. Equal in size to palate from the upper portion of Washakie A. Washakie A: The type skull of the species (Am. Mus. 1569) from the brown sandstones. Probably an aged male. Horn rudiments prominent, slightly rugose. Estimated basilar length 492 milhmeters; p'-m', 183. Washakie A: The type skull (Am. Mus. 1570). Adult male skull, occiput lacking. P>-m', 186 millimeters. Agrees closely in size with Am. Mus. 1511, from Bridger C or D. Washakie A: Skull with jaws (Am. Mus 2353) lacking pre- maxillae. Zygomatic breadth, 274 millimeters. The grinding teeth are of relatively small size (p'-m', 178 mm.). 4. Manteoceras uintensis Douglass Uinta C (lower levels) : The anterior half of a skull (Carnegie Mus. 2388), "from gray sandstone in red Uinta beds," the type of M. uintensis. A large male; grinding series, 240 millimeters (see below). Synopsis of progressive characters in the three succes- sive species of Manteoceras. — 1 . M. manteoceras: Levels, Bridger C 2 to D and Washakie A. Skull of medium size (basilar length 447-500+ mm.). Face relatively- short; zygomata stout; horn swelling prominent. 364 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA P'-m^ 176-186+ millimeters; postcanine diastema short; i^ much larger than i^; p" with deuterocone and tritocone poorly developed; p"* (ap. by tr.), 19 by 26 to 22 by 29 millimeters; m\ 28 by 29 to 32 by 33; m^ 35 by 37 to 41 by 40; m^, 36 by 39 to 39 by 43. 2. M.washakiensis: Level, Washakie A, upper part. Slvull somewhat larger (basilar length in supposed female 490 mm., estimated). Face relatively short; zygomata moderate; horn swelling inconspicuous (? 9 ). P'-m', 200 milhmeters (estimated); p^ with deutero- cone somewhat better developed; p* (ap. by tr.), 23 by 29 millimeters; m\ 35 by 38; m^ 41 by 43; m', 39 by 42. 3. M. uintensis: Level, Uinta C, lower part. Skull larger (basilar length not known). Face relatively long; female horn swelUng (?) absent; zygomata not stout, in inferior view forming a marked angle in front of the glenoid surface. Postcanine diastema long; disparity of i^ over i^ less marked; p'-m', 240 milli- meters; p^ with deuterocone slightly and tritocone markedly more advanced; p* (ap. by tr.), 26 by 32 millimeters; m', 40 by 38; m^, 52 by 46; m', 45 by 50. Range in size of Manteoceras manteoceras and two successive stages oj increase in size, in millimeters Manteoceras manteoceras M. wa- shakien- sis, 13165 M. uin- 12194 12678 12683 1511 12204 2353 1532 - 1569 1570 1545 2388 Skull: 500 '■294 240 255 187 447 »490 492 °310 °245 305 63 183 164 80 103 186 165 81 107 623 '350 "209 "320 -66 197 181 83 118 490 »290 Dentition : Pi-m3 1 176 160 79 100 15X8 18X16 17X21 20X26 28X29 35X37 36X39 184 168 82 104 16X11 22X19 19X25 20X28 27X°30 38X37 39X39 29X31 33X38 36X38 178 159 77 102 17X 9 18X18 18X23 21X28 30X31 38X39 35X38 »200 -183 83 116 15X 9 19X17 20X25 23X29 35X38 42X43 39X42 240 p2-in3 219 Pi-p* 101 Mi-ms P', ap. by tr 108 138 24X11 P-, ap. b3' tr _._ _ _ 22X18 20X25 23X29 33X34 42X40 42X42 21X19 19X24 20X28 31X33 38X41 38X43 "22X29 32X33 41X40 39X43 21X20 P', ap. by tr .. 24X29 P*, ap. bv tr . 20X27 29X31 38X38 40X39 26X32 M', ap. bv tr . . 40X38 M-, ap. by tr MS, ap. by tr 52X46 45X50 " Estimated. * Crushed. Numbers at heads of columns are those of the American Museum except the last (2388), which is of the Carnegie Museum. The geologic horizon and other facts concerning the specimens are given below: 12194. Intermediate molar proportions. Bridger C 2. 12678. Very old female. Intermediate molar proportions. Bridger C 5. 126S3. Young adult male. Smallest molar proportions. Bridger D. 1511. Male. Small molar proportions. Bridger (?). 12204. Very old female. Small molar proportions. Wa- shakie D 2. 2353. Female. Small molar proportions. Washakie A. The accompanymg table of measurements brings out the following facts : 1. In M. manteoceras there is a very considerable range in size: Am. Mus. 1545 is larger in total skull length than the type of M. wasliaMensis , but the first and second molars are smaller. 2. The molars in different specimens of M. manteo- ceras are either microdont (Am. Mus. 12194, 12683, 1511, 12204, 2353) or macrodont (Am. Mus. 1545, 1570), but other measurements, especially the dimen- sions of the premolars, do not confirm this division; 1532. Male. Large molar proportions. Washakie A. 1569. Type. Male. Large molar proportions. Washakie A. 1570. Paratype. Large molar proportions. Washakie A. 1545. Old male. Largest molar proportions. Bridger D (?). 13165. Female. Skull medium, cheek teeth large. Washa- kie A, upper levels. 2388. Type. Male. Skull and molars very large. Uinta C, lower level. it does not seem to be due to sex, nor, so far as known, to imply specific differences. 3. M. wasliaMensis, from the upper levels of Washakie A, is not much bigger in total skull length, but it is more progressive in the relatively large size of the molars. 4. M. uintensis, from Uinta C, is far more advanced than either M. manteoceras or M. washaJciensis in total skull length (inferred), length of face, and all dimen- sions of the dentition; but the molars are relatively more advanced than the premolars. EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 365 Manteoceras manteoceras Hay [Telmalotherium manteoceras (Osborn MS.) ; Telmatotherium vallidens Osborn, not Cope] Plates XVI, XVII, XXIX, XLVI, LI, LIII, LV, LXIII, LXVII; text figures 27, 29, 33, 87, 113, 215, 219, 255, 302-311, 323, 324, 380, 406, 408, 483, 508, 512-514, 516, 517, 521, 551-554, 556, 557, 566, 646-649, 661, 686, 701, 709, 712, 721, 723, 724, 745 [For original descriptions and type references see p. 177. For skeletal characters see p. 630] Type locality and geologic horizon. — Washakie Basin, Wyo.; Uintatherium- Manteoceras- Mesatirhinus zone (Washakie A). The most abundant material is from the Bridger Basin, Wyo., some from Bridger C but more from Bridger D. Specific characters. — Skull of medium size, basilar length, 447-500+ millimeters; cephalic indices, 58 to 68, face relatively short; zygomata stout; horn swelling prominent; p'-m', 176-186+ millimeters; postcanine diastema short; i^ much larger than i^; p^ with deutero- cone and tritocone poorly developed; p^ (ap. by tr.), 19 by 26 to 22 by 29 millimeters; m^ 28 by 29 to 32 by 33; m^ 35 by 37 to 41 by 40; m^, 36 by 39 to 39 by 43. The sTceleton. — The skeleton of Manteoceras is by no means so fully known as that of Palaeosyops. The feet are more slender than those of Palaeosyops leidyi but much more robust than those of Mesatirhinus. Intermediate proportions are seen throughout between those characteristic of Palaeosyops, the extremely broad-headed titanotheres, and of Dolichorhinus, the extremely long-headed titanotheres. In Manteoceras, therefore, the moderate breadth of the skulls (mesa- ticephaly, PI. LIII) is associated with moderate breadth of the feet (mesatipody). Progressive and specific characters: (1) Several muta- tions, subspecies, or substages from several different levels may be represented in the 20 or more specimens that have been referred to M. manteoceras; (2) as shown above, there is a considerable range in size between the smallest specimen (Am. Mus. 12683) and the largest; (3) some skulls have rather small grind- ing teeth in transverse measurement and are thus microdont; others have large grinding teeth and are thus macrodont, and this is not a sexual character; (4) in some the canines are more slender (figs. 308, 309), in others more robust, the form typical of the species being represented in Figure 311; but it is cer- tain that the canines in some lines become progres- sively obtuse and posteriorly cingulate and thus ap- proach the Oligocene types. From the detailed list of the materials given above it is seen that the known individuals from the upper levels of the Bridger Basin and the lower levels of the Washakie Basin are from a single geologic horizon — the Vintatherium- Manteoceras- Mesatirhinus zone. This horizon, however, represents a long period of time, but, owing partly to the slow rate at which the pre- molars in Manteoceras were evolving, the known specimens, although probably representing several different levels, do not present very marked progres- sive differences, except that Am. Mus. 12683, from Bridger D, is less advanced in the condition of the deuterocone of p^. Horns. — As shown in the carefully drawn detailed figures (Pis. XVI, XVII), the rudimentary horn con- vexity is borne chiefly upon an anterior spur of the frontals; it thus presents exactly the same relations as those observed in Protitanotherium emarginatum (figs. 318, 319, 374). In D. hyognathus (PI. XVII) the maximum horn convexity is on the posterior spur of the nasals, and the same is the case in the very much more rudimentary horn of Mesatirhinus petersoni. In another M. manteoceras skull (Am. Mus. 1545) the swelling and rugosity is shared partly by the nasals. In this stage of evolution, therefore, the osseous horn is, strictly speaking, a frontonasal horn. Some of the more aged specimens (especially Am. Mus. 1569) show a very faintly rugose condition of the surface of the bone on these horn bases. Proportions. — The width of these skulls (see table of measurements) is increased by the great out- ward arching of the zygomata posteriorly, the propor- tions, as presented in Am. Mus. 1569, being, length, condyles to incisive border, 492 millimeters, width 310. In other words, the zygomatic breadth is nearly three-fifths of the skull length, whereas in Mesati- rhinus petersoni the breadth is a little less than one- half the length, and in Dolichorhinus hyognathus the breadth is only a little more than one-third the length. Additional specific characters. — The other chief fea- tures of the cranium are as follows : (1) The rudimen- tary frontonasal horn swellings above described; (2) the widening of the nasals posteriorly; (3) the pos- terior spreading of the frontoparietal region; (4) the deep parietofrontal pit between the posterior portion of the supratemporal ridges, which have now almost replaced the sagittal crest; (5) the relatively broad, low occiput. In many details of structure, enumerated below, this skull unmistakably exhibits subfamily afHnity with Mesatirhinus petersoni, yet it differs from that species in many important features — namely, (1) the infraorbital ridge is incipient but not prominent, (2) the zygomata are stout and the zygomatic width of the skull is much greater than in Mesatirhinus, (3) the. basioccipital region is relatively broader and less elongate, and the same is true of the palate. The sTcull. — The superior view of the skull (fig. 304) shows several characters which are prophetic of the Oligocene Brontops: 1. The nasals are slightly expanded at the anterior extremities, measuring 63 millimeters (Am. Mus. 1569), then contracting slightly to 60 millimeters and again steadily expanding to 112 millimeters at 366 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA the junction with the frontals. The midlength of the nasals is 174 millimeters, as compared with 164 in the much smaller M. megarJiinus skull, showing that while the cranial portion of the skull has greatly increased in length, the nasal bones have not increased in length so fast as they have in MesatirMnus ; in other words, a retardation in the progressive lengthening of the nasals is observable and is more strongly expressed in M. uintensis; and this points toward the transfor- mation of the Manteoceras skull into the Oligocene titanothere type. In the contemporary D. Jiyogna- thus, on the other hand, which does not lead into an Figure 304. — Type skull of Manteoceras manteoceras Top view. About one-fifth natural size. Am. Mus 1569. Washakie Basin, Wyo., level A ( UMatherium-Manteoceras- Mesatirhinvs zone). After Osborn, Am. Mus. Nat. Hist^ Bull., vol. 7, flg. 8, 1895. H, horn swelling. Oligocene titanothere, the nasals attain the extraor- dinary length of 290 millimeters. 2. The frontal horn caps in the Eocene as in the Oligocene titanotheres overlap the outer sides of the nasals, so that the horn bases present upwardly and outwardly. 3. The orbits (Am. Mus. 1570) are 133 millimeters apart, and from the prominent triangular postorbital processes the narrow but distinctly rugose supratem- poral ridges converge backward into a broad, laterally expanded frontoparietal plate which prophetically represents the flattened summit of the Oligocene titanothere cranium. This plate flares laterally over the temporal fossae, as in many other species of titanotheres. Behind this point the supratemporal ridges converge to form a deep midparietal pit, which is apparently homologous with the vestigial pit ob- served in several species of the Oligocene Brontops; the supratemporal ridges again diverge, leaviag a narrow groove between the paired sagittal crest, which is from 18 to 29 millimeters in width. 4. A very important feature of the superior view, seen also in MesatirMnus, is the comparatively oval form of the openings left by the zygomatic arches, and the great backward stretch of the floor of the temporal fossa from the j miction of the zygomata with the skull of the occiput. The palatal view of the skull (fig. 303, B), best seen in three specimens La the American Museum, Nos. 1545, 2353, 1570, exhibits the following principal characters: (1) The base of the cranium, the mid- cranial region (postglenoid to orbit), and the face are all relatively longer than in Palaeosyops but shorter than in MesatirMnus; (2) the posterior nares open about the middle of the skull, opposite the interval between m2, ms; (3) the hard palate is gently arched; (4) the posterior narial space is elongate and nar- rowed by the decided median convexities of the pala- tines; (5) the pterygoids are greatly reduced as thin, elongate plates; (6) the relations of basicranial bones and foramina are as shown in Figure 303, B; (7) there are paired rugosities on the basisphenoid for attach- ment of the recti capitis muscles; (8) there is an elongate bridge (35 mm.) between the foramen ovale and foramen lacerum medium; (9) the inner portions of the occipital condyles are borne on the basioccipitals; (10) the paroccipital process is delicate. The anterior view of the cranium (fig. 305, B) clearly illustrates the decurved and thickened margins of the nasals, the prominence of the frontonasal horn, the deep lateral facial concavities, the character- istic structure of the premaxillaries, and the deep premaxillary symphysis. In the lateral view (figs. 307, 308) the superior profile is incipiently saddle-shaped, as in the Oligocene titanotheres, and we note that the skull descends from the occiput to the midparietal region, then arches gently upward to a point directly above the orbits, and then descends to the tip of the nasals. This facial convexity, combined with the lateral preorbital concavities, contributes to and is correlated with the prominence of the frontonasal horn. The facial concavity profile is similar to that of MesatirMnus and is totally different from the transversely convex preorbital section of Palaeosyops. Possible but doubt- ful evidence of a progressive shortening of the face is observed in the variable position of the infraorbital foramen. In most of the skulls (Am. Mus. 1570, 1511, 1587, 1545) there is a broad bridge of bone over EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 367 the infraorbital foramen, as in M. petersoni; in skull Am. Mus. 2353, on the other hand, in which the face appears exceptionally short (a condition possibly due in part to crushing), this bridge is abbreviated, the foramen issuing directly in front of the malar- lacrimal maxUlary bar. The maxillaries contribute the anterior portion of this bar. The projecting infraorbital shelf of M. petersoni is absent, being replaced by a prominent, more or less sharply convex Figure 305. — Skulls of Manteoceras manteoceras and Palaeo- syops leidyi Front view. One-fourth natural size. In Manteoceras (A) the horn region projects laterally above the facial concavity. In Falaeosyops {,B) the horn region does not so project and there is no facial concavity. longitudinal ridge, quite different from the more rounded suborbital bar of Palaeosyops or the broad, gentle convexity of T. cultridens and T. ultimum. Immediately below the orbit the malars are flat, and behind the orbit they are slightly concave; they give off the prominent postorbital processes and then gently arch outward with a convex exterior and a concave interior surface. The malars thus present two very striking differences from the Oligocene type: first, they are much shallower below the orbit and concave instead of convex externally; second, they are relatively slender and constricted behind the postorbital process. The zygomatic portion of the squamosal exhibits a wide superior as well as a broad lateral expansion, foreshadowing the decided develop- ment of this bone in the Oligocene titanotheres. Dentition in general. — The dentition as compared with that of MesatirJiinus and DolicJiorhinus agrees in the following characters: (1) The incisors are short- FiGUBE 306. — Skulls of Manteoceras manteoceras and' M. washakiensis Occipital and front views. One-fourth natural size A, M. manleoceras, from speci- mens in the American Museum, chiefly No, 1570, La Clede, Washakie Basin, Wyo., Washakie A. Nasals and occiput restored from No. 1669 (type) , Washakie Basin, Washakie A; canines and incisors from No. 1511, Biidger Basin, and No. 12678, Hemys Fork Hill, Bridger Basin, Bridger C 5. B, M. manteoceras, Am. Mus. 1587, Henrys Fork, Bridger Basin, level unknown. C, M. wasfialiensis, Am. Mus. 13165 (type), base of Haystack Mountain, east end, Washakie Basin, Washakie A. • crowned and rounded rather than conical, and the disparity of i' over i^ is less marked than in Telma- therium; (2) the sublanceolate canines are broadly obtuse at the base, taper rapidly at the summits, and have faint anterior and posterior ridges; (3) the pre- molars and molars are identical in general structure 368 TITANOTHERES OF ANCIENT WyOMING, DAKOTA, AND NEBRASKA but are of the mesaticephalic or subbrachycephalic type. The dentition differs sharply from that of Mesatirhinus and DoIicTiorMnus in the relatively short diastema behind the inferior canines and the retarded Figure 307. — Skull of Manieoceras manieoceras One-fourth natural size. Am. Mus. 1569 (type) and 1570. Washakie Basin, Wyo., level A Side view, view (reversed) of the tj^pe and paratype skulls development of the deuterocones and tritocones of the premolars. Incisors. — The inferior incisors as seen in Am. Mus. 1566 approach the transverse or slightly arched posi- tion observed in M. petersoni (Am. Mus. 1567) and in DoIicTiorMnus. The perfectly preserved crown of i2 indicates that this tooth is slightly larger than ii and nearly if not quite as large as is; the posterior face is smooth, with a faintly indicated basal cingulum. Canines. — The canines of the same jaw in the male are estimated as 40 millimeters in height, 21 antero- posterior, 21 transverse; they are implanted by stout fangs which cause the outer face of the ramus to bulge; faint anterior and posterior ridges bound the convex inner face of the canines; the diastema is much shorter than in DoIicTiorMnus (17 mm. in No. 1566). The superior incisors, partly preserved in Am. Mus. 1511, 1545, and fully preserved in Am. Mus. 12683, from Bridger D, are pointed, with a posterior basal cingulum slightly more promi- nent than in DolicTiorTiinus; in Am. Mus. 12683 they increase regularly in size from i' to i' and at first sight resemble those of TelmatTierium culfridens, but, as shown in Figure 309, they are distinguished by their slightly smaller size, weaker posterior cingulum, and less caniniformi^ The superior canines are robust, sublanceolate (that is, with anterior and posterior ridges), tapering and recurved; height 39 millimeters, transverse 26, antero- posterior 24; they are provided with very stout fangs. The postcanine diastema is very short, not exceeding 7 millimeters. Premolars. — The superior molar-premolar series, although entirely broken away in the type specimen, is superbly shown in five almost complete sets of teeth. In general, as compared with the grinders of MesatirMnus and DoIicTiorMnus, we observe the brachycephalic influence, indicated, first, in the rela- tive shortness and breadth of each of the teeth, and second, in the arching or posterior divergence of the series, which is much more marked than in Mesa- tirMnus. The series meas- ures from 176 millimeters in Am. Mus. 12683 to 197 in the old male No. 1545, as compared with 160 in M. megarMnus, and is about 20 millimeters greater than in DolicJiorMnus. The chief distinctions of the premolars from those of MesatirMnus appear to be as follows (Pis. LXVII, LXXII): In M. manieoceras p^~* are relatively broader, the deuterocones are not so wide anteroposteriorly and lie farther backward, giving a more oblique contour to the crown; the ectoloph is less hypsodont, its anterior convexity is much broader, and its posterior convexity is pronounced. The external cingulum, while variable, is vestigial in Am. Mus. 1511, 2353, and slightly indicated in Am. Mus. 1570, 1532. P' is a simple, elongate tooth, bifanged; the tritocone ectoloph is convex, and in certain specimens (Am. Mus. 1511) the tritocone nearly equals the protocone Composite side After Osborn, Am. Mus. Nat. Hist. Bull., vol.V, flg. 7, 1895. FiGUEE 308. — SkuU of Manieoceras manieoceras One-fourth natural size. Am. Mus. 12683, Sage Creek Spring, Bridger Basin, Wyo., level D. Ai, Anterior half of skull; A2, cross section through malar and m^, showing flattened external face of malar. in size; this tooth, nevertheless, still retains the ancestral character of elongation and the marked postero-internal position of the deuterocone. P^ is slightly broader than long and exhibits various degrees in the prominence and external convexity of the tritocone; the external cingulum in the less worn dentitions is well defined. No rudiment of the postero- internal cusp can be discerned except in one very large individual (Am. Mus. 1532), in which very faint indications of this fourth cusp are seen in p^ and p*. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 369 In p^, a larger tooth, we observe a more constant enlargement of the tritocone. The cingulum in all these premolars (except Am. Mus. 1532, the most pro- gressive specimen) fails to encircle completely the inner sides of the obtuse deuterocones. Comparative measurements of the premolars are given below. The inferior premolars are represented in Am. Mus. 1566, 1563, 2353. Of these. No. 1566 represents the least progressive stage. The premolar measurements here given relate to this specimen. Pi (ap. 12 mm., tr. 9) is a simple, laterally compressed cone. P2 (21 by 11) has an elevated protocone, slightly hollowed on its antero-internal border, with a rudimentary cuspule (=paraconid of molars) and a low postero- external cusp (=hypoconid of molars). In ps (18 by 12) the cusp analogovis to the metaconid in the molars is beginning to be constricted off from the protoconid and is defined by a prominent antero-internal con- vexity, and the anterior (trigonid) and posterior (talonid) crescents are beginning to be marked, but the entoconid, as in all middle Eocene titanotheres, is not yet developed. In p4 (20 by 14) we have a submolariform tooth lacking only the cusps analogous to the paraconid and entoconid but with a well- developed cusp analogous to the hypoconid. Molars. — The subquadrate sujJerior molars, which vary in longitudinal measurement from 100 to 118 millimeters (a wide range of variation), as well dis- played in five individuals, exhibit (1) progressive ex- ternal cingula with internal cingula in the valleys; (2) protoconules vestigial or wanting; (3) a marked angulation of the postero-internal border of m^ accompanied by a prominent elevatipn of the cingulum at this point, and in one specimen (Am. Mus. 1511) a small, distinct hypocone, which, however, may represent a metaconule. All the molars in the speci- mens at hand are too much worn to give the height of the ectoloph, which was undoubtedly elongate in the unworn condition and which slopes strongly inward. Comparative average measurements {ap. by tr.) of molars of Manteoceras, Mesatirhinus, and Dolichorhinus, in millimeters Manteoceras manteoceras (6 skulls) Mesatirhinus petersoni (4 skulls) Dolichorhinus hyognathus (4 skulls) Ml M2 29X31 38X38 38X40 25X26 31X32 32X34 35X35 43X44 M3 45X43 The inferior molars, well preserved in Am. Mus. 1566, 2353, 1563, are uniform in character, measuring from 111 to 118 millimeters, with an incomplete external cingulum which fdls the valleys and in No. 1566 rises behind the talonid into a rudimentary entostylid, or reduplication of the entoconid. Beside this reduplication we observe an incipient metastylid or reduplication of the metaconid. The teeth rapidly increase in length as we pass from mi to ma, the measurements in No. 2353 being mi 28 millimeters, m2 35, ma 53. The lower molars show a strong sub- family resemblance to those of Mesatirhinus but are distinguished by their somewhat larger size (mi_3 110 mm. in Am. Mus. 1566, as compared with 106 in the type of M. petersoni) and especially by their greater breadth (ms (tr.) 23 mm.; in M. petersoni 18). In Manteoceras also the cutting V's, as seen in crown view, form more acute angles than in Mesatirhinus. These differences are still more emphasized by com- parison with Dolichorhinus hyognathus, which has long, narrow molars, wide-angled V's, and a relatively small hypoconulid on ms. Jaws. — Many fine jaws belonging to this animal have been collected by the American Museum ex- peditions in the Bridger and W a- shakie Basins. Un- fortunately only a few are associated with the skulls, namely, Am. Mus. 1545, 1587, 12204, 2353. Another not associated is Am. Mus. 1566 (Bridger D). As shown in the plates and figures, the jaws, like the skulls, teeth, and feet, show certain charac- ters prophetic of the Oligocene titano- theres. These char- acters are always most clearly dis- played in the aged forms and are (1) the depressed lower °^^'S!S^: border of the angle; (2) the pit in the an- terior border of the coronoid behind ms; (3) the breadth and vertical elevation of the coronoid proc- ess and sharp recurvature at the summit; (4) the greater prominence and convexity of the chin. The coronoid process is more robust and less strongly recurved than in Telmatherium cultridens; it is decidedly more elevated and less falciform and recurved than in Dolichorhinus hyognathus. It is thus more of the type which we should expect to find as ancestral to the coronoid processes of the Oligocene titanotheres. Figure 309. — Incisors and canines of Manteoceras manteoceras Am. Mus. 12683, Sage Creek Spring, Bridger Basin, Wyo., level D; supposed male. B, Am. Mus. 12678, Henrys Fork Hill, Bridger Basin, level C 5; aged female. 370 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The peculiarities of the jaw of M. manteoceras (fig. 310, PI. LI) are as follows: 1. In most of the jaws there is a narrow postcanine diastema (18 mm. in No. 1566). 3. The powerfully rooted canines involve a sharp convexity of the bone opposite the alveoli, followed by a sharp depression in the outer face just below the postcanine diastema. 4. The chin, as seen from below, is broad and gently convex anteroposte- riorly and transversely, but on account of the great variation in both species it is difficult to express exactly the differ- ences in the horizontal rami between Manteoceras manteoceras and Mesati- rhinus petersoni. 5. The principal constant difference is the weaker chin of Mesatirhinus, in old jaws of which the lower border of the ramus is straighter than in Manteo- ceras; the ramus also seems stouter and deeper and rapidly increases in depth posteriorly from 54 millimeters behind Pa to 64 behind m2 and 87 behind ma. 6. In adult jaws the coronoid is of an entirely different shape from that of Palaeosyops, being elevated and broad at the top with the summit gently recurved, whereas in Palaeosyops it is pointed and well recurved at the top. It resembles that of Mesatirhinus but lacks the posterosuperior prolongation, and the superior portion of the process is flattened anteriorly with sharply angulate anterior external and internal borders. 7. The angle is produced decidedly downward and backward, and there is a rugose outer border in the old males. 8. The posterior border of the jaw between the angle and the condyle is somewhat incurved in some specimens but nearly straight in others. Measurements of lower jaws of Manteoceras manteoceras, in millimeters Figure 310. — Lower jaw of Manteoceras One-fourth natural size. A, M. manteoceras, Am. Mus. 1566, Bridger Basin, Wyo. level probably C or D. B, M. manteoceras, Am. Mus. 1563, Bridger Basin, level probably D; very progressive lower jaw (front part corrected from No. 1560). C, M. uintensist. Am. Mus. 2033 (reversed), White Elver, Uinta Basin, Utah, Uinta C; doubtfully referred lower jaw. Pi-mj Pi-P4 Mi-m3 Condyle to incisive border Condyle to bottom of angle Depth of ramus below m3 Length of symphysis- Am. Mus. 1566; Bridger C orD 182 73 110 360 165 86 109 Am. Mus. 2353; Washa- kie A 195 76 117 96 2. The opposite incisor series are placed more trans- versely than in the premaxillaries above, in which they are more convergent. Detailed cTiaracters. — Probably as a sex- ual character the skull itself is not so large as that of the largest Bridger specimens, the estimated basilar length (490 mm.) being less than in the large male M. manteo- EVOLUTION OF T£E SKULL AND TEETH OP EOCENE TITANOTHERES 371 greater than in M. manteoceras and with complete internal cingula; superior canine in female (?) obtuse, recurved, with heavy posterior cingulum; p'-m' 200 millimeters (estimated), p^ with deuterocone somewhat better developed, p* (ap. by tr.) 23 by 29; m' 35 by 38, m^ 41 by 43, m' 39 by 42. Face relatively short, zygomata moderate, horn swelling inconspicuous. The female type skull (Am. Mus. 13165) of this species was found by the American Museum expedition of 1906 at the base of Haystack Mountain, at the summit of the exposures of level Washakie A. Its decidedly progressive characters beyond those of M. manteoceras, from Bridger D and Washakie A, per- fectly accord with its somewhat higher geologic level. These are displayed chiefly in the canine, premolar, ceras (Am. Mus. 1545) from Bridger D, where the length is 423 millimeters. Similarly, from sexual causes the horn rudiments and facial concavities are not pronounced. The posterior sagittal crests are characteristically bifid, or deeply grooved superiorly, terminating anteriorly in the parietal pit which is so distinctive of this species. The occiput is distin- guished by the very decided prominence of the occipital pillars. Dentition. — It is the teeth which afford the most marked distinctions of this species. The Cannes (ap. 23 mm., tr. 21) are abbreviate, measuring 24 millimeters in length, the tips being worn off. P^ is a compressed, conical, bifanged tooth, measuring (ap. by tr.) 15 by 9 millimeters. P^ (19 by 17) exhibits marked external convexities and a weak ex- ternal cingulum. As there is considerable variation in the strength of the cingulum in M. manteoceras it is un- certain whether the weak cingulum is progressive or not. The deuterocone is more advanced in develop- ment than in the average M. manteoceras. P' (ap. 20 mm., tr. 25) exhibits the tetartocone fold somewhat more conspicuously than ;'n the most progressive Bridger D specimens. P^ (23 by 29) is progressive in transverse measurement and in the development of a low, barely perceptible te- tartocone swelling. M' (35 by 38) exhibits a prominent internal cingulum, which is almost continuous around the lingual side of the pro to- cone. M^ (42 by 43) shows a strong development of the cingulum (progressive), a and molar teeth. The horns are inconspicuous, prob Figure 311. — Skulls of Manteoceras manteoceras and M. washakiensis Side view. One-fourtli natural size. A, if. washakiensis , Am. Mus. 13165 (type); base of Haystack Mountain, east end, Washakie Basin, Wyo., upper levels of Washakie A. B, M. manteoceras, Am. Mus. 12678; Henrys Fork Hill, Bridger Basin, Wyo., level C 5; supposed female skull; back of skull slightly raised to correct the vertical crushing. crenulation of the enamel, and an elongate ectoloph. M^ (39 by 42) is slightly inferior in size to m^, the cingulum is most pronounced, and there is a well- developed hypocone ridge (progressive) but no dis- tinct hypocone. Manteoceras .washakiensis Osborn Plate LXVII; text figures 121, 306, 311, 717 [For original description and type references see p. 182] Type locality and geologic horizon. — Base of Haystack Mountain, Washakie Basin, Wyo.; summit of Uinta- therium- Manteoceras- MesatirMnus zone (Washakie A). Specific characters. — Skull somewhat larger than in M. manteoceras (basilar length in supposed female 490 mm., estimated). Superior molars and premolars ably because the specimen represents a female, and they are smooth rather than rugose. The small size of the canines is also a sexual character, but the form of the canines is very different from that of the specimens in Bridger D and Washakie A in their approach to the obtuse form characteristic of the Oligocene Brontops. Progressive characters. — (1) Canines short, obtuse, recurved; (2) internal lobes of p^ and p^ broadening, with shelf for development of deuterocone; (3) a tetartocone spur observed in p', as in most progressive Bridger specimens; (4) very distinct internal cingulum on m'~''; (5) true molar series relatively longer as compared with the premolar series than in M. manteo- ceras, in which an average of six skulls gives the 372 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, , AND NEBRASKA length pi"* as 76 per cent of that of m'~^ but in M. washaJciensis p^"* is only 71 per cent; in other words, the molar series is relatively longer, which might be expected, as there is a steady general increase in the relative length of the molar series as we pass from the lower Eocene Eotitanops to the Oligocene Brontofherium. Jaws of Manteoceras washakiensis? from Washakie A and B. — In the jaw from the base of Washakie A (Am. Mus. 13176) doubtfully referred to M. wasTia- Iciensis the dental measurements are all larger than in the well-preserved jaw of M. manteoceras forming Am. Mus. 1566 (p2-m3 182 mm. as compared with 168), and the premolars are distinctly more progressive. The large jaw from the Bridger (? level D) (Am. Mus. 1563) referred to M. manteoceras agrees nearly in size with Am. Mus. 13176. It is noteworthy that in all the explorations of the Washakie B and Uinta B levels few or no remains refer- able to Manteoceras have been found. The single exception, and this of doubtful character, is the jaw employed by Cope as the first cotype of his species " Palaeosyops" vallidens. This specimen (Am. Mus. 5098), consisting of the two incomplete rami, from Mammoth Buttes, Bitter Creek, regarded as an upper Washakie level, apparently represents the Washakie B stage of Manteoceras. (1) The measure- ment of p2-ni3 (187 mm.) agrees almost precisely with that of a M. manteoceras jaw (Am. Mus. 2353) as- sociated with a skuU; (2) the measurements of ms (ap. 57 mm., tr. 23) indicate a slightly longer tooth than that in Am. Mus. 2353 (54 by 23). The char- acters of the teeth and jaws are otherwise the same. Manteoceras uintensis Douglass Plate LXIII; text figures 131, 132, 310, 312, 313 [For original description and type references see p. 186] Type locality and geologic Jiorizon. — About 5 miles northeast of well 2, Uinta Basin, Utah; gray sandstone in lower part of Uinta formation {Diplacodon-Pro- titanotJierium-Epihippus zone, Uinta C). The per- sistence of Manteoceras in Uinta C is surprising because that level also furnishes the next higher stage in the phylum, namely, Protitanotherium. This persistence is partly explainable by the fact that M. uintensis is apparently an aberrant side branch with a decided elongation of the muzzle and to some extent of the tooth rows, or grinding series; in other words, it gives certain indications of dolichopy, whereas the main line {Protitanotherium) is mesaticephalic. Specific characters.- — SkuU larger than in M. manteo- ceras (basilar length not known) ; muzzle relatively long, horn swelling (?) not larger than in M. manteoceras; zygomata not stout, in inferior view forming a marked angle in front of the glenoid surface. Postcanine dia- stema long (28 mm.), postcanine constriction very marked; superiority in size of i' over i^ appearing less marked than in M. manteoceras; p'-m^ 240 millimeters; p^ with deuterocone and tritocone more advanced than in M. manteoceras; p* (ap. by tr.) 27 by 34; m\ 37 by 38; m', 49 by 52. That this species should be referred to the genus Manteoceras is indicated by the round-topped incisors, the robust, recurved canines, the twin convexities of the premolar ectolophs, the broad, subhypsodont m', with large parastyles and mesostyles, and the widely arched zygomata. From the Bridger and Washakie Basin species of Manteoceras the present one is dis- tinguished by its larger size, more dolichocephalic appearance, shorter free nasals, slightly more progres- sive premolars, and more pronounced postcanine con- striction. All these characters serve also to dis- tinguish M. uintensis from Sthenodectes incisivus (Douglass), which has very large, "cupped" incisors, long, lanceolate canines, and very advanced premolars. Affinities with Protitanotherium emarginatum and P. superbum are indicated by the general form of the incisors and canines, short nasals, and broad molars; but weU-marked differences from these forms are seen in the more elongate face, the larger size of the incisors, more pronounced postcanine constriction, and absence or arrested condition of the horn swelling. A remote analogy to Dolichorhinus is seen in the lengthening of the face, of the postcanine diastema, and of m^, as well as in the broad arching of the incisor series. These dolichocephalic features in a mesati- cephalic skull illustrate the subfamily kinship of Manteoceras with Dolichorhinus. The skull as a whole must have been large, for the distance from the premaxillaries to the glenoid region of the squamosal is given as 430 millimeters, as com- pared with 335 for the same measurement in M. manteoceras, an increase of nearly 25 per cent. The premaxillaries in correlation with the large size of the incisors are angulate superiorly; the free nasals are short (85 mm.), a progressive feature; the face is long (255 mm., as compared with an average of 184 in M. manteoceras). The horn swellings were certainly not larger than in M. manteoceras — a surprising fact, because the large canines and incisors indicate male sex. The infraorbital canal is broad; the zygoma is deep in its middle portion, but apparently the buccal swelling was slight or absent. Just back of the orbit the malar was slender but not so slender as in M. manteoceras; in inferior view the infraorbital portion of the malar formed a low ridge which was somewhat more pronounced than in M. inanteoceras but did not form a distinct shoulder as it does in Mesatirhinus and Dolichorhinus. The posterior nares open opposite the metacone of m^, whereas in M. manteoceras they sometimes open opposite the para- cone of m'. Passing to the dentition, we note that the incisors are intermediate in form between those of Manteoceras manteoceras and of Protitanotherium emarginatum, EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHEEES 373 although nearer the former in shape and much larger than in the latter. The anteroposterior and trans- verse dimensions of the incisors are, i', 18 by 14 millimeters; i^, 19 by 17; i^ 22 by 20. The incisors in general resemble those of M. manteoceras in their rounded front faces and obtusely pointed tips but differ in the form of the posterior cingulum, which is now represented by a very large, obtuse basal rim. The posterior face of i', i^ shows a median vertical ridge. The posterior face of the large i' is much worn. The canine is very stout, with widely protruding and divergent fangs; crown measurements (ap. by tr.), 27 by 20 milhmeters ; it is followed by a large post- canine diastema (26 mm.), which distinguishes this species from M. manteoceras, and also by a marked postcanine constriction. Premolars. — P' (ap. 22 mm., tr. 11) seems to be a somewhat more compressed, elongate tooth than in the preceding species, in which the pos- terobasal cingulum is produced upward along the median line into a very low incipient cusp. In p^ (25 by 21) the deuterocone is still confined to the postero-internal corner of the tooth but is a little more advanced than in M. manteoceras; the tritocone is also slightly more developed but not ^ so much as in either StJienodectes incisivus or Doli- chorMnus. The external cingulum opposite the tritocone is rounded. In p^ (24 by 29) the deutero- cone has a rather backward appearance as com- pared with the same cusp in M. washakiensis, but the internal cingulum is pronounced, although still incomplete opposite the middle of the deutero- cone; the protocone is still much larger than the tritocone; the "parastyle" is pronoupced; the ecto- loph convexities are marked; the external cingulum is pronounced except where it "festoons " the proto- cone convexity. In p^ (28 by 33) the deuterocone seems less robust than in M. wasJialciensis; the cin- gulum is robust but does not surround the deu- terocone; the "parastyle" is prominent, and the protocone convexity broad at the base; the trito- cone is at least no bigger than in M. wasJialciensis; the external cingulum is a broad, rounded ridge opposite the tritocone. Molars. — The molars (m^-m^, 138 mm.) are some- what larger than those of M. wasJialciensis but other- wise agree fairly well. The external cingulum is very pronounced opposite the valleys. The molars show a marked asymmetry of the external V's, the anterior V being more widely open and the posterior having a short posterior limb. The antero-internal cingula are heavy but not complete around the inner sides of thepro- tocones (contrast M. wasJialciensis). In m^ the postero- internal corner is less angulate than in M. wasJiaJciensis. The anteroposterior and transverse dimensions are, m\ 37 by 38 millimeters; m^ 48 by 48; m', 48 by 52. Other measurements of M. uintensis are given in the table above. Manteoceras uintensis? From the base of Uinta C in 1895 Peterson obtained a very large skull, apparently female (Am. Mus. 2029), which unfortunately is too aged as well as. too much crushed and imperfect to afford distinctive characters for definition. It is somewhat smaller than the type of M. uintensis. If more complete it might be found to Figure 312. — Type skull of Manteoceras uintensis One-fourth natural size. Carnegie Mus. 2388. "About 5 miles northeast of well 2, Uinta Basin; from gray sandstone in red Uinta beds, lower portion of horizon C." Ai, Side view, nasal region crushed; A2, palatal view, crushed laterally. represent a female of M. uintensis (with which it agrees in the retarded evolution of the premolars) or a species transitional between Manteoceras wasJiaJciensis and a higher stage of evolution. Size or metatrophic characters are truly progressive and undoubtedly correlated with other characters of distinct specific value. Its total length, condyles to incisive border, is about 555 millimeters, as compared with 523 in the largest skull of M. manteoceras found in Bridger D. The grinding teeth, p^-m^, measure 227 millimeters, as compared with 203 in the largest of the specimens of M. manteoceras from Bridger D and 240 in M. uintensis. Its progressive zygomatic brachycephaly is indicated by the widely arching zygomata, which attain a transverse width of about 360 millimeters. 374 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA the relations of width to length thus being width 360, length 555. The reference to Manteoceras is con- firmed by the obliquely flattened form of the infra- orbital portion of the malars, a character by which this animal may readily be distinguished from the con- FiGURE 313. — Upper canines and incisors of Manteo- ceras uintensis One-haU natural size. Carnegie Mus. 2388 (type). "About 5 miles northeast of well 2, Uinta Basin; from gray sandstone in red Uinta beds, lower portion of horizon C." ttemporary Telmatherium ultimum, though there is no infraorbital shelf. The proportions of this part of the skull indicate an elongation of the facial region (as in M. uintensis), a feature certainly not charateristic of the line leading to Protitanoiherium. The animal is undoubtedly a fe- male, as indicated by the relatively small canine teeth. A lower jaw (Am. Mus. 2033, fig. 310, C) found in the same region and at first referred to Telmatherium ultimum (cf. above) agrees in size with this specimen. It also belongs to a female animal and is of the mesaticephalic rather than dolicho- cephalic or brachycephalic type. The canines are relatively small and recurved. M3 is a relatively short (63 mm.) rather than elongate tooth as in Telmatherium ultimum (76 mm.). Protitanotherium General characters. — In their phylogeny the animals known as Protitanotherium are among the most interesting of the titanotheres that lived in the Uinta Basin, because of their evident relationship to Man- teoceras on the ancestral side and to Brontops and other Oligocene titanotheres on the descendant side. The profile figure of the horn region of Manteoceras manteoceras, P. emarginatum, two young skulls of the Oligocene Brontops hrachycephalus, Allops marshi, and Brontops rohustus illustrates the resemblance (fig. 712). These upper Eocene animals are robust, massive, vigorous, and well protected both by their powerful tusks and by their rapidly developing horns, which are far more prominent than those of any of the earlier Eocene titanotheres. Therefore the suggestion of Hatcher (1895.1, p. 1084) that this animal should be called Protitanotherium was eminently appropriate. Progressive characters. — The phyletic increase in size of the animals of this series is best illustrated by comparison of the adult jaws of M. manteoceras, P emarginatum, and P. superbum (figs. 310, 315, 321). The preservation of the ancestral phyletic character in these jaws is certainly very striking. The change is chiefly proportional, or quantitative. The differen- tial or generic distinctions are to be found especially in the teeth and in the horns. Specific stages. — These protitanotheres are known to include two stages — (1) P. emarginatum Hatcher, originally described as Diplacodon emarginatum, found 'y Figure 314. — Restoration of Protitanotherium emarginatum By Charles E. Knight. About one-ninth natural size. near the base of Uinta C, or the "Diplacodon beds" of Marsh, and distinguished from the following stage chiefly by its inferior size and more brachyodont teeth, and (2) P. superbum Osborn, a much larger animal, whose remains were probably found at a higher geologic level. Horns. — The bases of the horns preserve the Eocene anteroposterior elongation. This elongate oval form is in marked contrast to the obliquely oval, triangular, or transversely elongate form of adult Oligocene titanotheres. In very young Oligocene titanotheres, however, the resemblance in the elongate oval horn swellings to those of Protitanotherium is very noticeable (fig. 374). Size. — In the species P. superbum these animals attain a size considerably surpassing that of many of the smaller forms in the lower Oligocene levels. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 375 Protitanotherium Hatcher Plates LXVIII, LXIX; text figures 24, 29, 87, 112, 128, 314^321, 371, 374, 375, 408, 409, 594-596, 647-649, 701, 712 [For original description and type references see p. 176. For skeletal characters see p. 656] Generic characters. — Horns relatively large, elliptical LQ section, with anteroposterior diameter greatly Comparison. — We at first note the incipient loss of the piercing function of the incisor teeth. The lateral superior incisors are still large, but the median incisors are reduced. This may indicate that these animals were given to browsing and that the tongue and lips were increasingly used for the prehension of food, while the incisors became functionless and gradually aborted. Figure 315. — Lower jaws of Protitanotherium and Brachydiastematherium le-fourth natural size. A, P. emaTginaium, Princeton Mus. 11242 (type); Uinta Basin, Utah, Uinta C; region of angle and ma supplied from Am. Mus. 2028. B, P. superbum, Am. Mus. 2501 (type), reversed; Uinta C. C, B. tTansilvamcum, front type of lower jaw; upper Eocene (?) of Andrashiza, Transylvania. exceeding the transverse. The incisor series numeri- cally typical, -1; canines relatively large, robust and recurved; p^ submolariform but without entoconid; P3, p2 transitional. Materials. — These animals are known from three specimens referred to P. emarginatum and three referred to P. superbum. The lower grinding teeth are fully known, but the upper grinding teeth are only partly known. 101959— 29— VOL 1 27 We observe in comparison with Manteoceras that the nasals have taken on the broad, quadrate character which distinguishes the nasals of certain of the lower Oligocene titanotheres, such as Brontops and Menodus. The horns are intermediate in evolution between those of Manteoceras and of Brontops. The incisor teeth still retain the proportions observed in Manteoceras, but the median upper incisor is acquiring the rounded form characteristic of most OUgocene titanotheres. 376 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA FiGnRE 316. — Type skull of Protitanotherium emarginatum Less than one-fourth natural size. Princeton Mus. 11242, Kennedys Hole, 8 miles north of White Elver and 25 miles east of Ouray Agency, Uinta Basin, Utah; Uinta C. Provisional reconstruction of skull and lower jaw, front part directly from the type. Figure 317. — Type skull of Protitanotherium emarginatum One-fourth natural size. Princeton Mus. 11242, Kennedys Hole, 8 miles north of White Elver and 26 miles east of Ouray Agency, Uinta Basin, Utah; Uinta C. Ai, Side view of front part of skull; Aj, front view of front part of skull and lower jaw. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 377 The canines are intermediate in form between the Manteoceras and Brontops types, perhaps nearer Brontops, and were evidently robust fighting weapons. Unfortunately the form of the superior and posterior parts of the skull is still unknown. D Figure 318. — Nasal region in three specimens of Protilanotherium Top view. One-fourth natural size. A, X), P. emarginatum, Princeton Mus. 11242 (type); B iuperbumf, Carnegie Mus. 2855; C, P. superbumf, Princeton Mus. 11213. Protitanotherium emarginatum Hatcher [Diplacodon emarginatus Hatcher, p. 177] Plates LXVni, LXIX; text figures 24, 29, 87, 112, 314-320, 374, 375, 408, 648, 649, 712 Type locality and geologic horizon. — Kennedys Hole, 8 miles north of White River and 25 miles east of Ouray Agency, Uinta Basin, Utah; Uinta formation {Dipla- codon-Protitanotherium-EpiMppus zone, Uinta C). Specific characters. — Pi-ms 294 millimeters (esti- mated), pi with a small talonid; ii small, round-topped; is large, bluntly pointed ; ia much larger than is. Lower canine more erect, recui'ved, and abruptly swelling at the base. Postcanine diastema short (27 mm.) but relatively longer than in P. superbum. Lower pre- molars and molars more brachyodont, with sloping curves. P. emarginatum appears to be closely allied to P. superbum but is apparently a lower stage, distinguished by its smaller size, more brachyodont premolars and molars, and more erect recurved lower canines, which swell more rapidly at the base. Materials. — This species is represented by the type (see below), by the finely preserved anterior portions of a jaw in the Yale Museum (No. 635 D), and by a fairly well preserved jaw in the American Museum (No. 2028). The jaw in the Yale Museum is especially valuable because it includes, besides the incisors, canines, and portions of the pre- molars, a complete ma, a tooth which is imperfect in the type. Skull. — The type of the present species, discovered by Hatcher himself, is the an- terior portion of a skull and lower jaw (Princeton Mus. 11242). Hatcher noted the greater size of this animal than Dipla- codon elatus as indicated by the length of the grinding teeth (294 mm., as compared with 244). So far as preserved the dorsal surface of the skull is concave anteroposte- riorly and suggests the broad, fiat frontal region and flattened parietal vertex of the Oligocene forms. The nasal openings are high and deeply incised. The horns are composed of the frontals overlapping the nasals; they are placed longitudinally and directed upward, outward, and forward. The nasals are broad, strong, and rather short, firmly coossified, concave inferiorly; they measure in free length 107 millimeters and in greatest breadth 123. The specific name P. emarginatum, refers to the fact that the nasals are "emarginate anteriorly," but they are not deeply indented in the midline, as in a larger pair of nasals also described by Hatcher (fig. 318) which may be referred provisionally to P. superbum. The premaxillaries are well developed and separated anteriorly by a deep median notch, below which they are firmly coossified. The maxilla- ries are expanded at the base of the canines and decidedly con- stricted between these teeth and p'. The postcanine diastema measures 37 millimeters, and back of this the maxillaries ex- pand rapidly in order to ac- commodate the large posterior premolars and molars. The in- fraorbital foramen was probably situated just above p*, as in Palaeosyops, Limnohyops, Tel- matherium, and Manteoceras. In Mesatirhinus and Dolichorhinus it is more nearly above m^ Figure 319. — Sections of the nasals and horns of Protitano- therium einar gi- natum Princeton Mus. 11242 (type). One-seventh natural size. 378 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Dentition. — The statements made below refer to the type specimen unless otherwise indicated. The su- perior incisors in general are of the M. manteoceras type but exhibit several differences, which, on the whole, are progressive toward the Oligocene titano- theres of the genus Brontops. The median pair of superior incisors (i^) are distinctly the smallest of the series and are assuming the degenerate conical form seen in the Oligocene species of Brontops,'^'' a change that is effected especially by the reduction of the posterior cingulum and the rounding together of the anterior and posterior faces, with the consequent loss of prehensile or cropping function. The inferior series are all still pointed, jDosteriorly cingulate, and functionally of the cropping type; 12 is the largest of the series and ia is the most reduced. The superior incisors are nearly in a transverse line, but i^ is slightly behind i^. The incisors are separated from the terior faces, and with low posterior U-shaped basal cingula. The median incisors (ii) are somewhat compressed laterally (height 15 mm., ap. 14) but exceed in size the lateral pair. The second incisors (i2) are much larger (height 18, ap. 18) and the lateral incisors (ia) are the smallest (height 12, ap. 12, tr. 12) and the simplest, although still incisiform. The canines are distinguished by robust, rounded fangs, which diminish rapidly into forward-directed, tapering, pointed crowns, again suggesting those of M. manteoceras, especially by the posterior swelling at the base and the sublanceolate, internally flattened apex with faint anterior and posterior ridges. In the superior canines the diameters at the base are trans- verse 26 millimeters, anteroposterior 28; height 49. The inferior canines exhibit much the same form (ap. 32 mm., tr. 28, height 52), the base of the crown sloping Figure 320. — Lower jaw of Protitanotherium emarginatum One-fourth natural size. Partial reconstruction of anterior part of jaw to ms from Princeton Mus. 11242 (type), Kennedys Hole, 8 miles north of White River and 25 miles east of Oiu-ay Agency, Uinta Basin, Utah, Uinta C; ms and posterior part of jaw from Am. Mus. 2028, White River, Utah, Uinta C. canines at the sides by a narrow diastema (9 mm.) and separated in the median line by a diastema (10 mm.) similar to that observed in Mesatirhinus mega- rhinus. The lateral incisors (i^) are large pointed teeth (height 21 mm. side, 17 front; ap. 17) with oblique posterolateral basal cingula and rather sharp lateral cutting edges. The second incisors (i^) are much smaller (height 12 mm., ap. 14) with heavy posterior basal cingula connected with the apex of the tooth by a median ridge which divides the somewhat concave posterior surface. The median incisors (i') are still smaller (height 10 mm., ap. 11), with subspherical crowns and posterior median ridges rising to unite with the apex of the crown and obscure the cingulum except on the posterolateral side. The transverse extent of the inferior incisors is 69 millimeters; they aU exhibit pointed crowns, with uniformly convex anterior faces, more concave pos- " In the Oligocene genera probably the median pair of superior incisors (iO and he lateral pair of inferior incisors (ij) had disappeared, leaving i>, i' and ii, ia (see p. 448). backward into a deep, powerfully implanted fang, the crown diminishing rapidly as it rises to a rounded, sub- lanceolate apex. The premolar-molar series are separated by a short diastema from the canines in both jaws (27 to 30 mm. above, 23 below). A postcanine diastema of varying length is seen in Telmatherium ultimum, T. altidens, Manteoceras uintensis, Diplacodon elatus, and Protitano- therium superhum, an indication that it is independ- ently preserved in different phyla. Of the superior teeth unfortunately p^ only is preserved. This is a bifanged tooth measuring (ap. by tr.) 20 by 11 milli- meters, with a simple protocone, a sessile or rudimen- tary posterior heel, and a postero-internal cingulum anci concavity. The alveolus of p" shows that it was a broader and much more advanced tooth than that in Manteoceras manteoceras. The inferior grinding teeth measure about 294 millimeters in length, as compared with 180 to 192 in M. manteoceras. In the type the lower premolar series measures 103 millimeters on the left side, in which pi is abnormal EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 379 There is a marked asymmetry of this tooth on the two sides of the jaw, the right tooth being much smaller than the left and of a form normal in upper Eocene titanotheres. The abnormal pi is much swollen, a tendency seen also in the other cheek teeth and even more emphasized in the Oligocene titanotheres. The normal pi (ap. 16 mm., tr. 11) is compressed, sub- conic, with a posteriorbasal lobe that is better devel- oped than in earlier forms. P2 (ap. 25 mm., tr. 16) is in about the same stage of evolution as in P. superhum but is more brachyodont; it exhibits a protoconid relatively much more depressed than in M. manteoceras, while the postero-external cusp (hypoconid) is rela- tively more elevated, and a rudimentary internal cus- pule ( = paraconid) begins to appear. This is therefore a much more progressive tooth than the po of M. manteoceras but is clearly derivable from it. In pg, though the anterior crescent ( = trigonid) remains larger than the posterior crescent ( = talonid), the measurements (ap. by tr.) being 27 by 17 millimeters, the antero-internal cusp begins to be well defined; no median internal cusp corresponding to the metaconid of the molars appears, the tooth being less progressive in this respect than in P. leidyi but derivable from the conditions observed in ps of M. manteoceras. P4 dif- fers from the true molars chiefly in its smaller size (ap. 30 mm., tr. 21), in the somewhat greater elevation of the anterior lobe ( = trigonid), and in the absence of a distinct postero-internal cusp ( = entoconid) . The external cingula are obsolete on p2, ps and much reduced on P4. The inferior molars of the type are characterized by shallow internal valleys and an incomplete external cingulum, which is inflected in the valleys, by a well- defined posterior cingulum, by prominent paraconid and metaconid, by a considerable elevation (hypo- conid) of the crown (26 mm.) in m2. Metaconid folds are present as in many other titanotheres. The meas- urements (ap. by tr.) are mi, 46 by 26 millimeters; m2, 57 by 34 (estimated) . M3 is incomplete in the type ; in another specimen (Am. Mus. 2028) it measures 78 millimeters, and in a third specimen (Yale Mus. 635 D) 79. In Protitanotherium superhum this tooth measures 98 millimeters. Jaw of Protitanotlierimn emarginatum. — The type jaw of P. emarginatum. (Princeton Mus. 11242) ex- hibits the anterior half of the ramus and symphysis The second jaw (Am. Mus. 2028) preserves the pos- terior half but lacks the coronoid and condylar proc- esses. The third jaw (Yale Mus. 635 D) includes the symphyseal portion only. The type jaw exhibits a very massive symphysis, 159 millimeters in length, 80 millimeters across the narrowest portion of the chin below, with the charac- teristic postcanine constriction seen in M. manteoceras. The ramus is thickened (35 mm.) below mi and in- creases very rapidly in depth from 92 millimeters behind ps to 126 behind m2. The progressive increase of the ramus in depth posteriorly is evident in this series as well as in Telmatherium ultimum, T. altidens, Palaeosyops, etc. It is more pronounced in forms with relatively large molars. The mental foramen of the type jaw is single and placed directly below the posterior fang of p2. In the American Museum jaw of P. emarginatum (No. 2028) the depth is 90+ millimeters behind pa^ 108 behind m,, and 144 behind ms; the distance from the back of ms to the posterior border of the angle is 186 millimeters. The lower border is crushed, the distortion concealing its natural contour, but there appears to be a slight up curve below the coronoid, with a broad downward and backward sweep of the angle, and the posterior border of the angle appears to rise to the condyle with a slight incurvature. Protitanotherium superhum Osborn Plate LXIX; text figures 128, 315, 318, 321, 371, 593, 6i7 701 For original description and typs references see p. 185. For skeletal characters see p. 655] Type locality and geologic horizon. — White River, Uinta Basin, Utah; Uinta formation (Diplacodon- Protitanotherium-EpiJiippus zone, Uinta C, probably higher levels). Specific cTiaracters. — Very large (pi-ms, 318 mm.). Lower canines in males very robust, relatively nearer to the midline than in P. emarginatum; pi distinctly double-fanged; postcanine diastema abbreviated (about 30 mm.); premolars in about the same stage of complication as in P. emarginatum, but premolar and molar cusps more steeply sided (that is, more hypso- dont), p4 submolariform, ps, p2 transitional; external cingulum on ps, p4 a little clearer; true molars very large (mj-ms, 210 mm.); ms with hypoconulid sharply constricted at base. Materials. — This species is at present known from the type jaw (Am. Mus. 2501), discovered by Peterson in 1895. There are also two upper molar teeth re- corded as belonging to the same individual, as well as a pair of nasals in the Princeton collection (No. 11213). These nasals (fig. 318) are distinguished from those of P. emarginatum by their superior size and by a deep incision in the median line anteriorly. Comparisons. — The relative measurements of P. superium, P. emarginatum, and Teleodus avus are shown below. Measurements of Proiitanotherium and Teleodus, in millimeters P. emargina- tum (upper Eocene) P. superbum (upper Eocene) T. avus (lower Oligocene) P,-mj - --- - -- ■294 -187 "■99 27 78 318 210 105 31 99 Mi-m3 - - Pi-p, _ (p!-pO 106 Transverse posterior lobe M. . -- 99 380 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA P. superhum is distinctly of the Manteoceras and Protitanoiherium phylum. It is, however, more pro- gressive toward the Brontops stage in that it is larger, that its canines are extremely robust, and that p2_4 are a little longer but mi_3much longer than in P. emargi- natum. We observe that Teleodus (Brontops) avus of the Oligocene has grinding teeth similar in size to those of P. superhum. Inferior dentition. — The canines and grinding teeth are preserved in the type jaw, but the incisor series is wanting. They represent a specific progression upon those of P. emarginatum in their greater size; in the abbreviation of the postcanine diastema; in the lengthening and broadening of the true molars, a progressive tendency that may be followed from Eotitanops through Manteoceras and Protitanotherium up into the Oligocene titanotheres ; in the arrested length of the premolar series; and in the more pronounced hypsodonty of the cusps of the premolars and molars. Considered in detail, the inferior canines are seen to be relatively closer together than those of either 31. manteoceras or P. emarginatum, which are presumably near the ancestral types of P. superhum; but they retain the characteristically robust fangs and the stout recurved crowns with gen er all 3^ rounded section, faint anterior and posterior cingula. The basal crown measurements of the canines are anteroposterior 32 millimeters, transverse 31. The postcanine diastema is relatively reduced and uneven on the two sides — ■ namely, 30 millimeters on the left, 20 (estimated) on the right. The premolar series (PI. LXIX) is closely continuous, measuring 105 millimeters. Pi of the left side was apparently somewhat smaller than pi of the right. Nothing remains of the crown of this tooth except the laterally compressed subsecant talonid. P2 (ap. 28 mm., tr. 16) is distinguished from that of M. manteo- ceras by the deepening of the crescents and the more decided accent of the internal cusps, which are analo- gous to the paraconid, metaconid, metastylid, and entoconid on the molars. P3 (ap. 28 mm., tr. 18) is a slightly longer and decidedly broader tooth than in P. emarginatum; it is little if any more progressive in the development of the cusps analogous to the paraconid, metaconid, metastylid, and entoconid of the molars, which, being less worn down than in the type of P. emarginatum, produce at first the impression that they mark a higher stage of evolution, but the diJSfer- ences seem to be due largely to difference in wear; however, the entoconid of the right side only is some- what better developed than in P. emarginatum. P4 (ap. 33 mm., tr. 21) is still more decidedly molari- form, the median internal cusp ( = metaconid) being much more prominent than in either of the preceding teeth, a feature foreshadowed in M. manteoceras. The true molars are decidedly longer and broader than those of P. emarginatum. This progression in size and especially in width points toward brachy- cephaly. There is a somewhat stronger accent of the paraconids than in P. emarginatum, but this may result from the greater wear in the type of that species, which would depress the metaconids and entoconids nearer to the level of the paraconid. The metastylid ridges do not appear very prominent. The external cingu- lum tends to festoon the sides of the protoconids and hypoconids in a faint line, whereas in P. emarginatum the cingulum is straighter and is thus (by comparison with other titanotheres) seemingly more advanced than in P. superhum. The outer surface of the ecto- loph is entirely smooth on the median portion of the lobes — that is, the cingulum has disappeared. The measurements, in millimeters, are as follows: Mi, ap., 52; tr., 27 through trigonid, 30 through talonid. M2, ap., 63; tr., 35 through trigonid, 36 through talonid. M3, ap., 95 (estimated); tr., 40 through trigonid, 38 through talonid. Superior molars. — The second and third superior molars of the same individual (PL LXIX) are almost certainly associated with the type lower jaw; they have the proper dimensions to fit the lower molars, and in this jaw, as in others, they show more dentine than the lower teeth; they are quadrate in form, m- measuring 57 by 57 millimeters and m^ 62 by 62. They exhibit imperfectly developed external cingula and an internal cingulum , which faintly festoons the inner cusps of the crown. The features of m^ are the complete wearing out of the prefossette, the somewhat median position of the protocone, and the somewhat detached and anteroposteriorly compressed hypocone; it is note- worthy that, as in other titanotheres, while the ex- ternal crescents are extremely worn the internal cones are very slightly worn, the protocone barely exhibit- ing exposure of the dentine at its apex. M^ shows the bottom of the prefossette, a very large protocone, and an angulate hypocone region, in which, however, there is a sessile cingulum but no rudiment of the hypocone proper. Comparison of teeth of Protitanotherium with those of other genera. — Protitanotherium is at once distin- guished from Teleodus avus of the lower Oligocene by the character of the incisors, which in Protitanotherium are large and more or less flat-topped and in Teleodus smaller, with ovoid or hemispherical tops. The canines of Protitanotherium are more robust, especially at the base; those of Teleodus are more slender, erect, and evenly sloping. The premolars of Protitanotherium are in a lower stage of evolution — that is, they are less molariform than those of Teleodus and Brontops. The jaw. — The dimensions of the jaw of P. superhum (type) considerably surpass those of Brontops hrachy- cephalus, from the lower Titanotherium zone, the length from condyle to incisive border being 580 millimeters and the depth of the angle below the condyle 270, as against 490 and 220, respectively, in a small B. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 381 brachycepJialus (Am. Mus. 1495). The general contour of the jaw in these two species, however, is somewhat similar; there is the same long, slightly convex chin; the lower border in profile is convex below the grinders, slightly concave below the coronoid, and is produced downward and backward into the angle, a concave border rising from the angle to the condyle; and the stout, somewhat recurved coronoid processes have a heavy anterior and gently convex anterior border. This jaw, as compared with earlier forms, certainly resembles in its main features and proportions those vertically oval extension for articulation with the postglenoid process. The broadening and downward extension of this postglenoid facet on the inner side of the condyle is a striking progressive feature, which was probably acquired by all late Eocene and lower Oligocene titanotheres. The anterior border of the rotula extends nearly straight across, as in Manteo- ceras and probably also as in other phyla. The posterior border of the jaw arches gently forward below the condyle and then suddenly expands backward into the downward and backward produced angle. Figure 321. — Lower jaw of ProUlanotherium superbum One-fourth natural size. Am. Mus. 2501 (type), reversed; White River, Uinta Basin, Utah; Uinta C. The coronoid is somewhat altered by crushing. of P. emarginatum, Manteoceras manteoceras, and Mesa- tirMnus megarhinus. The symphysis is very massive, extending 158 millimeters anteroposteriorly, with 100 millimeters as the least transverse measurement of the chin; it is gently convex anteroposteriorly and somewhat more decidedly convex transversely; the postcanine con- striction is relatively less decided than in M. manteo- ceras; the jaw increases rapidly in depth from 109 millimeters behind ps to 124 behind m2 and 163 behind m^. In view of the relatively short diastema behind the canine and the rather rapid rise of the coronoid border behind ms, it appears that this jaw is progressively shortening and deepening. The coronoid is stout, gently recurved, and relatively less elevated above the postcoronoid border than in M. manteoceras or Brontops hracJiycephalus. The condyle is greatly extended transversely (106 mm.) and narrow anteroposteriorly (24 mm.), therefore exhibiting a rather sharply convex rotula, except on the inner side, where it exhibits a deep, Measurements of type loioer jaws of ProUlanotherium and Brachydiastematheriiim, in millimeters Is, anteroposterior I3, transverse C, maximum anteroposterior diameter C, maximum transverse diameter. C, lieight of crown Postcanine diastema, maximum. _ Postcanine diastema, minimum. _ Pi-p4, anteroposterior Pi, anteroposterior P2, anteroposterior P2, transverse * Pj, anteroposterior P3, transverse ' Tt, anteroposterior P), transverse ' Ml, anteroposterior Ml, transverse ^ B. transil- p. emar- vanicum ginatum 22 12 20 12 38 32 31 28 -40 52 12 35 3 + 25 107 "99 18 16 26 25 "17 16 31 27 '■22 17 38 30 -28 21 50 46 »30 26 105 « Estimated. ^ Transverse measurements are made through anterior lobe. 382 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA TRANSITIONAL TITANOTHERES IN THE EOCENE OF EUROPE Brachydiastematherium Biickh and Maty Plate LXX; text figures 100, 315 [For original description and type references see p. 166] Geologic Jiorizon. — Recorded as lower Eocene, but more probably middle Eocene (Abel), upper Eocene (Osborn), or even lower Oligocene (Stehlin). Generic and specific cliaracters. — Brachycephalic. Size large, about that of Protitanoiherium superium. Dentition: ly, Cx, P4, M^. Lower incisors large) heavily cingulate posteriorly, is larger than 12; post- canine diastema very short; canines stout, with short, heavily cingulate crowns; pi compressed, P2-P4 pro- gressive, submolarif orm ; premolar entoconids rela- tively well developed. Brachydiastematherium transilvanicum Bockh and Maty Materials. — The type species, B. transilvanicum, horn Andrashaza, in Transylvania, Hungary, is represented by a fragmentary lower jaw. As shown by every detail of the dentition this animal was unquestionably a titanothere, much resembling Protitanotherium, and not, as its describers supposed, a relative of Palaeo- therium. Geologic age. — The age was originally recorded as lower Eocene, but it is probably upper Eocene. The question of the geologic age of this specimen is one of the most important in the chronology of the titano- theres. Pavay, its discoverer, as well as Bockh, its describer, and A. Koch, who studied the strata in which the type was found, assigned a lower Eocene age to the species, and their opinion was accepted by Dep^ret, who placed the genus among the upper Ypr^sien fauna, which is correlated by Osborn with the lower part of the middle Eocene Bridger of North America (Bridger B), which contains the relatively primitive Palaeosyops pdludosus. This animal was found in the same beds as Prohyra- codon orientale Koch. In his monograph "Die Sauge- thiere des schweizerischen Eocans" Stehlin (1903.1, p. 125, note) remarks: " Ich hege indess einen starken Ver- dacht, dieser Prohyracodon mochte, wie Koch selbst friiher annahm, oligocanen und nicht mitteleocanen Alters sein." Schlosser (1901.1, p. 27) points out that Prohyracodon is not, as Koch had believed, a forerunner of Hyracodon but is closely related to the aceratheres (hornless rhinoceroses). Abel (1910.1, p. 24) appears to be doubtful as to the geologic age of Prohyracodon but considers it the most primitive of the European rhinoceroses, more primitive than Meninatherium, which is Aquitanian. From Abel's figure of Prohyracodon Matthew (letter, 1914) infers that it is of lower Oligocene or at most of upper Eocene age. It is closely allied, as Schlosser and Abel state, to the earlier Oligocene aceratheres, and it is much more progressive than Hyrachyus or Amynodon. Brachydiastematherium should therefore be considered of lower Oligocene or upper Eocene age. Characters. — Every detail of the dentition shows that, as compared with the American titanotheres, B. transilvanicum is in an upper Eocene stage, closely simi- lar to that of animals found in horizon C of the Uinta Basin. The indications are that titanotheres migrated from some northern center at about the same time into eastern Europe and into North America. Brachydiastematherium agrees with the upper Eocene Protitanotherium superbum in general appearance and in the dimensions of pi to mi. The canines have a peculiar very heavy curved internal posterior cin- gulum ridge, and the crown seems shorter and more recurved than in P. superbum. All the premolars appear to be in a slightly higher stage of evolution, and the pronounced external cingula are horizontal rather than festooned. The postcanine diastema is much shorter, an indication of a higher specialization. From P. emarginatum this species differs not only in the more advanced evolution stage of the premolars and shape of the canine but apparently also in the fact that is is the largest of the series, if Bockh and Maty's identification of this tooth as is is correct. The in- cisors are also larger and more heavily cingulate posteriorly. Brachydiastematherium differs from Telmaiherium altidens especially in the obtuse shape of the canines and in the much more progressive ■pi-p4. The large size of the heavily cingulate incisors suggests, however, the possibility that the genus under consideration may rather be an ofi^shoot of the Telmatherium than of the Manteoceras-Protitanotherium series. From Stheno- dectes incisivus, which it resembles in having three very large incisors on each side of the jaw, Brachydiaste- matherium differs in the much more progressive evolution stage of the inferior premolars, p2-p4- Derivation from Sthenodectes or from a nearly related form is suggested by the general appearance of the incisors, canines, and grinding teeth. The incisors of B. transilvanicum, with their broad i posterior cingula, parallel those of the Brontotherium type among the American Oligocene phyla. Brachydiastematherium is completely transitional between the Eocene and Oligocene titanotheres, in so far as it retains six large lower incisors of Eocene type in company with very progressive lower pre- molars of Oligocene type. The measurements (estimated from the original illustrations) in comparison with those of the nearest American titanotheres show that the incisors, canines, and p4 are all very large, the diastema very short, the grinders broad. On the whole, the evidence indicates that Brachy- diastematherium represents a distinct European phy- lum, which closely paralleled certain upper Eocene American titanotheres in many respects but was distinguished by the combination of three large lower incisors; heavy, blunt canines; very short diastema; EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 383 very progressive premolars; and broad molars. The skull was probably brachycephalic — that is, it had broad zygomata. subfamhy doiichoehininae riggs PHYLA AND RELATIONS These Eocene titanotheres branch from the same an- cestral stock as that of Manteoceras. Some are doli- chocephalic, and some are mesaticephalic. The horn swellings are developed chiefly on the nasals, partly on the frontals. Facial region laterally compressed; elongate symphyseal union of premaxillaries; orbits prominent; infraorbital processes more or less promi- nent. First occurring (Mesatirhinus) on upper levels of the Bridger Basin (C and D) and lower level of the Washakie Basin (A), reaching a dolichocephalic cli- max {DolichorMnus) in Washakie B and Uinta B 1 or terminating in dwarfed mesaticephalic forms (Metarhinus) and other collateral branches. Following Eometarhinus of Bridger A, Huer- fano B, the subfamily Dolichorhininae contains the following phyla or separate series: 1. Mesatirhinus; probably ancestral to Dolichorhinus. 2. Dolichorhinus; extremely dolichocephalic, becoming extinct. 3. Metarhinus; aberrant, small to dwarfed; broad nasals. 4. Sphenocoelus; little known, perhaps a branch of Mesatirhinus. A closely related subfamily, Rhadinorhininae, contains 5. Bhadinorhinus; nasals short, pointed; possibly ances- tral to Megaceropinae. In their common ancestral characters these animals exhibit closer affinities to the Manteoceras phylum than to either the Palaeosyops-Limno- hyops phylum or the Telmatherium phylum. They possess in common small canine tusks and rudimentary but progressively developing horns and thus do not appear to have been vigorous fighters, their relation to other animals doubt- less being defensive rather than aggressive. They possess long and rather straight rows of grinding teeth, usually narrow, and the zygomatic arches are slender and not widely projected. They are thus readily distinguished from their broad-headed con- temporaries, such as Manteoceras. The known members of the series geologically are the two species of Mesatirhinus of Bridger C and D, each of which gives rise more or less directly to the extraordinarily large, long-headed Dolichorhinus of the upper beds of the Washakie Basin and the middle beds of the Uinta Basin. The known species of Meta- rhinus are confined to the sandstone of the fluviatile deposits of the Uinta Basin, a fact which suggests that they may have been small aquatic animals. At the other extreme stand the species of Bhadinorhinus, readily distinguished by short, pointed nasals and the absence of infraorbital shelves and exceptionally in- teresting because of their apparent resemblances to the great Megacerops phylum of the Oligocene. A simple key to the skeletal characters of these mid- dle and upper Eocene animals is as follows: A. Nasals elongate, spreading laterally, decurved; prominent infraorbital processes (Dolichorhininae); face bent down- ward: 1. Mesatirhinus; primitive, dolichocephalic, horns rudi- mentary, feet elongate. 2. Dolichorhinus; progressive, hyperdolichocephalic, horns . prominent, feet abbreviate. 3. Metarhinus; mesaticephalic to subdolichocephalic, nasals expanding, dwarfed in size. 4. Sphenocoelus; hyperdolichocephalic, basieranial pits. B. Nasals abbreviate, pointed, no infraorbital shelves (Rhadino- rhininae): 5. Rhadinorhinus; dolichocephalic, facial region upturned. Figure 322. — Phjdogenetic relations of the species of Metarhinus, Mesatirhinus, Dolichorhinus, and Rhadinorhinus The author's theories (1919) as to the phylogenetic relations of these five phyla are expressed in Figure 322. HISTORY OF •DISCOVERY AND CLASSIFICATION The separation of these five distinct phyla has been an extremely long and difficult undertaking, beginning with the work of Cope in 1872 and ending in 1919. Even now the position of Sphenocoelus and the relation- ship of Bhadinorhinus and Megacerops await elucida- tion. The chronologic record follows: 1872. Cope describes, under the name " Palaeosyops vallidens," fragmentary upper and lower jaws from the Washakie Basin. This little-known species, probably from Washakie B, is now provisionally regarded as a primitive stage of Dolichorhinus, namely, D. vallidens. 1889. Scott and Osborn describe as "Palaeosyops hyognathus" a large lower jaw from Washakie B. This now proves to be Dolichorhinus hyognathus. 1891. Earle describes as "Palaeosyops megarhinus" a small skull from Washakie A, recognizing, however, 384 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA its diflerences from the typical Palaeosyops. This is now known as MesatirMnus megarhinus. 1894. Peterson explores horizon B 2 of the Uinta Basin and discovers a remarkable long-skulled form. 1895. This long-skulled form is described by Osborn as " Telmatotherium cornutum," which is now known to be a synonym of DolocJiorMnus hyognatJius. 1895. Osborn also describes a smaller form from Uinta B 1 as "Telmatotherium diploconum." This is now known as Rhadinorhinus diploconus. 1895. Osborn also describes, from Uinta B 2, SpJieno. coelus uintensis, a form that still remains problematical. 1895. Earle soon afterward points out the ances- tral relationship of "Palaeosyops megarhinus" to " Tel- matotherium cornutum," an affinity now recognized as that of Mesatirhinus to Dolichorhinus. 1895. Hatcher recognizes " Telmatotherium cor- nutum" as a new genus, namely, Dolichorhinus, pos- sessing horns, but not directly ancestral to any of the Oligocene titanotheres. 1894-1906. American Museum explorers in the Bridger and Washakie Basins, under Peterson, Mat- thew, and Granger, bring together good material of the "Palaeosyops megarhinus" type. 1908. Osborn reviews the narrow-skulled or doli- chorhine Eocene titanotheres . with the following principal results: {a) Hatcher's term DolicTiorhinus is adopted, and D. cornutus is shown to be a synonym of D. hyognathus, both occurring in Washakie B. A new species, Doli- chorhinus intermedius, is described. (h) Earle's Palaeosyops megarhinus from Bridger B and Washakie A is made by Osborn the type of the new genus " Mesatirhinus," ancestral to Dolichorhinus. The new species Mesatirhinus petersoni is described from Bridger C. (c) A related group, including small Eocene tita- notheres with slender limbs and relatively short, narrow skulls, from Uinta B 1 and Washakie B, is recognized by Osborn as the distinct genus Metarhinus, including M. fluviatilis, M. earlei, and [?] Telmatothe- rium diploconum. (d) The opinion is expressed that Metarhinus and Dolichorhinus represent the long-skulled form of the same stock that gave rise to the relatively broad- skulled Manteoceras. 1909. Douglas describes two new species of Doli- chorhinus {D. heterodon, D. longiceps) from Uinta B 2. 1912. Riggs greatly extends our knowledge of the dolichorhines of Uinta B 1 and revises and expands the species Metarhinus, Mesatirhinus, and DolicJio- rhinus, establishing the new subfamily Dolichorhi- ninae and basing the new genus Rhadinorhinus on the type R. abbotti, including also the " Telmatotherium dip- loconum" of Osborn. 1919. Osborn describes Eometarhinus from the upper part of the Huerfano formation, representing an extremely primitive ancestor of Metarhinus. The original and the present determination of the synonymy of these species is thus as follows: Palaeosyops vallidens = Dolichorhinus vallidens. Palaeosyop.g hyognathus = Dolichorhinus hyognathus. Palaeosyops megarhinus = Mesatirhinus megarhinus. Telmatotherium cornutum = Dolichorhinus hyognathus. Telmatotherium diploconum = Rhadinorhinus diploconus. COMPARISON WITH MANTEOCERAS There are in Manteoceras, Mesatirhinus, and Doli- chorhinus resemblances which prove that these animals sprang from the same stock. They appear specially in the comparison of the skulls of M. manteoceras and Mesatirhinus megarhinus; in other words, the ancestral and atavistic characters of Mesatirhinus are those which it has in common with Manteoceras, among which are (1) preorbital concavities; (2) nasals long, decurved, truncate distally; (3) posterior nares com- pressed, or narrow space between the palatines and pterygoid plates; (4) zygomatic arches shallow; (5) occiput broadly depressed; (6) pit in the parietal vertex of the Manteoceras skull represented by a long slit in the Mesatirhinus skull; (7) angulation of the malars of Manteoceras represented by the suborbital shelf of Mesatirhinus. Their ancestral affinity to Manteoceras is also seen in (8) the position of the horns above the preorbital concavities; (9) the elongate form of the horn rudiments. There is a decided departure from the position of the horn rudi- ments of Manteoceras (PI. XVII) — namely, in that in the Dolichorhininae the horn swelling is chiefly a pro- tuberance of the nasal bones, whereas in Manteoceras the horn swellings are chiefly on the frontal bones, the nasofrontal suture of the dolichorhines being pushed back by the remarkable elongation of the nasals. (10) Another distinction is that in Manteoceras the horn swelling is decidedly in front of the orbit, whereas in Mesatirhinus it lies more directly above the orbit. Other differences appear in connection with the fact (11) that the face is relatively longer in the dolicho- rhines than in Manteoceras. The face is relatively longer than in Manteoceras. , Correlated with this is the fact that in Metarhinus, Mesatirhinus, and Dolichorhinus the grinders are farther forward with reference to the orbit than in Manteoceras — that is, in the members of these groups the postorbital process of the malar lies above the mesostyle of m^, and in Manteoceras it lies above the parastyle of m^ Similarly in Dolichorhinus the lacrimal lies above the mesostyle of m^, in Manteoceras above the mesostyle of m'. In Dolichorhinus this relation appears to spring as much or more from the backward displacement of the orbit (especially of its upper border) as from the forward displacement of the molar series. This oblique backward displacement of the orbit may have been correlated with the in- creased size of the nasofacial muscles, and with this factor may also have been correlated the hypertrophy EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 385 of the posterior end of the nasals, the reduction of the anterior prong of the frontals, and exclusion of the frontals from the horn swelling. The conclusion is that the dolichorhines sprang from the same stock as Manfeoceras but that they dolichorhine phylum (MesatirJiinus-DolichorMnus) afford the finest examples we have yet discovered, ex- cepting only among the Equidae, of the changes in both skull and teeth which are correlated with pro- gressive dolichocephaly, accompanied by progressive Figure 323. — Top view of the skull in the Manteoceras-Dolichorhinus group One-eighth natural size. A, Manieoceras manteoceras; B, Metarhinvs earlei; C, MesaiiThmus petersoni: D, Dolichorltinus Jiyognatlius. diverged and radiated along lines of their own into persistent mesaticephalic and extreme dolichocephalic types. cyptocephaly," or bending of the face downward on the cranial axis, as in many other grazing quad- rupeds. Combined with this slowly acquired and DOLICHOCEPHALY AND CYPTOCBPHALY IN THE MESATIRIIINUS- DGLICHOEHINDS PHYLUM Figure 324. — Palatal views of the skull in the Manteoceras-Dolichorhinus group One-eighth natural size. (See fig. 323.) only imperfectly attained cropping and grazing adap- tation the cranium also converges toward the Oligocene titanothere type, as observed in the flatten- ing of the top of the skull {Dolichorhinus), the rise of the horns {Mesatirhinus, Dolichorhinus), the incipient metamorphosis of the premolar and molar tooth Elongation of the crowns of the grinding teeth, or subhypsodonty, generally characterizes the dolicho- rhines, from which we infer that they fed on harder kinds of food and may have adopted grazing as well as browsing habits. The members of the central " Cyptocephaly (Kuirru, ut^aXij) is a comparatively new term (first employed by Osborn as cytocephaly) 386 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA pattern (RJiadinorJiinus) . Yet so far as we know these long-headed animals of the Mesatirhinus- DolichorMnus phylum were not destined to survive and give rise to any Oligocene titanothere but rather to terminate in an excessively specialized type. CONVERGENCE OR KINSHIP CONTROL These dolichorhines afford an illuminating illustra- tion of kinship control in the simultaneous evolution of different character groups. Some of these character groups are predetermined or controlled by ordinal, others by family, others by generic affinities with related titanotheres. The general expression of kin- ship control may be summarized as follows: 1. Perissodactyl or ordinal kinship is shown, among other characters, in the independent progressive com- plication of the premolar-molar teeth, three of the premolars tending to acquire the pattern of the molars, although this pattern is less perfectly developed in other Perissodactyla. 2. Titanothere family kinship is indicated in the independent progression of the development of naso- frontal horns, the flattening of the top of the cranium. FiGUBE 325. — Leidy's cotypes of Palaeosyops ( = Mesaiirhinus) Junius Natural size. Crown view of premolar and molar in the museum of the Acad. Nat. Sci. Philadelphia. Bridger (?)B; level doubtful. A, Eight fourth lower premolar; B, posterior part of third lower molar. the obliteration of the sutures on the top of the cranium, and the elongation of the middle portion of the cranium. 3. The subfamily kinship to the Manteoceratinae is indicated in the persistent subtriangular shape of the horns, in the development of an infraorbital shelf, and in the contracted posterior nares and broadening nasals. In their progressive dolichocephaly, a tendency that strongly affects the middle region of the skull between the orbits and the auditory meatus, these animals (Mesatirhinus-Dolichorhinus) are partly inde- pendent of their subfamily, family, or ordinal relations and follow an extreme adaptive direction of their own in the elongation of the midcranial region and of the teeth. In this special adaptation to their partial grazing habits the dolichorhines further parallel certain of the Equidae and other grazing animals, such as the cattle, in their cyptocephaly. (See figs. 213, 214.) The incisor teeth further acquire deep posterior pits, or pockets, analogous to the pits that are developed in the incisors of the upper Oligocene Equidae and that tend to become typical cropping teeth. DIVERGENT OR INDEPENDENT EVOLUTION OF CHARACTER GROUPS IN THE DOLICHORHINES The independent evolution of these four or five groups of characters as observed in the skull alone obviously affords only a partial picture of the play and interaction between the vast number of contem- poraneous processes that are involved in the evolution of the members of this phylum. If we could similarly compare all parts of the vertebral column and of the limbs, we should probably discover many additional illustrations of this law of the evolution of groups of characters under the influence partly of kinship and partly of independent adaptation. The principle of independence or divergence is well illustrated in the skull. In Figure 302 the skulls of Manteoceras and the MesatirMnus group are compared as seen from the side. The palatal view of Manteoceras, Metarhinus, MesatirMnus, and DolichorJiinus (flg. 324) brings out the resemblances and contrasts between these four forms. The superior view (fig. 323) also brings out the wide progressive divergences between these undoubtedly related forms. We may also compare superior views (PI. LXXX) of the skulls of Metarhinus and Rhadinorhinus, show- ing how the latter departs from the other members of this dolichorhine group in the abbreviation and point- ing of the nasals and in the reduction of the infraorbital processes. PROGRESSIVE DOLICHOCEPHALY IN MESATIRHINUS- DOLICHORHINUS The dolichocephaly, which is the chief progressive character of the MesatirTiinus-Dolichorhinus phylum, is beautifully illustrated in the accompanying series of illustrations (figs. 339, 340). A very important fact (see the following table) is brought out by the cranial indices and ratios in this series of species — namely, that while the skulls lengthen and become relatively narrower, the facial portion is not relatively abbreviated as it is in the Manteoceras phylum, because the faciocephalic index is the same in the terminal member of the series, D. JiyognatJius, as in M. megarJiinus, the most primitive member of the series. In other words, in Manteoceras the face is abbreviated; in the dolichorhines it is not. Cranial indices of MesatirMnus and Dolichorhinus M. megarhinus M. petersoni D. superior D. longioeps D. intermedins D. hyognathus ( = cornutus) D. fluminalis 56-59 51-52 52 41-46 43-46 45 48 49 49 51-53 41 39 38 35-37 38 36 EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 387 In other words, while the ratio of breadth to length falls from 59 to 45 per cent, the ratio of the length of the face to the entire length of the cranium, or facio- cephalic index, remains at 48. This is a very impor- tant distinction, because progressive abbreviation of the face is characteristic of all the true Oligocene titanotheres correlated with dolichocephaly but is not found in these dolichorines. We observe other corre- lated dolichocephalic changes in the skull — namely, (1) the occipital condyles are set broadly apart; (2) the external auditory meatus becomes widely open; (3) wide spaces arise between the cranial foramina; (4) the horn rudiments rise chiefly on the nasals and less On the frontals; (5) the nasals have a long exposure on the top of the skull; (6) there are correlated changes in the teeth. The teeth also show the following correlations with doliochocephaly : (1) The opposite molar-premolar series become parallel; (2) the palate is narrowed, elongated, and arched; (3) the individual molar teeth are elongated or laterally compressed; (4) the pre- maxillary and mandibular symphyses become greatly elongated; (5) the jaws become long and slender, and there is an increasing distance between ms and the posterior border of the jaw, the coronoid becoming relatively low and backwardly recurved, its anterior edge oblique rather than transverse, and the chin shallow and sloping (hence the term hyognaihus, or hog-jawed, applied by Scott and Osborn); (6) simi- larly the inferior molars become elongate; (7) the whole upper tooth row shifts forward with reference to the orbit. The recently discovered EometarJiinus is described on pages 200, 419, 420. Mesatirhinus Osborn Plates XVII, L, LXXI, LXXII; text figures 26, 27, 33, 122, 211, 217, 219, 265, 302, 327-331, 333, 339-341, 483, 511-514, 516, 520-523, 526, 558, 559, 562-571, 586, 647, 656, 661, 686, 702, 713, 716, 724, 745 [For original description and type references see p. 182. For slieletal eliaracters see p. 636] Geologic horizon. — Bridger C and D and Washakie A. Generic characters. — Middle Eocene titanotheres of small but increasing size; basilar length, 354-485 millimeters. Mesaticephaly progressing to dolicho- cephaly. Horns incipient, borne chiefly on the nasals; prominent infraorbital malar shelf; nasals elongate, laterally recurved; cranium profile convex; face de- flected; sagittal crest gradually broadening; no sec- ondary palate. Humerus short; tibia relatively long; carpus and tarsus narrow; astragalus with elongate neck; metapodials relatively elongate. Geologic distribution. — There is the Eometarhinus of Huerfano B (Bridger A), and the Mesatirhinus Junius of Bridger B. In Bridger C and D, also in Washakie A, there first appears a rich array of small titanotheres, which are readily distinguished from the contemporary species of Palaeosyops, Telmatherium, and Manteoceras by the generic characters enumerated above. These animals are related on the one side to Metarhinus and on the other, by progressive changes, to Dolichorhinus, and the phylum is therefore regarded as central. The phylum is divided into the smafler, more primitive species Mesatirhinus megarhinus and the partly con- temporaneous, more progressive species M. petersoni. These species are contemporaneous in Bridger C 5, and both animals are found in Washakie A, which is evidence that they are contemporaneous and not successive species. At the summit of Uinta B 1 occurs the larger and more progressive "Mesatirhinus" superior, with partly flattened cranium. This animal is here referred to Dolichorhinus. In the Uinta region Metarhinus is so abundant in the fluviatile sandstones of Uinta B 1 that the horizon is named the Metarhinus zone. The animals disappear at the summit of this zone in the " Metarhinus sand- stones." The synopsis of these species is as follows : Mesatirhinus Junius (Leidy)?, Bridger B, a diminu- tive animal. (See fig. 325.) Mesatirhinus megarhinus (Earle), Bridger C and Washakie A. Skull small (about 354 by 170 mm.); cephalic index, about 53; faciocephalic index, 48; palatal crests narrow; nasofrontal horns incipient; premolars in less advanced stage. Mesatirhinus petersoni Osborn, Bridger C and D and Washakie A. Skull of intermediate size (about 425 by 205 mm.); cephalic index, 49; parietal crest narrow; faciocephalic index, 48; premolars in more advanced Mesatirhinus (Dolichorhinus) superior Eiggs, sum- mit of Uinta B 1. Skull larger (485 by 240 mm.); cephalic index, 52; faciocephalic index, 48; parietal crest spreading; cranium flattened on top. It is important to note that although these three species succeed each other progressively and this pro- gression leads directly to Dolichorhinus, there is no proof of direct phyletic succession. 388 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements oj Mesatirliinus megarhinus, M. petersoni, Dolichorhinus superior, and Metarhinus fluviatilis, in millimeters M. megarhinus M. petersoni D. supe- rior, Field Mus. 12188 (type) Princeton Mus. 10008 (type) Am. Mus. 1514 Am. Mus. 12202 Am. Mus. 1523 Am. Mus. 1571 Am. Mus. 1509 Am. Mus. 1566 Am. Mus. 12184 (type) tilis, Am. Mus. 1500 (type) Pmx— condyles - "■354 °402 386 "■425 485 "355 Mx— condyles - - 342 "180 210 92 "ISO 183 '■212 205 "220 "255 89 250 "180 "200 Face ... °170 195 223 178 157 140 91 19 23 205 "205 "170 202 "190 Nasal-postorbital frontal process _ P"-m3 . . ... ...... 168 147 190 156 141 90 18 23 195 154 138 87 18 140 125 80 16 22 147 132 83 17 22 147 132 82 17 22 156 139 89 18 23 184 105 146 P2-m3 . 130 Mi-m3 16 85 P*, ap. - ... - 17 P*, tr 23 * Estimated. 10008. Washakie Basin. 1514. Washak;ie A. 12202. Bridger C 5. 1523. Bridger C or D. 1571. Washakie A. 1509. Bridger D. 1556. Bridger D. 12184. Bridger D 3. 12188. Uinta B 1. 1500. Uinta B 1. The figures show that M. petersoni, most specimens of which are from the higher levels of Bridger D, is considerably larger in all measurements than M. megarJvinus. Both are much larger than the type of Metarhinus fluviatilis from Uinta B 1. Mesatirhinus Junius (Leidy) Text figures 91, 325 [For original description and type references see p. 159] Type locality and geologic horizon. — The type lower molar of M. Junius, according to Leidy's description, was found near Fort Bridger, Wyo., at a geologic level that Granger places in Bridger B. If this geologic level is correct M. Junius is the oldest known species in the Mesatirhinus phylum. Its geologic age must, however, be regarded as indeterminate. Specifl.c characters. — A doubtfully referred specimen, imperfectly known. M'-m^, 69 millimeters. A dimi- nutive Mesatirhinus or Metarhinus. Materials. — The type specimen (Acad. Nat. Sci. Philadelphia) is very fragmentary. The only other material that may be referred even provisionally to this species is a diminutive set of teeth, including ml to m^ (Am. Mus. 12686), from level D 5 of the Bridger. The teeth present generic resemblances to those of Mesatirhinus megarhinus but are far smaller than in any known upper Bridger, Washakie, or Uinta titanothere, m'-m' measuring only 69 millimeters, as against 85 in Metarhinus fluviatilis . The teeth are less hypsodont than in allied species; m' is relatively very small and m' very quadrate. A comparison of the lower molar with the type ms of Palaeosyops Junius Leidy leaves the specific identity doubtful. The com- parative measurements are as follows: Measurements of teeth of species of Mesatirhinus, in millimeters M.jimius?, Am. Mus. 12686; Bridger D 5 M. mega- rliinus, Am. Mus. 12202; Bridger C 5 M. fluvia- tilis, Am. Mus. 1500 (type); Uinta B 1 M. peter- Mus. 1656; Bridger D M. peter- soni, Am. Mus. 1612; Bridger D Mi-ms Ml, ap Ml, tr 69 21 22 36 83 85 23 90 26 27 Ms, ap 46 Mesatirhinus megarhinus (Earle) Plate LXXII; te.xt figures 106, 217, 324, 326, 328, 330, 331, 508, 510, 558, 560, 561, 685, 737 [For original description and type references seep, 170. For slieletal ciiaracters see p. 637] Type locality and geologic horizon. — Washakie Basin, Wyo.; probably Washakie A. The species is also recorded from Bridger Basin, Wyo., Uintatherium- Manteoceras- Mesatirhinus zone, level Bridger C ( = Washakie A). Specific characters. — P'-m^, 140-147 millimeters; true molars, 77-83. Cephalic index, 56-59. Cranial length, premaxillaries to condyles, 354 millimeters (estimated); facial region rather short (about 170 mm.); faciocephalic inde-x 48 (estimated); occipital condyles broad; premaxillary symphysis short; infra- orbital shelf prominent. Tetartocones on p^, p* very rudimentary. Nasofrontal horns incipient. Materials. — The type (Princeton Mus. 10008) is badly preserved and unfortunately lacks the occipital condyles, which appear to be relatively broader in Mesatirhinus than in Dolichorhinus. The geologic level of the type is not definitely recorded, but is probably Washakie A. EVOLUTION OF THE SKULL AND TEETH OF EOCENE •TITANOTHBRES 389 The material in the American Museum referred to this species includes the following: From the Bridger Basin, skull, No. 12202 (level C 5); palates, Nos. 12206 (level C 5), 1519, 1523 (level unknown); lower jaws, Nos. 1520, 1551, 12207 (level C 5), 12199 (level C 5); from the Washakie Basin (level A), palates, Nos. 1513, 1514; lower jaws, Nos. 1575, 1577. Mesadrhinus petersoni Osborn Plates L, LXXI, LXXII; text figures 26, 27, 33, 122, 211, 217, 219, 255, 302, 327-331, 333, 339-341, 483, 611-514, 516, 520- 523, 526, 558, 559, 562-571, 586, 647, 656, 661, 686, 702, 713, 716, 724, 745 [For original description and type references see p. 182. For skeletal characters see p. 641] Type locality and geologic Jiorizon. — ■ Cattail Springs, Bridger Basin, Wyo.; Uintatherium-Manteoceras-MesatirMnus zone (Bridger), level D 3. Also re- corded from Bridger C (?) and Washakie A. Specific cliaracters. — P^-m^, 154-157 millimeters; true molars 87-90. Skull length, premaxillaries to condyles (esti- mated), 402-425 millimeters; cephalic index, 51-52; preorbital facial region (estimated), 195-205 millimeters; facio- cephalic index 48. Other characters as in M. megarhinus — that is, broad occipital condyles, infraorbital shelf, etc. Materials. — The type skull (Am. Mus. 12184) is from Bridger D 3 (fig 327). Comparison of this animal with the type of M. megarhinus can leave no doubt that we have to do here with a more advanced stage of evolu- tion. The skull is longer, the preorbital region especially. The grinding teeth occupy more space, and there is an average advance in all the premolar rectigradations, which prove that these differences in form and size are not merely due to fluctuations of size or differences of sex. Other specimens (in the American Museum except as noted) referred to this species are, from the Bridger, skulls Nos. 1509 (level D) and 1556 (level D; now in British Museum), lower jaw No. 1567, lower jaw No. 12191 (level C 2); from Washakie A, skull No. 1571 and lower jaws Nos. 1512, 13178. Of these No. 1571, from Washakie A, fortunately has associated with it the fore foot, radius, ulna, astragalus, and pelvis. Another valuable skeleton (Am. Mus. 11659) is recorded from Bridger C 5, and a well-preserved forearm and manus in the Prince- ton Museum (No. 10013) came from Bridger C or D of Henrys Fork, Wyo. From Washakie A comes a very progressive doli- chocephalic specimen (Am. Mus. 1651) consisting of the three upper molars, which are strongly compressed laterally and measure collectively 96 millimeters. This specimen is provisionally referred to this species and appears to be an important and interesting transitional form leading into DolicJiorJiinus. General characters of Mesatirhinus megarhinus and M. petersoni. — It is impracticable to describe M. Figure 326. — Type skull of Mesatirhinus megarhinus One-fourth natural size. Princeton Mus. 10008, Washakie Basin, Wyo., level Washakie A?. A i. Side view (reversed) ; As, palatal view; A3, top view. megarhinus and M. petersoni separately, because it would involve duplication of description. Sexual characters: Unfortunately the imperfect preservation of the canine teeth does not admit of the sharp separation of males and females that is possible for many of the series of skulls. Comparison of the teeth in the more perfectly preserved jaws, however, shows that the canines were decidedly smaller in the females than in the males. 390 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Cranial elongation with age: There is considerable evidence that cranial elongation is not only a progres- sive but an age character — that is, one which appears in advanced years and through the "law of accelera- tion" will appear in earlier years of subsequent gener- ations. For example, the space between the glenoid fossa and m^ elongates with age, and correlated with it is the elongation of the ramus of the jaw between ma, the anterior border of the coronoid process, and the condyle. pas P"~ Figure 327. — Tj'pe skull of Mesatirhinus petersoni One-fourth natural size. Am. Mus. 12184, Cattail Springs, Bridger Basin, Wyo., Bridger D 3. Ai, Side view; Ai, top view. Slcull. — The general characters of the skull are as follows: (1) Skull as a whole elongate, with consider- able space between the glenoid process and the occipital condyles, leaving the auditory meatus open, as con- trasted with the contemporary Palaeosyops, in which this space is abbreviated; (2) zygomatic arches rela- tively straight, slender, and gently arched outward; (3) on the malars an infraorbital shelf, which ap- parently increases progressively; (4) sagittal crest sessile and narrow as compared with LimnoJiyops laticeps; (5) nasals long, expanding and decurved dis- tally; (6) premaxillary symphysis more abbreviate than in Telmatherium; (7) face moderately bent upon cranium, parietals convex in side view; (8) postorbital process of malar above posterior part of m^. On comparing the side views of Telmatherium cultridens and of Mesatirhinus petersoni we see that in the former the premaxillary is stouter, vertically deeper anteriorly, and extends posteriorly farther up on the maxillary. In T. ultimum this is much more pro- nounced. In Mesatirhinus, in correlation with the smaller incisors and more slender maxUla, the pre- maxillary is shallower vertically, and the sym- physeal surface is more delicate. The skull of members of Mesatirhinus petersoni ex- hibits many marks of general affinity to those of their collateral relative Manteoceras manteoceras. Among these are (1) the shape of the symphyseal union of the premaxillaries ; (2) the narrowing of the postnarial space between the pterygoids; (3) the sutural rela- tions of the nasals, frontals, maxillaries, malars, and lacrimals, as seen in side view, with the exception of the position of the horn rudiment; (4) the concavities at the side of the face slightly above and in front of the orbits; (5) just above these concavities the promi- nent convexities of the nasals at their junction with the frontals above the orbits, extremely interesting as a very early stage of horn evolution and prophetic of the distinct horn base of Dolichorhinus; (6) presence of a long and narrow pit in the anterior portion of the sagittal crest. Among the most significant resemblances to Man- teoceras are also the similarity in the base of the craniima; the slender zygomata, constricted back of the orbit; and the underlying similarity in the denti- tion in spite of differences of proportion. The skull differs markedly from that of Manteo- ceras, however, in the presence of infraorbital shelves and in its greater dolichocephaly. It also differs from Manteoceras in the shape of the occiput, shape of the skull top, and especially in the dentition. Its closer affinities, therefore, are with Dolichorhinus. The detailed characters of the teeth exhibit a direct dolichocephalic adaptation of those of the Manteo- ceras type. The community of type, again, is due to a community of ancestry, the two lines running together perhaps prior to Wind River and Huerfano time. More in detail: The superior view of the skull (fig. 328) exhibits the characteristic anterior expansion of the nasals, which measure transversely (Am. Mus. 1556, M. petersoni) anterior region 59 millimeters, mid- region 43, posterior region 84; the total length is 167. The nasals are separate anteriorly but firmly coalesced posteriorly; the lateral convexity just in front of their junction with the frontals (figs. 327, 328) represents the rudimentary stage in the evolution of the horn. The frontals are expanded above the orbits (91 mm., tr.), gradually contract posteriorly, and are bounded by the EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 391 prominent supratemporal crests, which arise from the postorbital processes and in some adult individuals converge in the form of two broadly convex ridges into the short and narrow sagittal crest (10 mm.)- Between these ridges there is a median depression. The suture between the frontals and parietals can apparently be made out in the Princeton skull (No. 10041, Mesatirhinus petersoni?). The parie- tals are best observed in the same skull and in Am. Mus. 1509 (-M. petersoni). In the superior view of the skull as figured we observe also the short symphyseal union (42 mm.) between the premaxillaries (much more abbreviate than in Dolicho- rhinus), the prominent infraorbital shelf on themalars, and the slender section of the zygomatic arches. In the inferior view of the skull of M. petersoni (fig. 328) we observe the converging incisive borders of the premaxillaries, the relatively narrow and transversely arched palate, which measures 152 milli- meters from the incisive foramen to the posterior nares. The palatal portion of the palatines measures 70 millimeters in the midline and converges anteriorly; on either side of the posterior nares the convex inner surfaces of the palatines con- verge, and on the inner side of the narrowest portion of this postnarial space are placed the slender ptery- goids, which are well defined. The conformation of this entire region is very characteristic of this genus as well as of M. manteoceras . The basioccipital region is best exhibited in the Princeton skull (No. 10041, M. petersoni), a very distinctive feature being the wide separation (28 mm.) by a plate of bone the foramen ovale and foramen lacerum medium, the same plate measuring but 17 millimeters in the contemporary Limnohyops lati- ceps. The conformation of this important region of the skull, as well shown in Figure 333, includes the following noteworthy features: (1) The deep groove extending backward and inward on the inner side of the postglenoid facets, believed to have lodged an extension of the meniscal cartilage, as in the horse; (2) the prominent basioccipital and basisphenoid; (3) the narrow bridge of bone between the foramen condylare 101059— 29— VOL 1 28 and the foramen lacerum posterius; (4) the continuity of the foramen lacerum medium and foramen lacerum pos- terius; (5) the peculiar inward extensions of the con- dylarfacets; (6) the general elongation of the basicranial axis; (7) the open nature of the auditory meatus. nf One-fourth natural size, tain, Henrys Fork, B FiGUEB 328. — Skull of Mesatirhinus petersoni Ai, Side view (reversed); Britisli Mus. (formerly Am. Mus. 1556), Big Bone Moun- idger Basin, Wyo., Bridger D; occipital region restored from Am. Mus. 1509, Big Bone Mountain, Henrys Fork, Bridger Basin, upper Bridger; and Princeton Mus. 10041; Incisors and canines from Am. Mus. 1571, Washakie Basin. Az, Top view; occipital region from Princeton Mus. 10041. As, Palatal view; details of pterygoid region from Am. Mus. 1509 (see above); incisors from Am. Mus. 1571 (see above) and 1514 (Af. mcgarhinns) , La Clede Meadows, Washakie Basin; suture between basioccipital and ex- occipital from Am. Mus. 12202 (J/. mejarUnus) , Summers Dry Creek, Bridger Basin, Bridger C 5, lower level. In the lateral view of the skull of M. petersoni (figs. 327, 328), we observe especially the horizontal suture connecting the maxillaries with the nasals, the lateral compression of the sides of the face at this point, the rudimentary horn convexities of the nasals, the scalelike overlap of the nasals by the frontals, the participation of the maxillaries in the anterior portion of the infraorbital shelf, the vertical extension 392 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA of the lacrimals, the postorbital processes on the frontals and malars, the slender malar portion of the zygomatic arch, the comparatively slight upward Figure 329. — Skulls of Mesaiirhinus petersoni One-fourth natural size. A, Front view of skull in British Museum (formerly Am. Mus. 1566); Big Bone Mountain, Henrys Fork, Bridger Basin, Wyo., upper Bridger, level D, B, Occipital view of Am. Mus. 1509; Big Bone Mountain, Henrys Fork, Bridger Basin, upper Bridger, level D. cm'vature of the squamosal portion of the zygoma, the incipient arching of the parietal region, the greatly elongate and not deeply vertical temporal fossa, the wide space between the postglenoid and post-tympanic processes, the relations of the frontals, parietals, occipitals, and squamosals, and the formation of the temporal fossa. The occiput is relatively broad and low, measuring (Princeton Mus. 10041, M. petersoni?) 107 millimeters transversely by 85 vertically. There is a deep de- pression in the superior portion of the occiput; the occipital condyles are widely divergent superiorly on either side of the foramen magnum. The anterior view of the skull of M. petersoni (fig. 329, A) best illustrates the characteristic form and symphyseal junction of the premaxillaries, the stout lateral decurvature of the nasals, and the postero- lateral horn rudiments on these bones. Dentition; influence of dolichocephaly. — In general the teeth show the dolichocephalic tendency, although they have not reached the extreme of elongation seen in the species of DolichorJiinus; they also are to be contrasted with those of the more mesaticephalic M. manteoceras. Thus it may be noted that Manteo- ceras and Mesaiirhinus are separated by strong differ- ences in the premolars and also in the molars. The premolars of Mesatirhinus are distinguished from those of Manteoceras as follows: (a) They are rela- tively longer as compared with their width; (b) in crown view p^-p"* appear more circular than in Man- teoceras in consequence of the deuterocones being farther forward and the postero-internal part of the crown more evenly rounded out; (c) the tritocones are, on the whole, relatively larger and flatter externally; (d) the external cingula are better defined opposite the tritocone; (e) the protocone ribs on the ectoloph are more pronounced and narrowed; (/) the protoco- nules and tetartocones are better developed. Between typical members of M. megarhinus and M. petersoni the differences are of a progressive character — that is, in M. petersoni the deuterocones and tritocones are larger, the tetartocones and ectoloph ribs are much more pronounced. Incisors. — The superior incisors (fig. 330) are ar- ranged to form a forward-pointed arch — that is, the opposite series are less parallel to each other than in T. cultridens and less transverse in position than in Palaeosyops. The series is short-crowned, with convex anterior and convexo-concave posterior faces; the posterior cingulum foreshadows the marked develop- ment of the cingulum in DolicTiorhinus. A note- worthy character is that i' is less caniniform than in TelmatJierium. Canines. — The canines are subround in section rather than laterally compressed as in TelmatTierium. The enameled crown area measures vertically 36 milli- meters and in base diameter 18 millimeters in certain specimens of M. petersoni. In the smaller specimens of M. megarhinus the crown measures 26 millimeters Figure 330. — Incisors, canines, and prema.xillae of Mesatirhinus One-balf natural size. A, Crown view of ^f. megarhinus, Am. Mus. 1514, La Clede Mead- ows, Washakie Basin, Wyo.; B, side view of M. petersoni, Am. Mus. 1671, Washakie Basin. in length. The inferior canines (Am. Mus. 1576, 1575) are more slender and rounder toward the tip, with more feebly indicated anterior and posterior EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 393 ridges, a feature which at once distinguishes them from the lower canines of T. cultridens, in which these ridges are prominently marked. Premolars. — The superior premolars especially ex- hibit the progressive rectigradations in the new cus- pules, which, as well as the progressive changes of proportion, are subject to slight fluctuations. In the more advanced American Museum specimens {M. petersoni) the first superior premolar is separated from the canine by a narrow diastema, whereas in the less advanced Princeton Museum type of M. megarJiinus, which represents a less dolichocephalic stage, it is in actual contact with the canine. The detailed characters are as follows: P' is bifanged, with a simple protocone, strongly compressed laterally, in which the proportions are typically 14 millimeters anteroposterior by 9 transverse; in the more pro- gressive specimens (M. petersoni) the tritocone (a rectigradation) is seen as a rudimentary swelling of the posterior base of the crown, which is less con- spicuous in M. megarJiinus. P^ is a highly character- istic tooth, suboval or slightly compressed trans- versely, the proportions (ap. by tr.) varying in different specimens from 15 by 13 to 16 by 16 millimeters; the proportions of this tooth are those correlated with mesaticephaly progressing into dolichocephaly; it is typically tricuspidate (protocone, deuterocone, tritocone); a generic feature is the excess of the large conic protocone over the small, externally flattened tritocone; the ectoloph is slightly cingulate (M. petersoni) but lacks the strongly accented cingulum around the base of the tritocone seen in T. cultridens and T. validum. P^ is naturally a more progressive tooth, the breadth exceeding the length (ap. 14 millimeters, tr. 18, M. megarhinus ; ap. 17, tr. 20, M. petersoni), the tritocone and protocone com- ponents of the ectoloph being more subequal, the basal external cingulum opposite the tritocone being more accented, and the deuterocone being more directly internal in position. P* still further marks this progression toward the molar type in its dimensions — 17 by 22 millimeters (ap. by tr.) in M. megarhinus, as compared with 18 by 24 in M. petersoni. The external cingulum, varying in both species, is either partially indicated (Am. Mus. 1523, 1571) or extends across the outer face of the crown (Am. Mus. 1514, 1556); the less progressive individuals (Am. Mus. 1523, 1513, M. megarJiinus) pass into more progressive stages (Am. Mus. 1556, 1509, M. petersoni) in which a faint rudi- ment of the protoconule is observed in p^, p^, and in Am. Mus. 1556 {M. petersoni) even a faint elevation of the tetartocone is observed (a rectigradation). Similarly the convex external rib of the protocone becomes more marked. In comparing the premolar series in all these specimens it is seen that the external cingulum exceptionally almost or quite embraces the ectoloph, but that the internal cingulum never completely embraces the deuterocone, as in the type of MetarJiinus fluviatilis. The premolar cingula are on the whole as progressive or more progressive than those of T. cultridens and D. vallidens. Another important pro- gression is seen in the premolar ectolophs — namely, in certain specimens, Am. Mus. 1556, 1509 {M. petersoni) the tritocone ectoloph is flat, as in D. vallidens (Cope), whereas in other specimens, Am. Mus. 1571 (if. petersoni), 1513 {M. megarJiinus), 12184 (type of M. petersoni), the tritocone ectoloph is more conic, as in M. manteoceras. Comparative measurements of the superior pre- molars are given in the table on page 388. The inferior premolars are more or less perfectly represented in six jaws in the American Museum collection, none of which, however, are certainly associated with skulls. Pi is a small, conic or slightly flattened tooth, separated from the canine by a diastema 8 to 12 millimeters in length; a slight diastema (4 mm.) also separates it from P2; pi is a typically single, rarely bifanged tooth, with a narrow, laterally com- pressed, recurved, pointed crown (9 by 6 mm.). P2 is a bilobed tooth and elongate, but relatively less so than in T. cultridens — in fact, it is slightly more pro- gressive than in that species; the typical measure- ments are 18 millimeters anteroposterior and 9 transverse; anterior to the elevated protocone is the beginning of the anterior valley and a rudiment of the antero-internal cusp ( = paraconid) ; the much more depressed talonid similarly consists of a shallow, rudimentary crescent, opening inward. P3, like its fellow in the upper jaw, shows more equal anterior and posterior lobes, on which the crescents and internal valleys and the cusps corresponding to the paraconid and entoconid of the molars are more accented; the typical proportions in M. megarJiinus are 17 by 9 millimeters; in M. petersoni the typical proportions of p2 are 19 by 10, but this tooth has only a rudiment of the prominent internal cusp corresponding with the metaconid of the molars. P4 is still further advanced or submolariform, having a prominent internal cusp corresponding to the meta- conid in the molars, each lobe consisting of two fairly defined crescents ; it differs from mi in its smaller dimen- sion {M. megarJiinus 17 by 11 mm., M. petersoni 19 by 12) and in the nonelevation of the postero-internal cusp (entoconid). The cingulum is practically rudimentary or wanting in all these teeth. Molars. — The MesatirJiinus or generic characters of the superior molars (PI. LXXII) are seen in the fol- lowing features: (1) The slight excess of anteropos- terior over transverse diameters, especially in the more dolichocephalic M. petersoni; (2) the high, sharply pointed protocone (unworn height, 6 mm.); (3) the high, elongate external cusps (height of unworn para- cone, 23 mm.); (4) the very sharp para-, meso-, and 394 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA metastyles; (5) the reduced but still persistent proto- conules (sometimes vestigial, Am. Mus. 1519, 1513, M. megarhinus); (6) the anterior and posterior cingula; FiGTjRE 331. — Lower jaws of Mesalirhinus One-tourth natural size. A, if. megarhinus, Am. Mus. 1520, Bridger Basin, Wyo., level unknown, B, M. petersoni, Am. Mus. 1512, La Clede Meadows, Washakie Basin, Washakie A; symphyseal region restored from Am. Mus. 1575 (.M. mega- rhinus), La Clede, Washakie Basin; canine from Am. Mus. 1551 (M. megarhinus). Twin Buttes, Bridger Basin. C, M. petersoni, Am. Mus. 1667, Bridger Basin, level unknown; lower jaw of an old animal. D, M. petersoni, Am. Mus. 13178, north of Haystack Mountain, Washakie Basin, Washakie A; lower jaw of an old animal. (7) the vestigial metaconule seen in m^ only in certain specimens (Am. Mus. 1556), the majority exhibiting no trace of this cusp; (8) the serrate external cingulum at the bottom of the ectoloph valleys, especially in the more progressive specimens. The superior molar series measures from 87 to 91 mUluneters in M. petersoni and from 77 to 83 in M. megarJiinus. The inferior molar series measures from 94 millimeters in M. megarhinus to 104 in M. petersoni. The inferior molars are characterized by faint serrate, noncontinuous external cingula, which follow the curvature of the crown inward between the outer lobes, as distinguished from the cingula in P. paludosus, which form a straight line along the base of the outer border of the tooth. In the long, narrow ma {M. megarlhinus, ap. 43 mm., tr. 19; M. petersoni, ap. 46, tr. 19) a serrate internal cingulum rises on the inner side of the hypoconulid but does not ascend so prominently as in T. cultridens. This hypoconulid is progressively conic in form; it is slightly more conic, more median in position, and less sharply crescentic or cupped on the inner side than in T. cultridens; but in certain specimens (Am. Mus. 1512, 1577) it has the moie crescentic form of the T. cultridens type. In some molars (Am. Mus. 1512, 1575, 1520) faint rudi- ments of the metastylid fold are seen, but as a rule the internal valleys are open and smooth. Other teeth are too much worn to determine the presence or absence of the metastylid fold. Lower jaws of M. megarJiinus and M. petersoni. — The lower jaw of these animals is represented by a large number of separate jaws belonging to both species (see below). These jaws taken together afford very complete knowledge of the progressive, age, and sexual characters. There is a very marked disparity in size between the smallest (Am. Mus. 1520, M. megarhinus) and the largest (Am. Mus. 1512, M. petersoni). Comparative measurements of Mesatirhinus and Metarhinus, in millimeters Pi-ms Pj-ms Mi-m3 Ms, anteroposterior Incisive border to angle. Mesatirhinus megarhinus, Am. Mus. 1520 (Bridger D?) 162 146 94 43 Mesatirhinus petersoni. Am. Mus. 1512 (Washakie A) 176 ■160 102 46 325 Metarhinus fluviatilis. Am. Mus. 1946 (Uinta B 2) 161 157 102 46 The coronoid rises rather rapidly behind m3, with a more or less rounded or angulate anterior border and with nearly parallel anterior and posterior contours until near its summit, when it suddenly curves back into a decided posterior hook. (Am. Mus. 1512, fig. 331.) EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 395 The condyle in M. petersoni is well raised (144 mm.) above the lower border of the angle and extends 107 millimeters behind ms; it is more extended antero- posteriorly and less transversely than in M. manteo- ceras. The angle is very similar to that of Eotitanops borealis on a larger scale, with a slender and slightly incm-ved posterior border. The lower border of the ramus is concave below the coronoid, convex below the grinders, and rises gradually into a laterally com- pressed chin gently rounded on the inferior surface. The ramus increases in depth posteriorly. The symphysis measures 69 to 80 millimeters, and as seen from below the chin is sharply contracted to a width of 40 millimeters behind the canines. Below mi the rami attain in males the width of 20 millimeters. On the whole this is a progressive development of the E. borealis type of jaw, the chief difference being the broader coronoid. Age characters. — In an aged, somewhat larger, more elongate, and perhaps more progressive jaw (Am. Mus. 1567), from the Bridger, there is a wider space (130 mm.) between the condyle and the posterior fang of ms, the condyle itself is wider (57 mm.) and less extended anteroposteriorly, the gentle rounding of the posterior border of the angle seen in E. borealis and the typical M. megarJiinus changes into a more decided, angulate projection of the postero- inferior border. Seen from behind, the border of the angle is marked by a sudden sharp inflection about two-thirds of the distance below the condyle occipital condyles measure 98 millimeters transversely, as compared with 86 in M. petersoni. The width across the postglenoid processes is 183 millimeters, as compared with 150 in M. petersoni. This cranium may possibly belong to an animal with a dentition such as that which we have referred below to D. vallidens. A progressive jaw from Washalcie A. — A specimen (Am. Mus. 2355) from Washakie A at Glove Springs consists of the rami incomplete posteriorly, but includ- ing all the teeth. It belongs to the dolichocephalic Mesatirhinus series. It is much larger than the most advanced jaw of M. petersoni from Washakie A. The measurements are compared below: Figure 332. — Lower jaw of Mesatirhinus sp. with deciduous dentition The COndvle One-half natural size. Am. Mus. 12211, Summers Dry Creek, Bridger Basin, Wyo., Bridger C. is more transversely extended. A small lower jaw (Am. Mus. 12211), from Bridger C, has deciduous incisors 1 and 2 and three deciduous premolars in place, with some of the replacing teeth below them. The incisors are chisel-shaped, some- what like the adult incisors of Lambdotherium. The fourth deciduous premolar is more molariform than its successor, especially in the somewhat better devel- opment of the entoconid. The chin is very sloping. (See fig. 332.) Mesatirhinus sp. Large progressive sTcuil (fig. 333). — There is interest- ing evidence (Princeton Mus. 10041) of the existence in Washakie B (?) of a much larger animal than M. petersoni, progressive at least in size toward Doliclio- rJihius vallidens. It differs from MesatirTiinus superior in the narrow sagittal crest. Unfortunately only the occiput is preserved. It ex- hibits in the parietal profile a pronounced convexity; the parietal crests are also broadly divergent anteriorly and rounded, suggesting those of SpTienocoelus. The superior dimensions are indicated by the follow- ing comparisons: The occiput measures 90 millimeters in height, as compared with 80 in M. petersoni; the Measurements of Mesatirhinus and Dolichorhinus, in millimeters Pi-ms- Pz-ma- Mi-ms. 175 164 103 192 177 112 Washakie B: D. hyogna- thus, Prince- ton Mus. 10273 (type) 240 213 120 The postcanine diastema in this specimen is -long (32 mm.). As in MesatirTiinus the canines are incurved as well as recurved. Mesatirhinus ( = Dolichorhinus) superior Riggs Reference may be made here to the skull of M. { = Dolichor7iinus) superior, which is fully described below (p. 405). This animal is intermediate in form between the two genera Mesatirhinus and Dolicho- rhinus, so that it might be placed in either genus with equal propriety. The skull and hypocone on m' of M. superior incline us to connect this skull with species of Dolichorhinus. 396 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Dolichorhinus Hatcher Plates XVII, XXIX-XXXII, XLVI, LII, LIII, LV, LXXI- LXXVII; text figures 27, 33, 105, 110, 125, 133-137, 140 210, 214-220, 254, 255, 302, 322-324, 335-337, 339-353, 483' 508-511, 520, 521, 579-585, 588-591, 647, 661, 685-686, 71l| 724, 733, 737-740, 742, 743, 745 [For original description and type references see p. 177. For skeletal characters see p. 645] Generic cTiaraders . — Animals of relatively large size, extremely dolichocephalic; cephalic indices 43 to 47; face relatively long; faciocephalic index 48 to 51; postorbital process situated above m'; summit of cranium broadly flattened; space above small brain chamber filled with large air sinuses; occiput low and broad; relatively prominent supraorbital horn swell- ings on nasals; axis of face and middle part of skull bent downward. Astragalus of the long-necked type. Figure 333. — Imperfect cranium of Mesaiirhinus petersoni One-fourth natural size. Princeton Mus. 10041, Washakie Basin, Wyo., Washakie, i view; As, occipital view; As, top view; A4, basal view. DolichorTiinus, appropriately named by Hatcher in reference to the elongation of the nasal region, is a titanothere in which we observe the dolichocephalic extreme. The genotype species, D. Mjognathus , from Uinta B 2, is connected by -transition forms in Uinta B 1, such as Dolichorliinus longiceps and especially D. superior, with the advanced structural stages of Mesa- iirhinus, namely, M. petersoni, so that there is no question that DolicliorMnus is a descendant of certain species of Mesatirhinus. Besides the highly progres- sive D. hyognathus and the more primitive D. longiceps there are several species of Dolichorhinus less clearly defined, a fact which indicates that this was a domi- nant and highly diversified form during the period of deposition of the river sandstones and flood-plain deposits of the levels Washakie B and Uinta B 1 and B 2 (see below). History of discovery. — This animal first became known through Cope's personal exploration of the Washakie Basin exposures of 1872, which yielded his cotypes of "Palaeosyops vallidens" ; this species ap- parently represents a distinct stage of Dolichorhinus, but unfortunately it is still known only from an im- perfect lower jaw and some upper teeth. The next discovery was that of Scott, Osborn, and Speir, of the Princeton expedition of 1878, consisting of the large lower jaw which in 1889 Scott and Osborn made the type of the species "Palaeosyops hyognathus." The third step was marked by Peterson's discovery in 1894 on behalf of the American Museum of Natural History of several skulls and parts of skeletons in the Uinta Basin. These skulls aroused unusual interest because of the presence of well-developed horn bases above the eyes, in reference to which Osborn named the animals Telmatotherium "cornutum." He first considered that they represented a direct progressive transition from "Tel- matotherium vallidens" {= Manteoceras) toward the Oligocene titanotheres, but, as Hatcher pointed out in 1895, the horn development in these animals is a paral- lelism rather than a direct approach to the Oligocene titanotheres, for accom- panying these horns are other characters which exclude the animals from such ancestry. Hatcher accordingly separated the species as a distinct genus, Doli- chorhinus. It was long believed that Dolichorhinus was confined to the Uinta Basin level B 2, to which Osborn gave the name Dolichorhinus cornutus zone. The animal certainly occurs in Uinta B 2, especially in the river-deposited sandstones, A. Ai, Side jq very great abundance and may be con- sidered as the dominant titanothere type of this deposition because it so far outnumbers all other types. The next step in discovery was made by the Ameri- can Museum expedition of 1906 in the Washakie Basin, during which Paul Miller found a beautifully preserved skull and jaws of a Dolichorhinus associated with parts of the skeleton and specifically identical with the type of D. cornutus prevailing in Uinta B 2. This discovery, together with evidence previously found, demonstrated the synchronism of the Washakie B 2 and the Uinta B 2 deposits. Further comparison of the jaws of this Washakie specimen with the type jaw of "Palaeosyops hyognathus," also from Washakie B, demonstrated that the species D. cornutus is a syn- onym of the earlier-described D. hyognathus. Exact EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 397 study and comparison of all these specimens resulted in the opinion that "Palaeosyops vallidens " also belongs to the Mesatirhinus-Dolichorhinus group. Among the crania that were at first included within the single species D. cornutus are two stages — an earlier stage, to which the name D. intermedius may be given, and a later stage, D. hyognathus. Akin to and possibly to be regarded as "mutations" of these stages are the species D. heterodon and D. longiceps of Douglass. Geologic range and faunistic parallels. — The type of Dolicho- rhinus cornutus { = 'hyognathus) was found by Peterson in the sandstone at the top of Uinta B 2. The genus thus ranges downward through 700 feet of deposits to the type locality of DolicJiorhinus longiceps, repre- senting a long period of geo- logic time, in which we should expect considerable evolution of structural type as well as considerable changes in the con- temporary mammalian life. In Uinta B 1, for example, Doli- chorhinus longiceps is associated with MetarJiinus fluviatilis and M. riparius. It is noteworthy, however, that Dolicliorldnus sel- dom occurs in the same sand- stone with Mefarhinus, a fact indicating that these animals occupied somewhat different local habitats. It is also a striking fact that Manteoceras does not occur at all in Uinta B 1 or B 2 nor has it been found in Washakie B, while its relative MesatirJiinus occurs quite abundantly. This would appear to prove that DolicJio- rhinus, MetarMnus, and Mesa- tirliinus had different habitats and habits from either Mante- oceras or Telmatherium, and that the conditions existing during the period of deposition of Uinta B 1 and B 2 were particularly favorable to the preservation of Dolichorhininae — namely, DolicTiorhinus , MetarMnus, and Mesatirhinus. Among other ungulates no repre- sentatives of the Equidae or Tapiridae are found mingled with the dolichorhines. The hyracodont or light-limbed division of the rhinoceroses is repre- sented by rare remains of Triplopus. The amphib- ious division of the rhinoceroses is represented by quite abundant remains of Amynodon. Among the Amblypoda, or giant quadrupeds, Eohasileus is very abundant and characteristic of the Dolichorhinus zone. Among the Artiodactyla the ancestral elothere Achaenodon occurs in the lower levels close to Doli- FlGTJRE 334.- Geologic section of the Bridger formation in the Washakie Basin chorJiinus, and Protelotherium occurs in the uppermost levels. The giant flesh eaters Mesonyx and Harpago- lestes are characteristic of this life zone. In general, the occurrence of the majority of these dolichorhine titanotheres in river sandstones associated with the remains of other fiuviatile or river-border types, such as Amynodon, Achaenodon, and possibly Eohasileus, 398 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA tends to favor the view that the doUchorhines which frequented the river borders were subject to being washed into the sandy deposits during periods of flood. apparently indicating no increase in speed. Dolicho- rhinus longiceps may be described as dohchocephahc and brachypodal. (See p. 652.) Figure 335. — Restoration of Dolichorhinus longiceps By E. S. Christman, based on the mounted skeleton in the Carnegie Museum. One-fifteenth natural size. The bodily proportions of the dolichorhines were similar to those of the existing forest-living pigs of Africa. Habits of Dolichorhinus. — We may compare Dolicho- rhinus remotely with Hippidium, an aberrant South American Pleistocene horse, in which an excessively The muzzle was rather expanded, or truncate; the face was not so long as that of other titanotheres. The incisors were arranged in a semicircle and made some approach in form to the cropping incisors of the ruminant. These teeth were also partly cupped to Figure 336. — Skull and lower jaw of Dolichorhinus hyognaihus One-Iourth natural size. Skull, Am. Mus. 1851; lower jaw, Am. Mus. 1856. Both from White River, Uinta Basin, Utah, level Uinta B 2. long skull is combined with exceptionally short meta- podials, in contrast with those of typical horses. So far as we can judge from very sparse evidence, the feet of Dolichorhinus were surprisingly short, facilitate prehension, as in the lower Miocene species of the horse. The diastema behind the canine tooth is longer than in other titanotheres, as in typical herbivorous forms. The canines in the males were EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 399 moderately long, recurved, sharp edged, and may have been used in fighting, as by the existing camels; the offensive power of the front teeth was less, however, than in the short-jawed Palaeosyops. The cheek teeth, concerned in the comminution of food, were relatively long crowned, with pointed cusps, and constituted an elaborate cutting and triturating appa- ratus. The movement of the more slender mandible was partly vertical, partly oblique, since the wearing of the cheek teeth gives evidence of an oblique shear- ing action. Adapted to these conditions were the length and proportions of the chewing muscles and their angles of action. (See below for details.) It is therefore probable that since the food evidently required finer cutting and better trituration than the food of Palaeosyops, Dolichorhinus was either a browser on harder materials or a grazer, perhaps coming out from the forests at night into the open grassy places or searching for smaller twigs, like the Indian rhinoceros {R. unicornis). The bend- ing down of the facial upon the cranial axis is a characteristic of many grazers, whereas the bend- ing up of the facial axis is generally characteristic of browsers. Directing attention, on the other hand, "to the progressive backward shifting of the hinder border of the posterior nares to what is known as the "sec- ondary palate," Riggs (1912.1, p. 36) has advanced the hypothesis that DolichorMnus was a river-fre- quenting form which perhaps fed upon submerged plants, like the moose. The backward shifting and closure of the hinder border of the palate is an ob- vious advantage to animals feeding partly in the water and is characteristic of many water-living forms. General characters of the genotype, D. hyognathus. — The elongate skull, the broad, flattened, and suture- less cranial region, the elongate nasofrontal horns are characters partly of progressive dolichocepha- ly, partly of family affinity to the Oligocene forms. The features of the main line of Dolichorhinus are the extreme narrowing and lengthening of the skull and zygomatic arches, the convex upward arching instead of a concave saddle shape of the skull top, the broad infraorbital shelf, the shal- low jaws, the parallel series of grinding teeth, and especially the extremely long, narrow nasals. The horns are borne chiefly on the nasals, as in Mesatirhi- nus, in contrast with Manteoceras, in which they are borne chiefly on the frontals. The occiput is low, possibly in correlation with the bending down of the cranium. In palatal view we observe the marked backward extension of the posterior nares and the formation of a secondary palate. The jaw is dis- tinguished by its long, slender, recurved coronoid process and its depressed angle. These characters combine to constitute this animal one of the most peculiar and distinctive of the whole titanothere At a first glance the long skull suggests that of a horse, but a closer examination shows that, although both are dolichocephalic, the resemblance is entirely superficial; the horse has a primitive short cranium (brachycrany) and an enormously long face (dolichopy) or preorbital region. Dolichorhinus has an elongate, highly modified cranium (dolichocrany) and postor- bital region and a relatively short face (brachyopy). As compared in detail with the skull of a horse that of Dolichorhinus furnishes an instructive minsrlins: of craruzan. FiGUBE 337. — Skulls of Dolichorhinus hyognathus (A) and modern horse (B) One-sixth natural size. Tliese show analogous and divergent adaptations to grazing habits. A-A', Basicranial axis; B~B^, basipalatal axis. convergent resemblances to other long-headed un- gulates and divergent hereditary differences. Among the convergent resemblances in Dolichorhinus are (1) the lengthening of the whole skull, especially of the face; (2) the bending down of the anterior half of the skull; (3) the backward prolongation of the palate; (4) the semicircular or cropping arrangement of the incisors; (5) the prominence of the orbits; (6) the forward extension of the masseter muscle, the anterior slip in Dolichorhinus being attached to the infra- orbital shelf. Among the divergent hereditary differences charac- teristic of the titauotheres and shown in Dolichorhinus 400 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA are (1) the lengthening of the middle part of the skull; (2) the development of supraorbital horn swellings; (3) the character of the teeth; (4) the shape of the 4o6 Dolichorhinus fluminalis Dolichorhinus cornufus, type Sfhenodectes incisivus, iype Dolichorhinus heferodon Dolichorhinus hyognathus (cornufus) Sfhenodectes incisivus Rhadinorh/nus diploconus Dolichorhinus longiceps, type Dolichorfilnus superior, type Mefarhinus riparius, type Metarhinus earlei Telmalotherium Dolichorhinus longiceps Rhadinorhinus abboffi Metarhinus fluviafiiis, type Sphenocoelus uintensis, type Metarhinus cristatus, type (?Dolichorhinus longiceps) Metarhinus riparius -" DiplojCodoTh ~f£r'~ zone idZA. ATrvynodorhskeL.Am.M:us.N9 J933 •noo Lorv scutdstoThe^JAmynodon. irvt^rmedius ' yProtelotTierium, uintense I EohCLSii^LLS < Stylinodon. I ProtylopiLs . EohasUeus- ^°J)_olic/iorkinus zone s^tndstone^ Harpct^ol^stes EobcLsileus uintensis, type FieldMus. 12170 JHobasileiis Triplopus Mesonyjc obtuside/is ? Triplopus In the comparison of numerous dental series we observe that the male teeth are somewhat larger, including the robust, sharp-edged canines, whereas the female jaws are more slender and the ca- nines smaller and rounder, with shorter enamel caps. Sex apparently does not affect the de- velopment of the osseous horns, which are practically similar in the male and female skulls. Synopsis of species. — The following sum- mary gives the principal features of the species assigned to Dolichorhinus: UPPER LEVELS D. hyognathus (Osborn) = Z). cornutus Osborn. Summit of Uinta B 2 and middle of Washakie B 2. Cranium large, most progressive, broad and convex, length 550 millimeters, breadth 240, cephalic index 46, faciocephalic index 53; broad secondary palate; horns well developed. D. fluminalis Riggs. Summit of Uinta B 2. Dis- tinguished by extreme backward prolongation of secondary palate. Length, type skull, 520 millime- ters, breadth 233, cephalic index 45, faciocephalic index 48. D. intermedius Osborn. Uinta B 2. A broad form with elongate skull; length 485 millimeters, breadth 223 (estimated), cephalic index 45 (estimated), facio- cephalic index 49; horns less prominent; secondary palate less extended posteriori}'. D. heterodon Douglass. Summit of Uinta B 2. Similar to D. intermedius. Length 487 millimeters; cephalic index not determined; faciocephalic index 50. NO MAMMALS RECORDED Figure 338. — Geologic section of the Eohasileus-Dolichorhinus and Meta- rhinus zones in the Uinta Basin lower jaw; (5) the broadening of the top of the skull; (6) the relatively short crowns of the grinding series. As a whole the grinding series is short (206 mm.) in proportion to the length of the skull, the molar index being 38. Correlated with molar dolichocephaly the inner and outer cones of the molar teeth are closely approximated and the crowns are elongated and narrowed. In adaptation to harder kinds of food the crests and cones are elongate or subhypso- dont; both the parastyles and mesostyles are very sharp and prominent. The backward and downward prolongation of the- bony palate is a very distinctive feature. In early stages {D. intermedius) the secondary palate is rudimentary and lies much above the plane of the primary palate; in later stages it descends and lies on the same plane as the primary palate, also extend- ing very far backward (D . fluminalis) . LOWER LEVELS D. longiceps Douglass. Base of Uinta B 2. Very abundant; more primitive; probably ancestral to D. hyognathus. Horns incipient. Large size, length 545 millimeters, breadth 260; cephalic index of type 47; faciocephalic index 48; cranial roof narrow, less arched. D.? vallidens (Cope). Washakie B(?). Imperfectly known teeth, more primitive than those of D. hyo- gnathus. D. (Mesatirhinus) superior (Riggs). Summit of Umta B 1. Smaller and more primitive. Horns very rudimentary. No secondary palate. Summary of cranial indices in Dolichorhinus Species Cephalic index Faciocephalic inde.t D. hyognathus. Am. Mus. 13164, 9 _._ 46 53 D. hyognathus. Am. Mus. 1851, 9 (type of Telmatherium cornutum) 43 51 D. intermedius, Am. Mus. 2001 »45 49 D. intermedius, Am. Mus. 1837 (type)__ -41 49 D. heterodon, Carnegie Mus. 2340 ' 50 D. fluminalis. Field Mus. 12205 (type)__ 45 48 D. longiceps, Carnegie Mus. 2347 47 48 D. longiceps, Am. Mus. 1852, 9 42 48 D. superior. Field Mus. 12188 52 48? The extremes of these specific indices are also presented above. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES Dolichorhinus vallidens (Cope) Plate LXXIV; text figures 95, 341, 353 [For original description and type references see p. 362] Geologic horizon. — Washakie Basin, Wyo., level 401 Washakie B. than in D. hyognathus; hypoconulid of m^ elongate; in general more primitive than either D. intermedins or D. hyognathus. As shown above, the imperfectly preserved upper and lower jaws, the co types of Cope's original descrip- tion, were not found together. Nevertheless it now FiGUBB 339. — Skulls showing progressive dolichocephaly in the Mesalirhinus-Dolichorhinus phylum Side view. Ono-fourth natural size. A, Mesatirhinus petersoni, British Mus. (formerly Am. Mus. 1556), Big Bone Mountain, Henrys Fork, Bridger Basin, Wyo., Bridger D; B, Dolichorhinus superior, Field Mus. 12188 (type), Uinta Basin, Utah, Uinta B 1; C, D. longiceps, Carnegie Mus, 2347 (type), Uinta Basin, Uinta B 2; D, D. hyognathus, Am. Mus. 1851, White River, Uinta Basin, Uinta B 2. Specific characters. — P'-m^ 185 millimeters (esti- mated); mj-m,,, 123; ectolophs of superior premolars with a broad basal spreading of the protocone con- vexities; tritocones more flattened than in D. inter- medins; p^, p^ of same proportions asm D. hyognathus; lower premolars less compressed and more primitive appears probable though not certain that the lectotype lower jaw (Am. Mus. 5098) and at least one of the original upper dentitions (Am. Mus. 5097) do pertain to the same species. Doubtful reference. — The reference of these types to Dolichorhinus is provisional; if the jaws are correctly 402 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA referred the cranium is apparently much less dolicho- cephalic than that of D. hyognathus or D. longiceps. Comparison with Dolichorhinus superior is also diffi- cult and unsatisfactory; in D. superior the premolars appear to be different in contour, also their cingula are not so heavy; the measurements of the superior teeth (p'-m^) in these two species are approximately assigned this animal to Palaeosyops. Osborn at first regarded it as belonging to the genus Manieoceras but subsequently recognized the prevailing dolichoce- phalic characters and placed the animal near Dolicho- rhinus. Lectotype lower jaws of D. vallidens (Am. Mus. 5098). — Comparison with typical lower jaws of D. FiGTJKE 340. — Skulls showing progressive dolichocephaly in the Mesatirhinus-DoUchorhinus phylum Top and palatal views. One-eightli natural size. A, Ai, Mesatirhinas petersoni, British lilus. (formerly Am. Mus. 1556), Big Bone Mountain, Henrys Fork, Bridger Basin, Wyo., Bridger D; B, Bi, Dolichorhinus superior, Field Mus. 12188 (type), Uinta Basin, Utah, Uinta B 1; C, Ci, D. Usngiceps, Carnegie Mus. 2347 (type), Uinta Basin, Uinta B 2; D, Di, J), hyognathus, Am. Mus. 1851, White River, Uinta Basin, Uinta B 2. pn^. Primary border of the posterior nares; pn', secondary border of the posterior nares. the same — 185 millimeters (estimated) in D. vallidens and 182 in D. superior. The hypocone on m^ prob- ably absent in D. vallidens, is present and strong in D. superior. The upper teeth of D. vallidens (paratype) are structurally ancestral to those of Diplacodon, but so also are the upper teeth of Mesatirhinus petersoni. History. — The species was at first referred by Cope (1885.1, p. 700) to the genus Palaeosyops. Earle also hyognathus from both Washakie B 2 and Uinta B 2 shows that D. vallidens was a smaller animal and some- what more primitive in the details of the lower pre- molars. (See fig. 353.) The chin is only partially preserved and with it the root of the right canine, which is stouter than in sup- posed females of D. hyognathus. The first lower pre- molar, as indicated by the alveolus in Cope's drawing EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 403 (1885.1, pi. 52, fig. 3), appears to have had but one root. P2 is shorter anteroposteriorly and less compressed than in the typical D. hyognathus; its posterior V is also smaller as compared with the protoconid and less sharply developed. P3 is not preserved. P4 is also less compressed, the posterior V lower and more primitive in form than in D. hyognaihus. The true molar series is considerably shorter, but the posterior half of ms and especially the hypoconulid are relatively longer and more compressed. The space between ms and the ascending ramus was less. Comparative measurements are as follows : Measurements of Dolichorhinus vallidens and D. longiceps? , in millimeters Front of canine to hinder border of ma Front of pi to hinder border of mj Length of true molar series P2, ap. by tr P4, ap. by tr M3 (anterior lobe) , ap. by tr M3, length of hypoconulid D. vallidens, Am. Mus. 6098, lecto- type jaw 230 146 123 19X11 23X13 55X20 14 D. longiceps?. Am. Mus. 1852 275 165 139 23X11 25X15 60X23 15 The specimen under consideration is distinguished from jaws of M. manteoceras by the longer molar series and more elongate hypoconulid on ms. Upper teeth of the paratype of Dolichorhinus vallidens (Am. Mus. 5097). — The characters of the premolar ectolophs are so constant in all the many specimens of D. hyognathus that the marked differences which they present in the paratype of D. vallidens, approaching as they do the characters of the Mesatirhinus premolars, appear to establish the specific separation. The whole series of upper grinding teeth (p'-m^) of D. vallidens is estimated at 185 millimeters, as compared with 177 in D. intermedins and 206 in D. hyognathus. Comparison with Dolichorhinus hyognathus. — As noted above, the superior grinding series is shorter than that of D. hyognathus (185 mm. (estimated), as compared with 206), and the detailed anteroposterior and transverse measurements of the crowns of the only perfectly preserved teeth, p^, p'*, are practically identi- cal with those of the average D. hyognathus, as shown below : Measurements of upper premolars in species of Dolichorhinus, in millimeters P^ anteroposterior P^ transverse P^, anteroposterior P^i transverse P', internal lobe, anteroposterior P*, internal lobe, anteroposterior D. inter- raedius, Am. IMus. 1837 (type) D. valli- dens. Am. Mus. 5097 (paratype) 13 15 9 10 18 20 20 22 16 19 19 22 15 9.8 20 22 21 23 The linear ectoloph measurements of the true molars are intermediate between those of D. intermedins and D. hyognathus. The ectolophs of the premolars of D. vallidens (fig. 341, B) afford the most distinctive character — namely, the broad festoon and the basal spreading of the convexities of the protocone, a primi- tive character which relates these teeth to the Mesati- rhinus stage. The deuterocones of p^"* are more D A Figure 341. — Upper premolars of Mesatirhinus, Dolichorhinus, and Metarhinus Outer side view. Natural size. A, Mesatirhimis petersoni, British Mus. (formerly Am. Mus. lo5G), Big Bone Mountain, Henrys Fork, Bridger Basin, Wyo., Bridger D; B, Dolichorhinus vallidens, Am. Mus. 5097 (paratype), reversed, Mammoth Buttes, Bitter Creek, Washakie Basin, Wyo., Washakie B; C, DolicliorJiinus hyognathus, Am. Mus. 1850, White River, Uinta Basin, Utah, Uinta B 2; D, Metarhinus fiuviatilis. Am. Mus. 1946, White River, Uinta Basin, Utah, Uinta B 1. primitive and the tritocones are more flattened than in' the D. intermedius type. These characters tend to show that so far as indi- cated by the paratype D. vallidens is somewhat more primitive than either D. intermedius or D. hyognathus. Comparison with M. petersoni and other forms. — The superior grindiag teeth of this paratype resemble those of certain specimens of M. petersoni on a larger scale. The progressive distinctions are (a) the pres- ence of a cingulum on the inner side of p'; (6) the quite complete cingulum on the inner side of p^ and p* and the somewhat more flattened and elevated ectolophs of p^~^, which are less elevated, however, 404 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA than in D. Tiyognathus; (c) the external cingula of the molar teeth are a shade more prominent; [d) the protoconule and metaconule have nearly vanished, al- though inconspicuous vestiges still persist. The faint rudiments of the tetartocone folds of the premolars are less marked than in certain specimens of M. petersoni. The ectolophs of the superior premolars are readily distinguished from those of M. manteoceras by the flattening of the tritocones. The ectoloph of p^ is simple and sharply convex. That of p- consists of a prominent protocone convexity which spreads downward into a broad cingulum bound- ing the base of the tritocone (the same region in D. Jiyognathus is strongly constricted); the tritocone ectoloph is nearly flat or very slightly convex. In p' the protocone ectoloph is a convex ridge spreading toward the base into a cingulum, while the tritocone ectoloph is very gently convex but without a rib. P* has the convexity opposite the apex of the proto- cone, while the outer face of the tritocone is slightly convex and the basal cingulum is nearly continuous across the ectoloph. The above-mentioned features enable us to distinguish the upper premolars from those of Manteoceras and of both D. intermedius and D. Tiyognathus. Summary. — In the lectotype lower jaw of D. rallidens the premolars are decidedly more primitive than in D. hyognathus. In the paratype upper dentition of D. vallidens the premolars are somewhat more primitive than in D. hyognathus. It is thus not certain that the lectotype and paratype belong to precisely the same stage of evolution; but, on the other hand, there is no evidence that they are specifi- cally distinct. Comparative measurements, in millimeters, showing progressive proportions of skull and teeth of Dolichorhinus Premaxillaries to condyles End of nasals to middle top of occi- put Face, anteroposterior. Cranium, anteropos- terior Transverse zygomata Pi-m3 P2-m3 P2-p< Mi-m3 P', ap. by tr P^, ap. by tr M', ap. by tr M2, ap. by tr M3, ap. by tr D. supe- rior, Field Mus. 12188 (type) 224 184 I), intermedius Am. Mus. 1837 (type) 475 230 236 '190 179 165 57 109 Am. Mus. 2001 475 241 248 '■223 177 164 57 105 12X10 19X2120X25 31X32| 30X? 39X38 36X37 39X39 D. val- lidens, Am. Mus. 5097 D. het- erodon, Carne- gie Mus. 2340 (type) 487 492 245 240 189 173 114 .22X25 .133X32 .|42X4I .[39X38 D.flu- minalis. Field Mus. 12205 (type) 520 233 171 105 D."lon giceps," Carne- gie Mus. 2347 (type) 590 °270 -285 -264 197 178 60 115 15X10 D.lon- giceps?. Am. Mus. 1852 573 270 283 230 202 ■185 62 118 15X11 21X2920X26 35X?36X33 39X40J41X39 39X?i40X37 D. hyognathus Am. Mus. 13164 580 '290 250 205 186 65 119 18X11 23X29 36X33 43X42; 42X42 Am. Mus. 1850 593 '260 320 208 185 61 120 16X9 22X25 36X30 41X38 43X35 Am. Mus, 1851 550 570 280 288 »240 208 187 65 122 15X10 23X27 34X34 43X43 45X? Am. Mus. 1845 290 215 '565 Field Mus. 12167 (D. 'cornu- tus") 310 131 25X30|. 35X33|. 47X41. 47X41. 285 214 135 575 270 298 231 206 186 62 123 15X10 22X27 35X33 42X40 44X39 ' Estimated. 12188. Uinta B 1. 1837. Female. Uinta B 2. 2001. Uinta B 2. 5097. Washakie B. 2340. Uinta B 2 (upper level). 12205. Uinta B 2. 2347. Uinta B 2 (low level). 1852. Female. Uinta B 2. 13164. Washakie B 2. 1850. Male. Uinta B 2. 1851. Female. Uinta B 2 (type of Telmatotherium cornutum) . 1845. Uinta B 2. 1848. Uinta B 2. 12167. Uinta B 2. The above table shows the dolichocephalic propor- tions of the cranium proper and of the true molars and the smaller dimensions of D. intermedius and the intermediate proportions of the type of D. heterodon. The type of D. longiceps and the type of D. "cornutus" agree well in size with the skulls of D. hyognathus. The well-preserved skull from the Washakie Basin (Am. Mus. 13164), which is referred to D. hyognathus, does not differ greatly in measurements from the type of Dolichorhinus "cornutus" and the other Uinta B specimens. The skulls of D. hyognathus, from Uinta B, show a considerable difference in size, ranging from the relatively small skull No. 1852 to the very large skull No. 1845. Measurements of the lower jaws of these species are given on page 416. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 405 Dolichorhinus (Mesatirhinus) superior Riggs Plates LXXV-LXXVII; text figures 137, 339, 340 [For original description and type references see p. 190] Type locality and geologic Tiorizon. — Uinta Basin, Utah, MetarMnus zone, top of the " Metarhinus sand- stones," summit of Uinta B 1. D. superior comes from a horizon 200 to 400 feet above that at which D. longiceps is commonly found. The type was found in the same ledge and associated with one of the more advanced stages of MetarMnus {M. earlei). Thus D. superior is contemporary with a more advanced stage of development of DolicliorTiinus and with the last of the Metarhinus phylum (Riggs). Specific characters. — "Skull 485 by 255 millimeters, molar series 182 millimeters, nasals free to a point over last premolar, infraorbital process present, arches slender anteriorly, nasals infolded at margins, sagittal area expanded, canines small, p^ and p^ oblique to axis of series. Molars relatively small, strong hypo- cone on m^, posterior nares opening opposite the anterior margin of last molar." (Riggs, 1912.1, p. 26.) Materials. — The only specimen known is the type skull in the Field Museum (No. 12188), described below. This important form is transitional between Mesatirhinus and Dolichorhinus. On the whole it appears to be a primitive species of the genus Doli- chorhinus. The original description by Riggs is as follows : This genus [Mesatirhinus], reported for the first time from the Uinta formations, is apparently indigenous to the Bridger and Washakie Basins. It is represented in the Field Museum collections by a single specimen — an incomplete skull collected by Mr. J. B. Abbott from the top of the Metarhinus sandstones near gilsonite vein No. 2. The right arch is wanting, together with the basioccipital and condyles. The dentition is anatomi- cally complete excepting the incisors. The skull presents striking similarities with the earlier representatives of Dolichorhinus. From the dorsal view, the nasals, facial, and supracranial regions appear very similar, though the cranial region does not have the pronounced down- ward curve characteristic of Dolichorhinus. In the palatal view more marked differences are noticeable. The premolars are more primitive, the molars smaller, and the posterior narial opening is unmodified. In these characteristics the specimen in hand resembles D. heterodon ^' from upper Uinta B more closely. However, it differs from that species in having a strong hypocone on the last molar and in the whole facial profile. In our present knowledge of these many closely related forms, this species may be regarded as the largest and most highly specialized representative of Mesatirhinus. This animal occurs geologically at the very summit of Uinta B 1 (upper A of Riggs), fully 300 feet above the first occurrence of Dolichorhinus longiceps. This fact is important, because otherwise it would certainly be considered the direct ancestor of Dolichorhinus , since it affords a complete structural transition to this genus, as shown in the comparative outlines displayed in Figure 339. This is another very interesting in- '8 Douglass, Earl, Carnegie Mus. Annals, vol. 6, p. 310, 1910. stance of the survival of a primitive stage side by side with a progressive stage. We have an analogy in existing nature in the survival of the hippopotami of Liberia and the Nile regions of Africa, namely, H. liheriensis and H. amphihius, the former extremely primitive, the latter rather progressive. Although the profile and the top views (figs. 339, 340) of the cranium of D. superior are closely similar to those of D. longiceps, the palatal view is less similar because of the entire lack of the secondary palate, which in its various stages of development is so characteristic of Dolichorhinus. In D. superior, moreover, the horn cores are even more rudimentary than in D. longiceps. There is a wide orbital-nasal area, and a sharp downward curve of the nasals. The species is also related to M. petersoni in its cephalic index, which is 52 as compared with 47 in D. longi- ceps — in other words, the skull is less dolichocephalic than that of the typical Dolichorhinus. The opening of the posterior nares is opposite the margin of the second molar tooth, or in the same posi- tion as the primary nares of Dolichorhinus. The crowns of the molar teeth are somewhat shorter or more brachyodont than in Dolichorhinus. The molar cephalic index, or ratio of the length of the grinding series to basilar length of skull, is estimated as 38, the same as in D. hyognathus. Dolichorhinus intermedius Osborn Plate LXXIII; text figures 125, 342, 343 [For original description and type references see p. 184] Type locality and geologic horizon. — Uinta Basin, Utah; Eohasileus-Dolichorhinus zone (Uinta B 2). Specific characters. — As compared with D. hyo- gnathus, of inferior size; p'-m^ 179 millimeters; m'"', 109; length, premaxillaries to condyles, 462; trans- verse zygomata, 190 (estimated); cephalic index of type 41, of paratype 45; faciocephalic index 49. Secondary palate present btit less developed than in D. hyognathus; infraorbital shelf of malar relatively narrow; premolars less progressive with subconic deuterocones; all cingula less robust; nasals more pointed or less expanded distally. This species when described in 1908 was regarded by Osborn as a structural ancestral stage, or ascending mutation toward the typical D. hyognathus. It now appears to be a dwarfed and somewhat more primi- tive form, which thus coincides in some of its char- acters with D. longiceps (the true ancestor of D. hyognathus) except that the horn bases appear to be more distinct. It might perhaps be regarded as a side or dwarfed phylum related to or identical with the D. heterodon of Riggs. Materials. — The type is the skull Am. Mus. 1837, representing the main characters of this species. Another skull (Am. Mus. 2001) is somewhat less typical. These skulls are recorded from Uinta B 2. 406 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA The more exact level, however, is not stated. Only from thehr less progressive condition does it appear probable that they belong to a somewhat lower geologic stage than the typical D. Jiyognathus. Proportions. — These animals are smaller than those referred to D. Jiyognathus. The type skull belongs to a young adult female with canines proportioned as in the females of the type species. The total length (462 mm.) is somewhat inferior to that of the older animal (Am. Mus. 2001), in which the length is 485 millimeters, as compared with an average of 550 in D. Jiyognathus. Similarly the superior grinding series measures 179 millimeters, as com- pared with 156 in Mesatirhinus peter- soni and 206 in D. hyognathus. Comparison with D. (cornutus) hyo- gnathus. — The crania are of inferior di- mensions throughout. The nasals are nar- rower anteriorly; the horns are less prom- inent and are borne entirely on the nasal bones; the flattened vertex of the skull in the parieto-occipital region is relatively narrow. The incisors, as shown by i', pre- served in Am. Mus. 2001,are deeply pitted or pocketed, posteri- orly. P' is a small, simple tooth, less broadened anteriorly than in any speci- mens of D. hyogna- thus. The common characters as com- pared with B. hyo- 1837 (type), White River, Uinta Basin, Utati, gnathuS in P^-p'' are : (1) The crowns are less hypsodont throughout; (2) the protocone con- vexities on the ectolophs are more convex or less sharply constricted; (3) the tritocone convexities, on the other hand, are somewhat more prominent; (4) the deuterocones are more rounded or conic, and there is less prominence of the internal cingula and of the protoconules. All these characters indicate a lesser degree of progression. Additional note on Doliehorhinus intermedius. — Three skulls in the Carnegie Museum from Uinta B 2 (middle levels), Nos. 3094, 3095, 3096, collected by Doctor Douglass, are referred to this species. The principal dimensions of two of these are given below: Measurements of skulls of Doliehorhinus intermedius, in milli- meters Figure 342. — Skull of Doliehorhinus intermedius Top view. One-tourth natural size. Am. Mus. 1837 (ty: Uinta B 2. Pmx-condyles Transverse zygomata (estimated) Pi-m3 Pi-p* M"-m3 472 170 176 70 103 462 Doliehorhinus longiceps Douglass Plates XXX-XXXII, LXXIII, LXXV-LXXVII; te.xt figures 135, 136, 254, 335, 339, 340, 343-346, 353, 589-591, 724 [For original description and type references see p. 188. For sjceletal characters see p. 651] Type locality and geologic horizon. — Uinta Basin, Utah; Eobasileus-Dolichorhinus zone (Uinta B 2). Geologic range 300 to 400 feet. Specific characters. — Horn bases small; cranium large, 530-550 millimeters; breadth, 264-240; cephalic index 44-47; secondary palate in early stage of development, lying above level of primary palate; cranial vertex narrow posteriorly; premolars with relatively feeble internal cingula. Geologic distribution. — The type skull of this prim- itive and clearly defined species, recorded by Douglass as "700 feet below Uinta red beds," would place the type well down in Uinta B 2. The four skuUs (Field Mus. 12175, 12176, 12193, 12200) collected by Riggs extend from the lower to the upper portion of Uinta B 2 or the "upper Metarhinus beds" of Riggs. These specimens are somewhat smaller and less specialized than the type; they vary in length from 525 to 560 miUimeters. Type.-^^he. type skuU of Douglass has been dis- torted from right to left and from above downward, so that the left upper part is tilted and overhangs the left temporal fossa and orbits. The right pre- maxillo-maxillary rostrum is flat, and the general wearing plane of the left tooth row is tUted toward the right, while the parietofrontal vertex above the squamosal region is squeezed up into a long antero- posterior convexity. This distortion makes it difficult to determine what are the real structural differences from D. hyognathus, but the judgment of Douglass in separating this species is fully confirmed by the skuUs discovered by Riggs in Uinta B 1 and B 2. One skuU (Am. Mus. 1852), presumably that of a female in regard both to measure- ments and to characters, appears to bridge over the differences between this species and the type of D. cornutus { = JiyognatJius), as shown in the following measurements : EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 407 Measurements of Dolichorhinus longiceps and D. hyognalhus, in millimeters Tip of nasals to occipital crest (lateral) Premaxillary to condyle Transverse zygomata Face, anteroposterior (premaxil- lary tp postorbital process, frontal) Cranium, anteroposterior (post- orbital frontal to condyle) Pi-m3 Pi-p< M'-mS P', ap. by tr P^ ap. bytr M', ap. by tr M2, ap. by tr M', ap. by tr D. longi- ceps, Car- negie Mus. 2347 (type) 590 '555 264 "270 ■■285 197 79 115 15X11 21X29 35X ? 39X40 39X ? D. hyognnttius Am. Mus 1852 (a transitional form) '550 230 270 283 202 78 118 20X26 36X33 41X39 40X37 Am. Mus. 1851 (typo of Tel- matotlie- rirua cornutum) 577 550 '240 280 288 208 82 122 15X10 23X27 34X34 43X43 45X45 Thus these measurements indicate that as com- pared with the type of T. cornutum { = D. Jiyognathus) the type of D. longiceps is somewhat broader and its tooth dimensions sUghtly less, except that p** is wider. Field Museum sJculls. — The four skulls as described by Riggs (1912.1, p. 33) are somewhat smaller, less specialized than the type, and range in length from 525 to 560 millimeters. One of the largest, a finely preserved skull, is shown in Plate LXXVI. There is little evidence of incipient horn cores. The nasals overhang the margins of the premaxillaries, which are somewhat narrower than in the type of Douglass. Compared with the type of D. intermedins, the smaller D. longiceps skull (Field Mus. 12193) approaches closely in size; the dental series is similar in length; the premolars are more advanced in structure. In the palate there is a ridge between m^ and m^ corre- sponding to the primitive position of the posterior narial border, which is bridged over by the outgrowth of thinner plates from the lateral margin of the palatal bones so that the nares have receded to a point behind the hamular processes of the pterygoids; the plates of this secondary palate are, however, so thin that they are often broken through, so that the secondary border of the posterior nares can not be precisely determined. The secondary palate in this species is pierced by a pair of foramina; its posterior extension is an enfoliate process free from the lateral walls and probably attached to the inferior margins of the vomer (Riggs). A mandible associated with the incomplete skull of D. longiceps (Field Mus. 12200) is relatively strong, curved in the ramus, and broad at the angle. The skeleton of this same specimen, which was found near the base of Uinta B 1, is described on page 651. The detailed measurements of these skulls are given in the following table: Measurements of DolicJiorJiinus iy Riggs, in millimeters D. longi- ceps, Car- negie Mus. 2347 (type) D. cornu- tus. Field Mus. 12167 D. flumi- nalis, Field Mus. 12205 (type) Skull: Length, incisors to condyles Breadth across arches Breadth above orbits ; Postorbital process to condyles Last molar to condyles Length of free nasals Greatest breadth of nasals Postglenoids to condyles (median line) . Length of molar-premolar series Length of molar series Length of crown of canine Diameter of crown of canine Length of diastema Narrowest point in sagittal area Breadth of orbitonasal area Mandible: Length, condyles to incisors Height, condyles above angle Length of molar-premolar series Length of molar series Length of crown of canine Diameter of crown of canine Depth of ramus from base of ps Depth of ramus froiia base of ma '545 260 595 285 '310 300 152 81 197 115 140 214 135 40 22 16 550 247 '134 305 263 170 79 122 212 131 24 17 14 52 '560 255 130 305 280 160 530 240 '535 295 258 132 264 259 520 233 116 137 57 121 200 124 115 192 121 67 114 198 122 171 105 32 18 400 159 209 123 29 16 70 57 » Estimated. 101959— 29— VOL 1- 408 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 343. — Skulls of Dolichorhinus From White River, Uinta Basin, Utah, level Uinta B 2. One-fourth natural size. A, D. iniermedius, Am. Mus. 1837 (type), reversed; B, D. heterodon, Carnegie Mus. 2340 (type), reversed; C, D, longkeps, Carnegie Mus. 2347 (type), "from the lowest level at which fossils were found in horizon B" (Uinta B 2). Measurements of Dolichorhinus heterodon, D. longiceps, and D. hyognathus, in millimeters Pmx-condyles Transverse zygomata Mi-m' P^ ap. by tr M', ap. by tr M2, ap. by tr M3, ap. by tr D. hetero- don, Carnegie Mus. 2340 (type) 487 240 114 22X25 33X32 42X41 39X38 "485 «225 119 21X26 32X33 42X42 42X42 Carnegie Mus. 2347 (type) "555 »264 115 21X29 35X ? 39X40 39X ? D. hyogna- thus. Am. Mus. 1851 (type of Telmato- therium cornutnm) 550 -240 122 23X27 34X34 43X43 45X ? Additional observations on Dolichorhinus longiceps. — A skull in the Carnegie Museum (No. 2865) referred by Mr. Peterson to D. longiceps is associated with a complete fore limb and other parts of the skeleton. It was found at a low level in Uinta B 1. It differs from the type of D. heterodon in having a larger m^; it appears to be smaller than the type of D. longiceps in skull dimensions but somewhat larger in the second and third upper molars. Mr. Peterson's description (1914.3) of this skull, with the mandible and hyoid bones, is in substance as follows : The specimen (No. 2865) consists of the greater portion of the skull, the posterior part of the mandible of the left and frag- ments of the right side, the hyoid arch, the cervical vertebrae, two dorsal and two lumbar vetebrae, together with the fore limb and foot practically complete. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 409 Cranium and mandible. — The cranium is somewhat smaller than in the type of Dolichorhinus longiceps, the sagittal area of the parietals is more compressed laterally, the zygomatic portion of the squamosal is slenderer and less expanded laterally, and the basicranial axis has a greater bend. These characters together with the slightly larger teeth constitute the most marked differences in the two crania compared, but that they should be regarded as of specific value is rather questionable. The base of the skull has received some crushing fore and aft, a fact to which the greater curvature of the basicranial axis may partly be due. The sudden downward bend of the occiput of Dolichorhinus heierodon, the flatter frontal region, the smaller preorbital ledge, and the smaller and more delicate nasals seem to separate that species more widely from the present specimen. Further- more, the difference in the geological horizons in which D. heierodon and the present specimen were found is to be consid- ered. The former came from horizon "Lower C, " while the latter was found in the lower part of horizon "Upper A" of the Uinta sediments. The high coronoid process and its sudden backward turn at the top, so characteristic of the mandible of Dolichorhinus, is well shown in this specimen. The angle is much compressed laterally, the temporal fossa is located high up but is quite deep, and the horizontal ramus has but small vertical diameter. Measurements Milli- meters Length of skull from anterior border of the orbit to top of occiput 365 Anteroposterior diameter of upper molar series 125 Transverse diameter of frontals at postorbital processes 145 Depth of mandible at ms 71 Length of stylohyal, approximately 168 Anteroposterior diameter of basihyal, median line 15 Hyoid arch. — The hyoid arch may best be compared with that of the tapir, because in that genus there is apparently no extended anterior appendix or process such as is seen on the basihyal of the horse or the rhinoceros. However, the bone as a whole, especially its anterior border, is relatively heavier than in the tapir. The thyrohyal is unfortunately broken off on both sides. This element was perhaps relatively less developed than in Tapirus ierrestris. The ceratohyal is also unfortunately broken off at the upper end, but its length was no doubt pro- portionately equal to that of the American tapir, while the shaft is less constricted anteroposteriorly. The epihyal is not present; this bone no doubt was nodular in character, as is the case in Tapirus terrestris. The anterior portion of the shaft of the stylohyal is rounder in cross section than in the tapir or the horse, but the upper end is flattened and terminates in enlarged processes, the superior attached to the hyoidial portion of the temporal bone and the inferior somewhat more obtusely rounded, extending downward and outward. This riblike upper end of the stylohyal is more suggestive of the rhinoceros or the horse than of the tapir. (See figs. 344 and 345.) Dolichorhinus hyognathus (Osborn) [Telmatherium cornutus Osborn] Plates XVII, XLVI, LII, LIII, LV, LXXI, LXXII; text figures 27, 33, 105, 110, 215, 217-219, 254, 255, 302, 336, 337, 339-341, 346-353, 483, 511, 520, 521, 579, 580, 582-585, 588, 647, 661, 686, 737, 743, 745 [For original description and type references see pp. 169, 173. For skeletal characters see p. 645] Type locality and geologic Tiorizon. — Uinta Basin, Utah; summit of Eohasileus-DolicTiorliinus zone (Uinta B 2). This animal is very abundant within its known geologic range through the upper 200 feet of Uinta B 2. The type specimen of D. cornutus ( = 7iyognathus) and most of the crania in the American Museum collection were found by Peterson in the upper or " Amynodon sandstones," at the summit of Uinta B 2, but the animal has also been recorded by Peterson 150 feet below the summit of B 2. It is not thus far recorded in Uinta C A single specimen has been found in the Washakie Basin, Wyo., on the 180-foot level of Washa- kie B 2. Specific characters. — Skulls large, 550 by 240 to 595 by 285 millimeters; relatively narrow, cephalic indices 46 to 43; face relatively long, faciocephalic index 53 to 51; grinding series p'-m^, average 206 millimeters; molar-cephalic index 38; horn cores very prominent; face decidedly bent down on cranium — that is, cypto- cephalic; secondary palate broad and nearly on the same plane with the primary palate; premolar ecto- lophs more hypsodont ; premolar protocone convexities sharply ridged; molars with prominent cones and crests; vestigial protoconules; hypocones of m^ very distinct. Figure 344. — Skull referred to Dolichorhinus longiceps? , side and top views One-sixth natural size. After Peterson. Carnegie Mus. 2865. This animal, from Uinta B 2, represents the most advanced stage known of this series. It appears to be a progressive descendant of D. longiceps from the base of Uinta B 1. The crania are somewhat larger in all dimensions than those of D. longiceps or D. inter- medius, and the parieto-occipital vertex is broader. Synonymy. — The male jaw, type of D. hyognathus (Princeton Mus. 10273), was found by the Princeton expedition of 1878 in Washakie B 2. When compared with the female skull and jaw (Am. Mus. 13164) found at the 185-foot level of Washakie B 2, it can not be separated specifically. Thus they must both be referred to D. hyognathus. These specimens in turn closely resemble in form and measurement the females in Uinta B 2 which were first referred to D. cornutus. Thus D. cornutus can not be separated specifically from D. hyognathus. 410 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Materials.— 1:^0 other Eocene titanothere is repre- sented by so rich cranial material as this species. The numerous skulls and jaws, although uniformly recorded from the DolichorTiinus (cornutus) Tiyognathus zone (Uinta B 2 and Washakie B 2), were undoubtedly Figure 345. — Hyoid apparatus of Dolichorhinus longiceps? (1, 3) compared with that of a modern tapir, Tapirus ter- restris (2) One-haU natural size. After Peterson. The two upper figures show a front view, the three lower a side view. This almost unique fossil belongs with the skull and jaws previously figured (fig. 34-1) and other bones comprising the specimen Car- negie Mus. 2865. bh, Basihyal; th, thyrohyal; ch, ceratohyal; eh, epihyal; sh, stylohyal. Compare the hyoid bones of Brontops sp. (Am. Mus. 518, fig. 425). found at somewhat different levels and may represent different stages of mutative progression, although it seems impracticable to separate them into species. These crania are enumerated below. Washakie B 2: Princeton Mus. 10273, type of D. hyognaihus; jaw of an aged animal. Am. Mus. 13164, skull and jaws of a young adult female; m' slightly worn, associated with parts of skeleton, fore limb, and parts of vertebrae. Uinta B 2: Am. Mus. 1850, skull of a young adult male; m^ just appear- ing. Am. Mus. 1845, skull of a young adult; m^ in place but unworn. Am. Mus. 1851, skull (type of Telmatotherium cornutum) of aged female; m^ well worn. Am. Mus. 1852, skull of young female; m^ slightly worn. Am. Mus. 1848, very old skull; m^ greatly worn, sex inde- terminate. Am. Mus. 1843, anterior part of skuU of large size, asso- ciated with complete backbone and humerus. Am. Mus. 1849, parts of skull and fragments of skeleton doubtfully recorded as of base of Uinta B 2. Field Mus. "l2167, skull from Uinta B (1 or 2). Jaws. — Besides the type of D. hyognaihus from Washakie B 2, we have the jaws Am. Mus. 13164, associated with a skull. Also from Uinta B 2 we have thirteen jaws more or less completely preserved, in- cluding Am. Mus. 1834, 1836, 1840, 1852, 1854, 1855, 1856, 1857, 1858, 1941, 2008. STcull. — The afiinities of the skull of these animals to that of Mesatirhinus petersoni are apparent in many details of structure, but, as above noted, there is a very marked progressive advance, which is bridged over partly by the intermediate stages of D. inter- medius and D. longiceps. The affinity to M. ( = Doli- chorhinus) superior is still closer. The full descrip- tion which follows is based principally on the female skull Am. Mus. 1851 (type of Telmatotherium cornu- tum) and the superb skull and jaws, also of a female (Am. Mus. 13164), from Washakie B 2. In the superior aspect (fig. 346) we are immediately struck by the extraordinary elongation of the nasals, which extend behind the line between the orbits and occupy a little less than one-half of the entire length of the skull. The longitudinal suture persists between the nasals and is traceable a short distance back between the frontals. These bones expand to 129 millimeters immediately above the orbits and are convex both anteroposteriorly and transversely; the line of junction between the frontals and parietals is obliterated. The vertex is here arched both trans- versely and longitudinally. The supratemporal ridges, now wholly lateral in position, follow the superior border of the supratemporal fossa; the top of the cranium is expanded slightly to 108 millimeters and then contracts to 62 millimeters just in front of the junction with the occipitals. This flattened arching and spreading of the vertex of the skull naturally differs both according to age or growth and the progressive stage of evolution which the skull represents. The superior view also displays the comparatively long and slender zygomatic arches, which reach a maximum width of 245 millimeters and an average width of 231, as compared with 550, the total length of the skull. Horns. — The nasals diverge suddenly into the osseous horns, which lie directly above the orbits, whereas in M. manteoceras the horns lie in front of the orbits; they present an outward-directed elongate- oval convexity, to the posterolateral portion of which only the frontals contribute. On the vertex between EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 411 the horns are gentle longitudinal valleys separated by median longitudinal convexities; the horn bases thus actually rise decidedly above the surrounding surfaces and overhang the orbits. A biologic fact of interest is that the horns appear to be developed as strongly in the female as in the male skulls and are not at this stage a distinctively sexual character; in none of the crania are they distinctly rugose, as in some of the male crania of M. manteoceras . Under these horn swellings, which are 127 millimeters apart, the nasals nent paired eminences, as in Am. Mus. 13164. The extreme elongation of the posterior nares is unique among perissodactyls. The pterygoid plates of the alisphenoid are elongate and depressed on either side of the long and narrow postnarial depression. The palatines do not crowd into the postnarial space as in M. manteoceras. The infraorbital malar plates con- stitute a very prominent shelf, the anterior part of which is shown by the sutures to be composed of the maxillaries. To this prominent infraorbital shelf Figure 346. — Skulls of Dolichorhinus One-fourth natural size. A, D. longkcps, Carnegie Mus. 2347 (type), Uinta Basin, Utah, Uinta B B, B. hyognatlius, Am. Mus. 1851, White Eiver, Uinta Basin, Utah, Uinta B 2. narrow to 66 millimeters, then broaden again to 76 at the widest point near their extremities. Palatal aspect. — As seen from below (fig. 347), the elongation of the palate, in which the palatine and maxillary plates take about equal share, is a most striking feature. The posterior nares open behind . m^. A kind of secondary palatal plate is formed by the backward and upward extension of the dorsal surface of the palatine. In this compressed post- narial chamber the maxilloturbinals appear as promi- was probably attached an anterior slip of the masseter muscle, as in many other mammals with weak zygo- mata. Behind these projections the malars are seen to present a long and comparatively narrow edge. The lacrimals are larger and have a broader extension on the face than in any other species. The lacrimal tubercle is preserved in one skull, as in the Palaeosyops series. Among the most striking results of progressive dolichocephaly are those seen in the conformation of 412 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA the articular facets for the condyles of the jaw. The postglenoid processes are narrow and tuberous; the glenoid facets are not transverse in position but directed obliquely forward, as if their outer borders were drawn out anteriorly by a stretching of the skull. In M. manteoceras these glenoidal facets are more directly transverse. The whole basicranial region is Vplam.ms.plj VlpfJUJO Figure 347. — Skull of Dolichorhinus hyo- gnathus Palatal view. One-fourth natural size. Am. Mus. 1S51 (type of Telmaiotherium cornutum Osborn). Supple- mentary details from Am. Mus. 1845. Both specimens from White Kiver, Uinta Basin, Utah; Uinta B 2. correspondingly elongate, the bridge of bone separat- ing the foramen ovale and the foramen lacerum medium now measures 42 millimeters as compared with 27 in M. petersoni or 17 in the brachycephalic LimnoTiyops laticeps. The anterior aspect of the skull (fig. 348) exhibits the extraordinarily long premaxillo-maxillary rostrum, which extends horizontally backward into the floor of the narial chamber, the total length being 144 milli- meters. The infraorbital shelves are well shown. The infraorbital foramina are deep and narrow; the nasals are vertically decurved at the sides, so that they form three sides of a square. The projection of the nasal horns, although the animal is a female, is admirably illustrated. The lateral aspect of the skull (fig. 349) shows that the midcranial concavity characteristic of Manteo- ceras and the Oligocene titanotheres is replaced in this species by the prominent frontoparietal convexity, the lowest point of the cranium in the upper profile being above the orbits between the horns. The suture between the maxillary and the nasals is similar in form to that in related species, the nasofrontal junc- tion being above the orbit. The failure of the frontals to send forward a spur overlapping the enlarged nasals is well shown. A broadly concave space (68 mm.) separates the narial notch and the orbit, and the infraorbital foramen issues 33 to 40 millimeters in front of the orbit. The maxillaries contribute to the anterior portion of the infraorbital process, the chief convexity being formed by the malars. Below the orbits the malars are gently concave, as in the related species of this genus. Other characteristic dolicho- cephalic features are the limited vertical extent of the zygomatic portion of the squamosal, the great fore and aft thickening of the postglenoid processes, and the widely open external auditory meatus. The occipital view (fig. 348), best shown in Am. Mus. 1845, is highly characteristic. The height of the occi- put, 142 millimeters from the basioccipital to the supraoccipital crest, approximately equals the width across the middle of the occiput. Above the foramen magnum two prominent ridges diverge and terminate in tuberous convexities in the upper lateral portions of the crest. The interior structure of the skull, including that of the narial and cranial cavities, is shown in Figure 254. Dentition in general. — The grinding teeth are more or less perfectly preserved in most of the crania and jaws; three specimens afford a complete knowledge of the upper and lower cutting teeth. Incisors. — The superior series has a semicircular arrangement (Am. Mus. 1851); the median pair are separated by a considerable diastema (12 mm.). The incisors increase in size regularly from i^ to i'. They exhibit convex anterior faces, more flattened posterior faces, with a median convex ridge. The posterior cingulum rises to form a distinct cup in i^ less marked in i^ and i'. P is fully incisiform (in contrast to its caniniform shape in the contemporary Telmatherium) ; it exhibits a narrow antero-external cingulum besides the postero-internal, obliquely sloping cingulum; it differs from other incisors in its more elevated crown. A narrow diastema (9 mm.) separates i' from the canine. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 413 The inferior incisors are also arranged in semicircular series, have obtusely pointed crowns and evenly convex anterior faces, slightly recurved concave posterior faces, and a pronounced median rib, which expands into the basal cingulum. The transverse measure- ments of the broadest part of the crown in Am. Mus. ap. 18 mm., tr. 15) the anterior and posterior ridges are less prominent, but a large, obtuse posterior basal cingulum is observed. The lower canines are more obliquely placed and recurved than in T. cul- tridens. There is a conspicuous antero-internal cingu- lum, but the lingual side of the base of the crown is pglscf. •p. fy. s^ FiGUKE 348. — Skulls of Dolichorhinus hyognathus Front and occipital views. One-fourth natural size. A, Am. Mus. 1851 (type of Telmatotherium cornutum Osborn), front view. B, Am. Mus. 1815, occipital view. Both specimens from White River, Uinta Basin, Utah; Uinta B 2. 1856, a female, are respectively i', 14 millimeters; i^, 17; i', 16. The entire breadth of these teeth in this specimen is 72 millimeters. Canines. — The sexes are sharply distinguished by the size of the lower canines : in the males the enameled crown of the tusks measures vertically 41 millimeters, smooth in the middle basal portion only; elsewhere it is cingulate. The posterior cingular ridge is slightly less acute than in T. cultridens. In the upper canines also the difference between the sexes is sharply marked, the male tusks in Am. Mus. 1850 measuring (ap. by tr.) 24 by 20 millime- FiGURE 349. — Skull of Dolichorhinus hyognathus One-fourth natural size. Am. Mus 1851 (type of Telmatotherium cornutum Osborn). White Eiver, Uinta Basin, Utah, Uinta B 2. X, y, Section lines in Figure 255. in the females only 27. The fine male tusks pre- served in Am. Mus. 1850 (ap. 24 mm., tr. 21) are laterally compressed, with sharply defined antero- internal ridges and somewhat less prominent posterior cutting ridges. No internal basal cingulum is observed in this specimen. In the female tusks (No. 1856, ters and having a vertical height of 42, whereas the female tusks of the type (Am. Mus. 1851) measure 18 by 15 and have an estimated crown height of 27. Upper premolar-molar series. — The distinctive ecto- loph characters of p^-p* are a sharp vertical protocone rib or ridge, a flattened to gently convex tritocone, and 414 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA a well-elevated ectoloph. In general this series is long, averaging 202 to 208 millimeters in length. The true molars are very strongly dolichocephalic in certain specimens (as in Am. Mus. 1850), in which the antero- posterior considerably exceeds the transverse measure- ment of each tooth. In other specimens, however (as in Am. Mus. 1851, type), the anteroposterior and transverse diameters are more nearly subequal. In most specimens the external cingulum is sharply defined, although there is considerable variation in this respect also. The ectoloph cusps and internal cones are subhypsodont, or elevated (paracone 34 mm. high, protocone 23 mm.). The internal cusps (the deuterocones of the premolars and the proto- cones and hypocones of the molars) throughout the series are rounded at the apices and are decidedly Figure 350. — Upper incisors and canines of DoKchorhinns hyognathus One-half natural size. A, Am. Mus. 1S51, White River, Uinta Basin, Utah, Uinta B 2, side view; B, Am. jMus. 1845, White Biver, Utah, Uinta B 2, crown view. convex internally as compared with those in the Telmatherium phylum. Vestigial protoconules appear in the molars of Am. Mus. 1850. Small, more or less cingulate hypocones appear variably in m^ A well- defined and sometimes broad internal cingulum extends around the lingual side of the premolars in the more progressive specimens (Am. Mus. 1850, 1851, 1852). P^-p* in crown view appear more subcircular in outline than in Telmatherium. Premolars. — Of the superior premolars, p' is sepa- rated by a slight diastema (15 mm.) from the canine and is a bifanged, narrow, laterally compressed tooth (ap. 16 mm., tr. 9) with convex buccal and more concave lingual faces. The posterobasal lobe is becoming well defined. P^ is more subcircular than in M. petersoni, its proportions being 20 by 19 millimeters, a condition which is due to the more anterior position of the deu- terocone. The protocone and tritocone are subequal, , but the protocone is much more prominent externally. Figure 351. — Lower incisors and canines of Dolichorhimis hyogna- thus One-half natural size. Am. Mus. 1856, White River, Uinta Basin, Utah, Uinta B 2; crown view. the tritocone being stdl nearly flat. A rudimentary protoconule is observed (Am. Mus. 1850). In p' (ap. 20 mm., tr. 21) the inner portion of the crown is broader, a rudimentary protoconule is seen, a slight spur foreshadowing the tetartocone extends back from the deuterocone, the protocone exhibits a narrow but sharply convex buccal face, the trito- cone is slightly convex, with a basal cingulum. In p^ the ectoloph rises to 21 millimeters, the external cingulum is more continuous, and the inner side of the crown is relatively broader, the crown measuring 22 by 24 millimeters. In these premolars (p', p*) the tetartocone rudiments are of the faintest character; in well-worn teeth they are not perceptible at all. 1>. hyognathus has less progressive tetar- tocones than T. ulti- mum but is very highly specialized in respect to the peculiar sub- circular form of p^-p^. D. hyognathus presents a considerable advance beyond D. vallidens and some advance beyond D. intermedius in the elevation of the ectoloph as a whole; the increased symmetry of the protocones and trito- cones, especially in p^, which is a much more progres- sive tooth than in D. vallidens; and the more nearly subcircular form of p^-p^. A specific dolichocephalic feature of the inferior pre- molars is the spacing of pi in the midst of the long dia- stema between the canine and p2, as seen especially in the type of D. hyognathus, in which this diastema measures 52 millimeters, the diastema in front of pi measuring 25 and that behind 14. These diastemata naturally increase as the individuals advance in age, and they are affected by individual growth, by the stage of evolution, and by the sex. In the fe- male (Am. Mus. 1856) the total dia- stema between the canine and p2 is 42 millimeters, nearly in the center of which lies pi. The lower pre- molars are well preserved in Am. Mus. 1856, from which the following descriptions and measurements are taken: Pi is a laterally compressed tooth, measur- ing (ap. by tr.) 15 by 8 millimeters, with the posterobasal lobe well defined and bearing a dis- tinct cusp. In p2 (ap. 23 mm., tr. 11) the protocone, which is somewhat less prominent (16 mm.), rela- tively, exhibits the antero-internal concavity and a rudiment of the antero-internal cusp ( = paraconid); the posterior lobe (=talomd) is more distinctly of the Figure 352.-Left upper canine of Dolichorhinus hyognathus One-half natural size. Am. Mus. 1850; ex- ternal view. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 415 molar type and has a rudimentary fold analogous to the metastyhd. In ps (ap. 23 mm., tr. 12) still further progress is made, there being quite a decided antero-in- ternal valley and a well- defined posterior basin. In P4 (ap. 24 mm., tr. 15) we find a submolariform tooth including a high metaconid and rudimentary paraconid but lacking the distinct en- toconid. Molars . — The superior molar series varies in length from 118 to 131 millime- ters and in addition to the characters enumerated above exhibits a very broad and prominent antero-inter- nal extension of the cingu- lum, the crown of m^ being broader in front (43 mm. through parastyle) than it is behind (32 mm. through metastyle) . In some speci- mens (Am. Mus. 1852) the hypocone of m' rises as a small but sharp and distinct cusp but is not so prominent as in the type of Rhadino- rhinus diploconus. In other specimens (Am. Mus. 1851) it is less prominent and more cingulate. The proportions of the molars are given in detail in the table (p. 416). The inferior molar series measures 138 millimeters in the female (Am. Mus. 1856). A very distinctive feature is the infolding of the external cingulum between the outer lobes of the tooth, which is, however, less mai-ked in the type than in most other spec- imens. In the Princeton type of D. Tiyognatlius (Princeton Mus. 10273) the only molar preserved is mg, which measures (ap. by tr.) 64 by 25 millimeters, agree- ing almost exactly with specimens in the American Museum. There is no proof of sexual inferiority in the female grinding teeth (ma, ap. 62 mm., tr. 28), and we should not expect it, because the females require as much food as the males or more. In the type mg there are rudimentary folds on the inner valleys, and the hypoconulid has a crenulate internal cingulum. Figure 353. — Lower jaws of Dolichorhinus A, D. valUdens, Am. Mus. 5098, one of the cotypes of " Palaeosyops " mllidens Cope, here regarded as the lectotype; Mammoth Buttes, Bitter Creek, Washakie Basin, Wyo.; Washakie B?. B, D. hyognaOim, Am. Mus. 1S56; White Eiver, Uinta Basin, Utah, Uinta B 2; ooronoid from Am. Mus. 1852 {Z>. Jonskeps?), White River, Uinta Basin, Utah, Uinta B 2. C, D. hyogmthus, Princeton Mus. 10273 (type); White Kiver, Uinta Basin, Utah, Uinta B 2; coronoid and angle restored from Am. Mus. 1852 (D. longiceps?) . A one-half natural size; B and C one-fourth natural size. In the more perfectly preserved mg of a female speci- men (Am. Mus. 1856) the external cingulum is very feebly marked except opposite the external valleys. 416 TITAJSrOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The internal valleys are more prominent and con- spicuous, and the hypoconulid is reduplicate. In the more worn m2 of the same specimen (ap. 43 mm., tr. 25) these internal valley folds have been worn away, and the same is true of the still smaller mj (ap. 35 mm., tr. 20). The external cingulum is more sharply accented in some specimens (as in Am. Mus. 1855) than in others. Jaw of DolichorTiinus hyognathus. — The somewhat fractured and crushed type jaw (Princeton Mus. 10273) presents the distinctively long and shallow ramus and shallow sloping symphysis that are char- acteristic of this species. (See fig. 353.) The measure- ments given below serve to determine the variations due to age, to sex, and in a measure to the progressive evolution of this type. Measurements oj Dolichorhinus hyognathus, in millimeters Washakie B 2 Am. Mus. 13164, V Princeton Mus. 10273, . hyognathus. The glenoid facets exhibit a long, obliquely transverse exten- sion, so characteristic oiD. hyognathus; the postgienoid processes also have a peculiar obliquity. In front and to the outer sides of the postglenoids the squamosals are deeply concave. Conclusion. — The peculiar pits remove this animal from any other known genus or species; it is possible that they are not con- stant characters. The various distinctive characters, while somewhat extreme, ap- pear to be paralleled or foreshadowed in the member of the Mesatirhinus-Dolichorhinus series. Eometarhinus Osborn Text figures 156, 355 [For original description and type reference see p. 200] This recently discovered Eometarhinus is recog- nized as ancestral to Metarhinus and is thus the ear- liest known member of the Metarhinus phylum. Geologic horizon. — The type specimen was found 205 feet below the top of the Huerfano formation, in Huerfano B {Eometarhinus-Trogosus zone). Generic chai^acters. — Small; ancestral to Metarhinus; with rudimentary frontonasal horn; nasals elongate; overhanging premaxillaries, decurved as in Metarhinus; no infraorbital shelf; characters apparently interme- FiGUEE 355. — Type skull of Eometarhinus huerfanensis, from Huerfano B One-half natural size. A, nasals, top view; Ai, anterior nasal sections; A2, posterior nasal sections; B, 0, palatal 1 side 1 ' witli crown view of dentition. 1 rudiments. diate between those of the Metarhinus and Mesati- rhinus phyla. Type species. — Eometarhinus huerfanensis. (See below.) Original description. — Osborn writes (1919): This new genus and species from the upper Huerfano is founded upon the anterior portion of a skull (Am. IMus. 17412) 420 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA representing an animal widely distinct in aU its characters from the contemporary P. foniinalis and more closely related to the other group of middle Eocene titanotheres to which Metarhinus, Mesaiirhinus, and Dolichorhinus belong. The most surprising character in an animal of this geologic antiquity is the very rudimentary osseous horns at the junction of the nasals and frontals, indicating the horn rudiment, a very ancient character in this phylum. The nasals are long, arched, de- curved, and revolute on lateral borders, thus resembling the rhadinorhinal type in the metarhine group. The malars below the orbits are prominent. There was probably no infraorbital shelf, as in Rhadinorhinus. The comparative measurements of Eometarhinus, of Mesaiirhinus megarhinus, and of the contem- porary Palaeosyops foniinalis are shown below. Eometarhinus huerfanensis Osborn Text figures 156, 355 [For original description and type reference see p. 200] Type locality and geologic horizon. — The type speci- men was found near the Huerfano-Muddy divide, 3 miles west of Gardner, in the Huerfano Basin, Colo., 205 feet below the top of the Huerfano formation, in the EometarJiinus-Palaeosyops foniinalis zone (Huer- fano B). Specific characters. — Inferior in all measurements to M. megarhinus. Premolars with small deuterocone. Pl-m^ 124 miUimeters; p^-p*, 53; ml-m^ 72. Materials. — This species is known from the type skuU (Am. Mus. 17412) and from two referred specimens — a fragment of a lower jaw (Am. Mus. 17013) with the first and second molars preserved (Osborn, 1919.494, fig. 7, B), and the right and left fourth lower premolars with a fragment of a canine (Am. Mus. 17416). Both of these specimens are from approximately the same level as the type and from the same general locality. Doubtfully referred to this species are three fragmentary upper molars (Am. Mus. 17415), found 3 miles north of Gardner, on the lowest level of the upper Huerfano (Huerfano B), or 200 to 300 feet below the level of the type. General characters. — The principal skull characters are noted above under the generic description. The, dental formiila is normal. The premolars are small, apparently very simple in pattern. The antero- posterior diameters of the molars appear to exceed the transverse; as they are in fractured condition, no accurate measurements can be taken. Apparently a hypocone on m^ Measurements of Eometarhinus, Mesaiirhinus, and Palaeosyops, in millimeters Eometartiinus, Am. Mus. 17412 Mesatirhinus megarhinus, Am. Mus. 12202 Palaeosyops fontinalis, Am. Mus. 17425 Pi-ms P2-m3 Mi-m3 ?■•, anteroposterior 124 109 72 14 »18 21 147 133 83 17.5 23 25 26. 5 28 31 "146 "131 83 16.5 21. 5 M', anteroposterior M', transverse 23 26 M3, anteroposterior M^, transverse 25.5 29 34 A tibia, found in association with the type skull, measures 275 millimeters (estimated) in extreme length. Metarhinus Osborn Plates LII, LXXI, LXXIV, LXXVIII-LXXX; text figures 123, 124, 138, 139, 219, 302, 323, 324, 341, 356-361, 404, 407, 509, 521, 522, 573-578, 647, 745. [For original description and type references see p. 183] Animals of medium or small size, divergent in structure, and probably different in habit and habitat from members of the Mesatirhinus-DolicJiorhinus series, hence the name Metarhinus; perhaps of fluviatile or semiaquatic habit; skull with rudimentary horns, elongate expanding nasals, orbits prominent; opposite sides of the upper jaw firmly united, proportions mesaticephalic; persistent sagittal crest and narrow occipital condyles. Geologic horizon. — This group of small animals is at present found only on one geologic level — namely, Uinta B 1 and Washakie B 1, which may be Icnown as the Metarhinus zone. It originally sprang from the same ancestral stock as Mesatirhinus megarhinus, but in these "metarhines" dolichocephaly was arrested and mesaticephaly persisted. The four or five known species exhibit a considerable number of characters in common which afford ground for regarding them as a related natural group of the smallest titanotheres of the period; they are truly dwarfed forms. The skull is very broad across the orbits, which are notably prominent; immediately in front of the orbits the face contracts; the nasals are long and expand distally. There is some evidence that the phylum divides into two lines, consisting of broad-headed and narrow- headed forms that run parallel throughout the period represented by Uinta B 1. The remains of these animals are very abundant, and the genera Metarhinus and Dolichorhinus are equally well represented (Riggs, 1912.1, p. 24) in Uinta B 1. The remains are usually associated in the same ledges and were apparently deposited under the same conditions. In one ledge of sandstone a large part of an articulated skeleton of D. longiceps was so mingled with the skeleton of a young Meta- rhinus that it was at first mistaken for a young animal of that genus. In other ledges, however, Metarhinus is very abundant and Dolichorhinus is absent. Other genera occurring in the same life zone are the more rare Rhadinorhinus, the giant amblypod Eohasileus, the small hyracodont Triplopus, and two large creo- donts, Mesonyx and Earpagolestes. Occasionally crocodiles and numerous turtles are found, also beds of fresh-water clams. Reeds, leaves, and branches of trees are abundant in the upper sandstones of the Metarhinus zone. Geologic occurrence in channels. — It is evident that our knowledge of this Metarhinus zone fauna is con- fined to that of the intrusive sandstone ledges of stream EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITA.NOTHERES 421 origin, because fossils are rarely found in the shales which alternate with these ledges (Riggs, 1912.1, p. 24). Skulls are often embedded in the semigravelly layers and have their narial or orbital cavities filled with pebbles which could be carried only by rapidly flowing water. Another evidence of stream action lies in the complete dissociation of the various skeletal elements; seldom are the lower jaws associated with the skulls, or so many as two vertebrae found articulated. In the exceptional instances where the whole skeletons are but little disturbed they are found embedded in the fine-grained homogeneous sandstone apparently deposited in more quiet water, such as deep pools or eddies. This prevailing mode of occurrence supports Os- born's theory that these animals were of semiaquatic or fluviatile habits, as indicated by tne name given to the type species of the genus, M. fluviatilis. Generic characters. — Small titanotheres, basilar length 355 to 415 millimeters. Persistently mesatice- phalic; postcanine diastema short; orbits very promi- nent, with projecting infraorbital shelves; nasals expanding distally; narial opening deeply recessed at sides; premaxillary symphysis greatly elongated; snout moderately broad; rudimentary horns on fronto- nasal suture; sagittal crest high and thin. Incisor teeth small, cingulate; canine teeth small, pointed, recurved; grinding teeth subhypsodont; pz'emolars rather progressive; hypocone of m^ present or absent; hypoconulid of ms small, conic. Historical notes. — The type species of this genus was recognized by Osborn (1908.318) from a specimen found in Uinta B 1 , which had been confused previously with Mesatirhinus megarhinus. This is a very small animal, perhaps the primitive member of the series. At the same time Osborn recognized in Washakie B a second species, M. earlei, distinguished from M. fluvia- tilis by its much greater size and the lesser prominence of the orbits. After the successful expedition of 1910 in the Uinta Basin, Riggs (1912.1) added M. cristatus, an animal of intermediate size but of the same proportions as M. fluviatilis, also M. ripa?-ius, an animal of larger size with an apparently longer and narrower skull. Osborn placed in the genus MefarJiinus the species M. diploconus, which Riggs (1912.1) on excellent groimds removed to the new genus RhadinorMnus. It is possible that the Palaeosyops Junius of Leidy, from Bridger B, a very diminutive form, represents an ancestral form of this metarhine phylum. Is Metarhinus diphyleticf — Riggs (1912.1, p. 27) regards the genus as including two phyla, one contain- ing M. fluviatilis and M. riparius, which were more primitive and had longer heads and larger canines, the other containing M. earlei and M. cristatus, which were relatively shortheaded and had somewhat more progressive teeth. The restudy of these forms by cephalic indices partly sustains Riggs's opinion, but indices can be depended upon only when a con- siderable number of skulls can be measured, because the effect of crushing is deceiving. The indices actually taken are as follows: M. earlei, 63, 61, 60, 60, 60; M. cristatus, 60 (estimated); M. fluviatilis, 58, 56; M. riparius, 55, 51. It would appear from these indices that M. earlei contains the forms with broadest heads and that M. riparius contains the forms with narrowest heads. There are also other characters which may divide these animals into two phyla, as shown below. Phyla of Metarhinus M. riparius M. earlei, M cristatus, M. fluviatilis More elongate skulls. Broad-faced skulls. Hypoeone on m' constant. Hypocone on m^ variable. Canines larger (? males). Canines smaller (? females) . Frontals narrow. Frontals broad. Supracranial areas lyre-shaped. V-shaped sagittal area. Smaller molars. Molars relatively large. The synopsis of these species in chronologic order is as follows: Metarhinus fluviaiilis Oahorn. Middle of Uinta B 1; skull small, moderately broad (length 352 mm., breadth 205; cephalic index 56-58); a hypocone on m^. Metarhinus earlei Osborn. Summit of Uinta B 1 and Wash- akie B; skull somewhat larger (length 405 mm., breadth 255, or 338:245); somewhat broader (cephalic index 60-63); rudiments of a secondary palate; no trace of hypocone. Metarhinus cristatus Riggs. Lower level of Uinta B 1; type skull of intermediate size (length 385 mm., breadth 240; cephalix index 60); hypocone present, cingulate; similar to M. fluviatilis. Metarhinus riparius Riggs. Summit of Uinta B 1; skull larger (length 406 mm., breadth 210, or 406 : 210; cephalic index 51-55, estimated); apparently longer and narrower; a hypocone on m^ Metarhinus fluviatilis Osborn Plates LII, LXXI, LXXIV; text figures 123, 341, 356-358, 404 [For original description and type references see p. 183. For skeletal cliaracters see p. 644] Type locality and geologic Jiorizon. — White River, Uinta Basin, Utah; Metarhinus zone (Uinta B 1); abundant. Specific characters. — Relatively short, broad skull, length 352 millimeters, breadth 205, or 355: 200; cepha- lic index 56 to 58. Eye socket small; circumorbital ridges prominent; premaxillary symphysis elongate; basicranial region short; sagittal crest high and prominent. Grinding teeth subhypsodont, m^ with a rudimentary "cingulum hypocone" in the type. Geologic distribution. — Uinta B 1 is the horizon of the type specimen (Am. Mus. 1500) and of the referred specimen (Am. Mus. 1877). Similar but somewhat more progressive forms showing the same 422 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA diminutive measurements and probably representing higher mutations or species occur in Uinta B (Uinta B 2 of this monograph) according to Peterson's field rec- ords (1893)— namely, Am. Mus. 1946, 1864, 2059 (a tiny jaw), 1865. Riggs, however (1912.1, p. 21), reports no species of Metarhinus in Uinta B (Uinta B 2 of this monograph), nor have we found any specimens referable to M. fiuviatilis in upper levels of Uinta B 1 . It is possible that the line between B 1 and B 2 is not drawn at the same point by these two observers. General characters and Tiabits. — The type skull (Am Mus. 1500) is supplemented by a crushed skull (Am. Mus. 1877). Of the two skulls known neither in- cludes the complete nasals nor affords a knowledge of the nasofrontal horn. We can not therefore speak positively as to this character. A further knowledge of these animals is afforded by Am. Mus. 1946, from Uinta B 1 (see geologic note above), which includes a palate with superior teeth associated with a jaw and parts of the pes and of the limbs. Also recorded from Uinta B 1 we find a series of upper teeth, which differ from those of the type in that the external cingulum is absent and the tetartocones on p^ are somewhat more advanced. From Uinta B 2 also are recorded the American Museum jaws 1865 and 2059. The latter is a very small jaw. This animal is by far the most diminutive of the known upper Eocene titanotheres — in fact, it may be described as a dwarfed form. The specific name, M. fiuviatilis, was assigned by Osborn on the ground that the animal was probably a river-living animal. In size it is appreciably smaller than the known specimens of M. earlei, although the teeth are larger than those of the doubtfully referred Metarhinus Junius from the Bridger. Specific distinctions. — Distinctions from Metarhinus megarhinus are foimd in both the progressive and the adaptive characters. In its progressive characters, although it is an animal of inferior size, M. fiuviatilis presents an advance upon M. megarhinus in the increased hypsodonty of the molar teeth, the increased strength of the external cingulum, the decidedly sharp and compressed parastyle, the upward curvature of the anterior.portion of the grinding series, a character pointing toward the Oligocene brontotheres. The protoconids or anterior crests of p2_4 are ridged, and the cusps corresponding to the metaconid are better developed. The tooth progression is thus parallel with that of Dolichorhinus in certain respects, divergent in others. The adaptive characters are most interesting. We observe especially that the narial openings are carried very far back on the sides of the face, so that a very narrow space is left between the orbits and the narial notch (a feature observed also in M. earlei and Rhadinorhinus diploconus). The region across the orbits shows unusual breadth (partly attributable to crushing), because the orbits as a whole are prominent and the circumorbital ring appears to be elevated and the eye sockets themselves are small. This feature is analogous to that in the Oligocene brontotheres, in which the orbits are small. We might therefore conjecture that these animals had adopted aquatic habits, because although the orbits are invariably prominent, the eyes tend to become smaller in all swimming ungulates. A pes provisionally referred to M. fiuviatilis (from Uinta B 1) has slender metapodials, which would indicate cursorial rather than amphibious habits. The habitat must be left an open question until the skeleton becomes definitely known. Correlated with a relative shortening and broad- ening of the skull is the narrowing of the occipital condyles. It is difficult to discover the reason for the elongation of the maxillary symphysis. The symphysis appears to be elongate partly because of the deep recession of the narial openings. Detailed description. — The skull of this species exhibits a great number of interesting characters, because it presents a wide departure from the skull of all other Eocene titanotheres except those of the little group to which it belongs. It is a cm-ious mix- ture of adaptive, progressive, and conservative charac- ters. Among the last may be mentioned the relative elongation and height of the sagittal crest. The skull is sharply characterized specifically by the combination of the above characters with a prominent infraorbital shelf, a prominent antorbital bridge, and a prominent postorbital process. The superior view of this peculiar skull exhibits the elongate symphyseal union of the premaxillaries, recalling that of Dolichorhinus, the broadened posterior portion of the nasals, the deep lateral depression of the face, or antorbital fossae in front of the orbits, the prominent lateral projection of the orbits, the relatively short supratemporal ridges, which rapidly unite posteriorly, the prominent, narrow sagittal crest. The general resemblance of this aspect of the skull to that of Rhadinorhinus diploconus is quite apparent. The palatal view brings out dis- tinctive features. The posterior narial space is long and narrow with parallel sides, the palatines not projecting inward as in Manfeoceras manteoceras. As compared with that of R. diploconus it appeal's to afford the following important differences: In M. fiuviatilis the basicranial region appears to be less dolichocephalic, the zygomata are relatively heavier, the infraorbital shelf (wanting in R. diploconus) is very prominent. The anterior view of the skull exhibits a prominence above the orbit which possibly represents a supraorbital frontonasal horn element; it may, however, be due to crushing. The lateral aspect of the skull is equally distinctive, as it exhibits the deep narial notch separated from the orbit by a very narrow interval, the prominent antorbital bridge concealing the infraorbital foramen, and the sharp and deeply EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 423 depressed infraorbital shelves. The peculiar elevation of the front of the upper jaw may be partly increased by crushing but appears to be correlated with a similar upward flexure of the front part of the lower jaw (figs. 356, 358). The third molar is somewhat farther back with reference to the orbit than in Doliehorliinus. Dentition of type and referred speci- mens. — Some of the "referred" specimens are recorded from Uinta B 1 and may well belong to a more progressive species than M. fluvia- tilis. The following description of the dentition includes the referred Uinta B 1 specimens which may belong to more progressive muta- tions or species than the type of M. fluviatilis. The teeth of this diminutive titanothere are represented by the very much worn and somewhat crushed series of the type (Am- Mus. 1500, Uinta B 1) and by the perfect grinding teeth of Am. Mus. 1946, Uinta B 1, with which is associated the lower dentition ; also by the complete dentition of an- other jaw (Am. Mus. 2059, Uinta B 1) and possibly by a more frag- mentary upper jaw (Am. Mus. 1864) from Uinta B 1. Incisors : The six upper incisors, as seen from the lower side, are arranged in a semicircle, much as in DolicTiorhinus. They have smooth, gently convex anterior faces and in- crease gradually in size from i' to i', a tooth which is separated from the canine by a narrow dia- stema. The inferior incisors as exhibited in Am. Mus. 2059 are intermediate between the Mesa- tirJiinus megarhinus and Dolicho- rhinus types, pointed by wear, in- creasing in size very gradually from ii to is, and having smoothly con- cave posterior surfaces bounded by a sessile cingulum. Canines: The superior canines are also of the M. megarhinus form and although much crushed in the type exhibit evi- dence of smoothly rounded sides swelling toward the base of the crown and accented by very deli- cate anterior and posterior ridges. The coronal measurements (ap. by tr.) are 17 by 16 milli- meters. In Am. Mus. 1946, although probably a male, 101959— 29— VOL 1 30 the canines are still smaller (ap. 18 mm., tr. 16). The inferior canines in jaw Am. Mus. 1946 are finely pre- served except at the tips, measuring 15 by 14 millime- ters ; they exhibit a strong antero-internal cingulum at the base. In the other jaw (Am. Mus. 2059), obviously FiGTJEE 356. — Skull of Metarhinus fluviatilis One-fourth natural size. Am. Mus. 1500 (type); White River, Uinta Basin, Utah; Uinta B 1. Ai, Side view (crushed downward); Aj, palatal view; As, top view; Ai, occipital view. a female, the canines are much smaller (height 18 mm., ap. 13, tr. 12), with strong antero-internal cingulum, slightly recurved apex, and somewhat ex- panding base. The incisors and canines as a whole seem to foreshadow the short swollen type seen in the Oligocene Brontotheriinae. 424 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Superior premolar-molar series: The grinding series, although of diminutive dimensions (145 mm. in the type, which includes p', and 137+ mm. in the longi-' tudinally crushed cotype), are really more progressive in character than those of M. megarhinus in the fol- lowing respects: (1) P' has a subquadrate instead of elongate subtriangular crown, the trito- and deutero- cones being subequal; (2) m^ has an elevated postero- internal cingulum, which forms a subfunctional hypo- cone in the type; (3) the ectolophs of p^ p*, as well as of the molars, are relatively more elevated. Premolars: The premolars of the type measure (ap. by tr.), p^, 13 by 14 mm.; p^ 15 by 19; p*, 17 by 23. The internal cingula are well defined and progressive, completely surrounding the deuterocone of p^ and nearly surrounding the deuterocone of p*. The external cingula of p^ p' are not well marked across the protocone but are very pronounced opposite the tritocone; in p* they are strongly developed oppo- site both cusps, and the protocone "rib" was also pronounced, all marks of relatively advanced speciali- zation. The coronal pattern is exhibited much better in Am. Mus. 1946, in which the premolar measurements Figure 357. — Right lower premolars (pi-pi) of Metarhinus fluviatilis Crown view. Natural size. Am. Mus. 1946; White River, Uinta Basin, Utah, Uinta B 1. (ap. by tr.) are, p', 15 by 17 millimeters; p^ 15 by 20; p*, 17 by 23. The most significant features of the premolars in this specimen (No. 1946) are as follows: (1) The deuterocones are large and give a well filled out subquadrate inner contour to p^ p*; (2) the tritocones are very large and progressive, especially in p', p*) and have subflat ectoloph faces except in p^, which has a more convex tritocone ectoloph; (3) the proto- cones (antero-external cusps) have large, sharply defined external ribs; (4) no tetartocones are .yet present; (5) the external cingula are very advanced, rising into prominent parastyles, faintly continuous across the protocone base in p% p*, and better de- fined opposite the tritocone; the external cingulum of p* at the base of the tritocone surrounds a sharp protuberance, emphasizing the gentle protuberance seen here in M. megarhinus; (6) the internal cingula are well defined but still incomplete opposite the deuterocone, the posterior cingulum of p^, p* very broad; (7) the internal cones of the premolars and molars are relatively elevated, and they have very thick enamel, these conditions causing the partly worn tips to be sharply truncate. Allowing for differences in wear and for some differ- ences in level, we conclude that, from the evidence of the premolars. No. 1946 is related to or referable to M. fluviatilis. The inferior premolars, measuring 59 millimeters in Am. Mus. 1946, are also more advanced than those of M. megarhinus. Pi is more advanced than in that species; it is more elongate, has a larger posterobasal swelling, a lower, rounder tip, and a faint rudiment of the anterior valley. There is a faint external cingulum on the anterior and posterior end. Its measurements are 10 by 6 millimeters (ap. by tr.). P2 is also more advanced than in Mesatirhinus, with a lower trigonid, a somewhat higher talonid, and better-defined an- terior and posterior valleys. Its measurements are 16 by 95 millimeters. In P2-Pi the protoconid forms a high, blunt transverse ridge, extending internally into the cusp analogous to the metaconid. In ps (ap. 16 mm., tr. 10) the molariform tendency is still more pronounced, the posterior lobe being stronger, with rudiments of the internal styles appearing. P4 (ap. 18 mm., tr. 13) exhibits a somewhat more ac- cented external cingulum, and the cusp analogous to the entoconid is much more elevated than in Mesa- tirhinus and only less elevated than in the molars. Molars: The lower molars do not differ greatly from hose of M. petersoni, except that they are of slightly smaller size (98 to 103 mm.), have the external cingula a little better defined, are perhaps slightly more hypso- dont, and have a more conic hypoconulid on ma. In M. fluviatilis, as in M. megarhinus, the hypoconulid of m3 is of small size and variable (or progressive) from a crescentic to conic form. Lower jaws of Metarhinus fluviatilis; type and re- ferred specimens. — The type skull lacks the jaw. The superior grinding series (Am. Mus. 1946) is associated with the jaws, and they are also found in the nearly perfect jaw of Am. Mus. 2059 and in the left ramus of Am. Mus. 1865, which is from Uinta B 1 . The diminu- tive jaw forming Am. Mus. 2059 barely exceeds in length the jaws of certain specimens of Eotitanops horealis, but the rami are relatively deeper, and the dentition is of course far larger and more advanced. Measurements of the lower jaws of specimens of Mesatirhinus and Metarhinus are given in the ac- companying table: EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHBRBS 425 Measurements of species of MesatirJiinus and MetarTiinus, in millimeters [Specimens in Am. Mus. except 12195, which is in the Field Museum] Incisive border to angle Incisive border to condyle Posterior border of jaw to ms- Depth below ma Pi-m3 Mi-m3 Mesatirhinus M. mega- rhinus, Bridger No. 1520 162 94 M. petersoni, Bridger No. 1567 No. 1512 132 74 168 325 112 83 172 103 325 325 -90 79 169 106 M. sp., Uinta B, No. 1859 "SSO 350 90 86 195 MIS M. fluviatilis, Uinta B 161 102 M. riparius Uinta B 1 (upper Metarhinus beds), No. 12195 280 285 78 62 157 338 172 110 These jaws are distinguished by the laterally con- stricted chin, correlated with the constricted premax- illaries above; also by the slender rami with the lower borders slightly downcurved below the grinders and upcurved below the coronoid, extending back into an angle which is produced posteriorly, as in Mesati- rhinus. With the exception of the chin, the jaw, so far as known, was of the Mesatirhinus type. The an- terior border of the coronoid is decidedly angulate in character; the coronoid itself rises with nearly parallel anterior and posterior borders to a gently recurved tip. These characters suggest those of a dwarfed col- lateral of M. megarhinus. In the supposed female (Am. Mus. 2059) the ramus appears much deeper and more slender than in Am. Mus. 1946. The type of Heterotitanops parvus Peterson — is it a young Metarhinus? — A very young, perhaps a fetal skeleton in the Carnegie Museum (No. 2909), the type of Heterotitanops parvus Peterson (figs. 150-152), may possibly represent Metarhinus fluviatilis. It was found lower down in Uinta B 1 than any other known mammalian remains. The type of Metarhinus flu- viatilis was also found by Mr. Peterson in Uinta B 1. According to W. K. Gregory its reference to Meta- rhinus or to some closely allied genus is indicated by the following facts : 1. The deciduous cheek teeth, in size and appear- ance, bear to the adult dentition of Metarhinus fluvi- atilis much the same relation as the deciduous denti- tion of Oligocene titanotheres (PI. XXIII) bears to the permanent dentition — that is, the deciduous molars are more molariform than the permanent premolars, and they are elongate anteriorly and have more widely open external V's and less prominent mesostyles. 2. The large upper and lower grinding teeth that have not wholly emerged from the alveoli are prob- ably not m' and mi, as supposed by Peterson, but dp* and dp4. The anteroposterior diameters as given by Peterson are dp* 21 millimeters, dp4 25, dimensions nearly as great as in m' and mi of the Metarhinus fluvi- atilis type and apparently large enough for dp* and dp4 of that species. The resemblance of dp*, dp4 to m' and mi of that species seems sufficiently striking to indicate congeneric relationship. The measurements of dp^ Figure 358. — Lower jaws of Metarhinus One-fourth natural size. A, M. fiumatilis. Am. Mus. 2059, White River, Uinta Basin, Utah, Uinta B 1; B, M. earlei. Am. Mus. 13179, northwest point of Haystack Mountain, Washakie Basin, Wyo., Washakie B. are also not inferior to those of dp4 in Mesatirhinus sp. (Am. Mus. 12211). 3. The deep lateral excavation of the anterior nares, which leaves a very narrow bridge of bone be- tween the narial sinus and the orbit, a feature char- acteristic of Metarhinus and its near aUies, is strongly marked also in the specimen under consideration. 426 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 4. In the side view the form of the lambdoidal ridges of the occiput is not dissimilar to that of Meta- 7-hinus ; however, these ridges do not unite above into a narrow median crest as they do in MetarTiinus fluviatilis but form a flattened vertex which suggests that of Dolichorhinus intermedins. Possibly the median crest of the adult M. fluviatilis may be derived by the Figure 359. — Lower jaw of Metarhinus? (Rhadinorhinus?) sp. One-fourth natural size. Am. Mus. 1859; White River, Uinta Basin, Utah, Uinta B 1. upgrowth of these lambdoidal ridges toward the mid- line. At any rate the form of occiput seen in Hetero- titanops is not seen elsewhere outside the subfamily Dolichorhininae. In brief the animal named Eeteroti- tanops parvus may provisionally be regarded as the young of Metarhinus fluviatilis. The postcranial skeleton has been well described by Peterson and is chiefly interesting as illustrating the highly progressive or caenogenetic character of the slceleton, which foreshadows the adult in the expansion of the scapula, of the proximal end of the humerus, and of the ilia, as well as in the great relative size of the thorax and in the general proportions of the limb bones. Metarhinus earlei Osborn Plates LXXVIII-LXXX; text figures 124, 219, 302, 358, 361, 407, 521, 522, 573-575, 577, 647, 745 [For original description and type references see p. 183. For slseletal characters see p. 644] Type locality and geologic horizon. — North side of Haystack Mountain, Washakie Basin, Wyo.; Meta- rhinus zone (Washakie B 1). Also abundant in Uinta Basin, Utah, at the summit of the Metarhinus zone (Uinta B 1), in the " Metarhinus sandstones" of Riggs. Specific characters. — Skull (Am. Mus. 13166, type), length 393 millimeters, breadth 240, or 388:245, or 405:255; cephalic index 60-63. Occipital condyles narrow (78 mm.), premaxillary symphysis elongate, nasals elongate, spreading distally, prominent in- fraorbital shelf . Type p'-m', 167 millimeters. Molar series broad and low crowned, no hypocone on m^; canines slender, diastema short. This animal is readily distinguished from M. fluviatilis by its superior size and by the lesser prom- inence of the orbits. Geologic distribution and materials. — The type of this species is a skull (Am. Mus. 13166), fortunately discovered by the American Museum expedition of 1906 in Washakie B 1. The nasals and the naso- frontal horn region of the type are broken away. A jaw belonging to another individual (Am. Mus. 13179) was also found in Washakie B 1 and agrees approximately in measurement with this skull. It may prove to belong to the same species. In the Riggs collection of the Field Museum there are two skulls (Nos. 12169 and 12187), also two lower jaws (Nos. 12178 and 12189). These four specimens are recorded from the uppermost " Metarhinus sand- stones, " or the Metarhinus zone (Uinta B 1). Skull. — The first feature of note in the type skull is the mesaticephalic proportions, the measurement being 240 millimeters across the zygomata and 393 from the condyles to the symphysis — that is, the width is a little less than two-thirds the length, whereas in Mesatirhinus petersoni skulls the width is only a trifle more than one-half the length. Correlated with this feature is the relative narrowness of the occipital condyles (78 mm.). The occipital region is also distinctive because of a deep pit on the upper part of the occiput and a pair of hooldike processes ttu-ning inward on the borders of the occipital crest (fig. 361); these falciform, incurved, overhanging borders are quite distinctive. The sagittal crest is short. The infraorbital shelf is prominent but Figure 360.- -Skull and deciduous teeth of type of Heieroiiianops parvus One-half natural size. Alter Peterson. Carnegie Mus. 2909 (type), White River, Uinta Basin, Utah, Uinta B 1. Possibly a young Metarhinus. A, Skull; B, right upper deciduous molars (dp^, dp^ dp^); the anterior tooth is probably the permanent p'; C, left lower deciduous molar (dp(). slender. There is no hypocone on m^. We are espe- cially struck by the prolongation of the premaxillary symphysis, as illustrated in Figure 361, and the resem- blance to Dolichorhinus. The well-preserved basi- cranial region, which is also of mesaticephalic character, indicates the affinities of this species to Mesatirhinus megarhinus and more remotely to M. petersoni. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 427 The fine skull in the Field Museum, No. 12187, illustrates the very peculiar constriction of the nasals in front of the orbits and their distal expansion, also the rudimentary horn bosses and curved nasals in lateral view. Riggs notes that the Uinta Basin representatives of M. earlei are somewhat broader headed and more mas- sive than the Washakie Basin type and that in both the Field Museum skulls the hypocone on m' is wanting; there is also a slight elongation of thepostca- nine diastema. (Riggs, 1912.1, p. 30.) Dentition. — A feature distin- guishing this animal from both M. megarJiinus and M.fluviatilis is seen in the double convexities (protocone and tritocone) of the ectolophs of the superior pre- molars, as in certain skulls of M. petersoni. The superior molars exhibit prominent ectolophs and greatly elevated internal cones. The internal cingula of p^, p* are large and progressive, extending completely across the internal base of the protocone. The external cingulum of p* also extends nearly across the base of the tritocone. The deutero- cone of p^ is larger than in M. petersoni. The crowns as a whole are laterally compressed. This hypsodont character is an advance on the condition observed in M. petersoni. Jaw. — A jaw from Washakie B (Am. Mus. 13179) provision- ally referred to this species, as shown in Figure 358; exhibits proportions which are distinc- tively mesaticephalic, like those of the skull. It is of very su- perior size, and the ramus is of much greater depth than in the jaw referred to M.fluviatilis (fig. 358). The canines are short, rounded, and recurved. The hypoconulid of ma is conic and posteriorly cingulate, with a detached cuspule on the base of the inner side. Additional ohservations on MetarJiinus earlei. — An excellent skull in the Carnegie Museum, No. 3098, found 190 feet above the bottom of Uinta B 1, has the dimensions following. Millimeters Basilar length 410 Zygomatic width 243 Cephalic (zygomatic) index 60 P'-m3 162 pas. pa-'' '"^ Al Figure 361. — Type skull of Metarhinus earlei One-fourth natural size. Am. Mus. 13166 (type) ; north side of Haystack Mountain, Washakie Basin, Wyo., Washakie B 1. Ai, Side view: A2, palatal view; A3, top view; A4, maxillo-premaxillary symphysis, dorsal view; As, occipital view. pi-p< 63 M'-m3 98 The infraorbital shelf is almost as small as it is in RhadinorMnus, but the nasals are broad. 428 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements of Metarhinus fiuviatilis, M. cristatus, M. earlei, and RhadinorJiinus diploconus, in millimeters Am. Mus. 1600 (type), Uinta B 1 Am. Mus. 1946, Uinta B2 M. cristatus, Field Mus. 12194 (type) M. earlei, Am. Mus. 13166 (type), Washakie B 1 Am. Mus. 2055, Uinta B2 Am. Mus. 1863 (type), Uinta B 2 Pi-m3 p2-m3 Mi-s P^, anteroposterior P*, transverse Transverse condyles.. Pmx to condyles Transverse zygomata. 145 132 85 18 23 140 131 84 92 23 »385 240 167 156 102 -21 26 °76 393 240 19 168 156 103 20 26 "76 '' 440 * Specimen crushed. This table shows the marked inferiority in size of M. fiuviatilis as compared with E. diploconus and M. earlei, as well as the close agreement in most measurements between M. earlei and the type of E. diploconus. M. cristatus is intermediate in size between M. fiuviatilis and M. earlei. Measurements of species of Metarhinus, in millimeters M. fiuviatilis, Am. Mus. 1500 (type) M. riparius Field Mus. 12186 (type) M. cristatus. Field Mus. 12194 Am. Mus. 13166 i.type) Skull Length, incisors to condyles Breadtli across arches Breadth between orbits Postorbital process to condyles Last molar to condyles Length of free nasals Greatest breadth of nasals Postglenoids to condyles (median line) . Breadth across condyles Greatest depth of arch Length of molar-premolar series Length of molar series Length of crown of canine Diameter of crown of canine Length of diastema Narrowest point in sagittal area Breadth of orbitonasal area 352 205 112 214 107 39 406 >> 210 <> 114 205 189 »128 68 87 79 55 160 93 24 18 11 10 41 406 220 107 212 193 115 68 89 82 51 155 93 29 20 10 385 240 145 215 195 390 237 137 198 182 388 245 142 405 255 220 220 90 75 92 169 104 Mandible Length, condyles to incisors Height, condyles above angle Length of molar-premolar series . Length of molar series Length of crown of canine Diameter of crown of canine Depth of ramus from base of ps- Depth of ramus from base of ms. 338 133 172 110 34 21 52 39 330 125 162 102 » 32 19 49 60 184 120 74 73 79 50 158 100 29 15 42 130 170 103 30 16 11 17 50 330 157 171 107 "315 135 168 110 340 148 170 105 ' Specimen distorted. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 429 Metarhinus cristatus Riggs [Compare M. fluviaiilis Osborn] Plates LXXVIII, LXXIX; text figure 139 [For original description and type references see p. 191] Type locality and geologic horison. — Uinta Basin, Utah; Metarhinus zone (Uinta B 1), lower levels. Specific characters. — Skull, estimated length 380 millimeters, breadth 240; molar series 94; frontal region broad; sagittal crest long and high; zygomatic arches relatively heavy. Molars short crowned, no hypocone on m^. Materials. — Known only from the type (Field Mus. 12194), a single skull lacking nasals and premaxillaries. The type (fig. 139) was described by Riggs under the impression that it came from a much higher geologic level than that at which the type of M. fiuviatilis was found. This geologic record has now been corrected to show that M. cristatus was found at nearly the same level as the type of M. fiuviatilis — the lower half of Uinta B 1, or the Metarhinus zone. M. cristatus may therefore be the same animal as M. fiuviatilis. It is true that there is no hypocone on m^ in M. cristatus, but this element is described as a mere "cingulate hypocone" in M. fiuviatilis. The disparity in size between the two types is not very great, the breadth across the zygomatic arches being 205 millimeters in M. fiuviatilis and 240 in M. cristatus. The character of the sagittal crest and of the orbital region in the two types is quite similar, although it would appear that in the M. fiuviatilis type the orbits are somewhat more prominent. The difference may be sexual, but the size of the skull of M. cristatus is intermediate between that of M. fiuviatilis and M. eirlei (see measurements above). Its breadth, or cephalic index, is possibly greater, although its length is merely estimated. It certainly contrasts sharply with the long-headed M. riparius. Metarhinus riparius Riggs Plates LXXVIII, LXXIX; text figure 138 [For original description and type references see p. 191] Type locality and geologic horizon. — Uinta Basin, Utah; "Metarhinus sandstones" at summit of Meta- rhinus zone (Uinta B 1). Abundant in both lower and upper levels of Uinta B 1. Specific characters (Riggs). — Skull long and narrow, length 405 millimeters, breadth 210, cephahc index 55, 51; anterior cranial region expanded, sagittal crest short; interorbital region relatively narrow and rounded; rudimentary horn cores above orbits; man- dible straight in the ramus ; canines large, lower canine long and recurved ; molar series relatively short, 88-93 millimeters, hypocone usually present on m^. Geologic distribution and materials. — The type of this species (Field Mus. 12186) was found in the "Meta- rhinus sandstones " at the summit of Uinta B 1 , but the animal also occurs abundantly in the lower levels, associated with the broad-headed M. cristatus and with Dolichorhinus longiceps (see table, above). The author describes it as the most common species in this life zone. Four skulls, two associated lower jaws, and one isolated jaw in the Field Museum collections are referred to it — namely. Field Mus. 12174, 12183, 12191, 12195, 12196. General characters. — The type of these species is a laterally crushed skull, a condition which greatly increases its apparent dolichocephaly; this character, however, seems to rest substantially on other speci- mens, the cephalic indices ranging from 51 to 53. It is described as the long-headed Metarhinus riparius and may possibly be ancestral to the aberrant animal described above, known as Sphenocoelus, which it resembles in the following characters: (a) Relative dolichocephaly; (&) form of the occipital and especially of the temporal crests; (c) wide separation of the post- glenoid and post-tympanic processes. It is possible that this represents a branch, M. riparius-Sphenocoelus phylum, which may also be represented in the Prince- ton occiput (Princeton Mus. 10041) from Washakie ?A. The hypocone on m^ is not invariably present although seen in the type and in the examples of the species from higher levels. The canines (Riggs, 1912.1, p. 29) are said to be strong, whereas in the broad-headed species, M. cristatus and M. earlei, they are reduced in size. Metarhinus? sp. Text figure 359 A lower jaw (Am. Mus. 1859), from Uinta B 1, is larger than that of any known Metarhinus or Rhadi- norhinus but probably represents a relative of those genera. It differs from the various species of Doli- chorhinus in being stouter and in having a shorter tooth row. Comparative measurements are given above. Another lower jaw (Am. Mus. 2355), from "Glover Spring," level Washakie B, apparently represents a large Metarhinus of uncertain specific reference. It is probably not referable to Dolichorhinus vallidens. SUBFAMILY RHADINORHININAE (=?MEGACEROPINAE) Animals of medium size; divergent in structure from members of the Mesatirhinus or Metarhinus phyla. Skull with nasals contracted and pointed distally; cranial profile concave instead of convex; skull cyptocephalic ; facial region — that is, grinding series — somewhat upturned; infraorbital processes not prominent ; frontonasal horns wanting or retarded in development; traces of terminal nasal horns in one species. Molars subhypsodont; premolars slightly progressive. This group of small titanotheres is contemporaneous with Metarhinus, Mesatirhinus and Dolichorhinus Ion- 430 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA giceps in Uinta B 1 . It is a well-defined branch from the more typical Dolichorhininae and, as first observed by Gregory, has the right combination of characters for a remote ancestor of the Megaceropinae. Such an ancestral relationship, however, awaits confirmation by discovery in Uinta C. RJiadinorJiinus certainly does not lead either into Diplacodon or into Protitano- therium. Riggs observes (1912.1, p. 41): Rhadinorhinus apparently represents a side branch from the early Metarhinus stem. The facial and cranial regions and the zygomata are similar. The nasals indicate an early specializa- tion in another direction. The dentition is more highly spe- cialized than that of Dolichorhinus and in some respects resembles it. The posterior nares open a little farther forward than those of Metarhinus. The mere trace of an infraorbital process also removes it somewhat from the long-nosed, low- ground titanotheres. The grounds for relating this phylum to that of Megacerops { = Symborodon) are stated fully below. DolicTiorhinine affinities. — Eemote relationship to the dolichorhines {MesatirTiinus-DolichorMnus) rather than to Manteoceras is indicated by resemblance in the following characters: (a) Form of temporal and sagittal crests; (6) form of zygomatic arches in top view; (c) marked preorbital concavity and projecting orbits; (d) slight projection of infraorbital portions of malar; (e) similar conformation of palatal and basicranial region; (/) rudiments of a secondary palate; (g) incisors somewhat resembling the dolichorhiue type; (h) premolars and molars in all features doli- chorhine with added peculiarities. These animals differ widely from the typical doli- chorhines, however, in the relatively short, pointed nasals and in the concave or saddle-shaped profile of the cranium as seen from the side but even more distinctly in the upbending of the grinding series, which carries with it an upturning of the anterior portion of the face, a feature very distinctive of Brontoiherium and especially of Megacerops, as shown in Figures 401-403. Rhadinorhinus Riggs, 1912 Plates LXXIV, LXXX, LXXXII; text figures 109, 141, 322, 359, 362-364, 401-403, 405, 647 [For original description and type references see p. 192] Geologic horizon. — MetarJiinus zone (Uinta B 1); also Eohasileus-Dolichorhinus zone (Uinta B 2). Generic characters. — Titanotheres with slender sub- dolichocephalic skulls; cephalic indices 47-52; nares deeply recessed laterally; orbits not prominent; nasals tapering or pointed and abbreviate anteriorly; in- fraorbital shelves rudimentary or wanting. Molars with elevated hypocones; premolars somewhat pro- gressives; hypocone of m' present or rudimentary. History. — As shown above the first of these animals known was originally described by Osborn in 1895 as " TelmatotTierium diploconum," but it was subsequently (1908.318) transferred by the same author to Meta- rhinus diploconus. The type lacked the nasal bones. The superior specimen found by Riggs, in 1910, a skull with the nasals preserved, justified his creating the new genus and species Rhadinorhinus abhotti, belonging to Uinta B 1 , or a lower geologic level than that of R. diploconus (Uinta B 2). The species appear to be distinguished as follows : Rhadinorhinus abhotti Riggs, Uinta B 1; skulls somewhat more primitive and of smaller size; length 426 millimeters, breadth 224, cephalic index 52; hypocone on m' vestigial. Rhadinorhinus diploconus (Osborn), skulls of somewhat larger size, length 440 millimeters, breadth 210, cephalic index 47; hypocone on m' large. The indices of these species, which are of limited reliance because based on single specimens, are re- markably similar, as follows : Indices of Rhadinorhinus Cephalic Facio- cephaUc Molar- premolar- cephalio Molar- cephalic 47 °52 43 44 38 38 23 24 Rhadinorhinus abbotti Riggs Plate LXXX; text figures 141, 403 [For original description and type references see p. 192] Type locality and geologic horizon. — Uinta Basin, Utah; Metarhinus zone (Uinta B 1). Rare. Specific characters. — Skull mesaticephalic, length 426 millimeters, breadth 224; cephalic index 52. P^-m^ 164 millimeters; m'-m^ 104. Nasals shorter than premaxillaries, thickened at suture, and tapering toward a terminal rugosity; posterior nares opposite m^; sagittal crest long and narrow. First and second incisors with semioonical crowns; postcanine diastema short. Materials. — The type of this interesting species (Field Mus. 12179) was discovered by Mr. J. B. Abbott, in whose honor it was named by Riggs. No other specimens are recorded from Uinta B 1 . From the author's original description the following citations may be made: In its general proportions the skull of R. ahhotti (PI. LXXX) is similar to that of Metarhinus riparius; somewhat longer and more slender in the zygomatic arches; the skull suggests a longer-limbed and more active animal; the skull is slight in construction, the arches more slender; the frontal region is rounded; the postorbital processes elongate; the nasals are convex on the inferior surface owing to the thickening at the sutural line; the facial and palatal aspects differ widely from those of Meta- rhinus riparius i the anterior narial opening is high and the nasals are not infolded at the sides; they terminate EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 431 above the anterior margins of the canines so that the premaxillaries are exposed when the skull is viewed from above. There is no antorbital fossa, as in M. fluviatilis; the muzzle is broad and the canines evidently elongate. The teeth are longer in the crowns (that is, more hypsodont) throughout than in MetarMnus. The author continues (1912.1, p. 37): The molar teeth are long in the crown, having inner cones nearly equal in height to the eetoloph. The molar-premolar series is well preserved and unworn in the type specimen. The canines are broken at the alveolus; half of the incisors are pre- served. The dentition as a whole is more highly specialized than that of Metarhinus. The first and second incisors have short, rounded, semiconical crowns indented by cups on the posterior surface. The third incisor has a more elongate crown (23 mm.), the cup is suggested by a prominent cingulum on the posterior margin. An interval of 6 millimeters separates the third incisor from the canine. The latter is 18 millimeters in diameter and circular at the alveolar section. The elongate third incisor would indicate a long-crowned canine. The first premolar is a simple, blunt cone with an internal cingulum and emplanted by two roots. Premolars ^' ^' * increase steadily in length of eetoloph and deuterocone; strong internal cingula persist. The last three have taken on the subrectangular out- line indicating a stage in dental specialization similar to that of Sthenodectes. The molars are long-crowned; the protocone increases steadily in height from first to third. The hypocone is more prominent in the second and reduced to a vestige in the cingulum of the third. The entire molar-premolar series is more curved [that is, in a vertical plane] than in any other Uinta titanothere. Additional observations on RTiadinorhinus ahhotfi. — A fine skull in the Carnegie Museum (No. 2866, Uinta B 1) has the following measurements, which are very close to those of the type: Measurements of Rhadinorhinus abhotti, in millimeters Pmx to condyles Transverse zygomata Postorbital process to condyles Last molar to condyles p'-m' Mi-m3 Field Mus. 12179 (type) Carnegie Mus. 2866 426 424 224 220 240 220 196 200 164 168 103 101 The infraorbital protuberance is small but distinct ; the nasals are not so sharply pointed as in the type. The superior maxillary symphysis is very long. This specimen, together with the type, clearly shows that Rhadinorhinus is an early offshoot from the Meta- rhinus stem (W. K. Gregory). Measurements of skulls of Rhadinorhinus, Mesatirhinus, and Sthenodectes, in millimeters Length, incisors to condyles Breadth across arches Breadth between orbits Postorbital process to condyles Last molar to condyles ^. Length of free nasals ,__^j__^^ Greatest breadth of nasals Postglenoids to condyles (median line) - Breadth across condyles Greatest depth of arch Length of molar-premolar series Length of molar series Length of crown of canine . Diameter of crown of canine Length of diastema : Narrowest point in sagittal area Breadth of orbitonasal area R. abbotti, Field Mus. 12179 (type) 426 224 134 240 196 102 84 38 164 103 R. diploconus, Am. Mus. 1863 (type) 440 '210 172 110 M. petersoni, Am. Mus. 12184 (type) 435 215 115 220 210 95 165 105 -20 10 M. superior, Field Mus. 12188 (type) 585 240 120 276 260 138 36 184 105 23 18 15 26 52 S. incisivus, Carnegie Mus 2398 (type) 488 305 170 261 197 207 130 56 25 94 S. mcisivus, Field Mus. 12168 460 300 160 262 190 101 211 132 47 27 o Estimated. Measurements of lower jaw of Sthenodectes incisivus {Field Mus. 12166) Millimeters Length, condyles to incisors 360 Height, condyles above angle 168 Length of molar-premolar series 215 Length of molar series 130 Length of crown of canine (estimated) 30 Diameter of crown of canine 19 Depth of ramus from base of ps 60 Depth of ramus from base of ma 84 Rhadinorhinus diploconus (Osborn) Plates LXXIV, LXXXII; text figures 109, 362-364, 401, 402, 405, 647 [For original description and type references see p. 173] Tyfe locality and geologic horizon. — White River, Uinta Basin, Utah; Eolasileus-Dolichorhinus zone (Uinta B 2), two specimens. 432 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Specific characters. — Skull mesaticephalic, length 440 millimeters; breadth 210 (estimated); cephalic index 47. Facial region upturned; basicranial region subdolichocephalic; premaxillary symphysis elongate; infraorbital shelf absent; malar rounded; preorbital region relatively abbreviate; occipital condyles rela- tively narrow. P'-m', 168 millimeters ; large hypocone on m' (?variable); marked upward flexure (cypto- cephaly) of premolar series and of incisive border; superior premolars somewhat progressive, with rudi- mentary tetartocone rectigradations ; molars elevated, with prominent protocones. Materials. — This peculiar animal, according to O. A. Peterson's record, is geologically more recent than B. ahhotti, since it occurs in Uinta B 2 in the Aj. Figure 362. — Type skull of Rhadinorhinus diploconus One-fourth natural size. Am. MuS. 1863 (type); White River, Uinta Basin, Utah; Uinta B 2. Ai, Side view, as partly reconstructed in 1895 by H. F. Osborn and R. Weber; drawing reversed. A2, Top view. Later reconstructions of this skull are shown in Figure 364. Eobasileus-DolichorTiinus zone. The type skull (Am. Mus. 1863) was discovered by Peterson in Uinta B 2 during the American Museum expedition of 1894. Our knowledge is partly supplemented by another skull (Am. Mus. 2055), also from Uinta B 2. History. — The specific name R. [Telmatotherium) diploconus was assigned by Osborn in reference to the large hypocone on the last superior molar of the type specimen, a character which is lacking in R. ahhotti, also in the second specimen from Uinta B 2 (Am. Mus. 2055). There is, therefore, some doubt whether the hypocone on m^ is constant. In the original description it was also stated that the naso- frontal did not possess a horn; there seem to be no certain indications of a horn swelling in the nasofrontal region. The extremities of the nasals are wanting. The small size of the canines in circular section may indicate that the type specimen was a female. The premaxillary symphysis is decidedly longer and more firmly united than in MesatirMnus megarhinus. Distinctive features. — In contrast with MesatirMnus and DolichorJiinus we observe that the frontoparietal profile is concave instead of convex; associated with this is the distinctively upturned facial region of the skull. The mesaticephalic proportions of the skull are indicated by the moderate transverse breadth of the zygomata, with an estimated width of 210 millimeters as compared with the total length of 440 millimeters from the symphysis to the condyles. Correlated with this is the relative narrowness of the occipital condyles as compared with those of M. megarhinus. The primi- tive elongation of the sagittal crest is comparable to that in M.fluviatilis and M. earlei. The animal also agrees with these species decidedly in the narrow- ness and abbreviation of the preorbital region. Like these animals, it may be described as narrow-snouted (a characteristic of Megacerops). The narial notch is also deeply recessed, so that there is only a short space at the side between the notch and the anterior border of the orbits. The postorbital processes of the frontals are very large. Beneath the orbit is found one of the most distinctive char- acters — namely, the simple, rounded form of the malars, which is in wide contrast to the oblique shelf of Manteoceras or the broadly project- ing shelf of all the other species of MesatirMnus and Metarhinus. R. diploconus differs from Metarhinus fl,uviatilis as foUows: (1) All the cheek teeth are more elongate anteroposteri- orly, hence the internal border of the molars is less oblique than in M.fluvi- atilis; (2) the internal cingulum of p* is not complete; (3) the postero-external shoulder on p"* is more prominent; (4) the skull top in side view is broadly concave (flatter in M. fluviatilis) ; (5) the zygomatic arch in side view curves downward more sharply. Variations. — Of the two skulls referred to R. diplo- conus, one (Am. Mus. 2055) is smaller and has smaller teeth than the type and lacks the hypocone on m'. Cyptocephaly . — The upturned face of R. diploconus suggests that of Megacerops of the Oligocene. It would seem that the skull in correlation was some- what saddle-shaped above, with the nasal region more elevated than the frontal. Features in detail. — The superior view of the type skidl (Am. Mus. 1863) lacks the extremities of the nasals; it exhibits the marked backward extension of these bones, the great prominence of the orbital ring, EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 433 the gentle supratemporal ridges converging from the very prominent postorbital processes into the long, narrow sagittal crest, the gentle outward arching of the zygomata. In the badly distorted palatal view of the same skull a resemblance to Mesatirhinus is never- theless evident. The distorted occiput exhibits an elevation of 97 millimeters and a deep superior con- cavity. In the lateral view (figs. 362, 364) we are struck by the downward, midcranial curvature of the profile (which appears more marked than in Mesati- rJiinus and Manteoceras) , by the very prominent conformation of the orbital region above described, by the narrow space, measuring only 45 millimeters, between the orbit and the narial notch. The malar, as in other titano- theres, forms the entire outer portion of the infraorbital bar, the maxillary entering into its anterior rim, as in MesatirTiinus and Manteo- ceras. It appears to lack the infraorbital shelf as noted above. The malar passes anteriorly into the narrow and decidedly convex and elevated bridge at the point of union with the lacrimals. It is narrow and flattened below the posterior portion of the orbit as it passes backward; it is thin on the inferior surface. The zygo- matic portion of the squa- mosal is moderately elevated and expanded. Dentition. — The materials consist of the type skull (Am. Mus. 1863) and of another skull (Am. Mus. 2055) con- taining alveoli of the cut- ting teeth and the worn crowns of the grinding teeth. Incisors: The superior incisor alveoli have the characteristic convergence of the opposite series, and so far as can be judged from the alveoli, which increase regularly from i' to i^, the canine fang (ap. 14 mm., tr. 12) is laterally compressed and is of small dimensions, like those of a female. The molar-premolar series are of the Mesatirhinus type; they measure 169 millimeters in length, as com- pared with 156 in M. petersoni. They are distinguished by several features, among them the very prominent pointed hypocone of m^, to which the specific name refers but which is lacking in Am. Mus. 2055. P^, p' exhibit the very rudimentary postero-internal or tetartocone ridges which are observed in T. ultimum and Dolichorhinus. There are other faintly indicated progressive characters, especially the comparatively deep medifossettes and postfossettes on the molars and premolars, correlated with the greater hypso- donty, and the nearly symmetrical protocone and tritocone convexities on the ectoloph of p^"'*, as in M. earlei. Premolars: More in detail: P' is separated from the canine by a narrow diastema (5 mm.) and is a f:/,l.a.(n. Type skull of Rhadinorhinus diploconus One-fourth natural size. Am. Mus. 1863. White River, Uinta Basin, Utah; Uinta B 2. Partly straightened, occipital crest has heen narrowed by crushing. Ai, Top view; M, palatal view. bifanged tooth; p^ is in a much more progressive stage of evolution than in M. petersoni, with its sub- quadrate form, anteriorly placed deuterocone, sub- functional tetartocone, well developed and slightly convex tritocone, the dimensions (ap. by tr.) being 16 by 19 millimeters; p^ is of similar pattern (ap. 17 mm., tr. 21), with nearly symmetrical protocones and tritocones, an anterior style, and a rudimentary pos- tero-internal cusp or tetartocone. In the develop- ment of the last-named cusp p^ is slightly more pro- gressive than p^ (ap. 20 mm., tr. 25); p^ is of the same pattern but entirely lacks any trace of the tetartocone. An internal cingulum nearly surrounds these teeth ex- cept opposite the deuterocone. 434 TITANOTHERBS OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Molars: M' (ap. 29 mm., tr. 27) is slightly longer than broad and thus like the other molars is of marked dolichocephalic type; the hypococone is very high and prominent. M^ (ap. 37 mm., tr. 34) exhibits faint external and incomplete internal cingula with a strong antero-internal cingular basin, a progressive feature of aU these molars. This basin, or "hypoconid fossa," indicates relatively advanced hypsodonty in the lower molars, which we might infer also from the subhypso- donty of the upper molars. Peculiar also is the sharp furrow dividing the proto- cones from the hypocones, and the prominence of the internal cingulum midway between the protocones and hypocones. M^ has an elevated ectoloph (28 mm.) and is also longer (37 mm.) than broad (35 mm.); (3) nasals small, pointed anteriorly; (4) malars and antorbital bar rounded, with very short space in front of antorbital bar; (5) orbits small, inset; (6) a deep recession at the sides of the nares, and nasals high set; (7) occiput with a deep concavity; (8) chin weak, concave below, sloping up to a plane higher than that of the grinding teeth; (9) ramus with coronoid process, etc. (see Am. Mus. 2059, Metarhinus fluviatilis, a diminutive copy of the Oligocene Megacerops type); (10) first and second upper incisors {R. ahhotti Riggs) with short, rounded subcorneal crowns; (11) canines of somewhat obtuse form, with swelling posterior cingulum; (12) lower premolars sloping upward anteri- orly with strong internal cingulum; (13) upper pre- molars relatively progressive, well-rounded contours , large tritocones and relatively progressive tetartocone rudi- ments; (14) tetartocone rudi- ments in p^, p' {R. diploconus) more advanced than in p*, pre- molars sub quadrate in form; (15) molars elongate or sub- hypsodont. Intermediate forms between R. diploconus which may be discovered in Uinta B 2 and Uinta C will determine the question whether this relatively primitive form is ancestral to the Megaceropinae. successors to the manteo- ceras-dolichorhinus group (eotitanotherium, DIPLA- codon) subfamily diplacodontinae (=?meno- DONTINAE, = ?BEONTOTHEKmf AE) [Eocene phylum Diplacodon] Including upper Eocene fore- „ . , , , runners of the Oligocene genera One-fourth natural size. Am. Mus. 1863. White River, Uinta Basin, Utah; Umta B 2. Straightened and recon- ° "^ . structed, except nasals and condylar region. Ai, Side view (reversed); A2, front view, partly straightened, but JilenoduS, BrOntotlieriUm. occiput probably too high and postorbital process of frontal too low; A3, occipital view, straightened. Primitivclv dolichocenhalic Figure 364. — Type skull of Rhadinorhinus diploconus its prominent hypocone constitutes a distinctive feature, but from its absence in Am. Mus. 2055 it may not be a valid and constant specific distinction. Is Rhadinorhinus ancestral to Megacerops? — The possible ancestry of R. diploconus to Megacerops first suggested itself to W. K. Gregory in 1903 and was carefully considered by him and by Osborn in sub- sequent years. There are, in fact, many features in which Rhadinorhinus diploconus appears to fore- shadow Megacerops, chief among which are the fol- lowing: (1) Saddle-shaped profile, high nasofacial region, depressed frontoparietal profile; (2) facial region relatively abbreviate, upturned — that is, an upward flexure of maxillaries and premaxillaries; progressively mesaticephalic. Precocious develop- ment of horns. Nasals narrowing anteriorly, curved downward at the tips. Second internal cones on the superior premolars precociously developed. (For progressive characters see Oligocene stages, p. 467.) The phyletic relations of the two animals now to be described are not certainly determined; they are pro- visionally placed in a separate subfamily (Diplaco- dontinae), although it is possible that they belong in one of the OHgocene subfamiUes (Menodontinae, Brontotheriinae). They include the Diplacodon of Marsh, discovered in 1873, and the Eotitanotherium of Peterson, discovered in 1912. EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 435 Eotitanotherium Peterson [Diploceras Peterson; cf. Diplacodon Marsh] Plate LXXXI; text figures 148, 149, 365-367, 372, 598-605, 647 [For original description and type references see p. 196. For skeletal characters see p. 656] Geologic Tiorizon. — Uinta B 2. In 1913 Peterson (1914.1) announced the surprising discovery, in the middle levels of the Uinta Basin, Utah, of a large titanothere having a pair of prominent bony out- growths above the orbits, which he proposed to name Diploceras. Peterson later gave it the name Eotitano- therium to replace Diploceras, which was preoccupied. Generic characters. — Large upper Eocene dolicho- cephalic titanotheres with very long, decurved nasals and well-developed, anteroposteriorly oval horns. Premolars decidedly progressive. P^, p* with large distinct tetartocones and reduced external and inter- nal cingula. The type of Eotitanotherium oshorni Peterson (Carnegie Mus. 2859), so far as known, conforms with the definition of Diplacodon as given below: Skull mesatlcephalic to dolichocephalic, zygomatic arches slender; superior premolars with flattened ectolophs and double convexities, p^-p"* progressive, quadritubercular — that is, with tetartocones; molars of elongate or dolichocephalic type — that is, laterally compressed. E. oshorni Peterson further agrees with Diplacodon elatus Marsh in the general small, obtuse form of the canine and in the presence of a wide post canine diastema. On the other hand, the type of E. oshorni appears to represent a distinct species or even a different genus from D. elatus, for although it comes from a lower geologic level (Uinta B 2) its premolars are decidedly more progressive in character, p', p* having the tetartocones larger and more separate from the deu- terocones, the external and internal cingula reduced, and the whole appearance of the crown more molari- form than in D. elatus. Comparative measurements of the two species are given below. Comparative measurements of Eotitanotherium and Diplacodon, P>-m3 Pi-p4 M'-m' P^, ap. by tr M', ap. by tr M2, ap. by tr_ .. M', ap. by tr Canine, vertical Canine, anteroposterior Postoanine diastema Nasals, length E. osborni, Carnegie Mus. 2859 (type) D. elatus, Yale Mus. 10320 (type) 246 250 97 90 146 158 29X37 28X34 40X43 41X42 48X50 62X42 60X55 60X57 25 17 28 24 119 Comparative measurements of Eotitanotherium and Diplacodon, in millimeters — Continued Pi-ma Pi-P4 P2-P4 Mi-ms Postcanine to hyloid of ms Postcanine diastema . Depth below ms . E. osborni, Carnegie Mus 2859 (type) 255 95 79 158 283 34 93 D. elatus, Yale Mus. 10320 (type) Whether the paratype skull (Peterson, Carnegie Mus. 2858) belongs with this species is somewhat doubtful. The greater size of the horns and the asso- ciated widening of the nasals in the paratype may well be a sexual difference, the paratype being possibly a male, the type a female. On the other hand, the paratype differs radically from the type in having the tetartocone of p* feebly developed, thus approach- ing D. elatus. Although Eotitanotherium, from Uinta B 2, is prob- ably allied to the somewhat later Diplacodon (Uinta C 1), it seems best to let it stand provisionally as a distinct genus, especially in view of the progressive character of p^ and p* in this older stage. Eotitanotherium osborni Peterson [Diploceras osborni Peterson] [For original description and type reference see p. 195. For skeletal characters see p. 656] Type locality and geologic horizon. — Duchesne River, near My ton, Uinta County, Utah; Eohasileus-Doli- chorinus zone (Uinta B 2). Specific characters. — P'-m^ 240 millimeters; m'-m^, 145. Median and lateral incisors small and "round- topped," approaching Oligocene type, with heavy posterior cingulum; lateral incisors more massive. Canines short, subconic, without cingula, postcanine diastema 28 millimeters. Premolars and molars with little or no external cingula; internal cingula reduced; p', p* highly progressive, submolariform, with large tetartocones (type) and two well-marked external con- vexities; dimensions of p* (ap. by tr.) 29 by 37 milli- meters. Upper molars wider than in D. elatus, m' with incipient tetartocones. Nasals long, tapering, decurved. Horn swellings low, elongate, oval in basal section. Lower jaw with deep ramus and deep symphysis. The type and paratype skulls of Eotitanotherium {Diploceras) oshorni were thus described by Peterson (1914.1, pp. 30-37): SKULL [Fig. 366] In comparing the recently discovered material with the best preserved remains of Protitanotherium (P. emarginatum Hatcher) a number of important differences are at once observed. The 436 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA nasals of the new species are longer, thinner, somewhat narrower (especially in specimen No. 2859); furthermore the lateral borders of the nasals are much less thickened, and instead of the broadly emarginated area at the free end of the nasals in P. emarginatum, the termination of the nasal of the present form has an abrupt downward turn resembling that of Megacerops coloradensis Leidy, and its anterior margin is very convex transversely, instead of concave, as is the case in P. emarginatum. Upon the whole the nasals of the species we are X, ^^»8««i«^ '■'*% Figure 365. — Skulls of Eotitanotherium osborni [ = fig. 365].) This varied development of the horn cores is no doubt due to se.xual differences, or possibly to individual variation. The premaxillaries extend well in front of the maxillaries and are separated in front, forming a deep median notch, as in P. emarginatum, so that the median pair of incisors are wide apart, while farther back they are firmly coossified and also solidly fused with the maxillaries. The infraorbital foramen is also of large size as in P. emarginatum and located above p^ as in the latter species. The maxillary is on the whole very robust and shows that it had advanced well toward the condition found in Diplacodon and Titanotherium. This is also true of the ]'ugal, the prominent lower border of which has the downward and backward sweep in front of and under the orbit, which is charac- teristic of Titanotherium. The zygo- matic arch, though widely expanded behind, is, however, less robust than in the Oligooene genus and agrees better with the type of Diplacodon elatum described by Marsh. The postorbital processes on the frontal and jugal are of large size, in this respect unlike Titanotherium. The postorbital process on the frontal of the latter genus is usually located farther back and is much smaller in proportion. The external portion of the glenoid cavity is preserved in No. 2858 and is somewhat less convex in the antero- posterior direction than in the latter genus. As in Titanotherium the anterior palatine foramina are small round openings, which in the present genus are situated farther back from the alveolar border of the incisors. The palate is of the deep concave form usually met with in the titanotheres, and the posterior narial opening ex- tends approximately as far forward as in the Oligocene genus, reaching to the posterior portion of m^. That the type of the skull was saddle- shaped is very evident from the material under study, but whether or not the characteristically broad superior aspect of the parietals and the heavy and broad occiput seen in Titanotherium had been attained to the same degree as the similarity of the anterior region in the two genera suggests might have been the case will not be completely known until the posterior region of the skull of the Uinta After Peterson. One-Iourth natural size. A, Type skull, with associated lower jaw, 9 , Carnegie Mus. 2859, Duchesne representatives of this phylum is River near IVIyton, Uinta County, Utah, Uinta B 2; B, paratype skull, tf , Carnegie Mus. 2858 (reversed), same fo^nd. It is highly probable that the locality and level as A. describing extend further forward. There seems to be a con- siderable variation in the development fo the horn cores; thus, in skull No. 2858 this protuberance appears to have a develop- ment comparable to that of some of the titanotheres found in the Ohgocene, while in specimen No. 2859 these osseous bosses are very much smaller, more conical, and in proportion more like those of P. emarginatum, in spite of the fact that the skull we are considering pertains to an old individual. (See PI. VII similarity presented by the anterior region will be preserved throughout the cranium, which will then reveal more exactly the features of a true titanothere than was anticipated. From the type of Protitanotherium emar- ginatum at Princeton University, Hatcher ^o was apparently able to determine that the sagittal crest is absent and that the dorsal surface of the skull is probably slightly concave antero- posteriorly. !• Am! Naturalist, vol. 29, p. 1085, 1895. EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 437 Measurements in millimeters No. 2858 No. 2859 Skull Diameter from incisors to posterior border of glenoid cavity 470 "ISO 67 55 260 "93 -67 <• 290 Diameter from incisors to anterior border of orbit _____ ''179 -67 Diameter from incisors to anterior border of posterior nares Transverse diameter at the canines Transverse diameter at diastema between the canines and the premolars Transverse diameter across the horn cores. _ Lower jaw Total length of jaw fragment- 256 86 67 136 380 Diameter from incisor to pi _ _ 67 »70 »80 92 " Approximate measurements. MANDIBLE [Fig. 3C5] The lower jaw is somewhat depressed by crushing, but allowing for this fact, it appears that the horizontal ramus of Diploceras osborni is shallower than in P. emarginatum. Char- acteristics which may further be noted are the more rounded under surface of the symphysis and the constriction of the lower jaws in the area between the canine and the premolars, which is greater than in P. emarginatum. As in the latter, the symphysis is strong and the mental foramen is large, located well down on the ramus, directly below P2. The lower jaw is broken off back of ms. DENTITION [Fig. 366] The upper incisors and canines are well preserved, though much worn in the two crania under description. The molar- premolar series is less completely preserved in No. 2858, while in 2859 the superior dentition is completely represented. The lateral incisor and the canine of the right mandible and the complete molar-premolar series of the left ramus are also present in the latter individual. As stated above, the median upper incisors are widely separated by the deeply excavated median notch of the pre- maxillaries. As seen in the illustration, the incisor series is placed well in front of the canine and the arc of the circle, which their arrangement represents, is more convex than in P. emarginatum. Their crowns are nearly circular in outline, covered with a heavy coat of enamel, bluntly conical, with a prominent cingulum at their posterior bases. They perhaps increase in size more gradually from i' to i^ than in P. emar- ginatum. The canine is relatively smaller than in the latter genus, which imparts a much lighter looking aspect not only to this region of the dentition but also to the entire outline of the anterior portion of the muzzle in the paratype. No. 2858, as well as in the type. No. 2859. Furthermore the crown of the canine (especially in No. 2859) is shorter, blunter, and the lateral ridges are less developed in the present species than in either P. emarginatum or Diplacodon elatum. D. elatum has the canine more nearly of the same proportion as in P. emar- ginatum. The diastema back of the canine is relatively longer and its border much thinner than in P. emarginatum, in which respect it is more nearly like Diplacodon elatum. The crown of pi is so much worn that its characters can not be made out. It is, however, of greater anteroposterior than transverse diameter and undoubtedly had a simple structure like that of P. emarginatum. P^ is also much worn, especially Figure 366. — Nasals and horn swellings of Eotitanotherium osborni One-fourth natural size. Ai, Carnegie Mus. 2859 (type), Duchesne River near Myton, Uinta County, Utah, Uinta B 2, top view; Aj, same, basal view; B, Carnegie Mus. 2858 (paratype), locality and level same as A, top view. along the external portion. The external face of the ectoloph is subdivided by a deep vertical groove and is much convex both anteroposteriorly and supero-inferiorly. This deep groove adds greatly to the anteroposterior convexity of the proto- and trito- cones. The general outlines of the tooth are less quadrate than in Titanotherium, which is apparently due to the lack of devel- 438 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA opment of the antero-internal angle in the species under con- sideration. In the type of Diplacodon datum p' is lost, while the external portion of p^ is broken off. In the present species the deuterooone of p^ is less ridgelike than in D. elatum, the two interal tubercles being somewhat better indicated and the ridge between them distinctly less developed. P' is more quadrate in outline than the preceding tooth and has two dis- tinct internal tubercles on the crown, which are separated by a shallow groove, while in Diplacodon elatum these tubercles are united into a solid internal ridge, revealing a distinct differen- tiation from what is seen in the present species [compare PI. LXXXll. On the other hand, p' both in the type we are describing and in D. elatum are similar, there being two internal tubercles, deutero- and tetartocones, the former considerably the larger.^" The more important differences in the dentition of the two forms, so far as they can now be compared, seem to be in the proportion of the canines, the difference in the length of the premolar series, and the detailed structure of p'. The greater length of the premolar series is naturally to be expected in a form from a lower geological level. The detailed characters of the molar series of the genera here compared present no differences of importance. The two Uinta forms agree in the obscure or feeble development of the cusp- like elevations on the anterior face of the molars near the inner angle, more conspicuously developed in Titanotherium. At the postero-internal angle of the cingulum of m' in the Oligocene Figure 367. — Two upper molars of Eotitano- therium {" Diploceras") osborni Peterson Crown view. One-half natural size. Carnegie IVIus. 2860a. These isolated teeth were found with the paratype, Carnegie Mus. 2S60; Duchesne River near Myton, Uinta County, Utah.; Uinta B 2. forms there is sometimes a distinct tubercle, which is indicated in the Uinta forms by only a slight swelling of the cingulum. In proportion the inferior incisor dentition is further in ad- vance of the canine than in P. emarginatum. Ii and i2 are repre- sented only by a portion of their roots buried in the symphysis. I3 has a very prominent cingulum posteriorly. Notwithstand- ing the much smaller size of the specimen, its crown has very nearly the same diameter as in P. emarginatum, which would indicate that the inferior incisors were possibly larger in propor- tion and more nearly equal in size. The crown of the canine is injured, but its diameters appear to be equal to those in the superior series, though relatively smaller than in P. emar- ginatum. Pi has a single root and a simple conical crown, which has not received any wear, due to its somewhat inferior position. Pj is submolariform and in its general characters does not differ from the same tooth in P. emarginatum. P3 is quite molariform, while P4 has a complete molar pattern. There is no difference in the general features of the lower molars in the two genera here compared, and in turn the molars of Diplacodon are on the whole quite similar in their detailed structure to those of the Oligocene genus. •» In No. 2858, the paratype, there is only one internal tubercle, the deuterocone which may by some be regarded as of sufficient importance to constitute a specific difference. For the present I prefer to regard this character as possibly representing a reversion. The proportion of the alveolar border occupied by the lower premolars of this species is in accord with the upper series — that is, of a greater anteroposterior diameter than in P. emar- ginatum and D. elalum.^^ Judging from the type (lower jaw) of Protitanotherium superbum Osborn, recently described,'^ that species also has the same proportion of the molar-premolar series as the two latter, while Telmatherium? altidens of the same publication has a longer premolar series and more nearly agrees with the present genus. Measurements, in millimeters Length of superior incisor series 1', anteroposterior diameter 1', transverse diameter I^, anteroposterior diameter I^, transverse diameter I' , anteroposterior diameter 1', transverse diameter Canine, anteroposterior diameter at the base. Canine, transverse diameter at the base Length of molar-premolar series Length of superior premolar series P', anteroposterior diameter P', transverse diameter P^, anteroposterior diameter P^, transverse diameter P', anteroposterior diameter P^, transverse diameter P*, anteroposterior diameter P*, transverse diameter Extent of superior molar series M', anteroposterior diameter M', transverse diameter M^, anteroposterior diameter M^, transverse diameter M', anteroposterior diameter M', transverse diameter I3, anteroposterior diameter I3, transverse diameter Canine, anteroposterior diameter, approxi- mately Canine, transverse diameter, approximately Length of inferior molar-premolar series Length of inferior premolar series Length of inferior molar series Pi, anteroposterior diameter Pi, transverse diameter P2, anteroposterior diameter P2, transverse diameter P3, anteroposterior diameter P3, transverse diameter P4, anteroposterior diameter P4, transverse diameter Ml, anteroposterior diameter Ml, transverse diameter M2, anteroposterior diameter M2, transverse diameter M3, anteroposterior diameter M3, transverse diameter No. 2859 No 34 11 10 12 12 15 14 19 18 246 101 19 12 22 25 30 31 33 38 ■146 38 45 52 54 67 51 14 12 17 14 255 94 160 14 10 24 14 28 18 29 20 38 26 49 30 78 32 " Professor IMarsh's measurement of the molar series of the type of D. elatum is 31 In remeasuring the molar series of Prof. IMarsh's type of Diplacodon elatum it would seem that he was in error in regard to the measurement, which should read 167 instead of 152 millimeters. 3! Osborn, H. F., New and little-known titanotheres from the Eocene and Ohgo- cene: Am. Mus. Bull., vol. 24, p. 615, 1908. EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 439 Diplacodon Marsh Plates LXXXI, LXXXII; text figures 99, 368-371, 406, 594, 595, 597, 723 [For original description and type references see p. 166] Geologic Jiorizon. — True Uinta formation (Uinta C). History. — The original and only known remains of Diplacodon elatus, an animal that has played a very large part in titanothere literatm-e, consist of a crushed palate with a full series of grinding teeth, found by the well-known prospector Samuel Smith on White River, Utah, August' 24, 1874. The geologic level is now known to be the lower part of Uinta C, a level higher than that of the species Eotitanotherium osborni, which we have just been considering. The specimen was described by Marsh in February, 1875, as representing a new genus, which he named Diplaco- don in reference to the double internal cones on the superior premolar teeth. Marsh immediately recog- nized the importance of this animal as a structiu-al connecting link between "Palaeosyops" and "Bronto- therium." He subsequently observed: "In the upper Eocene, both [Limnohyops and Palaeosyops] have left the field, and the genus Diplacodon, a very near relative, holds the supremacy." (Marsh, 1877.1, p. 31.) This animal was also regarded by Earle^ Scott, and Osborn as a direct connecting link between the Eocene and Oligocene titanotheres. Generic cTiaraciers. — Skull mesaticephalic to dolicho- cephalic; zygomatic arches slender. Superior premo- lars with flattened ectolophs and double convexities; p^~* progressive, quadritubercular — that is, with tetartocones; molars of elongate or dolichocephalic type — that is, laterally compressed. Horns. — In his original description Marsh stated that this animal was distinguished from the Oligocene titanotheres "by the absence of horns." As the skull in the type and only known specimen of D. elatus is badly crushed it is difficult to determine whether or not the type animal possessed rudimentary horns; if it had they were certainly not so large as those of the Oligocene titanotheres. In the related type of Eotitanotherium (Diploceras) oshorni Peterson there are well-developed horns with elongate oval bases. Progressive molar characters. — The type skull is too imperfectly known for us to distinguish all its con- servative and progressive characters. In the dentition the following progressive characters are observed: (1) Premolars with well-developed tetartocones; (2) complete internal cingula; (3) rudimentary fossettes on the crown surface; (4) premolar tritocones large and subequal with protocones; (5) p^ much more progressive than in Telmatherium ultimum or Manteo- ceras uintensis, with large, centrally placed deutero- cones and subquadrate rather than triangular contour; (6) molars with very distinct fossettes or pits on the crown surface near the ectoloph. 101959— 29— VOL 1 31 All these molar teeth characters point toward the Oligocene stages of the teeth of Menodus rather than of Brontotherium; on the other hand, the structure of the canines and incisors points rather toward Bronto- therium than Menodus. Diplacodon elatus Marsh Plates LXXXI, LXXXII; text figures 99, 368-371, 406, 597 [For original description and type references see p. 166] Type locality and geologic horizon. — Uinta Basin, Utah; Diplacodon-Protitanotherium-Epihippus zone, Uinta C 1, the true Uinta formation. Figure 368. — Type skull of Diplacodon elatus Palatal view. One-fourth natural size. Yale Mus. 10320; Uinta O I. Partial reconstruction of the under surface of the slcuU based on the type materials. Specific characters. — Skull mesaticephalic to doli- chocephalic. P'--m', 242 millimeters; molars elongate anteroposteriorly and subhypsodont; premolars short and broad; tetartocone rudiment on p^, tetartocones increasing in distinctness on p^ and p"*; m^ without hypocone; canines small in females. The type specimen (Yale Mus. 10320) is a female, as indicated by its small, rounded, recurved canines. The postcanine diastema is considerable, measuring 24 millimeters. The grinding teeth are laterally 440 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA compressed, and the zygoma is slender and decidedly feeble, indicating that the skull was dolichocephalic. The total dimensions of the grinding series are, pre- molars, 89 millimeters; molars, 152; premolar-molar series, 242. This is of smaller size as compared with the inferior grinding series of Protifanotherium emarginatum (304 mm.), of P. superbum (318), and of Telmatherium altidens (313). Progressive characters. — Among the details of pro- gressive character may be mentioned : (1) The cingulum pas. Tiis. tr Aj Figure 369. — Type skull of Diplacodon elatus (female) Upper jaw and zygoma. One-fourth natural size. Yale Mus. 10320; Uinta C 1. zygoma partly reconstructed; si, S2, sections; As, top view of zygoma, is continued on the inner sides of the crowns of the premolar teeth; (2) the premolar teeth are rendered quadrate internally by the expansion of the tetartocone shelf, and p^-p"* exhibit the progressive development of the tetartocone from before backward by constric- tion or budding from the deuterocone crest; (3) the fossette of the crown is seen in a rudimentary form in the premolars and very distinctly in the molars, a feature characteristic of all Oligocene titanotheres and clearly foreshadowed in Rhadinorhinus diploconus of the upper Eocene. Canines. — The canines are small, laterally com- pressed, and slightly recurved, but too much worn and fractured to clearly represent their form. Premolars. — As noted above, the deuterocone crest is convex on the median or lingual side and flat or concave on the outer or buccal side, as in Telmathe- rium. Another distinctive character is seen in the ectolophs, which are decidedly flattened, especially the outer surfaces of the tritocones, reminding us of the condition in Dolichorhinus; the external cingulum arises and festoons the protocones. The detailed proportions of the teeth (ap. by tr.) are, p^ 21 by 23 millimeters; p^ 25 by 29; j)\ 28 by 35. In p* the deuterocone is large and elevated (17 mm.) and the worn tetartocone is low (13 mm.) but almost entirely distinct. Molars. — In the molars the external cingula are faint, but the internal cingula are wanting. A characteristic feature is the antero-internal expansion of the cingulum, which makes the entire anterior border angular and prominent, especially as seen in m^ This cingulum ridge bounds the fossa for the metaconid and is clearly foreshadowed in Rhadino- rhinus diploconus. The fossettes are nearly worn out in m' and m^; both anterior and posterior fossettes appear, and in m' the anterior fossette is a deep, narrow pit. The proportions of the teeth (ap. by tr.) are, mS 41 by 44 millimeters; m^, 55 by 55; m', 60 by 55. These proportions are decidedly different from those in Protitanotherium. (Fig. 371.) The posterior cingulum of m^ is elevated, and a low, distinct hypo- cone swelling appears. The ectolophs exhibit compressed mesostyles. The ectolophs are moderately hypsodont; the pos- terior view of m^ shows that the mesostyle terminates in a horizontal ridge, as in many little-worn or unworn titano- there molars; it is especially strong in Rhadinorhinus diplo- conus. Comparison of Diplacodon ela- tus with middle Eocene titano- theres. — Diplacodon combines characters of Telmatherium and of Dolichorhinus but does not agree with either in all respects. Comparison with Telmatherium: Diplacodon shares with Telmatherium the tendencies toward dolichoce- phaly and toward the development of tetartocones, but it far outstrips Telmatherium in both features and differs very radically in others, as follows: The canines are small; p^ in Diplacodon is much more advanced and of different type; the tritocones of the superior premolars are flattened instead of rounded; the molars Ai, Upper jaw and teeth, with partly reconstructed. Figure 370. — Third and fourth upper pre- molars of Diplacodon elatus Natural size. Yale Mus. 10320 (type, reversed). Uinta CI. Outer side view. are much more elongate and have flattened ectolophs; the zygomata are much more slender. Comparison with Protitanotherium: These animals are readily distinguished from the mesaticephalic Protitanotherium in all the dolichocephalic propor- tions of the teeth; and from the radical differences in EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 441 the teeth it is safe to infer that the proportions of the skull also differed widely. Comparison with DolicJiorhinus : Resemblances to members of this genus are observed in the flattening of the premolar ectolophs, especially of the tritocones; A.M. 250i, type ^YaleMus.l03Z0,type FiGtTBE 371. — Upper molars of Diplacodon and Prolitanotherium compared One-half natural size. Superimposed contours of the first and second upper molars of the dolichocephalic D. elatus (heavy line) {Yale Mus. 10320, type) and the brachycophalic P. su- perbum (light line) (Am. Mus. 2501, type) . in the slenderness of the zygomatic arches posteriorly; in the elongate or dolichocephalic type of the grinding teeth; and in the small size of the canines. The most important progressive or divergent differences from Dolichorhinus are the great development of the tetar- tocones, especially on p*; the great breadth of p**; the absence of a broad infraorbital shelf on the malars; the apparent retardation of the horn rudiments. Comparison with RhadinorJiinus: Some resemblances between Rhadinorhinus and Diplacodon elatus led to the doubtful view that the two were related. These resemblances are seen especially in the propor- tions of the molars, which are of dolichocephalic type, and of the premolars, which are relatively broad. Among other characters common to the two species are the following: Molars subhypsodont; m^ of elon- gate, compressed form; parastyles and mesostyles sharp and delicate; hypocones of m', m^ set well in on crown; external cingula delicate; deep fossettes median and posterior. In the premolars we observe that the crowns are relatively broad; the tetarto- cones of p^, p' are somewhat progressive in B. diplo- conus; the tritocone is very large on p^-p*; and the medifossettes on p^-p* are deep. Type skull oj Diplacodon elatus. — The fractured skull affords only a few characteristic features. The main indications are of a dolichocephalic type, with slender zygomatic arches. The premaxillary sym- physis is apparently deep, measuring 92 millimeters from the incisive border to the lower border of the nasal notch. The extent of the palatines upon the hard palate was apparently rather narrow, the pos- terior nares opening directly opposite the interval between the second and third molars. There is the characteristic rugosity at the junction of the basi- sphenoid and basioccipital. As above noted, there is no conclusive evidence regarding the horn rudiments except that if present at all they appear to have been not very prominent. The most important feature by far is the slender and simple structure of the zygomatic arch (fig. 369). The malars give no evidence of the existence of an infraorbital shelf; on the contrary, this region was smooth, flattened, and not very prominent. Simi- larly, the squamosal portion of the arch is shown, giving a maximum depth of 30 millimeters and a Figure 372. — Facial region of Eotitanotherium os- borni and Brontotherium leidyi One-fifth natural size. A, E.oshorni, Carnegie IVIus. 2859 (type), Uinta B; B, B. leidyi, Nat. IVIus. 4249 (type), Chadron A. width of 37. The very slender zygoma was apparently nearly parallel with the sides of the skull, as in Doli- chorhinus, but as in Rhadinorhinus diploconus it lacked the infraorbital shelf; it also lacks the deep vertical expansion seen in Telma.therium. CHAPTER VI EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES SECTION 1. REVIEW OF THE ENVIRONMENT, GEO- LOGIC SUCCESSION, AND GEOGRAPHIC DISTRIBU- TION OF THE LOWER OLIGOCENE TITANOTHERES In order to facilitate an understanding of the evolu- tion of the skull and dentition of the Oligocene titanotheres, a brief resume of the matter presented in Chapter II is here given. The environment of the lower Oligocene titanotheres, described in Chapter II, was different from that of their Eocene ancestors. It consisted mainly of the broad flood-plain region east of the Kocky Mountains. The geographic range, however, extended westward into the broad upland plateaus west of the Rockies and northward over British Columbia into Asia as far west as the eastern part of Europe. The conditions in the Great Plains region appear to have been eminently favorable to the existence of the titanotheres, because the members of all the numerous branches into which this great family was divided show an increase in size, which is especially conspicuous in the males. Our ideas regarding the geologic levels of the species and the mutations of the members of the four sub- families are founded upon the original observations of Hatcher, who very care- fully recorded the vertical distribution of the types and other specimens in his great collection for the National Museum, which are fully enumerated else- where in this monograph. To the records of these specimens have been added records obtained from other museum collections. The faunistic subdivisions of the Titanotherium zone, which forms part of the White Eiver deposits and is of Chadron age (lower Oligocene), range in thickness from 150 to 200 feet, as follows: Feet Chadron C (upper or Brontops robustus zone) 30-50 Chadron B (middle or Brontops dispar zone) 70-90 Chadron A (lower or Brontops brachycephalus zone) 50-60 The lower Oligocene deposits of the Chadron forma- tion of the Great Plains were laid down on the irregu- larly worn surface of the Pierre shale (Upper Cre- taceous), which had been exposed to erosion for a long time. Consequently the deposition of the Titanothe- nwm-bearing beds was not uniform: it began at some points earlier than at others, and the total thickness of the Titanotherium zone accordingly ranges from 1 50 to 200 feet. There are also some discrepancies in the records, which are doubtless due to irregularities of deposition in the overflow and stream channel deposits. The known areas where deposits of the Titanothe- rium zone are exposed and where fossils occur are shown on the accompanying map (fig. 373). The change of form of the lower Oligocene titano- theres was almost as great while this 200 feet of Former land areas Former migration areas Figure 373. — Map showing the areas in which remains of titanotheres have been found (solid black) and areas in which, during Eocene and Oligocene time (oblique lines) titanotheres were probably in migration The general regions in which titanotheres have been found are the northwestern United States, the Gobi Desert (Mongolia), Burma, and southeastern Europe. sediment was being deposited as that of the Eocene titanotheres while 2,000 feet of sediment was being deposited. We infer that the average deposit of 200 feet of sediment in so many localities entirely deceives us as to the length of lower Oligocene time. These sediments were being laid down probably not for hun- dreds but for thousands of years. During this long period the titanotheres were certainly very abundant over the entire western plains. Without exception all the animals whose remains are found at the base of the Titanotherium zone were relatively small, and all had short and superficially similar horns. The great increase in size observed be- tween the animals of the beds of Chadron A and those 443 444 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA of Chadron C is made evident by a comparison of the outlines of the skulls (figs. 389-393) and lower jaws (figs. 395-397). A2 Figure 374. — Comparison of upper Eocene and lower Oligocene titanotheres, showing similar early stages in the evolution of the horns Ai, A?, Reconstruction of the skull and jaw of an adult Protitanotherium emarginatum, Princeton Mus. 11242; upper Eocene. Bi, Ba, Partly reconstructed skull ofa young Brontopsbrachycephalus, Nat. Mus. 4258; lower Oligocene. One-eighth natural size. In order to comprehend the extraordinary number and the extremely varied forms of the titanotheres that inhabited South Dakota in early Oligocene time we must first imagine the existence of a vast conti- nental region as the theater of evolution of these mi- grating animals, a region far larger than any of the comparatively small areas in which the fossil remains have been col- lected and which are shown by the black areas on Figure 373. A vast level or undulating country, consisting of great flat plains traversed by slow meander- ing streams, bounded on the west ,,- — "V by mountain ranges, valleys, and \ plateaus interspersed with active /' volcanic peaks but allowing free y' migration to the east, north, and /A, south — such was the environment ""' ' I \ of the Oligocene titanotheres. \^.'i; I'' J SECTION 2. INTRODUCTION TO [\ ■■"'' THE ANATOMY OF THE SKULL i AND THE DENTITION OF THE i OLIGOCENE TITANOTHERES I \ HORNS: TRANSFORMATION, ELON- 1 GATION Length oj the horns. — The grad- ual evolution of the horns in the Eocene titanotheres was followed by their more rapid evolution in the Oligocene titanotheres, until they became the dominant and central feature of the skull, con- ditioning its entire architectiu-e. With the development of the horns as powerful weapons are cor- related changes in the structure of the nasals, of the zygomatic arches, of the cranial vertex, of the occi- put, of the vertebral spines and in the entire anatomy of the anterior portion of the body. The primary divisions of the Oligocene titano- theres as determined by length of horn are as follows: Menodontine group (short-horned) : Teleodus, Brontops, Diploclonus, Allops, Menodus. Brontotheriine group (long-horned) : M egacerops {" Symborodon"), Brontotherium. First stage of development. — The horns in the Oligocene titano- theres, as in the Eocene Dolicho- rhinus and Protitanotherium, arise at the junction of the frontonasal suture, slightly in front of the orbits, overhanging the sides of the face (fig. 374). The primitive horn section is an anteroposterior oval. The longest diameter of all the earliest horn tips is parallel with the long axis of the skull. The anterior edge of the EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 445 oval extends into the sides of the nasals (N); the pos- terior edge of the oval subsides laterally toward the frontals, lacrimals, and maxillaries {M). Second stage of develop- ment. — A low transverse " connecting crest " arises at the median bases of the hornsand gives them a more or less distinct tri- lateral section consisting of an antero-internal face, an antero-external face, and a posterior face. These three faces are es- pecially distinct in spe- cies of Menodus, such as M. trigonoceras . In Mega- cerops, however, the con- necting crest is not typi- cally developed, so that no internal angle (C) appears {Megacerops acer, p. 545). Sections of the horn. — Each phylum takes on a characteristic horn sec- tion, which may be repro- duced by bending a cop- per or lead wire in the Figure 375 manner shown in Figure 376. The horn section is always cut at right angles to the perpendicular axis of the horn and as near the base of the horn as practicable. Each horn develops progressively a characteristic section at the base (fig. 399) and at the tip. The four chief types of horn sections are as follows: Short-horned: Anteroposterior oval, subtriangular, oblique oval, subtransverse oval : Brontops, Diploclonus. Subtriangular oblique, subtriangular transverse: Menodus, Allops. Long-horned: Subtriangular, suboval, transverse oval: Bronto- therium. Rounded, anteroposterior oval, flattened anteri- orly, rounded posteriorly: Megacerops. Forward shifting of horns. — The horns tend to shift forward and absorb the nasals. Thus the general correlation of horns and nasals is as follows: Short, triangular, oval horns, elongated nasals; Bron- tops, Menodus. Long, oval or rounded horns, abbreviated nasals: Bron- totherium, Megacerops. -Sections at base of horn in the six chief generic types of Oligocene titanotheres (B-G) and in the upper Eo- cene Prolilanolherium emargi- natum (A) A, ProtitanoiheriumemaTginatum, Princeton Mus. 11242; anteroposteriorly oval. B, Brontops dispar, Nat. Mus. 4290; rounded, obliquely oval. C, Diploclonus amplus, Yale Mus. 12015a (type); rounded, trans- versely widened, trihedral. D, Allops serotinus, Yale Mus. 42.')1 (type); angu- late, trihedral. E, Menodus giganteus. Am. Mus. 505 (neotype); rounded, trihe- dral. F, Megacerops acer, Am. Mus. 6348 (type): rounded, quadrate. G, Brontotkeriitm plutyceras, Harvard Mus. (type); transversely oval. All one-fifth natural size. In the extremely long-horned types, such as Bronto- therium, the horns shift forward until they overhang the anterior nares and finaUy the symphysis; they thus absorb the nasals but retain their base of sup- port on the greatly shortened maxillaries. Thus the nasal angle (iV) disappears, and the horns acquire a transverse oval section. Horns in females. — In skulls from the higher geologic levels of the Oligocene the difference between the horns of the two sexes is rather marked; in skulls from the lower Oligocene and from the Eocene the difference is less. There is reason to believe that the horns were at first alike in both sexes. In females the horns exhibit an arrested stage of development. This fact is most clearly shown by a comparison of two female skulls of Brontotherium (Am. Mus. 1005, 1006) with two male skulls (Am. Mus. 492, 1070). (See fig. 377.) In many females the horns are imperfectly ossified at the tips; in some they are pointed. In species of Menodus the "connecting crest" is more constant and more pronounced in males than in females, but in females of Brontotherium the connecting crest appears Figure 376. — Position of the standard sec- tions and contours of Oligocene titanothere skulls N, Nasal contour; M,median section of nasals and connect- ing crest; H, basal horn section; HN, oblique-longitud- inal section, nasals to horn tip; P, section across parietal verte.x; B, buccal section of zygoma. to rise almost to the summit of the horn, as in the female skull selected by Cope as the type of Menodus peltoceras ( = Brontotherium curtum) (fig. 478). The 446 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA male skull of M. giganteus (Am. Mus. 505, 1066, 1067; fig. 444) has stout triangular horns, whereas the female skull (Am. Mus. 506; fig. 445), although a very large specimen, has more slender horns, which are rounded in section, and has very slender canines. Similar sexual differences are observed in specimens of M. trigonoceras. Sport variations in the horns. — It is difficult to deter- mine whether the internal "hornlet," or branch horn, which appears on the inner side of the main horn in specimens belonging to at least three different phyla (Diploclonus, Menodus, Brontotherium) is a veritable progressive character — that is, a rectigradation — or merely a sport. This branching horn is believed to be a generic character of the animal called Diploclonus amplus by Marsh. Another example of internal Figure 377. — Male and female skulls of Brontotherium gigas A, Am. Mus. 1006, 9 ; B, Am. Mus. 492, -m3 P*, ap. by tr Sum of anteroposterior measurements of m'-m^ compared with sum of transverse measurements- 765 667 87 350 45 137 220 40X65 220 X 255 825 515 62 425 51 150 270 ■ 50 X 68 » 249 X 246 830 740 89 353 42 130 241 47X72 241X277 • Measurement taken from Am. Mus. 605. Lower molars. — The lower grinding teeth also indicate either the brachycephalic or the dolichoce- phalic proportions of the skull. This is especially witnessed in the third lower molar (PI. XXII), in which, for example, the relatively long, narrow form in Menodus contrasts with the relatively broad, robust form in Brontotherium. M3 is further dis- tinguished clearly in the different phyla by the form of the hypoconulid, or third lobe, which is more lophoid in the Menodus group, more crescentic in the Brontotherium group. Similarly the main crescents are somewhat more open in dolichocephalic molars and more closed or acute in brachycephalic molars. The cingulum is strongly developed on the lower grinders in members of the menodontine group and feebly developed or obsolete in members of the brontotheriine group. The molarization of the lower premolars proceeds step by step with the molarization of the upper premolars. Thus the premolars acquire the molar pattern slowly in the menodontine group and more rapidly in the brontotheriine group. The internal wall, especially of the third lower molars, develops sharp crests (metacristid, entocristid^ fig. 383), which are similar in form and position to those of certain other early perissodactyls, especially the paleotheres and chalicotheres; but, with the exception of Lamhdotherium, the titanotheres do not develop the metastylid and entostylid, cusps which in other perissodactyls arise by fissure of the metaconid and entoconid respectively. DEVELOPMENT OF THE SKUIL AND DENTITION Stage 1. — The earliest known stage (PL XXIII) is represented by a specimen in the Yale Museum, which is a lower jaw containing the deciduous teeth of a newly born animal. Three alveoli of deciduous incisors and two deciduous premolars (dp2, dp3)are in place; the latter are unworn. The third and last deciduous premolar had not yet erupted and much of it is buried in the jaw. The titanotheres, like many other ungulates, apparently had but three deciduous premolars on each side in the upper and the lower jaws. The position of the incisor alveoli was the same as in Teleodus avus (PL XIX, D) and suggests their identification as dii, di2, dis. The opposite incisors were separated in the midline, and dii lies much below the plane of di2. Perhaps this indicates a protrusile tongue. The first permanent premolar, Pi, is just emerging. Possibly the deciduous canine had been shed at an earlier stage. The deciduous premolars (dpa, dps) have heavy external cingula. The horizontal ramus of the jaw is very shallow; the ascending ramus relatively very heavy. Stage 2.- — Stage 2 is represented by a "calf" jaw with alveoli for three deciduous incisors and for the deciduous canines (Am. Mus. 510; PL XXIV, A), which is provisionally referred to Menodus giganteus. PROTOCONID HYPOCONUL •D ^ENTOCONID I METACONID PARACONID \ I METAC^RISTID | Figure 383. — Third left lower molar of Bronto- therium leidyi, showing the metacristid and entocristid Carnegie Mus. 93. One-half natural size It includes the alveolus of permanent pi of the left side. The deciduous premolars (dp2-dp4) are in place and slightly worn. They are more molariform than the permanent premolars that succeed them. Stage S. — Stage 3 is represented by a "calf" jaw of Menodus giganteus (Am. Mus. 509; PL XXIV, B). The root of i2 (?) is in place; the remaining front teeth are not preserved; the tip of the permanent canine is embedded in the jaw, and behind it is a root that may belong to pi; dp2-dp4 are in place; dp2 and dps are considerably worn, but not dp4. Permanent pi is in horizontal line with dp2, and although it is a very small tooth it is probably the one that is present in adult titanotheres. Mi lies nearly ready to cut the gum. Stage 4- — One of the yoimgest Imown titanothere skulls (fig. 384) is in the Musemn of the University of Wyoming (No. 4). It was collected by Mr. W. H. Reed in HeU's Half Acre, Natrona County, Wyo., from a low level in the Titanotherium zone. 452 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA The large size of the first true molar (ectoloph ap. 57 mm.) indicates a larger animal than 5. hracTiy- cephdlus; it is of a size equaled in B. dispar (Nat. Mus. 4290). The horn swellings are small knobs, not much larger than those of very old individuals of Manteoceras manteoceras. The horn swelling on each side consists of a protuberance on the anterior tip A2 Figure 384. — Development of jaws and teeth, stage 4 Skull and teeth of female calf, Univ. Wyoming Mus. 4, referred to Bronlops iispar. Skull and lower jaw, one-third natural size; Aj, upper teeth, one-half natural siz' lower teeth, one-half natural size. of the frontals, which is embraced in a corresponding excavation of the expanded posterior border of the nasals. The free portions of the nasals are short and rounded distally, with sharply decurved lateral borders. The sutures in the region of the horn — that is, the nasofrontal, lacrimal, and maxillary sutures — conform to the plan exhibited in the National Museum skull 4258 and in skulls of other young titanotheres. The upper jaw shows the permanent incisor i' (which is exposed in the specimen but was possibly beneath the gum in life) ; a tooth is just coming into place which is apparently p' of the adult; three deciduous premolars (dp^, dp', dp*) are in place. The lower jaw shows \2, (?) pi, and dp2, dps, dp4. M' and mi are buried in the jaws. Measurements of this specimen are as follows : Measurements of deciduous teeth 0/ Brontops dispar {Univ. Wyoming Mus. 4) Upper jaw Millimeters P'-dp< 132 Pi, ap. by tr 18X18 Dp2, ap. by tr 29X25 Dp3, ap. by tr 37X32 DpS ap. by tr 44X33 Dp*, tr. (across mesostyle) 40 Permanent m', ectoloph, anteroposterior. 57 Permanent m', ectoloph, height of meta- cone 45 + Nasals to middle of horn 75 Nasals, free breadth (estimated) 55 Pmx to condyles (rough estimate) 305 Lower jaw Front edge of symphysis to angle 284 Height condyle to angle 145 P,-dp4 128 Dpi, ap. by tr. (trigonid) 11X10 Dp2, ap. by tr. (trigonid) 31X15 Dp3, ap. by tr. (trigonid) 37X20 Dp4, ap. by tr. (trigonid) 49X25 Stage 5. — A more advanced stage is shown in a young jaw (Carnegie Mus. 124; PI. XXIV, C) referred provisionally to Brontops dispar. The permanent incisors ii and 12 are just coming in, while the deciduous incisors and canines have probably been shed. The permanent canine lies j ust below the surface. The tooth designated ? dpi although associated with mUk teeth, appears to be the permanent pi. The milk molars dpo-dp^ are worn. Ml is just protruding. M2 lies buried in the ascending ramus, below the coronocondylar sinus. Stage 6. — A later ontogenetic stage is illustrated in a remarkably complete skull and jaw in the Carnegie Museum (No. 116), which were de- scribed by Hatcher in 1901 (1901.1; figs. 385, 386). The specimen was found on Warbonnet Creek, Sioux County, Nebr., near the base of the Titanotlierium zone. The reference to Bron- tops hrachycepJialus is provisional. In the side and top views (fig. 385) it is seen that the horns are formed by the overgrowth of the frontals upon the nasals, as in the Eocene Man- teoceras. The lacrimal is expanded, and its outer ridge is continuous with the external ridge of the horn. The parietals extend forward upon the frontals. The interparietal is apparently distinct. The occiput is shown in Figure 386. EVOLUTION OF THE SKULL AND DENTITION OE OLIGOCENE TITANOTHERES 453 The construction of the skull conforms in its under- lying plan to that of Eocene titanotheres, differing chiefly in the shortening of the face, the lengthening of the midcranium, and the widening of the skull top. shed. The deciduous premolars (dp-, dp', dp'') are well worn. M' is fully in place. P^ p^ p' lie embedded; permanent p| are apparently represented in the well-worn teeth immediately in front of dpf. In FiGUBE 385. — Development of jaws and teeth, stage 6 Broniopsf brachycephalus?, Carnegie Mus. 116: young skull and Jaw. One-fourth natural size. After Hatcher. Level near the base of the Chadron formation (Titanotherium zone). Ai, Side view; An, top view. Dentition. — The deciduous teeth were about to be shed, and their roots protrude widely. In the upper dentition the deciduous incisors, di-(?), di'(?), are small and round topped. Behind them the permanent canine lies buried. The deciduous canine has been the lower jaw the milk incisors were probably in the gums; at least their alveoli must have been shallow. The permanent canine is beginning to come in. The milk molars are well worn, and mi is in place. P2, P3, p4 lie on descending levels in the jaw, the first 454 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Hearing eruption. M2 also lies embedded, and nis is beginning to form. Hatcher's original description of the skull is as follows : When viewed from above the frontals are much the more conspicuous elements. They are bluntly rounded posteriorly and are projected far backward beneath and between the lateral anterior projections of the parietals. Anteriorly the frontals are continued into two long lateral projections which extend beyond the orbits, overlie the posterior and lateral margins of the nasals, and give rise to the pair of horn cores that form such characteristic features in the Titanotheridae. The nasals are arched superiorly, concave inferiorly, with rather long posterior extensions interposed between the frontal horns. In the present specimen the nasals are very thin along their inner margins but much thickened externally and posteri- orly, where they give the chief support to the horns. They are slightly shorter than the premaxillaries and somewhat emarginate anteriorly. The parietals are not so broad as the frontals. They are deeply emarginate anteriorly and posteriorly, where they are separated by the interparietal portion of the supraoccipital Figure 386. -Occiput of young skull of Brontops brachycephalus? Carnegie Mus. 116. One-fourth natural size. The exoccipitals meet above the foramen magnum. The wide supraoccipital affords attachment to the power, ful ligamentum nuchae, the recti capitis lateralis, complexus, and other neol;: muscles. much as in Equus. From the above description it will be seen that the anterior border of the parietals overlies and incloses laterally the posterior border of the frontals, while the anterior border of the latter has a like articulation with the nasals, the relative position of these bones being similar to that of the shingles of a roof. The zygomata are rather broadly expanded and are composed about equally of the malars and squamosals. When seen from the side the skull appears somewhat low, with an abbreviated facial region and a rather long posterior portion. The occipital crest and anterior frontal regions are each somewhat elevated. The nasals appear rather deep and send down the inferior and posterior projection, which articu- lates by suture with the superior border of the maxillary. The infraorbital foramen lies wholly within the maxiUary. The max- illonasal suture is opposite the middle of the orbit. The lac- rimal is rather large. The malar is long and thin; anteriorly it has an extended contact with the maxiUary and posteriorly with the squamosal portion of the zygoma. The squamosal rises high above the external auditory opening and overlaps the side of the parietal throughout most of its length. There is a long, thin, transversely expanded postglenoid process and a shorter and proportionately stronger post-tympanic process of the squamosal. The tympanic is absent, having been lost from the present specimen. The periotic is present, and its para- mastoid portion appears externally between the post-tympanic and parocoipital process. Just above this there is another small bone which is continued into a long pointed process inserted between the exoccipital and the squamosal and pa- rietal; it probably became coossified later with the periotic, but in the present specimen it is seen as a separate bone, as shown in Figure I and in Plate VII. [See figs. 385, 386.) The exoccipitals are rather large and support the paroccipital process and the occipital condyles. The latter are ossified from two distinct centers, the articular portions bearing distinct epiphyses, as shown in the accompanying figures. Seen from behind the occiput is low and broad. The condyles are widely separated by the foramen magnum, which is much broader than deep. The condyles are entirely supported by the exoccipitals, which rise and meet in the middle line above the foramen magnum, entirely excluding the supraoccipital from any part in the formation of the superior border of that opening. The supraoccipital is very broad and low. The occipital crest is nearly flat above but broadly emarginate posteriorly. (See fig. 1 of the text, and PI. VIII.) Inferiorly the palate is seen to be formed anteriorly by the very short premaxiUaries, for the most part broken away in the present specimen, and by the maxillaries, between the posterior lateral extremities of which are inserted the palatines. These form the posterior median portion of the roof of the palate and send backward on either side a lateral projection along the inner sides of the maxillaries and pterygoids, which are continued nearly to the posterior end of the basisphenoid. The vomers are continued far back as a thin plate resting upon the pre- and basisphenoids and sending downward a thin, knifelike median bony septum. The basisphenoid in the present specimen is entirely free from the basioccipital, the suture being open, and the basioccipital had dropped out and was lost before the specimen was found. The absence of the basisphenoid and tympanic bones makes it impossible to describe and locate the various foramina of this region of the skull. Stage 7. — Still more advanced is the stage shown in Am. Mus. 497 (PL XXV, A), deciduous and permanent upper teeth of Menodus giganteus. The first tooth of the series is the permanent pS as shown by its exact agreement in measurements and in characters with the first premolar n adult skulls of M. giganteus (Am. Mus. 505, 506). PhylogeneticaUy this tooth may have been forced into association with the deciduous pre- molars dp^, dp' through the abbreviation of the muz- zle region and the consequent crowding backward of the permanent canine. In this specimen the perma- nent canine, which is still buried in the jaw, lies closely appressed against the second permanent premolar. The serial homology of this tooth as p^ is also estab- lished by its measurements, as compared with the adult Menodus giganteus. In this as in many other mammals there was probably no deciduous predecessor of p^ In the specimen here figured m' is coming into place. The identity of these teeth is also established by comparison with the adult M. giganteus, so that there is no doubt that dp*, dp', dp^ are correctly identified. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 455 Comparative measurements, in millimeters, of deciduous and permanent dentition of species of Menodus and Brontops [The measurements given in the first five columns were made on the ectolophl P> pj pS Ml Dp! Dp> Dp< Dpi M, P>-dp< Pi-dp4 22 23 32 35 45 46 72 71 77 76 M. giganteus, Am. Mus. 506, 9 63 45 46 »47 47 51 67 65 66 26 21 40 46 174 58 62 62 66 136 160 40 68 138 48 130 B sp ' 'Yale Mus "135 B. sp.?, Univ. Wyo. 4 ._- - 18 57 29 39 47 49 57 132 128 SUMMARY OF THE REPLACEMENT OF THE TEETH IN OLIGOCENE TITANOTHERES The following is a summary of the order of succes- sion of the upper and lower teeth as observed in the six juvenile stages represented in Plates XXIII and XXIV and in Figures 384 and 385. 1. The three deciduous incisors (di-^, f, f) have the same relative position as the permanent incisors (Ml i> f) ^ Teleodus avus. They were shed very early. 2. The deciduous canines, known only from their alveoli in one specimen (PL XXIV, A), were shed perhaps even earlier than the deciduous incisors (PI. XXIII, XXIV). 3. No evidence of deciduous predecessors of p' and Pi has been observed either in Eocene or in Oligocene titanotheres, and in these, as in other ungulates, there were probably only three and not four deciduous pre- molars. 4. The permanent p^ came into place soon after the deciduous dpf , f and functioned with the decid- uous series. 5. The first true molars (m^) came into place be- fore the deciduous premolars had been replaced. At later periods mf and mf came into place successively, so that in old animals m-^ is greatly worn, whereas mf is but little worn. 6. The fourth premolar (p|) follows the general mammalian rule of coming in late. In the Oligocene titanotheres, as in many other mammals, the second, third, and fourth deciduous premolars (dpf, f, f) are much more molariform than the permanent premolars (pf, |, |) which re- place them. Thus molarization of the deciduous premolars is observed even in the middle and lower Eocene titanotheres {LamMotherium, Palaeosyops) and is equally characteristic of the Oligocene titano- theres. In Menodus giganteus (Am. Mus. 497, PI. XXV, A) the third deciduous premolars (dpf) are like molars, dp' and dp* having prominent mesostyles and large, distinct tetartocones. 101959— 29— VOL 1 32 Figure 387. — Stages of wear in the adult upper grinding teeth of Oligocene titanotheres The following specimens, all drawn to the same length, show the progressive degrees of wear from the young adult (X) to the very aged (XV) ontogenetic stage of the dentition: X, Allops crassieornis, Nat. Mus. 4289 (type); XII, Brontops roiustus, (type), Yale Mus. 12048; XIII, Menodus giganteus. Am. Mus. 505; XIV, Allops serotinus. Am. Mus. 620; XV, Brontops brachycephaJus, Nat. Mus. 4947. (See p. 456 ) 456 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA STAGES OF WEAR OF THE ADULT GRINDING TEETH Seven early and adolescent stages in the ontogeny of the dentition have been described above. In the eighth and ninth stages (adolescent) the deciduous premolars (dp^ dp^ dp'') are gradually replaced by the permanent premolars p^, p^, p*. In the tenth ontogenetic stage (X) of the young adult the internal cusps of the second and third but not the fourth premolars are beginning to show the dentine through the enamel surface; the last molar is but little worn. In the very aged fifteenth ontogenetic stage (XV) XII. Twelfth ontogenetic stage: Broniops robustus Yale Mus. 12048 (type). X. Tenth ontogenetic stage: Alloys crassicornis, Nat. Mus. 4289 (type). AGE AND OTHER CHARACTERS COMMON TO BOTH SEXES OF TITANOTHERES OF ALL STRATIGRAPHIC LEVELS Age characters. — The age characters are naturally much more conspicuous in males than in females. In both sexes the adults of one geologic generation follow the general law of anticipating the advanced muta- tions or specific stages reached by adults of higher geologic levels. Thus the variability of a tooth on a Figure 388. — Skull contours showing extreme divergence between Menodus giganteus (A), a final term of the menodontine series, and Brontotherium platyceras (B) , a final term of the brontotheriine series In Menodus the opposite borders of the cranial roof diverge anteriorly, the horns are short and trihedral, the zygomata but little expanded, and the occiput not greatly produced backward. In Brontotherium the opposite borders of the cranial roof are nearly parallel, the horns very long and flattened, the zygomata widely expanded, and the occiput greatly produced backward. One-tenth natural size. nearly the whole enamel surface of the crowns of p'-m' inclusive has been worn away, so that the dentine is very widely exposed; in m^ both the pro- tocones and hypocones are much worn. The inter- vening stages show intermediate conditions, as follows (fig. 387) : XV. Fifteenth ontogenetic stage: Brontops brachyce- phalus, Nat. Mus. 4947. XIV. Fourteenth ontogenetic stage: Allops serotinus, Am. Mus. 520. XIII. Thirteenth ontogenetic stage: Menodus giganteus, Am. Mus. 505. lower geologic level is prophetic of its absence on a higher geologic level. This variability is especially displayed in retrogressive structures such as the degenerate incisor teeth in the Brontops series, as is shown by the following formulas: Juvenile incisors, I{i^ . Adult incisors, I|^}- . The incisive teeth tend to drop out in the adults, as observed in the type of Diploclonus tyleri Lull. Thus among the age characters are the foUowrng: (1) Increasing size and rugosity of the skull, arches, horns, and nasals; (2) distal expansion and rugosity of EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 457 the tips of the nasals; (3) loss of variable and vestigial teeth, incisors and premolars, in old age; (4) union of the anterior caudal vertebra with the sacrum to form four sacrals (Hatcher, 1893.1, p. 217). In general, growth is in a high degree differential — that is, proportions constantly change, as, for example, in the gain of width over length, in the rapid increase of the horns in length, and in the relative gain in the length of the nasals, although only in a few specimens have we sufficient material to measure these differ- entials in growth. Abnormal sport and reversional characters. — Among the abnormal characters may be observed the following : (1) Reversional or abortive protoloph and metaloph on the superior premolar teeth; (2) abnormal redupli- cation of tetartocones on superior premolar teeth ob- served in specimens of Brontops dispar, Allops cras- sicornis, Menodus varians; (3) progressive rectigrada- tional or anomalous reduplication of horns as observed in specimens of Diploclonus, Menodus, Brontotherium. SECTION 3. DIVISION OF THE OLIGOCENE TITANO- THERES INTO GROUPS AND SUBFAMILIES CHARACTERS OF THE SKUII AND TEETH OF THE MENO- DONTINE AND BRONTOTHERIINE GROUPS The following study of the characters of the skull and teeth enables us to divide all the highly varied forms of Oligocene titanotheres into two great groups, the menodontine and the brontotheriine, which sepa- rated from each other in Eocene time. (See p. 467.) These characters, which are much more pronounced in male than in female skulls, are seen to be the direct and indirect results of one or other of the opposing principles of skull and tooth transformation described on pages 254-262, 450, namely: 1. Dolichocephaly and dolichopy versus brachy- cephaly and brachyopy; cyptocephaly. 2. Differential transformation of canines (p. 448), premolars, and molars. (See figs. 405, 406.) 3. Differential development of horns, nasals, orbits, zygomata, etc. (See figs. 389-394.) Primary groups oj the Oligocene titanotheres MeDodontine group (Teleodus, Brontops, Diplo- clonus, Allops, Menodus) Brontotheriine group (Megacerops ("Symborodon"), Brontotherium) Canines - Opposite grinding series as seen in psflate view. Upward flexure of premolars as seen in side view. Antorbital region of skull Premolar series (length) Internal cusps of upper premolars Ectolophs of grinding teeth Hypocone of m^ Internal cingula External cingula Zygomata Horns Malar-Iacrimal bridge over infraorbital foramen. Anterior narial cavity Backward prolongation of occiput behind zygomata. Jaw Pi Convexity, top of parietals Orbit Skull vertex Skull vertex, side view Proportions of molar teeth Primitively pointed, recurved, not closely approximated toward median line. Not strongly arched anteriorly toward median line. Slight to moderate Relatively elongate Relatively longer Steep-sided (Menodus); moderately so (Brontops) . Somewhat more vertical Often surrounded by cingulum Sharp on premolar teeth Usually pronounced Slight to heavy, often deep Short, diverging obliquely outward and forward . Usually very broad Broad and open Moderate Angle sharply produced backward Somewhat more elongate Absent Medium to large Divergent anteriorly, convergent pos- teriorly. More concave More elongate Primitively short, bulbous, with swelling posterior cingulum, closely approxi- mated. Strongly to very strongly arched toward median Hne. Very pronounced. Abbreviated (Brontotherium) to very abbreviated (Megacerops) . Relatively shorter. Very low, robust, subcircular. Sharply depressed to crown. Very heavy, triradiate, continuous with cingulum. Often less developed and rounder. Reduced or absent. Broad to extremely expanded and flat- tened. Long, usually more erect. Usually very narrow. Becoming very high and narrow. Moderate to extreme. Angle less produced backward, more slender. Somewhat abbreviate. Pronounced. Medium to small. More or less parallel. More convex. More expanded transversely. 458 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Features of the jaws. — As most of the jaws found were entirely dissociated from the skulls, it is difficult to determine the generic and even more so the spe- cific references of many separate jaws. Observation should be directed first to the prevailing dolichoce- phalic or brachycephalic proportions, to the shallow- ness or depth of the ramus, to the slenderness or massiveness of the angular region, and especially to the depth or shallowness of the chin region. In general, members of the menodontine group have a deep symphyseal or chin region, whereas members of the brontotheriine group have a shallow chin. The generic diagnosis of remains that include a full series of teeth is relatively simple. Specific diagnosis is partly dependent on size. In order to associate a lower jaw with a skull the measurement should be Characters of the jaw that distinguish members qf the menodontine group from members of the brontotheriine group taken from the glenoid cavity to the front of the superior canine and from the mandibular condyle to the posterior face of the inferior canine. If all the other characters are properly determined and the progressive stages of the superior and inferior pre- molars correspond, this method of associating the jaws with the skulls is reliable. This method, however, can be used to advantage only on skulls and j aws that are not distorted by crushing. Usually the most practicable measurements are the following: Upper: Front face of canine to middle of posterior fossa of m^ (for hypoconulid of ma). Lower: Rear face of canine to tip of hypoconulid of ma. The following table shows the chief characters of the jaw that distinguish the members of the two groups : Horizontal ramus Symphyseal region in side view below men- tal foramen. Region of angle Incisors Pi in fully adult jaws- Diastema in front of pi Canines __ External cingulum of canines, premolars, molars. Upward flexure of premolar series. Menodontine group Deep Convex.. Barely pro- duced. I3 Present Present Slender. Not sharply defined. Shallow Very shallow.. Usually produced downward. Usually present Present, wide -. Slender to short, stout. Present in early types; lost in B. robustus. Slight or moderate. Shallow. Shallow. ? Produced, truncate. Present Present Slender to short, stout. Intermedi- ate. Slight or moderate. Allops Intermediate -. Intermediate to con- vex. Intermediate - l2_l.— -. Present or absent Absent Conic to compressed anteroposteriorly. Present-- - Very slight Deep... Fuller.. Produced backward and downward into a convex elbow. Pos- terior border obli- que. lo (typically) Present or absent Absent Conic - Strongly marked- Brontotheriine group Short, massive. Broad, posterior border vertical. 1 lo ?Absent.. 7 Absent. (?) Brontotberium Massive, deep poste- riorly. Very shallow. Broad, posterior border often vertical. Present or absent. Absent. SwoUen at base with massive p s t e r i or cingulum. Absent. Typically pronounced. EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 459 Allops walcotti Figure 389. — Skulls of the menodontine group Side view. One-twelfth natural size. Subfamily Brontopinae: A, Brontops bracJiycepMlus, °, a very small and primitive member of the Brontops phylum, with mcipient horn swellings, long nasals, and slender canines; Chadron A. B, Brontops dispar, an intermediate stage with moderately developed horns; Chadron B. C, Brontops robustus, final stage of this phylum; a very massive brachycephalic skull with stout forward directed horns, short, thick nasals, short, heavy occiput, and expanded zygomata; canines short and thick; Chadron C. D, Viplo- clonus bicornutus, referred to this genus partly because of the accessory horn swelling (h'). The skull contour suggests that of Menodus. E, Diploclonus amplus, a massive short skull recalling Brontops robustus but possessing an accessory horn swellmg and verv short nasals. Occiput long. Subfamily Menodonfinae: F, Allops walcotti, a primitive dolichocephalic form, supposed to be ancestral to the Allops phylum; Chadron A G, Allops marshi, in general contour intermediate between Brontops dispar (B) and Menodus (H), thought to be related to Allops serotinus and Allops crassicornis. H, Menodus giganteus, latest stage of the Menodus phylum; uolichocepnalic, horns trihedral in basal section, nasals long, zygomata not expanded, canme long, cheek teeth with sharp external cingula; Chadron O. In all members of this group the skull top in side view is deeply concave. The cheek teeth almost always have sharp external cingula. 460 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Megacerops copei " Megacerops acer Figure 390. — Skulls of the brontotheriine group Side view. One-twelfth natural size. Subfamily Brontotheriinae: A, Srontotherium leidyi, a primitive member of the Brontotkerium phylum with short horns transversely oval in section, long nasals tapering and decurved, canines short and swollen, two stout incisors, and cheek teeth with but little external cingulum; Chadron A. B, Brontotkerium gigas, a progressive brontothere with long erect horns transversely oval in basal section, nasals of intermediate length, zygomata widely expanded, occiput long, and a marked parietal convexity; Chadron C. G, Brontotherium curium, a highly advanced brontothere with very long horns far in front of the orbits and much flattened anteroposteriorly, nasals short, skull top long, and zygomata much expanded; Chadron C. D, Brontotherium (.peltoceras) curtum, a female brontothere with skull short and massive, horns very thick, high connecting crest, and nasals short. Subfamily Megaceropinae: E, Megacerops copei, long horns thick at the base, nasals long and thin, canines very short and swollen, premolar series upturned, and zygomata massive. F, Megacerops acer, horns of moderate length but very thick at the base, nasals short and thick, premolar series upturned, zygomata heavy, midparietal swelling prominent. Compared with the Menodontinae, members of this group usually have the skull top less deeply concave, and many have a parietal swelling. The horns are commonly transversely oval in section, the premolar series upturned anteriorly, and the canines in males short and swollen. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHBRES 461 Diploclonus amplus Allops serotinus Menodus giganteus Allops walcotti Allops marshi Brontops brachycephalus Brontops dispar FiGUBB 391. — Skulls of the menodontine group Top view. One-twelfth natural size. Subfamily Brontopinae: A, Brontops brachtjcephalus, a primitive stage witli horns very short and nasals long and tapering. The specimen is a female, and the zygomata are not much expanded; the skull top is rather slender, although the skull as a whole Is broad. B, BToniops dispar, an intermediate stage with short horns rounded in section, nasals and frontals broad, and zygomata expanded. The midparietal crest is constricted, as in many other members of this family. C, Diploclonus amplus, a highly specialized brachycephalic stage, resembling Brontops robustus in general proportions. The horns are w ide and flattened at the base and pointed at the tips and bear accessory hornlets on the antero-internal portion. The nasals are wide and short. Subfamily Menodontinae: D, Allops walcotti, a primitive dolichocephalic type (thought to be ancestral to the Allops phylum) with small horns elongate oval in section, nasals long and tapering, zygomata slender, and skull top narrow. E, Allops marshi, horns widely trihedral in basal section and directed outward and upward, skull top fairly broad, zygomata gently expanded. F, Allops serotinus, a specialized stage with long outward-directed horns, wide, short nasals, wide frontals, and moderately stout zygomata. G, Menodus giganteus, the terminal stage of the Menodus phylum, with skull dolichocephalic, skull top long, zygomatic expansion moderate, nasals long and distally wide, horns sharply trihedral in basal section, the connecting crest lying in the plane of the posterior face of the horns. 462 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA B i_^.XT/^XX:_/ A ic/^^\3 c Brontotherium (peltoceras) curtum ?q Brontotherium leidyi ?? Brontotherium curtum Figure 392. — Skulls of the brontotheriine group Top view. One-twelfth natural size. Subfamily Brontotheriinae: A, Broniotheriitm leidyi, a primitive stage with skull top long, small horns transversely oval at the tips, nasals long and tapering, frontal region wide, zygomata not expanded; Chadron A. B, Brontotherium (peltoceras) curtum, skull top long but zygomata arching widely, short massive horns confluent with the high connecting crest and pointed at the tips, nasals reduced and pointed. C, Brontotherium curtum, a very large male skull with widely expanded zygomata, widely flaring flat-oval horn.s, very short nasals, wide frontal region, occipital pillars widely expanded transversely. Subfamily Megaceropinae: D, Megacerops acer, skull short with widely expanded zygomata, horns cylindrical with but little connecting crest, nasals short and wide, skull top wide with parallel edges. E, Megacerops bucco, 9; resembles in general the preceding type but has larger horns. F, Megacerops bucco, J , a large animal with greatly expanded zygomata and rather feeble cylindrical horns. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 463 ^ Allops serotinus ■"■ Brontops brachycephalus " Brontops robustus Figure 393. — Skulls of the menodontine and brontotheriine groups Palatal view. One-tweirth natural size ■ ^ , „ i Menodontine group: A, Brontops brachycephalus, brachycephalic, grinding teeth of moderate width. B, Bronlops robustus, skull very large and brachycephalic, grinding teeth wide, incisors and canines massive. C, Allops serotinus, skull more elongate, grmding teeth of inter- mediate proportions. D, Menodus giganteus, dolichocephalic, grinding teeth elongate, tooth rows rectilinear. Brontotheriine group: E, Megacerops bucco, skull base very brachycephalic, grinding teeth broad, tooth rows curvilinear, fourth premolar sub- molariform, canines small. F, Brontotlierium curtum, skull very large, skull base long but tooth rows curvilmear, grmdmg teeth wide, fourth premolar submolariform. 464 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Brontops dispar Diploclonus amplus Allops walcotti Figure 394. — Skulls of the menodontlne and brontotheriine groups Front view. One-twelfth natural size. Meuodontine group: A, Brontops dispar, horns short and thick, suhcurcular in section and pointed, nasals of moderate size, zygomata moderately expanded, lateral incisors present, canines relatively long and conical. B, Brontops robustus, skull massive, horns comparatively short and transversely expanded, with thick rugose tips, nasals short and robust, zygomata heavy, two large incisors, canines short, thick, and conical. C, Diploclonus amplus, horns divergent, very wide at base and rapidly narrowing to the pointed tips, an accessory hornlet, nasals small, zygomata expanded. D, Allops walcotti, skull small and slender, horns very small, nasals broad, zygomata deep and slender. E, Allops serotinus, long divergent horns narrowing but little toward the tip, zygomata of moderate size, canines slender and pointed. F, Menodus giganteus, long divergent horns narrowing rapidly to pointed tips and trihedral in section, nasals expanded distally, zygomata relatively small and deep, canines (not shown) elongate pointed, incisive border edentulous. Brontotheriine group: O, Megacerops copei, J, horns long, erect, and cylindrical, with no connecting crest, nasals thin, zygomata not widely expanded (diminished by crushing), canines small, rounded, and close together, premolars upturned anteriorly. H, Brontotherium gigas, cf , skull very wide, zygomata enormous, horns long and wide, nasals of intermediate length, two incisors on each side, canines short and swollen. I, Brontotherium curium, 9?> horns short but very broad and massive, with high connecting crest, nasals short and narrow, zygomata not widely expanded, canines swollen. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHBRES 465 CHAEACTERS AND EEIATIONS OF THE SUBFAMILIES Each of the major groups described above divides into two separate branches to which Osborn gives the rank of subfamilies, as follows : Menodontine group (short-horned) : Brontopinae : Mesaticephalic (in females) to brachy- cephalic. Incisor teeth persistent. Menodontinae : Mesaticephalic, stenocephalic. In- cisor teeth reduced or wanting. Brontotheriine group (long-horned) : Megaceropinae : Brachycephaho. Incisor teeth re- duced or wanting. Brontotheriinae : Mesaticephalic to brachycephalic. Incisor teeth persistent. Each of these subfamilies includes one or more phyla, which in turn may embrace one or more genera. PolypTiyly. — The fact that the Ohgocene titano- theres separated into four subfamilies before the be- ginning of the Ohgocene epoch has been shown both by the distinct connection of two of these subfamilies with different Eocene ancestors and by the marked differences between titanothere remains that are found at the very lowest levels of the Titanotherium zone. Here the ancestors of the Menodontinae, of the Brontopinae, and of the Brontotheriinae are found to be quite separate and distinct. No Megacero- pinae have yet been found at these low geologic levels of the Oligocene. Summary of distinctions. — The means of distin- guishing the numerous branches and sub-branches of the great family differ somewhat from those em- ployed to distinguish the Eocene branches from one another. The proportions of the head and of the zygomatic arch, whether dolichocephalic or brachy- cephalic, still remain a distinguishing characteristic. Owing to the buccal expansion of the zygomatic arches the males of Menodus are technically mesati- cephaUc or even sub-br achy cephalic rather than "dolichocephalic," in spite of the fact that the skuU in general appearance and in form of the teeth is long and narrow, especially in contrast with the wide skull and grinding teeth of Brontops, Megacerops, and Brontotherium. The narrower Oligocene skulls may therefore be described as stenocephalic. As the horns become the dominant feature of the skull the main line of division first arises between the short-horned and long-horned titanotheres. The typi- cal shape of the horns, whether triangular, rounded, oval, or flattened, also becomes of very great value. With these weapons of offense are developed the gigantic swellings of the zygomatic arches by which the breadth of the skull as a whole is measured and cephalic indices are determined. A further differen- tiation is found in the presence or absence of incisor teeth and in the shape, size, and offensive character of the canine tusks. Wide divergence is seen also in the process begun in upper Eocene time — namely, the molarization of the premolars, or their transforma- tion into the molar pattern. In some lines of descent this process is accelerated, and in others it is retarded. Another distinction is in the development of the cingulum on the grinders. FiGtTBB 395. — Lower jaws of the Brontotherium phylum A, Brontotherium leidyi, Carnegie Mus. 93 (paratype); B, B. leidyi. Am. Mus.516; C, B. hatcheri. Am. Mus. 1070; D, B. gigas, Yale Mus. 12009 (type); E, B. gigas elatum, Yale Mus. 12061 (type of Titanops elatus); F, B. medium. Am. Mus. 1051. All one-twelfth natural size. 466 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Figure 396. — Lower jaws of the Brontops and Menodus phyla A, Brontops brachycephalus, Am. Mus. 1495; B, B.dispar, Nat. Mus. 4941 (type); C, B. robustus, Prince- ton Mus. 10061; D, B. robustus, Yale Mus. 12048 (type); E, Menodus torvus. Am. Mus. 6365 (type); F, M. trigonoceras , Nat. Mus. 4745; Q, M. giganteus, Am. Mus. 506; H, M. giganteus. Field Mus. P 5927. All one-twelfth natural size. Figure 397. — Lower jaws of the Diplo- clonus and Allops phyla A, Allops walcotti (?), Nat. Mus. 4247; B, A. marshi. Field Mus. P 6900; C, A. angustigenis, Ottawa Mus. (cotype); D, Diplodonus bicornutus. Am. Mus. 1476 (type); E, D. tyleri, Amherst Mus. 327 (type). All one-twelfth natural size. Figure 398. — Heads of Oligocene titanotheres, showing proportions, lip structure, and horns Modeled by Charles R. Knight. A, Brontops (bracbycephalic); B, Menodus (dolichocephalic); C, Megacerops (hyperbrachycephalio); D, Broniotherium (brachycephalic). EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES Groups and subfamilies of Oligocene titanotJieres 467 [See fig. 697, p. Menodontine group: Persistently short-homed titanotheres; teeth with persistent cingula i broad; grinding Brontotheriine group: Originally short-homed, progressively long-homed titano- theres; nasals pointed, reduced; grinding teeth with cingula reduced SUBFAMILY BBONTOPINAE Descendants of the Eocene Manteoceras and Protitanotherium. Subbrachy cephalic to hyperbrachy cephalic. Horns suboval, subcircular, transversely oval. One to two pairs of persistent incisor teeth. Premolar transformation retarded. Genera: Teleodus, Brontops, Diploclonus. SUBFAMILY MENODONTINAE Descendants of unknown Eocene ancestors, possibly related to Tehnatherium. Mesaticephalic to dolichocephalic and to brachycephalic. Horns typically subtriangular. Incisor teeth vestigial or reduced to one pair. Premolar transformation not retarded. Genera: Allops, Menodus. SUBFAMILY MEGACEROPINAE Descendants of unknown Eocene forms, possibly of Rhadinorhi- nus. Brachycephalic to hyperbrachycephalic. Horns rounded and without connecting crest. Incisor teeth reduced or vestigial. Premolar transformation accelerated. Genus: Megacerops. SUBFAMILY BEONTOTHBRIINAE Descendants of unknown Eocene forms. Mesaticephalic to brachycephalic. Horns transversely oval and progressively flattened. Incisor teeth persistent. Premolar transformation accelerated. Genus: Brontotherium. The members of these subfamilies are more or less clearly connected with ancestral forms in the Eocene, as shown in Figures 402-409. Further exploration, however, is needed to bridge over securely these lines of descent. Our present knowledge is about as follows : Genetic relations of Oligocene to Eocene titanotheres Eocene titanotheres OUgoceue titanotheres Subfamily Palaeosyopinae. Becoming entirely extinct in middle Eocene time. Subfamily TelmatheriLnae. Exhibiting some resemblances to the Oligocene Menodontinae but differing widely in the presence of large incisor teeth. Subfamily Manteoceratinae. Including two stages, Manteo- ceras and Protitanotherium, which exhibit _ many resem- blances to the Brontopinae but differ in the possession of very broad nasal bones. Subfamily Dolichorhininae. Composed chiefly of generic forms that became extinct in the upper Eocene but early gave off one branch. Subfaraily Rhadinorhininae, which shows some resemblance to Megacerops. Subfamily Diplacodontinae. Including the genera Diplacodon and Eotitanotherium, of uncertain relationships with the lower Oligocene Menodontinae and Brontotheriinae. No known descendants of the Palaeosyopinae. Subfamily Menodontinae. Exhibiting some resemblances to the Eocene Telmatheriinae; no known connecting forms. Subfamily Brontopinae. Including Teleodus, Brontops, and Diploclonus, which show many resemblances to the Eocene Manteoceratinae. Subfamily Megaceropinae. Including Megacerops, the "Sym- borodon"of Cope, which shows certain resemblances to Rhadi- norhinus and other very strong resemblances to Bronto- therium. Subfamily Brontotheriinae. Showing resemblances to the Eocene Diplacodon, Eotitanotherium, Rhadinorhinus. Direct Eocene ancestors unknown. Brontopinae. — The more heavily built titanotheres of the genus Brontops exhibit close resemblances to the Eocene Manteoceras and Protitanotherium. They pass through the lower Oligocene species of Teleodus and exhibit a wide adaptive radiation into the genera Brontops and Diploclonus. In general they have short, robust limbs, very broad skulls, and short, rounded or pointed horns and are provided with one or two pairs of cropping teeth. The incisors have rounded crowns; the canines are roundly pointed; the premolars are very slow in developing the second internal cones. Short limbs and broad, spreading feet give them a singularly graviportal and brachy- podal character, whtch is analogous to what is known of the limb structure of Manteoceras, of the Eocene. Menodontinae. — The long-limbed relatively cursorial animals typified by the genus Menodus agree with the Telmatheriinae in the very robust development of the 468 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA canines and of the grinding teeth but differ in the marked degeneration of the incisor teeth. In the loss of incisors they parallel the Megaceropinae. In contrast to the Brontops series the limbs and arches are slender, the bones of the girdles.are relatively high and narrow, and the feet are of the elongate, dolichopodal type. Thus the menodonts are relatively cursorial in contrast with the typically graviportal brontopines. Brontotheriinae. — The gigantic titanotheres of the subfamily Brontotheriinae appear to be related either to the Diplacodon or to the Ehadinorhinus with the elongation of the horns. The cheek teeth are less elongate than in Menodus and have more rounded crowns and nearly obsolete cingula, well fitted for cutting and crushing coarse vegetation. Megaceropinae. — The Megaceropinae are much smaller animals than the brontotheres, distinguished by skulls of intermediate proportions, with tall, rather slender and cylindrical horns placed well forward above the eyes. The incisors are absent. It appears probable that the snout was narrow and terminated in a pointed Up like that of the black rhinoceros of Africa. The face was very short, narrow, and upturned, bring- MENODUS MEGACEROPS BRONTOTHERIUM bnuJiycefhalus 'brndiyc^Jtalus MtngoruKems FiGUKE 399. — Sections at base of horn in five principal lower Oligocene phyla of titanotheres, arrang according to ascending geologic levels Drawn to the same scale. phylum of the upper Eocene.^^ They are distinguished by the precocious development of the horns, the rapid transformation of the premolar teeth, the stout, obtuse canines, the retention of two pairs of upper and lower incisor teeth that have cingulate crowns in contrast to the smooth, rounded crowns in the Bron- topinae. The horns early acquire at their extremities a transversely oval shape, which finally extends down to the base of the horn. The cranial vertex is ex- tremely long and narrow, but the great buccal proc- esses at the sides of the head develop pari passu " See pp. 434, 441, 469, 560. ing the mouth almost up to the level of the eye. The small obtuse canines were brought close together toward the median line. The cusps and cutting edges of the cheek teeth were even more rounded than in Brontotherium and entirely devoid of cingulum. POSSIBLE EOCENE ANCESTORS OF THE BEONTOTHEEIINE GROUP We may again consider the evidence and theories as to the transitions between Eocene and Oligocene titanotheres. (Compare Diplacodon, Rhadinorhinus, pp. 439, 441, 470-474.) EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 469 Diplacodon theory. — The imperfectly known skull of Diplacodon elatus from the base of Uinta C exhibits several resemblances to B. leidyi from Chadron A, and Osborn was at first disposed (1915) to regard Diplacodon as containing the ancestors of Brontothe- rium. (See pp. 439, 441.) Gregory at first regarded Diplacodon as intermediate between the RJiadinorJiinus- Metarhinus group on the one hand and Brontotherium on the other, but later he was inclined to regard Diplacodon as possibly related rather to the ancestors of the menodontine group, on account of the resemblances in the premolar-molar series between Diplacodon and the early members of the Brontops and Menodus phyla. (See fig. 406.) SECTION 4. OLIGOCENE GENERA ACCEPTED AS VALID IN THIS MONOGRAPH Generic name used Valid generic name and type species by Osborn in 1902 Menodus Pomel (M. giganteus), 1849 Titanotherium. Megacerops Leidy (M. coloradensis), 1870- Megacerops. Brontotherium Marsh (B. gigas, jaw), 1873- Brontotherium. Brontops Marsh (B. robustus), 1887 Megacerops. Allops Marsh (A. serotinus), 1887 AIlops. Diploclonus Marsh (D. amplus), 1890 Megacerops. Teleodus Marsh (T. avus), 1890 Megacerops. Menodus. — As is fully explained on pages 204-205, Pomel's name Menodus giganteus (1849) was based upon the first specimen of a titanothere made known to the scientific world, a fragment of a lower jaw, originally described by Prout in 1847. A comparison Figure 400. — Restorations of lower Oligocene titanotheres of the four principal genera A, Brontops rohustus; B, Menodus giganteus; C, Megacerops acen D, Brontotherium hatckeri. One-flftieth natural size. RJiadinorJiinus tJieory. — In 1902 Gregory observed that MetarJiinus fluviatilis and RJiadinorJiinus diplo- conus of the middle Eocene foreshadow Megacerops and BrontotJierium in the following characters: (1) Ante- rior nares very deep, nasals becoming shorter; (2) bridge over infraorbital foramen very rounded; (3) upward flexure of premolar series in side view and "curvilinear" effect in palate view; (4) canines small and sometimes swollen at base, lower canines set near each other; (5) premolars relatively advanced, tetar- tocones set well in toward center of crown; (6) lower jaw with spoutlike incisive region (cf. R. diploconus, M. earlei, M. fluviatilis). of a carefully drawn figure of this specimen given by Leidy (1854.1, pi. 16, fig. 1) with more complete material now available indicates that Menodus gigan- teus is the same form as that which was later named by Marsh BrontotJierium ingens. Accordingly, Meno- dus giganteus Pomel has priority over Marsh's name and has therefore been adopted in this work. Titano- tJierium Leidy, although formerly used by the present author instead of Menodus Pomel, is now regarded as a synonym of that name, for reasons given on pages 205-206. Megacerops. — -Leidy's Megacerops coloradensis was founded upon a fragment of a skull (including the 470 TITANOTHERES Or ANCIENT WYOMING, DAKOTA, AND NEBRASKA coossified nasals and "horns" ; see p. 208) of a titanothere that now appears to be congeneric with the skulls subsequently named by Cope Symiorodon iucco. The present author formerly used the name Megacerops for the generic group called Brontops by Marsh, but a re- examination of Leidy's above-mentioned type of Megacerops coloradensis has convinced him that this was an error. In the present monograph the name Megacerops includes the forms called by Cope "Sym- horodon bucco" and "S. altirostris." (See pp. 212, 215.) Brontotherium. — Marsh's Brontoiherium gigas, the genotype of Brontotherium, rested upon a certain lower Brontops, Allops, Diploclonus, and Teleodus are all now treated as distinct genera, although they were formerly regarded by the author as referable to Megacerops. SECTION 5. THE MENODONTINE GROUP SUBFAMILY BEONTOPINAE, INCLUDING THE PHYLA MAN- TEOCERAS, PEOTITANOTHERIUM, TELEODUS, BRONTOPS, AND DIPLOCLONUS STRATIGRAPHIC LEVEL AND DISTINGUISHING FEATURES The menodontine group consists of titanotheres of upper Eocene to lower Oligocene age that reached a Figure 401. — Skulls of Rhadinorhinus and Brontotherium Palatal view. A, Rhadinorhinus iiploconus, Am. Mus. 1863 (type); White River, Uinta Basin, Utah, Uinta B 1; two-ninths natural size. B, Brontotherium leidyi, Carnegie Mus. 93 (paratype); Chadron formation; one-sixth nattttal size. jaw in the Yale Museum (see p. 210), which the present author regards as congeneric with the flat-horned skulls that were later named by Marsh Titanops elatus, Titanops medius, Titanops curtus. Marsh, however, erroneously referred his type skull of "Brontotherium" ingens to the genus Brontotherium. "B. ingens" proves rather to be a synonym of Pomel's Menodus giganteus. Symhorodon Cope is unfortunately a synonym of Menodus Pomel, for the reason that the type species Symiorodon torvus (see p. 211) was founded upon lower jaws that appear to be congeneric with the type jaw of Menodus giganteus Pomel. climax in the upper levels of the upper Titanotherium zone. Related to the Eocene Manteoceras. Dis- tinguished by progressively broad heads (brachy- cephaly), short-crowned teeth (brachyodonty), and short or moderately proportioned feet (mesatipody) . Horns short, progressively shifting forward, of primitive trihedral section at the base, rounded to oval at the summits, progressively transverse oval. Nasals progressively reduced in length and broadening at the extremities. Incisor teeth with rounded crowns; one or two pairs persistent above and below. Canine teeth pointed, of medium length, progressively obtuse. Premolar evolution retarded. Zygomata, EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 47L especially in males, progressively expanding into con- vex buccal processes. Includes origiual contempora- neous phyla known as Teleodus, Brontops, and Diploclonus. These short-horned, ; broad-headed titanotheres, which have persistent incisor teeth, are very abundant in the lower, middle, and upper levels of the upper TitanotJierium zone. They present certain character- istics which seem to be shared most nearly by the animal that Marsh called Brontops dispar. This animal is represented by a large nmnber of specimens, which grade below into other specifically distinct forms, such as Brontops irachycepTialus, and above into larger forms, such as Brontops rohustus. It appears wise to retain as generic or subgeneric names the names The horns attain no very great length and are usually circular in section at the summit. At the base they are trihedral, rounded, or transversely oval in section. The nasals, unlike those of Menodus and Protitanotherium, are rounded anteriorly, progressively shortened in ascending mutations, and in most old individuals they expand at the extremities. At the very base of the Titanotherium zone Hatcher found a number of small, broad-skulled titanotheres that certainly belong to the Brontops phylum and that have been termed Brontops hrachycepTialus by Osborn, because they are surprisingly broad-skulled. Although they are perhaps not directly descended from any known upper Eocene form, such as Protitano- therium emarginatum, they have one striking feature FiGUKE 402. — Skulls of Bhadinorhinus and Brontotherium Side view. A, RhadiTwrhinus diploconus, Am. Mus. 1863 (type); White River, Uinta Basin, Utali; Uinta B 1; two-ninths natural size. B, Brontotherium leidyi, Carnegie Mus. 93 (paratype); Chadron formation; one- sixth natural size. proposed by Marsh to distinguish members of the two chief' phyla — namely, Brontops rohustus and Diploclo- nus arnplus. SUBFAIMILY CHARACTERS OF TELEODUS, BRONTOPS, AND DIPLOCLONUS COMPARISONS AND CONTRASTS Very precise observation of the animals ranged under the phyla Teleodus, Brontops, and Diploclonus shows that they possess a large number of characters in common which distinguish them more or less clearly from the members of the far more readily defined phyla Menodus, Brontotherium, and Mega- cerops. 101959— 29— VOL 1 33 in common — the short or rudimentary horns are placed directly above or slightly in front of the orbits and exhibit an elongate oval section at the base, pre- cisely like those of P. emarginatum. Such horns were adapted to a lateral butting motion of the head, and it is notable that they are always blunt, or have elongate oval tips. On the same low geologic level was also found a lower jaw containing three incisor teeth (an Eocene character), described by Marsh as Teleodus avus, which may belong to the same group as B. irachy- cephalus. Features of the horns. — Horns thWt are transversely oval at the tips belong to members of the Bronto- 472 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA tJierium phylum. The free portion of the nasal bones is correspondingly elongate; the nasals contract in width anteriorly. The canine tusks, especially in males, are elongate, pointed, and slightly recurved. It appears that all lower Oligocene titanotheres also have elongate nasals. The extremely primitive char- acter of the nasals and of the horns in the Teleodus- Brontops-Diploclonus group is correlated with a re- tarded stage in the evolution of the premolar teeth, a very characteristic feature which sharply distinguishes members of this group from members of the Megace- rops and Brontotherium phyla. The premolars (fig. 406) are even more retarded or simpler than those of iherium zone we note that the skulls referred to B. hrachycephalus progress in size and in general evolution. The horns shift forward somewhat on the face and become elongate; the base of the horn becomes longer in transverse diameter rather than in anteropos- terior diameter. The free portion of the nasals becomes shorter and spreads out distally. The pre- molar grinding teeth gradually become somewhat more complex in these stages of mutation, of change of proportion, and of rectigradation, which are pro- phetic of the next higher phase of evolution. It must be remembered that many of these skulls are crushed and distorted and represent differences in age and sex Figure 403. — Skulls of Rhadinorhinus and Brontotherium Top view. A, RJtadinorUnua abioiti. Field Mus. 12179 (type); Uinta B 1; two-nintlis natural size. B, Brontotherium leidyi, Nat. Mus. 4249 (type); Chadron formation; one-sixth natural size. Diplacodon elatus from the upper Eocene Uinta forma- tion, a proof that D. elatus was certainly not the an- cestor of Brontops. On the inner side of the superior premolar crowns we see a large anterior cusp (deutero- cone) followed by a low posterior ridge or small rudimentary posterior cusp (tetartocone). The muta- tions of B. IrachycepTialus in the lower beds were therefore very characteristic and clearly separable both from the upper Eocene forms and from succeed- ing Oligocene forms. Ascending mutations. — As we pass upward into the middle A and upper A levels of the lower Titano- as well as a number of progressive stages of evolution. Among the animals specifically classed as B. hracTiy- cephalus it is quite possible that ancestors of more than one subsequent phylum may be foimd, such as that leading to Diploclonus. Prophetic characters among the aged individuals of B. hracTiycepTialus point toward the much more robust titanotheres of the middle beds to which Marsh gave the names Brontops dispar and Brontops validus, the former name having the priority. As the following tables show, the Hatcher collection in the National Museum is so rich in forms that it includes a series EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 473 of transition stages, almost unique in mammalian paleontology, which connect the diminutive B. Ira- chycepTialus from level A, the base of the Titanotherium zone, with the large animals of level B known as B. dispar, a form distinguished not only by its larger size throughout but by rounded horns, shorter nasal bones, more robust buccal processes of the zygomatic arches, and somewhat more complicated premolar grinding teeth. It appears that B. dispar is highly characteristic and distinctive of the B levels, or middle beds, but that a form of B. dispar passes up into the base of the upper Titanotherium zone. Diploclomis phylum. — The existence of a second phylum is indicated by specimens that are probably from the upper parts of the lower and from the middle Titanotherium zone. The stratigraphic records in regard to them are incomplete. This is the species Diploclonus bicornutus (Osborn), which is remotely related to B. dispar and is distinguished, as the name indicates, by a duplication of the horns on the inner sides but especially by its very narrow, elongate nasals and the straight outer contour of the horns, as seen from the front. Nasals of somewhat similar type are observed in the animal from Assiniboia, Canada, which was named Menodus selwynianus by Cope. The most interesting biologic conclusion to be drawn from this assemblage of varied types is that in the lower and middle Titanotherium beds there were many kinds of small and'J'mid- dle-sized titanotheres more or less closely related to Brontops. The group is certainly diphyletic, possibly polyphyletic, and is there- fore all the more difficult of analysis. The type Brontops in the upper Titanotherium zone. — On the lower levels of Chadron C, the upper Titanotherium zone, are found the great animals to which Marsh gave the name Sronfops robustus, as well as the surviving members of the Brontops dispar series. At first this animal appears to be widely separated from Brontops brachycephalus and B. dispar, but like B. dispar, which Marsh also placed in the genus Brontops, B. robustus is represented by a large number of skulls in different stages of growth, and there can be no question that certain character- istics of age, growth, and sex of B. robustus are close to some of the variations in B. dispar, but no direct ancestor to the type stages is known. The more important ascending mutations will be expounded in the more precise study of the phylum which follows. Special distinctions from Menodus. — As already stated, these animals belong to the same group as the Menodontinae and are more closely related to Menodus than they are to either Megacerops or Brontotherium. It is therefore desirable to summarize their distinctive characters: (1) The males show brachycephaly, having a zygomatic index that ranges in general from 79 to 91 (mesaticephaly) ; the females have a zygomatic index that ranges from 64 to 69; (2) all retain one or two pairs of upper and lower incisors, which are full sized and round crowned; (3) the canines in males are progressively shortened in the ascending series, whereas in Menodus they are Figure 404.- -Lower jaws of Melarhinus fluviatilis and Brontotherium hatcheri , M. fluviatilis, Am. Mus. 2059; White River, Uinta Basin, Utah; Uinta B 1; two-ninths natural size. B, B. hatcheri, Am. Mus. 1070; Hat Creek, Nebr.; Chadron formation; one-sixth natural size. persistently long and pointed; (4) the tetartocones of the premolars are retarded in evolution, whereas in Menodus they evolve rapidly; (5) as the horns shift forward the nasals are progressively reduced in length — they are not quadrate but broadly expanded distally at their extremities; (6) the horns are typi- cally suboval or cylindrical in basal section and have rounded rather than trihedral tops as in Menodus; (7) the face progressively shortens, whereas in Meno- dus it remains persistently long; (8) in correlation with the shortening of the face the tooth row becomes bent upward anteriorly; (9) the zygomata expand progressively. 474 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA A phylum somewhat intermediate between Brontops and Menodus is Allops, from which members of the Brontops phylum are distinguished by their retarded tetartocones and by the fact that the horns in the males are upright or slightly divergent, in contrast with the depressed and widely divergent horns of Allops. Retarded evolution of the Brontops premolars. — The retarded molarization of the premolars is a conspicu- FiGXJEE 405. — Progressive evolution of the up- per premolars in Brontotherium and its prede- cessors A, Eotitanops borealis, Am. Mus. 14887, Wind Eiver; B, Palaeo- syops paludosus, Am. Mus. 13032, lower Bridger (althougli this genus is not in the Brontotherium series, it illustrates a stage of premolar evolution); C, JRhadinorhinus diploconus, Am. Mus, 1863, Uinta B; D, Brontotherium leidyi, Nat. Mus. 4249, Chadron A; E, Brontotherium gigas, Am. Mus. 492, Chadron C. All one-half natural size. ous character of this phylum. The following facts should be noted: 1. The exact stage of evolution of the tetartocones is partly obscured by the degree of wear, so that much worn teeth appear simpler in structure than unworn teeth, and the greatest degree of complication appears in the intermediate stages of wear. 2. All the specimens referred to the Brontops and Diploclonus phyla show essentially similar premolar characters; in the primitive forms the tetartocone of p* is a concave spur from the deuterocone, continu- ous postero-internally with the internal cingulum; the spur becomes more convex on the buccal side, less concave on the lingual side, and gradually loses its connection with the cingulum; the point of con- striction between the deuterocone and tetartocone moves forward so that the tetartocone enlarges at the expense of the deuterocone. In all species of Brontops Figure 406. — Progressive evolution of the upper pre- molars in Menodus and Brontops and their predecessors A, Eotitanops borealis. Am. Mus. 14887 (neotype). Wind Eiver; B, Manteoccras manteoceras. Am. Mus. 12683, upper Bridger (Telmatherium cultridens illustrates this stage even better); C, Biplacodon elatus, YalQ Mus. 11180, Uinta C; D, Brontops irachycephalus, Nat. Mus. 4258 (type), Chadron A; E, Menodus giganteus, Am. Mus. 505, Chadron C. All one- half natural size. the tetartocone never appears entirely distinct from the deuterocone, as it does in Brontotherium. 3. Within the species B. dispar there is considerable range of evolution in the progressive development of the tetartocone, possibly due to the crania having been found on different levels. 4. An interesting fact is that occasionally there are noticeable differences in the tetartocones on opposite sides of the same individual. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 475 5. Comparison reveals the general kinship oi Br ontops, Allops, and Menodus in the evolution of the premolars. The most important biologic fact disclosed is that the tetartocones show a somewhat different rate of v-yv^'^/-..V.^ J FiGUKB 407. — Progressive evolution of the lower pre- molars in Brontolherium and its predecessors A, EotUanops princeps, Am. Mus. 290 (type), Wind River; B, Palaeosyops paludosus, Am. Mus. 11680, lower Bridger (although this genus is not In the direct line, its premolars well illustrate this stage of evolution) ; C, Metarhinus earlei, Am. Mus. 13179, Washakie B; D, Broniotherium medium, Am. Mus. 1051, Chadron C. All one-half natural size. evolution, or retardation and acceleration, in each of these subphyla. A second fact of importance is that Brontops in its later stages is convergent in many characters with BrontotJierium. Ascending mutations between species. — That species referred to Brontops and Diploclonus are allied generically is shown by numerous skulls that com- bine features of two species, either in the same Figure 408. — Progressive evolution of the lower premolars in Brontops and its pred- ecessors A, EotUanops princeps, Am. Mus. 296 (type), Wind Riv- er; B, Manteoceras manteoceras. Am. Mus. 1556, upper Bridger; C, Protitanotherium emarginatum Princeton Mus. 11242, Uinta C; D, Brontops brachycephalus. Am. Mus. 1495, Chadron A. All one-half natural size. or in different phyla, such as B. hracTiycephalus and B. dispar (Nat. Mus. 4258, 1214) or D. bicor- nutuS) D. amplus, and type of D. tyleri, Amherst Museum. ITie Brontops-Diploclonus phylum as represented in the Hatcher collection oj 60 sJculls and jaws from the Chadron formation, in the United States National Museum Catalog No. Sex Genus and species Specimen Notes 4710 Female Male Male- SkuU.- -- 8733 Skull... Extremely large male, exceeding in size the type of B. robustus. Very typical of the species. 8732 do - Skull, anterior half and jaws. Skull- -- 8766 Male- do 8767 (?) do Skull . - 4943 Male -. do - Skull (typical) Skull-. 8315 Male do - Showing transition from B. dispar (type) to horns and canines. 8747 (?)- do Skull Skull Right ramus Skull and jaws Skull and jaws Skull Skull.-- Skull... 4696 1242 Male Male --.do--- ...- : do .. series. Transitional B. dispar to B. robustus. Agrees in "tooth measurements with type of B. robustus. Jaw of old animal. 4941 Male Male Male Male. Type specimen. 1217 4245 4248 Brontops (serotinus) dispar Marshl :-. Brontops dispar Marsh do Fine skull and jaws attached. 4253 (?) do - -. - Hyperbrachycephalic; old individual. Fine skull. 4703 Male- do Skull. . - 4706 Male.. do - -. Skull.. Do. 4738 Female do SkuU 476 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The Brontops-Diploclonus phylum as represented in the Hatcher collection of 60 sTculls and jaws from the Chadron formation, in the United States National Museum — Continued 8316 7749 7642 4944 4942 4939 8742 8746 8725 8748 4290 8792 8769 8796 4948 1241 8319 8779 8784 8785 8764 4258 ] 8776 1233 1235 4259 4274 4261 8738 8754 8739 4940 4947 8750 1214 1237 4246 8320 8782 1862 Female (?) (?) Female (?) Male Female Male Female Male- Male- (?)-- (?) — Female- Male,-. Female - Female- (?) Female - (?) Male--. Female - Male- (?)-.. Male- Female. Male. Male. Male- Male- Male- Male- Female. Genus and species Brontops dispar Marsh do -do. -do. -do. .do. .do- .do. .do. .do- Brontops? dispar Marsh. Brontops? Female (?) Male (?) Female Male (?) (?) (?)- Brontops (brachycephalus) dispar Marsh. Brontops dispar Marsh. do .do- .do- .do. -do. .do. .do- Brontops brachycephalus Osborn Brontops (?dispar) brachycephalus Os- born. Brontops (brachycephalus) ?dispar Marsh. Brontops (dispar) ?brachycephalus Os- born. Brontops brachycephalus Osborn do Skull Skull Broken skuU SkuU Skull SkuU SkuU SkuU SkuU (part of skele- ton). SkuU, anterior half, and jaws attached. Skull Pair of lower jaws Posterior part of skuU. Right ramus and symphysis. Lower jaws SkuU E, right jaw in matrix. Lower jaws. Young Pair of lower jaws.. Pair of lower jaws.. Right ramus, ?AUops marshi. Pair of lower jaws.. SkuU SkuU Pair of lower jaws- Large female (?) of progressive size. Canines lacking; measurements agree well. Equals Allops marshi? Medium size, adult. Measurements typical of female (rare) . Vigorous male. Teeth poorly preserved. Badly preserved and crushed. Vigorous male; young, m' just coming in; medium to small size. .do- .do- -do. .do- .do- Lower jaw Pair of lower jaws- SkuU. SkuU. SkuU SkuU Upper dentition. Skull, lower jaws, and femur. SkuU .do- .do- .do. .do. Brontops? brachycephalus Osborn . do Brontops brachycephalus Osborn . SkuU Upper dentition SkuU Lower jaw Lower jaw Lower jaws Anterior half of lower jaw. Imperfect skuU Very fine female. Transitional in meas- urements from B. brachycepha us to B. dispar. Advanced stage, approaching B. dispar. Advanced stage, approaching B. dispar. Measurements agree p ecisely. Advanced stage, approaching B. dispar. Type specimen. Measurements typical. Superior dentition of both sides; advanced in size. Vigorous male skull; measurements typi- cal. Canine alveoli small; molars inferior to type of B. brachycephalus. Paratype. Dentition of smaUer dimensions. Juvenile, imperfect. Measurements agree closely with B. brachycephalus. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 477 Out of a multiplicity of specimens we are able to select evidences of two or three directly successive phyla, as follows: I. Teleodus avus, an ancestral form to Broniops, from the lowest geologic levels. Teleodus primitivus, of doubtful relationship, preserving three lower incisors. II. Brontops phylum, occiput progressively prolonged back- ward behind zygomata; rounded horns vertically placed: C. Upper beds: B. dispar Marsh. B. Middle beds: B. dispar Marsh. A. Lower beds: B. brachycephalus (Osborn). Ila. Brontops phylum, less directly successive; occiput less extended backward, horns laterally overhanging maxiUaries, obliquely to transversely oval in basal section: C. Upper beds: B. rohustus Marsh. III. Collateral phylum, less directly successive; occiput ex- tended very far back, nasals narrow, progressively reduced, horns with internal hornlets. Levels largely undetermined: C. Upper beds: Diploclonus amplus Marsh. B. Middle beds: D. tyleri (LuU). B. Middle beds: D. bicornutus (Osborn). §.3 UPPER EOCENE M. giganteus ALLOPS MENODUS ' / B. brachycephalus ' A. walcotti M. heloceras \ t I ^ Teleodus avus ^^^ "^ *^ Protitanotherium nasals progressively broadening and abbreviating; horns moderately elongate, rounded, or transversely oval; facial region progressively abbreviated. Teleodus, a primitive stage. Brontops, extreme brachycephaly, single-horned. Diploclonus, brachycephaly to extreme brachycephaly, duplicate-horned. Figure 410. — Evolution of the horns in the Brontops phylum a, Brontops tmchycephalus, Nat. Mus. 4261 (type), Chadron A 1; b, B. brachy- cephalus, Nat. Mus. 1214, Chadron A 3; c, B. dispar, Nat. Mus. 4703, Chadron B 2; d, B. dispar, Nat. Mus. 4941 (type), Chadron B 2; e, B. robustus, Nat. Mus. 4696, Chadron C 2. All one-fourth natural size. These outlines show progres- sive increase in height and thickness of the horns; their gradual displacement forward, in front of the orbits; progressive thickening of the nasals and the confluence of their posterior upper border with the anterior border of the horns. Figure 409. — Phyla of the Brontopinae and Menodontinae, titanotheres of the short-horned group CONSPECTUS OF CHARACTERS OF THE SUBFAMTLT BRONTOPINAE Summary of general characters. — Titanotheres ex- tending through lower Oligocene time; incisor teeth 2-1, the crowns rounded; canines of medium size, pointed; premolars and molars cingulate; premolar transformation retarded; skuU progressively brachy- cephalic to hyperbrachycephalic, indices 71-91; FiQUHE 41L — Basal section of the horns in the Broniops phylum A, Broniops brachycephalus, Nat. Mus. 4261 (type), Chadron A 1; B, B. brachy- cephalus, Nat. Mus. 1214, Chadron A 3; C, B. brachycephalus, Nat. Mus. 4259, Chadron B 1; D, B. dispar, Nat. Mus. 4703, Chadron B 1; £, B. dispar, Nat. Mus. 4696, Chadron C 2; F, B. robustus, Yale Mus. 12048 (type), Chadron C 3. All one-fourth natural size. These outlines show a progressive change in the basal section of the horns from the obliquely oval section in B. brachycephalus through the rounded trihedral section of B. dispar to the transversely oval section of B. robustus. Summary of special cTiaracters. — If^. Third upper and second lower incisors the largest; second (or median) upper incisor frequently shed in adult; circu- 478 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND- NEBRASKA lar, smoothly rounded crowns ; incisors smaller tlian in Brontoiherium. Canines cf usually of medium size, anterior face 40, 42, 48 milli- meters, recurved, pointed crowns, often com- pressed anteroposteriorly; canines 9 more slender, shorter, and in section rounder. Opposite molar-premolar series rectihnear to arched. Upward flexure of premolars in side view progressive and especially pronounced in hyperbrachycephalic skulls; dental (pre- molar-molar, basilar) index cf 46-50. Premolars, tetartocone of p* typically re- tarded, often a concave spur; tetartocones of p^, p^ more progressive, but less so than in Menodus; external cingulum typically less prominent than in Menodus. Molars with internal cingula less prominent than in Menodus, crowns less hypsodont than in Menodus. Skull cf brachycephalic to hyper- e'dc ha Figure 412. — Progressive broadening of the nasals in the Bronto-ps phylum from B. brachycephalus, with narrow nasals, through B. dispar to B. robustus, with broad nasals a, Brontops brachycephalus, Nat. Mus. 4947, Ghadron A b, B. brachycephalus, Nat. Mus. 1214, Chadron, A; c, B. brachycephalus, Nat. Mus. 1258, Chadron B; d dispar, Nat. Mus. 4703, Chadron B; e, B. dispar, Nat, Mus. 4696, Chadron C;/, B. robustus, Am. Mus. 1083, Chadron C (?) brachycephalic, zygomatic index 73-91, skull 9 with narrow buccal arches. Nasals primi- tively elongate, progressively shortened and swelling at the extremities; nasal index 61-140. Horns cf primitively low, elongate, ovoid, becoming cylindrical {B. dispar), club- shaped {Brontops robustus) or with a small secondary branch {Diplodonus) , roundly tri- hedral (B. dispar) or broadly oval (Brontops rohustus). Face progressively abbreviate. CONSPECTUS OF CHARACTERS OF SPECIES The salient features of the several species are set forth in the following summary: Teleodus avus Marsh. The most ancient form re- corded from the base of the lower beds. Readily dis- tinguished by the three incisor teeth with rounded crowns in the lower jaw, although the type is some- what larger than B. brachycephalus. Phyletic position somewhat doubtful. Figure 413. — Lower jaws of Teleodus primitivus, Brontops brachycephalus, and Allops ualcottif A, Teleodus primitivus, Otta.wayias. (type); Cypress Hills, Saskatchewan; perhaps the smallest and most prhnitive known jaw of an adult Oligocene titanothere, with horizontal ramus long and moderately deep, chin gently convex, angle not projecting sharply backward, ascending ramus relatively broad, coronoid of moderate height, three incisors, and a prominent postcanine diastema. The external cingula of the cheek teeth are partly confluent with the ectolophs, whereas in all other Menodontinae they are sharply defined. B, Brontops brachycephalus. Am. Mus. 1495; compared with the preceding has a horizontal ramus very shallow anteriorly, angle projecting downward and backward, coronoid high, and molars relatively larger. C, Allops walcottif, Nat. Mus. 4247; differs frona the typical Brontops in having sharply defined external cingula and no diastema in front otpi (pi has dropped out). It differs from the known Menodus in possessing incisors. All one-fifth natui'al size. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 479 Teleodus primitivus (Lambe). Known from a jaw found in the Cyp ress Hills, containing three lower incisor teeth, of which i2 is the largest and ii the smallest. Brontops brachycephalus (Osborn). Very abundant in the lower beds, extending from the lower into the middle levels and passing into transitional forms in the upper levels. Readily distinguished by the broad, rounded skull, and very simple premolar teeth. Brontops dispar Marsh. Very abundant and characteristic of the middle levels. Transitional in structure between B. brachycephalus and B. robustus. Brontops robustus Marsh. An enormous animal characteristic of the lower levels of the upper beds. It presents in an extreme degree the progressive characters of this genus — great width of the skull combined with decided abbreviation of the horns as well as of the nasals and surprisingly backward development of the premolar teeth. Diploclonus bicornutus (Osborn). Closely related to Brontops dispar, from which it differs in its longer, narrower nasals and internal hornlets on the horns. Geologic level unknown, prob- ably the lower part of the middle beds. Diploclonus lyleri (Lull). Intermediate between D. bicornutus and D. amplus. Brachycephalic, with shortened nasals and well-developed internal hornlets. Geologic level propably mid- dle beds. Diploclonus amplus Marsh. Extremely brachycephalic, with short nasals; divergent horns with steep connecting crest. Probably from the upper beds. Conspectus of characters of jaw in the menodontine group Character Teleodus Brontops Diploclonus Allops Menodus Horizontal ramus Deep. Shallow. Shallow. Intermediate. Deep. Symphyseal region in Convex. Very shallow. Shallow. Intermediate to con- Fuller. side view below mental vex. foramen. duced. downward. truncate. downward into a con- vex elbow. Posterior border oblique. I3 IfT I2T IfT 1 (typically) . Present or absent. Pi in fully adult jaws Present. Usually present. Present. Present or absent. Diastema in front of pi.. Present. Present, wide. Present. Absent. Absent. Slender. Slender to short; Slender to Conic to compressed anteroposteriorly . stout. short; stout. External cingulum of Not sharply Present in early Intermediate. Present. Strongly marked. canines, premolars, defined. types, lost in B. molars. robustus. Upward flexure of pre- Moderate. Slight or moderate. Slight or mod- Very slight. Absent. molar series. erate. MEASUREMENTS OF THE BRONTOPS SERIES The species of the genus Brontops, like the species of Menodus, constitute a closely graded ascending series beginning in the lowest and running up into the latest beds of the Titanotherium zone. The range in the chief measurements of the successive stages may be summarized as follows: Measurements of Brontops, in millimeters Pi-m' Pi-p* Mi-m3 Pmx to condyles Zygomatic index Nasal length Horn length 340-376 132-161 215-236 743-843 77-87 52-90 130-210 310-345 123-145 195-215 660-687 78-87 85-90 155-198 B. brachy- cephalus, cf 265-297 101-123 160-180 580-610 72-?82 60-85 59-135 Hence the series of measurements exhibits small gaps between B. brachycephalus and B. dispar in the measurements p'-m', m'-m', in the basilar length (pmx to condyles), and in the length of the horns; but the two species are continuous or overlap in respect to the length of the premolar series, in the zygomatic index, and in the nasal length. On the other hand B. dispar is continuous with or overlaps B. roiustus in the measurements p'-m', p'-p*, m'-m^, as well as in the zygomatic index, nasal length, and length of horns; but there is a sharp gap between the two species in the basilar length, B. roiustus being a far larger type. 480 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Standard measurements in the Brontops phylum, in millimeters Upper teeth Skull Jaw and teeth a 1 S 1 o 1 p. i i a S3 a a i 1 o 1 •1 1 1 1 1 1 1 M s s w CM a 1 o i 1 1 'i •i a B. robustus, Am. Mus. 1069, c? 376 355 363 376 350 363 345 340 147 144 145 149 137 151 139 132 236 220 225 232 220 223 215 220 237 215 203 207 210 207 203 205 195 203 185 195 200 179 180 813 743 755 "843 765 795 795 640 620 600 508 667 628 615 76 83 79 675 "640 87 52 90 153 126 133 123 130 180 47 47 37 34 B. robustus?, Carnegie Mus. 314 B. robustus, Yale Mus. 12048, c? (type). B. robustus, Carnegie Mus. 315 42 36 87 79 77 635 76 149 207 140 174 210 385 136 250 40 — - 683 B. robustus, Princeton Mus. 11015, cf 48 41 26 385 137 240 43 B. robustus, Nat. Mus. 4696 "■85 77 125 124 -540 515 553 535 79 »80 78 730 250 »598 B. dispar, Nat. Mus. 4706, d' 345 "■315 »340 »340 330 "330 320 315 320 308 310 320 295 297 288 278 280 97 R 145 140 131 130 123 126 128 130 128 127 122 121 123 47 ?7 665 B. dispar, Nat. Mus. skull V 687 B. dispar, Nat. Mus. 4248, &.. -- _ 685 B. dispar, Nat. Mus. 4941, cf'Ctype) B. dispar, Nat. Mus. 4245, d' .. . _. 48 45 94 92 102 110 188 155 «365 41 30 27 685 655 B. dispar, Nat. Mus. 4253 B. dispar?, Nat. Mus. 4738, 9 35 24 660 650 660 640 670 643 600 596 "560 605 545 93 108 B. dispar, Nat. Mus. 8742, 9 .. . 435 565 428 520 420 B. dispar, Nat. Mus. 4290, cf (type of "validus"). - - 40 27 85 605 85 93 90 120 100 110 198 B. dispar, Nat. Mus. 8764, 9 . B. dispar, Nat. Mus. 4703, cf - - 45 30 87 65 585 162 B. dispar?, Carnegie Mus. 123, 9 B. brachycephalus, Nat. Mus. 1214, c?.- B. brachycephalus, Nat. Mus. 4274, cT.. B. brachycephalus. Am. Mus. 1495 B. brachycephalus, Nat. Mus. 8738, cf -- B. brachycephalus, Nat. Mus. 4259, cf .. B. brachycephalus, Nat. Mus. 8739, cf-- 39 - — 456 118, 171 118 171 — - 23 85 92 59 310 104 195 ''505 441 -470 110 114 113 "104 101 105 + 32 175 170 175 163 178 160 150 + 24 — - -500 60 93 135 648 610 B. brachycephalus, Nat. Mus. 4258, &..' 280 B. brachycephalus, Nat. Mus. 1862 i 273 B. brachycephalus, Nat. Mus. 4261, c? (type) - 280 43 28 440 72 «520 65 113 40 ---- '•580 «480 82? 85 102 81 143 B. brachycephalus?, Nat. Mus. 4940, 9 .1 265 «109 98 — ? 82 ? B. brachycephalus, Nat. Mus. 4947, 9 . 2.'i3 34 590 + 31 380 + 39 64 Percentage of change from B. brachy- cephalus (type) to B. robustus (type)_. + 25 1 EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 481 Measurements of sJculls and jaws associated with or referred to the Brontops phylum, in millimeters Brontops robustus Brontops dispar Brontops brachy- cephalus. Am. Mus. 1495 Teleodus avus, Yale Mus. 10321 (type) Teleodus Am. Mus. 10690, cf Yale Mus. 12048, d' (type) Princeton Mus. 11015, Nat. Mus. 1217, cP Nat. Mus. 4941, tf (type) Ottawa Mus. (type) Skulls and upper teeth : Anterior canine to hypoconulid fossa of third molar _ 307 345 215 529 -397 385 255 665 193 "310 288 171 443 313 310 195 »505 129 P'-m3 376 236 597 345 215 196 »330 210 M'-mS Symphysis to glenoid Lower jaws and teeth: Posterior canine to hypoconulid of third molar_ - _ 320 "270 Pi-ma -_. --_-.._ 385 240 230 "365 233 -610 o 273 Mi-ms _ 183 Symphysis to condyle " 450 Depth below third molar 99 SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES IN THE BRONTOPS-DIPLOCLONUS PHYLUM Teleodus Marsh Plate XIX, D>, D^; text figures 188, 204, 409, 413 [For original description and type references see p. 227] Generic characters. — Related to or identical with the lower Oligocene Brontops. Three lower incisors in each jaw, instead of two as in other Oligocene titano- theres; i2 the largest of the series; rounded or non- cingulate incisor crowns. Species. — Teleodus avus Marsh, Teleodus primitivus (Lambe). As the most primitive Oligocene genus known Teleodus should be distinguished from the most progressive upper Eocene species, Profitanotherium superhum, P. emarginafum, and other Eocene types. Teleodus is distinguished from Protitanotherium by three characters — (1) incisors smaller, round topped, not flat topped; (2) canines erect, more slender; (3) premolars more advanced in evolution. Teleodus avus Marsh {" Megacerops avus" Osborn, 1902) Plate XIX, D; text figures 188, 409 [For original description and type references see p. 228J Type locality and geologic horizon. — Chadron forma- tion, lower Titanotherium zone. Exact locality not published. Specific characters. — Dental formula I-j, Cx, P-j, M^. Of the three lower incisors in place on each side the middle one is the largest; there is a short diastema behind the lower canine, but no first pre- molar. P^-p* 106 millimeters; molars incomplete. Type, Yale Mus. 10321. As already indicated, this species is known only from the type lower jaw in the Yale Museum collec- tion and is of exceptional interest because it retains, either as a reversional or as a fixed generic or specific character, the full number (6) of lower incisor teeth, as in all the Eocene titanotheres, the formula thus being I^ as against I^qj- Consistent with this primitive morphologic condi- tion the type jaw was found by Hatcher near the base of the lower Titanotherium zone, and if the structural indications are supported by other speci- mens it may well remain as the type of a distinct genus, Teleodus Marsh; yet other structural characters relate it closely to Brontops and point rather to affinity to this genus. Dentition. — The summits of the incisors are ob- tusely rounded and noncingulate, as in Brontops; the lateral pair (I3) is so much reduced and crowded as to furnish conclusive evidence that it is the lateral incisors (I3) which first disappear in all the bron- topine titanotheres. The second incisors (I2) are relatively large — larger, in fact, than the median pair. 482 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA which in turn are a trifle larger than the external pair. The incisors of Teleodus avus thus differ widely from those of Protitanotherium emarginatum, since they are round topped and small instead of being flat topped and large. The canines are of medium size, rounded in basal section, with faint posterior cingula. The presence of only three premolars is also noteworthy. There is no alveolus for pi. P2 is separated by a narrow diastema from the canine. The combined measurement of the three premolars (106 mm.) is only 5 millimeters greater than that of the posterior molar, which shows in a marked way the inferiority of the premolar to the molar series. The jaw represents an individual slightly smaller than the paratype of Menodus proutii Leidy and larger than the jaws attributed to Brontops hrachycepJialus. The canines show that this specimen was a male, and the advanced wear of the molar teeth indicates that the animal was fully adult. The external cingulum is deficient opposite the widest part of the molar crowns, whereas in Menodus and Brontops the cingulum tends to surround the molars completely; in fact, through- out the series the external cingulum is not very promi- nent. The internal crest of the hypoconulid of ma is moderately developed and crenulate. Taken together the characters of the canines, of the incisors, and of the grinding teeth suggest affinity to Brontops, and for the present we may regard the genus Teleodus as of subgeneric rank. Teleodus primitirus (Lambe) Text figures 204, 413, A (For origina] description and type references see p. 235) Type locality and geologic horizon. — Cypress Hills, Saskatchewan; lower TitanotJierium zone. Specific cJiaracters. — ly, P^. Second lower incisor the largest, median lower incisor the smallest, crowns rounded. Premolars, 103 miUimeters (estimated); molars 183 (estimated). Lambe writes: In the Cypress Hills specimen the crowns of the incisors are of a depressed spherical shape, with a tendency to come to a rounded central point above. The second incisor is the largest, and the first is slightly smaller than the third, which is the most upright. The first is more procumbent than the second. Between the inner pair is a very decided interval, leaving a space of 6.5 millimeters between the crowns of the two teeth. The crowns of the canines are broken off, * * * and the right first premolar is lost from its alveolus. [Lambe, 1908.1, pp. 49, 50.] This apparently valid type represents the smallest and most primitive Oligocene titanothere known at the present time (1914). It is from the Cypress Hills of Saskatchewan. It differs from T. avus in its inferior size, in the presence of four lower premolars and in the proportions of the incisor teeth, in which is is smaller than ii. The rounded form of the crowns of these incisors relates them to those of B. irachycephalus. Another Brontops affinity is seen in the presence of an external cingulum on the grinding teeth. Lambe's detailed comparison with " Menodus" angustigenis Cope is as follows (op. cit., p. 50): Compared with Megacerops angustigenis (Cope) * * * M. ■primiiivus differs in the following respects: There are six incisors instead of four, and the breadth of the jaw between the canines, which are of smaller diameter, is relatively greater; the diastema between the canine and the first premolar is twice as long; the symphysis is of greater length, reaching back to a point almost in line with the posterior edge of the fourth premolar (in angustigenis in line with the anterior root of the corresponding tooth) ; the exterior cingula are much less de- veloped; the coronoid process is shorter. In both species the premolars have reached about the same stage of development toward the molar pattern, and the molars have very much the same proportions. In angustigenis the internal cingula are partially developed. In primitivus the mandible is shorter, proportionately deeper, and not so thick in the neighborhood of the alveolar border posteriorly. Brontops Marsh ("Megacerops phylum," Osborn, 1902) Plates XVIII, XX, XXI, XXIII, XXIV, XXXIII-XXXVI, XLIII, XLVII, LXXXII-CIII, CLIII, CLX, CXCV- CCXXIX; text figures 24, 28, 176, 179, 180, 195, 374, 375, 384-387, 389, 391, 393, 394, 396, 398-400, 406, 408-425, 482, 483, 510, 606-611, 616, 618, 622-024, 626, 628, 630, 635, 636, 639, 640, 650, 651, 653-655, 657-660, 665, 669, 673, 685, 686, 693, 699, 708, 712, 719, 727, 744, 746 [For original description and type references see p. 222. For skeletal characters see pp. 674-6761 Geologic horizon. — From lower to upper TitanotJie- rium zones. Generic characters. — Upper and lower incisors 2-1. Canines pointed, becoming obtuse. Premolars cin- gulate; tetartocones retarded. Horns of an elongate to transverse oval section, shifting forward. Skulls progressively brachycephalic. Incisors. — The Brontops phylum exhibits an assem- blage of characters that sharply mark it off from the main phyla of Brontotherium and Megacerops, as follows : In young animals there are two upper incisor teeth on each side (i^, i^), which in some specimens tend to be reduced in old age to one on each side (i^) by the loss of the median incisor; thus the incisive formula, ffx) clearly distinguishes these animals from Menodus and Megacerops, in both of which the in- cisors are vestigial, the formula being f^. In Bronto- therium the formula is the same (fEy,) but both upper incisors, in the males at least, are strongly persistent. Canines. — In the early stages of Brontops the canines are less elongate and less pointed than in Menodus, less robust and obtuse than in Brontotherium, but somewhat larger than in the feebly armed Megacerops; they are generally of a graceful, recurved form. Premolars. — As noted above, the premolar grinding teeth are decidedly retarded in complication — that is, in the assumption of tetartocones and a molar pattern. Thus the premolars of these animals from levels B and A can readily be distinguished from those of Menodus, which are advanced or progressive in com- EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 483 plication, as well as from those of Brontoiherium, which are rapidly progressive. The presence of both internal and external cingula is a character which Brontops presents in common with Menodus. Skull. — It is to be noted that the horns shift forward and evolve at the expense of the nasals, as in Mega- cerops and Brontoiherium, whereas in Menodus the horns are stationary in their relative position. The progressive brachycephaly of Brontops affects chiefly Specific cTiaracters. — Skull small, premaxillaries to condyles about 580 to 590 millimeters; males with strongly expanded zygomata, zygomatic index 82; transverse measurement about 480 millimeters. Females more mesaticephalic, zygomatic index 64. Horns very short ; outside length in transitional skulls (male), 85 to 135 millimeters; horn section antero- posteriorly elongate to roundly trihedral. Nasals elongate, not spreading distally; free length (esti- B mm, A m D -Sections and contours of skulls of Brontops brachycephalus Figure 414.- A, Nat. Mus. 4947, 9 (cotype) and B, Nat. Mus. 4261 (type) ; botb from Chadron A. In these more primitive members the horns are small bosses, rounded to elongate oval in basal section, the nasals are long and tapering, the parietal crest is narrow. C, Nat. Mas. 1214; upper levels of Chadron A; horns distinctly larger, rounded trihedral in section with external, internal, and posterior faces; nasals thicker and relatively shorter. D, Nat. Mus. 4268; base of Chadron B; an immature animal of more pro- gressive type with horns larger and oblique oval in section, nasals wider, zygomata stouter. All one-seventh natural size. (Of. flg. 376.) not only the zygomatic arches, which spring strongly outward, especially in the old males, but also the entire vertex of the skull. Brontops brachycephalus (Osborn) {Megacerops brachycephalus Osborn, 1902) Plates XXI, XLVII, LXXXIII-LXXXVIII; text figures 195, 374, 387, 389, 391, 393, 396, 399, 406, 408-419, 610, 611, 712 719, 744 [For original description and type references see p. 231. For skeletal characters see p. 676] Geologic horizon. — Lower Titanotherium zone (Chad- ron A, A 1, A 2, A 3). mated), 85 to 109 millimeters. Nasal index, 90 to 140. Grinding series, p'-m', in males 280 millimeters, in females 253 ; dental index in males 48, in females 43 ; true molars, in males' 178 millimeters, in females 150. Canines not strongly compressed anteroposteriorly, length of crown in males 34 to 40 millimeters, in females 34. Tetartocone of p* rudimentary (a con- cave spur from the deuterocone continuous with the internal cingulum)^ tetartocones of p^ and p' small, constriction separating tetartocone from deuterocone in the hinder third of the crown. Backward pro- longation of occiput moderate. 484 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Brontops iracJiycephalus is distinguished from its successor, B. dispar, by the much smaller size of all parts of the skull. From Allops marshi it is distin- guished by its somewhat smaller dimensions. From Menodus proutii it is distinguished especially by its shorter molar series and relatively wider premolars, with less progressive tetartocones. These differences are shown below. Measurements of skulls of Brontops, Allops, and Menodus, in millimeters B. braohycephalus A. marshi a" 9 (Nat. Mus. 4947) M. proutii Pi-m3. ... _ 265-297 101-123 160-180 590-610 72-82 (?) 253 105 150 590 64 300-335 119-135 185-205 630-675 64-71 333 pi-p4... .. . 135 Mi-m3 .. -- 203 Pmx to condyles Zygomatic index 628 (?) A female skull (Nat. Mus. 4947) that is referred to B. hrachycepTialus differs from the male skuUs in its Figure 415. — Upper canines and incisors of Brontops braohycephalus Nat. Mus. 4947, 9 1 canines subconic witti slender cingula; incisors (i', i-) small, with circular and rounded crowns. Natural size. smaller size and lesser width throughout. From the type of AUops walcotti this female skull differs in its smaller size, especially of p'-m^ (253:285 mm.). In the males of B. bracliyceplialus the fourth premolar is narrower than in the species of AUops. Materials. — The specific characters of B. hracTiycepJi- alus rest upon exceptionally complete material, most of which is in the National Museum. The type of this species is the adult male skull Nat. Mus. 4261 (PL LXXXVII), from the base of the lower Titanotherium zone. The paratype is the exceptionally aged female skull (Nat. Mus. 4947; Pis. LXXXIII, LXXXVII) foimd by Hatcher at the very base of the Titanotherium zone only 14.4 feet above the Cretaceous Pierre shale. A third specimen is skull Nat. Mus. 4940, also re- corded from the lower beds but from the upper level (A 3). A jaw (Am. Mus. 4247) agrees closely in size with this skull but may belong with Allops walcotti. A fourth specimen (Nat. Mus 1214, fig. 418) also comes from the upper level of A, or the lower beds, and it shows features of approach to B. dispar. A fifth skull (Am. Mus. 1495) is distinguished by its very small horns. General characters. — These small, broad-skulled ti- tanotheres are distinguished by the lateral arching and relatively broad section of the zygomatic buccal proc- esses. The nasals taper toward the extremities and are slightly decurved. The horns rise above the skull from 25 millimeters in the females to 60 in the male skull, the corresponding outside measurements beiug 85 and 102. The basal section of the horns (fig. 414) is highly characteristic, consistiug of a broad outer maxillary face, lying in an oblique plane, and a broad, highly convex inner face. The horns are obtusely roimded at the top. The superior iacisors (fig. 415) are two in number on each side in yoimg animals, but the iuternal pair, I^ are variable and likely to dis- appear in aged animals. The canines are elongate, pointed, with a posterior cingulum only and a strongly convex posterior face, varying in length from 33 millimeters in the females to 43 in the males. A very distinctive character is the structure of the premolars, the main internal portion of the crown being composed of the deuterocones, while the tetar- tocones are small and rudimentary, especially in p^ p*. In the type male skull the tetartocone of p^ on the right is a prolongation of the deutero- cone, concave and continuous with the cin- gulum on the lingual or internal border; on the left side the tetartocone is more circular. In Am. Mus. 1495 the tetartocone on p* is a concave spur from the cingulum; the tetar- tocone on p' is concave on the lingual side, convex on the buccal side, which is evidence that the buccal side develops faster. The pre- molar cingula are developed progressively in ascending levels but are never so broad as in Menodus. A primitive feature of m^ is the aborted metaloph extending inward upon the crown. There are incipient protostyles on the superior molars. Paratype jemale sTcull. — The very aged female skull (Nat. Mus. 4947) in the eleventh stage of growth, dis- covered by Hatcher in 1887 on the bottom level of the lower Titanotherium zone, is the paratype of this species. The palate is finely preserved (PL LXXXVII). It is by far the smallest and one of the most primitive skulls of the brachycephalic series; it measures only 590 millimeters from the occipital con- dyles to the premaxillae and 380 millimeters across the zygomata, while the total premolar-molar series is only 253 millimeters as compared with 320 millimeters in the lower jaw of the type of Teleodus amis. The nasals are rather elongate, contracting anteriorly. The horns are small, rounded knobs rising 25 milli- meters above the top of the skull from a point slightly anterior to the orbits — in fact, immediately in front of the preorbital malar bar; they are directed outward and backward, rugose at the tips and set wide apart, EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 485 overhanging the sides of the maxillae; the horn sec- tions are obliquely placed ovals that converge ante- riorly. Figure 416. — Reconstruction of crushed skull of Brontops brachycephalus Am. Mus. 1495; Chadron A; South Dakota. Ai, Skull fully adult, less brachycephalio than the type of B. brachycephalits, and one of the most primitive known of all Brontopinae. One- sixth natural size. Very small horns (fundamentally similar to those of the Eocene Manteoceras) consisting of swellings on the frontals which overlap the nasals. The surface of the horn swelling (Ai) is roughened and pitted as if for the attachment of a tough or even horny epidermal cap. A2, Horn region. One-third natural size. The upper teeth of this skull are shown in Figure 417; the lower jaw in Figure 413, B. The cranium proper is narrow, measuring 80 millimeters across the vertex; the occiput is broken away; the zygomata arch outward and give a brachycephalic character to the skull, although the measurements show that it is in reality mesaticephalic, the zygomatic index being only 64. This mesaticephaly is a female character. In section these arches are deep, thin bars. The posterior nares are placed very far back, opposite the posterior face of m^ The vomer ex- tends well back on the basisphenoid with a median keel in the portion lying between the pterygoids, extending back almost as far as the basisphenoid rugosity, as seen also in succeeding types. The ali- sphenoid canal, foramen ovale, f. lacerum medium, and f. condylare are clearly defined. The somewhat rugose junction of the basisphenoid and basioccipitals foreshadows the prominent projection for the rectus capitis muscle, which is so characteristic of succeeding species of Brontops. Dentition. — Of the incisors two (i\ i^) persist on one side and only one on the other. The premolars are completely worn down, but so far as one can observe the deuterocone constitutes almost the entire inner face of the crown "of p*. The hypocone of m' is a cingule connected with a very interesting abortive metaloph, as seen in some other titanotheres. The inner contour of the crown is very rounded, the primitive triangle being scarcely altered by the addition of the slight protostyle and hypocone. Type male skull. — The upper surface of this aged male skull (Nat. Mus. 4261, PI. LXXXVII, figs. 399, 410, 411, 414) is especially well preserved. Although compared with the female it is a relatively large and adult animal, in the ninth stage of growth, the horns rise only 85 millimeters above the nasals (outside meas- urement). They exhibit the primitive or original position of these structures — namely, very far back and immediately above the orbits, directed outward. The very robust section of the horn base is a male character distinguishing this from the above-described female horn. This skull is very interesting, moreover, because it illustrates the great breadth (480 mm., estimated) attained by the zygomatic arches in this low geologic level. This brachycephaly is a male character. The dental formula is I^~, P*~'. The premolars are dis- tinguished by narrow internal cingula and rudimentary tetartocones; the teeth exhibit faint external and de- cided internal cingula, but the most decided feature is the sessile character of the tetartocones, which occupy barely one-third of the inner face of the premolar Figure 417. — Left upper grinding teeth of Brontops brachycephalus Am. Mus. 1495. One-third natural size. These teeth agree well in form and measurements with those typical of B. brachycephalus. They difler from those of Allops walcotii (type) in minor points, and from those of Menodus proutii in being less elongate anteroposteriorly, especially the molars, which also have the internal eingulum less pronounced. The premolar pattern resembles that of aU the more primitive members of the menodontine group. (See flgs. 413, 416.) crowns. The incisors are absolutely transverse in position with perfectly smooth, rounded crowns; the canines are quite large. The American Museum specimen (No. 1495), the skull of a yoimg animal, also exhibits the character- 486 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA istic features of this species; the horns are somewhat smaller than in ProtitanotJierium emarginatum of the upper Eocene. They present an elongate oval section, with sloping anterior face. The zygomata are fairly developed and typical in section. The anteroposterior and transverse diameters of the molars are subequal. The tetartocone of p* is a concave spur from the the angle projects downward and backward; the coronoid is high; the molars are relatively larger. As arranged in the order of sequence from the base to the summit of the lower zone the remarkable series of skulls in the National Museum (see table on p. 480) gives us a complete series of ascending mutations toward the higher stages of B. dispar in the middle beds. Proportions of teeth of Brontops brachycephalus, in millimeters Antero- posterior Transverse c pi P2 PS P^ Ml M2 M3 23 23 28 32 38 49 59 61 20 20 32 43 54 49 59 63 Figure 418. — Skull (B) and horn region (A) of Brontops brachycephalus? Nat. Mus. 1214; Chadron A 3. The horns are more progressive than in the type of this species and in their trihedral section suggest Allops marsM, The dental measurements agree better with those of B, bra- chycephalus, although the premolar series is unusually long. The large canines probably indicate male sex. One-third natural size. cingulum. P' is set on a line with p^ (outer wall) and is separated by a diastema from the canine. The canines are procumbent, as in Nat. Mus. 4261, with faint external cingula. M^ has crenulate metalophs, m3 a hypoconulid not greatly different from that of Menodus. The jaw of this specimen is shown in Fig- ure 396, A. As compared with Teleodus avus this jaw has the horizontal ramus very shallow anteriorly; Transitional stages from Brontops hra- cJiycephalus to B. dispar. — A transitional stage is represented by a rather primitive skull (Nat. Mus. 4940), from the top level (A 3) of the lower beds, which is interme- diate in the length of its horns between the type of B. brachycephalus and some of the more primitive varieties of B. dispar. This skull taken as a whole is also somewhat more progressive in char- acter than the male type and female par- atype above described. The nasals are thin, elongate, and contract anteriorly. The horns present a decidedly oblique oval form at the base but round up toward the summit into a more cylin- drical section; they rise 60 millimeters above the midline of the skull, and since this animal is well advanced toward the eighth stage of growth, this is probably the maximum development of the horns in the males at this stage. The pos- terior nares open somewhat more ante- riorly than in the type of B. brachy- cephalus. There is but one incisor above on each side and the first superior premolar is rep- resented by its alveolus only on the right side, the alve- olus being closed on the left. It is important to note this reduction of the median incisor, or ii, and the shed- ding of the first superior premolar. The remaining pre- molars, p^-p*, show a slightly stronger development of the tetartocones than in the B. brachycephalus type and paratype, although in p* the deuterocone still constitutes the main internal portion of the crown, as in the type EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 487 of B. hrachycephalus ; in m^ the abortive metaloph ex- tends outward from tlie cingule without a distinct hypocone. A second transitional skull from upper A (A 3) (Nat. Mus. 1214; fig. 418) may also be considered transi- tional between B. iracJiycepJialus and B. dispar. It is in the seventh growth stage. The median pair of incisors (i|) has also dropped out entirely; the first superior premolar persists on one side and has dis- appeared on the other, so that the formula in both somewhat similar to that in Leidy's type of Mega- cerops; the horns show a more trihedral section at the base than in the preceding specimens owing to the incipient development of the connecting ridge (cr). A transitional stage from lower B is represented by the young male skull (Nat. Mus. 4258) found in B, or the middle levels — namely, 71.45 feet above the Cretaceous Pierre shale. It gives us valuable details of the architecture of the antei'ior portion of the face. Seen from above (Pis. LXXXIV, LXXXV) the A D Figure 419. — Sections and contours of skulls of Brontops hrachycephalus (A) and B. dispar (B, C, D) , Nat. Mus. 4259; B, Nat. Mus. 4941; C, Nat. Mus. 4703; D, Nat. Mus. 4290 (type of B. validus). All one-eighth natural size. The more progressive members of B. brachycephalus (A) approach B. dispar but are distinguished by their smaller size, small horns, low connecting crest, and relatively slender zygomata. In B. dispar (B, C, D), which is characteristic of Chadron B, the horns are much stouter with rounded basal section, the nasals thicker and shorter, and the zygomata more massive. The more progressive members (D) approach B. roiustus. Note the sunilarity of the buccal section in D to that of Am. Mus. 518, 9 (flg. 424; see also fig. 376, showing position of sections and contours). specimens is I', P*~', demonstrating absolutely the variability of p^ The marked primitive feature of the premolars is that while in p^, p^ the tetartocones are becoming more distinct and rounded, in p* the deuterocone still forms almost the entire inner surface of the crown. In this, as in the preceding types, the metaloph extends from the hypocone cingule of m' outward toward the metacone. The rugosity on the basisphenoid is very prominent; the nasals are of medium length in this specimen, expanding slightly at the sides, which is an age character, and their form is 101959— 28— VOL 1 34 frontals cap the nasals and constitute the entire tips of the horns. In side view, however (PI. LXXXIV), the nasals are seen to support the main anterior por- tion of the base of the horn, resting postero-inferiorly on the lacrimals, elements which are not quite so ex- tensive as represented in Plate LXXXIV, A^. Sub- sequently the lacrimal coalesces with the malar, and we speak of the ridge extending down from the side of the horns as the malar or external ridge. In palatal view the posterior nares open between m^ and m'. The vomer (PI. LXXXVI) extends well backward. TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The transitional skull just described (Nat. Mus. 4258) and another skull (Nat. Mus. 4259) belong to B. hrachyceplialus rather than to B. dispar, because, as shown in the table of measurements above, they agree far better with B. hracliycepJialus in tooth meas- urements, the horn sections (figs. 414, 419) agree more closely with those of B. hrachycephalus , and in their tetartocone development the premolars are more advanced than the type of B. hrachycephalus but less advanced than in the type and referred specimens of B. dispar. Review of primitive and progressive characters oi Brontops hrachycephalus. — Nasals: All the skulls ex- hibit nasals having a median terminal notch. The nasals are nari'owest distally in the fourth stage of growth and expand toward the extremities in the seventh stage; the progressive shortening and distal widening of the nasals are distinctive features of the Brontops phylum. In general, abbreviation of the nasals is correlated with brachycephaly. Horn growth: The sexual distinctions of size and form in the horns are well marked. The horns not only increase steadily in size in succeeding geologic levels but in course of individual growth they re- capitulate clearly the ancestral Eocene history of this species. In young bulls the horns present an elongate oval, as in the upper Eocene Protitanotherium emarginatum; as the males become older the antero- posterior diameter relatively decreases, the transverse diameter increases, and the horn gains in height, rising only 37 millimeters above the skull in the fourth stage. Skull growth: In the meantime the summit of the skull, or parietal vertex, does not perceptibly increase in width; the top of the occiput extends considerably back of the zygomata. The zygomatic arch sections are as highly distinctive as the horn sections, since the transverse diameters nearly equal the vertical diameters; the zygomatic arch is relatively broad and shallow and contrasts with that of Menodus heloceras, which is distinctly dolichocephalic. Teeth; variability of incisors: There are two well- developed incisors in the youngest skull transitional to B. dispar (Nat. Mus. 4258); the median pair has dropped out of another skull in the later part of the seventh stage (Nat. Mus. 1214); but both in- cisors persist in the still older type male (Nat. Mus. 4261). In all the male skulls the canines are promi- nent and pointed, measuring from 40 to 43 milli- meters in length in the males and resembling in form those of B. dispar. Pi'emolars: The first superior premolar is small; it exhibits an imperfectly formed internal crest and no tetartocone. The constitution of the remaining premolars, p^-p* (Nat. Mus. 4258, 1214), is mter- esting as exhibiting the relative rate of evolution of the internal cusps and bringing out the fact that the anterior premolars are more progressive than the posterior; this condition is adaptively correlated with the fact that the anterior premolars come into use earlier than the posterior premolars, as shown by Hatcher (1901.1, p. 261). Thus p^, although the smallest tooth, exhibits the largest tetartocone, form- ing a direct continuation of the crest of the deutero- cone; p^ has its tetartocone distinctly budded off, with conical sides; while in p* the tetartocone is a mere cingule (figs. 406, 417). Whereas in the typical B. hrachycephalus of the lower A levels the deutero- cone constitutes almost the entire inner surface of the crown, especially in p^"'*, and the tetartocones are rudimentary, in the later geologic stages (Nat. Mus. 4258, 4259) the tetartocones evolve into small, low, irregular-shaped cusps, leading into the condition found in the type of B. validus (Nat. Mus. 4290). The variability of the first superior premolar is shown, as mentioned above, in one of the older B. hrachy- cephalus skulls (Nat. Mus. 1214), in which it persists on one side but has disappeared on the other; this reduction and variability is seen also in B. dispar and in Menodus. A very important character consists in the fact that whereas the external cingulum on the premolars is variable and feebly developed in some skulls and more strongly marked in others (Am. Mus. 1495, Nat. Mus. 4258), the internal cingulum is con- stant but never very broad, as it is in Menodus trigonoceras . Molars: The ratio of molar length to premolar length in this evolution stage is as 175 to 114. In the molar series m^ also offers a very distinctive character: the hypocone is a cingule, and as in B. hrachycephalus an abortive crenulate crest (metaloph) extends from this toward the metacone; the strong cingule in front of the protocone known as the "pro- tostyle" is here a feeble, inward-directed ridge. Brontops dispar Marsh {Brontops validus Marsh; " Megacerops dispar" Osborn, 1902) Plates XXIV, XXXIII, LXXXVIII, XCIV; text figures 24, 180, 375, 384-386, 389, 391, 394, 396, 399, 409-412, 419, 421, 423, 609, 616, 719, 744 For original description and type references see p. 223. For slceletal characters see p. 664] Geologic horizon. — Middle beds (Chadron B) to lower portion of upper beds (C). Specific characters. — Skull of intermediate size in males; incisive border to condyles 660 to 685 milli- meters, in female 610. Males brachycephalic, zygo- matic index 73 to 87, average 80; zygomatic width 555 milhmeters (average). Females more mesati- cephalic, zygomatic index 60-71, zygomatic width 390 millimeters. Horns progressively shifting anteri- orly, more elongate, 162 to 200 millimeters. Basal section roimded, summits cylindrical. Nasals pro- gressively abbreviated (105 to 85 mm.), with for- ward-shifting horns, spreading distally. Grinding series 310 to 340 millimeters, average 329. Dental EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 489 index 46 to 49, average 47. Canines pointed and more or less compressed anteroposteriorly. Pre- molars more progressive; tetartocones p^"^, more advanced than in B. bracliycephalus. Tetartocone of p^ progressively developed from a concave spur from the deuterocone, continuous with the cingulum, to a rudimentary cusp. Backward prolongation of the occiput behind the zygomata very pronounced. This significant species connects closely with the most progressive stages of Brontops hrachycepJialus and with certain skulls which may be referred to Brontops rohustus, although the typical B. dispar does not appear to be directly ancestral to the typical B. rohustus. The range in size in skulls referred to Brontops dispar is given above. From Allops marsJii, as well as from Brontops hrachycepJialus , the skull of this species is distinguished especially by its greater robustness (zygomatic index 78-87, as compared with 64-71), larger horns, and shorter, thicker nasals, as indicated in the following measurements: Measurements of Brontops dispar and Allops marshi, in millimeters Pi-m3 P'-p4 Mi-m3 Canine: Vertical Anteroposterior. . Pmx to condyles Zygomatic width Cephalic index Nasal length ■_ Nasal breadth Horns, external length ! . dispar, Nat, Mus. 4290 (type of B. validus) 320 130 203 40 27 660 565 85 85 120 198 A. marshi, Am. Mus. 501 (type) 310 122 191 34 25 673 450 67 100 105 105 From Menodus proutii we observe that B. dispar is distinguished by its brachycephaly, by the greater relative width of its premolars and molars, and by the more retarded development of the premolar tetarto- cones, especially in p*. From Allops serotinus and A. crassicornis we observe that B. dispar is usually distinguished by its higher zygomatic index and by the lesser width of the pre- molars. The type of B. dispar, unfortunately a badly crushed skull, has less robust horns and nasals than the type of B. validus, but the general agreement in measurements as well as in dental characters appears to indicate that these two types belong either to the same or to very nearly related species. Geographic and geologic distribution. — According to Hatcher's record, the typical B. dispar skulls were found mainly in B or the middle Titanotherium zone, but they extend also into the lower levels of the upper beds, namely, lower C (Chadron C 1). The four ascending skulls in the National Museum most ac- curately recorded by Hatcher and Stanton in 1901 are from 55 to 71 feet above the Cretaceous Pierre shale, as follows: Skull 4258, Brontops bracliycephalus (transitional), 71.45 feet above the Pierre. SkuU 4259, B. brachycephalus (transitional), 55.6 feet above the Pierre. Skull 4290, B. dispar (type of B. validus), 62 feet above the Pierre. Skull 4703, B. dispar, 62 feet above the Pierre. Geologic variation. — Inconsistent with the general observation that the more progressive forms are found on higher levels is the fact that the specimen first named (Nat. Mus. 4258) is a very primitive form which is distinctly related to B. hracTiycephalus, as noted above. The type of Diploclonus tyleri, although very advanced, was also found at a very low level — namely, 35 feet above the Pierre shale, in 200 feet of beds belonging to the Titanotherium zone (Lull). These inequalities are certainly due to the uneven surfaces of the Pierre shale on which the White River was deposited. Materials of B. dispar. — This species is represented in Hatcher's collection for the Geological Survey by twenty-one or more skulls and several jaws now in the National Museum, including especially the follow- ing: The juvenile transitional skull (No. 4259) from lower B, or the middle beds, referred to B. brachyce- phalus; the male type of B. dispar (Nat. Mus. 4941) from middle B; four finely preserved male skulls — namely. No. 4703 from middle B, No. 4290 from middle B (this specimen is the type of Brontops validus), and Nos. 4245, 4248 from B. The two skulls last named are in the seventh and ninth stages of growth. The female sex is represented by No. 4738 from lower B. There is also a complete skull and ower jaw (Nat. Mus. 1217) from the upper zone (level C). This magnificent collection of well-recorded material in the National Museum enables us to determine positively the range of Brontops dispar from lower Chadron B to the base of Chadron C. In this long geologic range there are marked progressive advances in evolution. B. validus a synonym of B. dispar. — Careful com- parison of the measurements and characters of the type of B. dispar (Nat. Mus. 4941), from middle B, with the type of B. validus (Nat. Mus. 4290), also from middle B, together with the supplementary evidence furnished by the collection of skulls in the National Museum, shows that these nominal species are identi- cal. They both represent the brachycephalic, short- horned titanotheres of the middle levels. Although the type of B. validus has on each side two upper incisors and four premolars, a study of other materia 490 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA proves that both the median incisors and the anterior premolars may be shed at an early age. In other words, it is demonstrable in B. dispar, as in B. irachy- ceplialus, that the median incisors (i') are variable, that they are found in all young animals, that they may or may not drop out in later life. The lateral incisors (i^) are constant. The stage of evolution of the premolars in the type of B. validus is identical with that in the type of B. dispar and the sagittal sections of the skull in the two type specimens are closely similar. The horn and nasal sections in the two type specimens are essentially similar; the differences (fig. 419) are due to the lateral crushing and less advanced age of the skull of B. dispar. Chief distinctions from other species. — B. dispar is thus readily distinguished from B. brachycephalus by its greater measurements and more progressive characters throughout; from Allops marshi by the cylindrical section of its horns and by the great back- ward extension of its occiput ; and from the geologically succeeding B. rohvstus by the smaller size and cylindri- cal section of the horns and the backward extension of the occiput. Detailed characters oj the type of Brontops dispar. — The type of Brontus dispar consists of a male jaw and skull (Nat. Mus. 4941) with the occipital region entirely wanting (PL XCIII). The extreme lateral crushing of the skull obscures its real characters and actual resemblance to the type, which Marsh sub- sequently named Brontops validus. As association of the skull and jaw is rare and as this specimen rep- resents one of the predominant species of the middle zone, the type deserves full description and measure- ments in addition to those given in the comparative columns above. Measurements of type of Brontops dispar, in millimeters Inferior canine (partly worn) , anterior 41 Inferior canine, anteroposterior 30 Length of symphysis 159 Width of condyle - 137 Premolars and molars: p2 p3 P< ml m} m3 Superior series: Anteroposterior. _ 30 40 43 62 78 80 Transverse 39 60 60 72 80 90 pj pi Pi mi m2 mi Inferior series: Anteroposterior. . 31 39 45 63 77 108 Transverse 22 29 33 40 44 44 Teeth: Formula, 1\, P|f|. A stout lateral pair of incisors persists in both upper and lower jaws, though the animal is in the beginning of the eighth stage. The median incisors are wanting in both jaws. The canines are prominent and recurved, the superior canine is embraced by a posterior cingulum, while the inferior canine has a faint external cingulum sur- rounding all except the inner side. The anterior pre- molars (pj) have dropped out in both upper and lower jaws, being represented only by large alveoli; p^, p^ p* have moderately developed internal and vestigial external cingula and tetartocones in the middle stage of development. The hypocone is a prominent cingule on m'. The second lower premolar, P2, is an elongate simple tooth, the metalophid not being defined at all, and the hypolophid forming an incipient crescent; ps, p4 present double crescents and a rudimentary metastylid. The lower molars exhibit partly developed external cingula, reduced opposite the protoconid and hypoconid ; they also exhibit rudi- ments of the metastylid. The last inferior molar ip highly characteristic of the Brontops- Menodus group, a broad internal shelf extending inward from the hypoconulid, with a trace of a posterior cingulum beneath it. The much crushed jaw exhibits a shallow symphysis, a large mental foramen beneath pa. Plate XCIV gives its partly restored outlines. Skull: The skull exhibits the short, truncate, and cleft nasals, short horns with a subtrihedral section at the base, rounded malar, nasal and connecting ridges, and roimded faces. The posterior face is broad and imiformly convex, as in the other advanced members of this phylum. The horns have a charac- teristic angle of the outer border, leadiag into a rounded summit, as shown in the section. The zygomatic arches are robust and convex, but they are too much crushed for an accurate section. Type of Brontops validus. — The type of the syno- nymic species Brontops validus (Nat. Mus. 4290) is a male in the seventh growth stage. It is well figured in Plates LXXXVIII, LXXXIX of the present mono- graph. The principal measurements are as follows: Measurements of type of Brontops validus Millimeters Molar-premolar series 320 Condyles to premaxillary tips 660 Free portion of nasals, length 85 Free portion of nasals, breadth 120 Length of horn to nares 199 There are two incisors above and below ; the canines are large and pointed, indicating that this specimen was a male; the fourth premolar has a rather feeble tetartocone without a distinct valley separating it from the protocone; the hypocone of the third su- perior molar is a shallow ridge arising from the cin- gulum. The species is clearly characterized by the short, square nasals and horns borne well forward; but it can not be separated specifically from the pre- viously described Brontops dispar. General characters of B. dispar. — The measure- ments of the skulls referred to B. dispar are singularly EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 491 uniform and constant, as shown in the table above. In the males the superior premolar-molar series measures 310 to 345 millimeters in length. The basilar length, premaxillaries to occipital condyles, ranges from 660 to 685 millimeters, the male zygo- matic arches from 515 to 565, the horns from 155 to 200. In the females the dental and cranial measure- ments exhibit the usual sexual contrasts of inferiority. Main features of skull: The nasals are short but broad and thick in section in the type of B. validus, 85 to 103 millimeters in length and 100 to 143 milli- meters in breadth; in other males (Nat. Mus. 4245, 4703, 4706) they are somewhat long and narrow, resembling those in B. hracTiycepTialus and thus afford- ing a complete transition, as may be readily seen in Figure 419. The adult or growth changes are prophetic of the higher specific stages of this phylum. Thus in early stages of growth the nasals are some- what thinner and more narrow distally; in later stages of growth they thicken and broaden near the ends, while the median cleft deepens. The form of the horns is highly distinctive of this species; seen from in front (PI. XCIV) the lateral contours of the horns exhibit a strong convexity above the middle portion; from this point upward they suddenly contract into rounded, rugose tips; this rounding and pointing of the tips of the horns is a very distinctive feature as compared with the broad oval tips in members of the Brontotherium phylum. The basal section of the horn in the type skull of B. dispar (figs. 399, 419) approaches that of B. rohustus (fig. 421) in the round- ing out of the "anterior" or "nasal" angle or ridge and in the anteroposterior flattening; the posterior faces of the horns of B. dispar, however, unlike those in the type of B. rohustus, are directed obliquely inward toward each other and present a long, uniform convexity. Some horn sections are more trihedral because of the prominence of the nasal ridge. In lateral view the horns are seen to be slightly recurved and to be united, especially in the old males, by a deep and broad connecting crest. The zygomata present prominent convex buccal processes, which have a deep, thick section, similar to that of B. rohustus but somewhat less heavy. Details of male skull: The type skull of Brontops validus, "skull K," Nat. Mus. 4290 (Pis. LXXXVIII, LXXXIX, XCI, XCII; fig. 419), found at the 62-foot level above the base of the Titanotherium zone, in middle B, admirably exemplifies the distinctive features of the skull. It is in the seventh stage of growth. Seen from in front (PI. XCIV), the nasals are supported by a vertical bony septum, which in life may have been united by cartilage with a similar bony septum that extends up from the premaxillae (cf. Tapirella hairdi). The top of the skull is seen to be a larger development of the B. hrachycepTialus type (compare Pis. LXXXV and XCI), broad above the orbits, narrowing at the vertex, with a rugose supratemporal crest. In the middle of the parietal vertex of certain specimens (Nat. Mus. 4703) a very large central pit (PI. XCII) is observed. This is a vestige of the space between the convergent supra- temporal ridges and is similar to the median pit in some of the Eocene forerunners of Brontops — namely, Manteoceras. The occipital pillars are strong, but the rugosities at the summits are somewhat narrower than in B. rohustus. The palatal view (Pis. LXXXVIII, LXXXIX) also illustrates the brachy- cephalic character. The vomer extends back as a prominent keel upon the basisphenoid, which exhibits a robust, paired rugosity for the rectus capitis muscle at its junction with the basioccipital. The progressive size evolution of the skull is dis- tinguished first by the slight increase in length and great increase in width over that of B. hrachycepTialus, owing partly to the rapid expansion of the buccal processes but also to the broadening of the skull itself; second, by the elongation and strengthening of the horns as described above. Additional observa- tions on this specimen are given above. Dentition: The teeth show many characters in- herited from B. hracTiycepTialus. The formula (If^, P|^) exhibits hereditary variability, manifested in the occasional loss of one incisor and one premolar. The median incisor sometimes disappears before the seventh stage (Nat. Mus. 4941, 4703), or it may per- sist late in life (Nat. Mus. 4290, 4245). The canines are relatively long and rounded in section at the base, more or less compressed anteroposteriorly, foreshadow- ing the Allops sublanceolate canine, less elongate than in members of the Menodus phylum, and much less obtuse and recurved than in members of the Bronto- tTierium series. The first superior premolar is a fairly large and persistent tooth, but it also sometimes aborts and disappears early (Nat. Mus. 4703). The ectoloph of p' is well in line with the ectoloph of p^, p^ (contrast BronfotTierium) . In the advanced stages of B. dispar the transformation of the premolars has proceeded much further than in any specimen of B. hracTiycepTialus; the tetartocones of p^ and p^ are relatively distinct, but in p* the tetartocone is still a backward spur from the deuterocone, more rounded than in B. rohustus, but never sharply rounded off or prominent and always a much less distinct and sepa- rate cusp than in p'. In the type of B. dispar, in fact, the tetartocone of p* is only a little more advanced than in the typical B. hracTiycepTialus. The external cingulum of the superior premolars is fairly well marked; the internal cingulum is progressive or stronger than in B. hracTiycepTialus. On m^ the hypo- cone is a prominent cingule. Lower jaw (fig. 423): In the type jaw (Nat. Mus. 4941) vertical crushing has greatly diminished the depth of the horizontal ramus, but the symphyseal region was probably shallow. There was a diastema between pi (which has dropped out) and the canine. 492 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The canine is stout and conical; the incisors are sub- spherical; the external cingula are not as sharply defined as in Menodus. Female skulls: The females of this species (Nat. Mus. 4738) are, according to the principle expressed above, distinguished by smaller and much less robust canines; the greatly reduced canines, the more slender zygomatic arches, and the inferior dimensions of this skull and teeth as a whole constitute the distinctive sexual characters. The internal and external premolar cingula are quite as strong as in the male representa- tives of this species. The skull forming Nat. Mus. 4738 (fig. 391, A) presents rather slender zygomatic arches; the nasals, although imperfectly preserved, indicate a slender, unexpanded contour. This skull is in the eighth stage of growth, yet it retains two superior incisors on each Figure 420. — Restoration of Brontops rohustus By Charles R. Knight. Ahout one-ninth natural size. side. The canines measure only 35 millimeters an- teriorly as compared with 47 in the largest males. The protocones of m^, m' are rather low, are set well in from the inner margin of the tooth, and have a slight concavity on the lingual side near the top — an individ- ual variation. A peculiar feature of this specimen is the reduplication of the tetartocone of p"*, an anomalous character (see explanation above) also observed in B. hracTiycepJialus (Nat. Mus. 4259) and in the type of Alloys crassicornis as well as in the type of Menodus (Menops) varians. A character reversional to B. hracTiycephalus is the abortive crenulate metaloph of m', extending forward and inward from the hypocone. In palatal view the less robust character of the basi- sphenoid and the narrowness of the zygomata are well illustrated. Another character, which may be an individual variation, is the nonclosure of several of the sutures, the median suture between the nasals and frontals being still widely opened. Similarly the external portion of the mastoid is still well defined between the post-tympanic and the exoccipital proc- esses. The post-tympanic and postglenoid processes of the squamosal are not conjoined below. Extension oj B. dispar into upper beds, or C. — ■ Although B. dispar is especially characteristic of the middle Titanotherium zone, evidence of its occurrence in the upper beds even to a point a little below middle C is apparently indicated by skull No. 1217 in the National Museum. This fine specimen with the lower jaw attached (PI. XCIII) appears to represent a very large male of B. dispar, the length from condyles to premaxillaries being estimated at 702+ millimeters, m^-m^ 196, mi-mj 230. The detailed measurements of the grinding teeth, however, as in the large upper zone brontotheres, are smaller through- out than those of the large B. dispar, Nat. Mus. 4696, from the middle beds. Brontops robustus Marsh {" Megacerops robustus" Osborn, 1902) Plates XVIII, XX, XXXIII-XXXVI, XLIII, XLVII, XCV-CIII, CXCV-CCXXIX; text figures 24, 179, 387, 389, 393, 394, 396, 399, 400, 409-412, 420-423, 482, 483, 510, 608, 618, 622, 624, 626, 628, 630, 635, 636, 639, 640, 650, 651, 653-655, 657-660, 669, 685, 686, 693, 699, 712, 719, 727, 744 [For original description and type references see p. 222. For skeletal characters see p. 666] Geologic horizon. — Lower level of the upper Titanotherium zone (C). Specific characters. — Length of skull 765 to 813 millimeters, strongly brachy- cephalic, zygomatic index, 77 to 87, average 79; backward prolongation of occiput slight or absent; horns shifted forward and inclined forward, basal sec- tion broadly transverse oval, summit obtuse, outside length 207 millimeters (type); nasals abbreviate, free length 76 millimeters, breadth 149, nasal index 51 ; dental index 46, same as in B. brachy- cephalus and B. dispar, total grinding series 345 to 376 milhmeters; molars 215 to 236, average 223; canines rounded in section, obtuse, stout, length of crown 42 to 48 millimeters; tetartocones of p^"^ somewhat less developed than in typical B. dispar. This species marks the end of the Brontops phylum, toward the end of lower Oligocene time. It is related to B. brachycephalus but does not directly succeed any of the known mutations of B. dispar. General characters. — This is the most fully known species of titanothere. The fine type skull and skele- ton in the Yale Museum (Yale Mus. 12048) represent a middle-aged animal. Both were found in "Corral EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 493 Draw," near the base of Chadron C (upper Titanothe- rium zone), 60 feet below the summit, as recorded by Hatcher. This skeleton is so complete that almost all its parts were figured for this monograph in plates and woodcuts under the direction of Professor Marsh. Brontops rohustus appears to have been the predomi- nant animal during the deposition of the beds of the lower C 1 levels of the upper Titanotherium zone in South Dakota. It is a comparatively short-horned type, contemporaneous Vv'ith the early members of the that raises a doubt as to the invariable persistence of the median incisor. These and other characters relate this animal to Brontops hrachycepJialus and B. dispar, in which the median superior incisor is also variable. Convergence of Brontops rohustus to BrontotTierium. — ■ The typical B. rohustus is very easily distinguished, but there are at least six other skulls that simulate Brontotherium in the premolars, in the expansion of the zygomata, and in the flattening of the horns. In some of these also the skull vertex appears long, but B C Figure 421. — Sections and contours of skull of Brontops rohustus Nat. Mus. 4696; middle levels of Chadron C; either a very large and progressive B. dispar or a primitive B. robustus, with stout cylindrical horns far in front of the orbits, broadlv and roundly trihedral in section. Nasals short and vride. B, Am. Mus. 1083, referred to B. rohustus, although the horns are roundly trihedral in section. C, Yale Mus. 12018 (type); lower levels of Chadron C; very thick horns incUned forward and far in front of the orbits, in basal section very widely oval; nasals very short and thick, zygomata massive, and parietal crest wide. All one-eighth natural size. long-horned series of brontotheres, such as B. gigas. The long axis of the basal horn section tends to become directly transverse, and the posterior face is somewhat flattened; the nasals are still broad, resembling closely those in some specimens of B. dispar. The presence of two large upper incisors in the type skull of this species was believed by Marsh to be its most distinc- tive character; two upper incisors are observed also in skulls of the same species in the Yale and American Museums. The male skull (Princeton Mus. 11015), however, carries but one lateral superior incisor, a fact it never has the midparietal protuberance, and the nasals and horns are nearer to B. rohustus than to Brontotherium; also the internal cingula of the pre- molars are well rounded, and the canines are pointed at the tips. Observations on the measurements of Brontops ro- hustus. — The males of this species exhibit a con- vergent resemblance to Brontotherium in the buccal expansion of the zygomata, in the reduction of the external cingula of the premolars, in the progressive well-rounded premolar tetartocones of certain speci- 494 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA mens, and in the massive form of the canine. The internal cingula of the premolars, however, are pro- nounced and well rounded as in Brontops. The refer- ence of these specimens to Brontops roiustus rather than to Brontotherium is confirmed by the following contrasting measurements : Measurements of Brontops, Brontotherium gigas, and Bronto- therium platyceras, in millimeters Brontops robustus Brontotherium gigas Bronto- therium platy- ceras, Field Mus. 12161 Car- negie Mus. 313 Yale Mus. 12048 (type) Am. Mus. 492 Car- negie Mus. 341 pi-m3 . . 355 144 220 743 350 137 220 765 635 42 36 353 126 241 830 793 35 32 355 132 218 773 35 23 340 Pi-p* ... ... 120 M'-m' - - .... 223 880 Nasals to midvertex occi- 895 Canines: 47 37 36 Anteroposterior 29 In general Brontops rohustus is distinguished from Brontotherium by the greater length of the premolar series, by the shorter skull base and skull top, and by the larger canines, which are also more pointed at the tip. The horns are much shorter than in the flat- horned species of BrontotTierium, and the connecting crest is lower. The range of size in skulls referred to this species is indicated as follows: Millimeters SkuU, basilar length 743-813 Pi-m3 340-376 Pi-p^ 132-151 M'-m" 230-237 Zygomatic index 77- 87 Outside length of horns 130-210 The premolars and molars are wide compared with' those of Menodus: Brontops robustus Menodus trigonoceras. 40X65 to 46X69 43X51 to 41X55 73X84 to 90X89 72X70 to 82X79 Geologic and geograpJiic distribution. — So far as re- corded, all the known specimens of this species were obtained from the Titanotherium zone (Chadron for- mation) of South Dakota. It appears probable that the geologic level recorded for the type specimens by Hatcher, namely, lower C of the upper Titanotherium zone, is characteristic. Materials. — The type skull and skeleton in the Yale Museum (No. 12048) afford the best knowledge of the extreme characteristics of this type. In the American Museum there are two fine skulls (Nos. 1083, 1069) which confirm or supplement the characters observed in the type. In the Princeton Museum there are two male skulls, also found in "Corral Draw," South Dakota (Nos. 11439, 11015), associated withmany parts of the skeleton; also a valuable skull (No. 10061) which is transitional between B. robustus and Allops marshi in the disposition of the horns. In the Na- tional Museum there is an extremely brachycephalic, robust skull (Nat. Mus. 4253) of smaller size than the typical B. robustus, which may represent a primitive or transitional variety of this species. It agrees in all its principal characters with the type skull, although smaller and in many features more primitive. Slcull. — The most prominent cranial characters of this well-defined species are extreme brachycephaly, extreme abbreviation of the occiput, marked depres- sion of the midregion of the cranial vertex, rapid elevation of the summit of the occiput, downward and forward inclination of the zygomata. In female specimens of B. dispar the length exceeds the breadth by 200 millimeters; in the type of B. validus the length exceeds the breadth by 100 millimeters. In this skull also the length exceeds the breadth by only 100 millimeters. This feature stands in widest contrast with the dolichocephaly of the contemporary Menodus giganteus, in which the length exceeds the breadth by over 230 millimeters. The type skull, with its abbreviate occipital region, also contrasts with that of the contemporary Brontotherium gigas, which shows a decided backward extension of the occiput, especially in the males. The age or growth characters of this skull are similar to those in some of the collateral ancestors of this phylum — namely, expansion and not very marked elongation of the horns, which shift progressively forward and reach an extreme over- hanging position in the type. Old skulls exhibit a thickening of the connecting crests between the horns, also the distal growth, widening and rugosity of the nasals, and thickening of the buccal swellings of the zygomata. These buccal swellings are somewhat crushed laterally in the Yale type specimen ; but in all the skulls examined the buccal section appears to be smaller and more concave above, and on the outer border a less strong flaring out at the sides is observed than in the skulls of Brontotherium medium or B. curtum. In general, the skull apart from its much greater brachycephaly resembles on a grander scale that of Allops marshi, both in its superior and inferior aspects and in the form of the nasals, yet the retarded condi- tion of the premolar tetartocones, the extremely wide transverse sections of the horns, and, the relative obtuseness of the canines do not point to direct rela- tionship to A. marshi. Horns. — Horns are found, however (as in Am. Mus. 1083, 1069, Princeton Mus. 10061), which are transi- tional in position and basal section between those of Allops marshi and of this species; progressive varieties EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 495 of B. dispar also show more or less transition to this transversely flattened type of horn. Correlated with the general brachycephaly, the palate is broader and the grinding series more widely arched. Similarly we observe great breadth in the postglenoid and paroccipital processes. It is to be noted that the postglenoid and post-tympanic proc- esses unite late in life (Am. Mus. .1069), whereas in the contemporaneous species of Brontotherium {B. gigas and B. curtum) these processes unite early in life. The vomerine ridge is prominent, and there is a down- ward continuation, or septum, of the vomer tending to unite with the palatine and thus divide the posterior nares. This feature is observed also occasionally in B. dispar. Rugosities for the attachment of the recti capitis antici majores muscles are located at the junction of the basioccipital and basisphenoid (type skull and Am. Mus. 1069), another Brontops character. The external bridge over the infraorbital foramen is relatively narrow in contrast with its more primitive width, correlated with the longer face in A. marshi. The antorbital malar ridge is less rounded than in B. gigas and its congeners. There is a pair of small protuberances (k, k) on either side of the median line of the upper portion of the occiput, which very prob- ably were situated between the insertion areas of the median ligamentum nuchae and of the paired semispinalis capitis muscles. These protuberances are also observed in A. marsJii, B. gigas, and B. curtum. The great strength of the occipital pillars and the broadly rugose flare of their summits contrast with the narrow ridges observed in B. dispar. The hyoid elements, represented in Figure 425, include the stylohyals, 265 millimeters in length, which are expanded superiorly and exhibit a long, flattened shaft and a slight distal expansion terminat- ing in cartUage; the ceratohyals, 80 millimeters in length, expanded at both extremities. The epihyals were probably almost vestigial, as in other peris- sodactyls. In the horse they consist of a small nodule of bone embedded in the cartilage that con- nects the ceratohyal with the stylohyal. Dentition. — The variability in the upper incisors has already been mentioned. The formula is probably If^. The incisors exhibit the strongly convex, smoothly rounded crowns so characteristic of Teleodus avus, B. hracTiycepTialus , A. marshi, and B. dispar. The canines are quite distinctive of this species. In the males they are large and robust, recurved, pointed, and some specimens (Am. Mus. 1069) possess a strong posterior cingulum, thus being readily distin- guishable from the canines of the contemporary Meno- dus and Brontotherium. The diastema behind the canine of the Yale type skull is unusual, although it is seen occasionally in the earlier Brontops hrachycepha- lus and appears to be a somewhat distinctive specific character, since it is also observed in the two male skulls (Am. Mus. 1083, 1069). The premolars are highly distinctive in their retarded stage of evolution; p^ exhibits a rudimentary postero-internal cusp; the tetartocones of the succeeding p^"^ are rather feebly developed; in p* the tetartocone is a mere spur; the external cingula are barely indicated, and the internal cingula of the premolars are rounded and crenulate. In the molars also the cingula are irregularly de- veloped. The hypocone of m^ is variable in form, either cingulate (Am. Mus. 1083) or a small, acute cusp (Am. Mus. 1069). A reversional feature of interest is a crenulate crest or abortive metaloph connected with the hypocone, recalling the similar structure in B. irachycephalus. Skull Am. Mus. 1069 exhibits on m^ a well-defined "crochet" and " antecrochet, " and also a minute "crista." As Figure 422. — Skull of Brontops rohusius Top view. One-ninth natural size. Am. Mus. 1069; resembles the type of B. robustus in its extreme brachycephaly. The horns are shorter and thiclier at the tips. The midparietal opening is seen also in certain skulls of B. dispar. observed above, the grinding series tend to rise anteriorly. Additional details of the type sTcull. — In reference to the characters noted above, it is important to observe that the top of the type skull is somewhat crushed from front to back, increasing the concavity along the top line and the spread of the vertex. The grinding series are in the ninth stage of wear. It is in this skull that we observe extraordinary breadth and relative shortness, extremely robust zygomata, horns extremely far forward and oval in transverse section, well overhanging the abbreviated nasals, and the arching character of the opposite grinding series. The chief measurements are as follows: Top of occiput to tip of nasals 635 millimeters, transverse 496 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA A, BTontops dispar, Nat. Mus. 4941 (type) ; depth of horizontal ramus greatly diminished by vertical crushing, but the symphyseal re- gion was probably shallow. A short diastema between pi (which has dropped out) and the canine, which is stout and conical. In- cisors subspherical, external cin- gula not so sharply defined as in Menodus. B, B. Tobustus, Princeton Mus. 10061; symphyseal region crushed laterally, ramus with long, straight lower border, angle pro- duced. Two well-developed in- cisors, canine stout, diastema in front of pi (which has dropped out), external cingula not sharply defined. C, B. Tobustus, Yale Mus. 12048 (type); ramus long and shallow, symphyseal region very shallow, angle very large and produced downward, coronoid high, verti- cal truncate. Incisors stout, ca- nines very stout and short, diastema in front of pi, external cingula reduced, crowns of pre- molars and molars less hyp.^odont than in Menodus. All one-fifth natural size. Figure 423. — Lower jaws of Broniops dispar and B. robustus EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 497 width of zygomata 667, transverse width of occipital condyles 225, condyles to tip of incisors 765, molar- premolar series 350, true molars 220, p'~* 137; anterior enamel of canines, worn, 42; anterior diameter of canines, 36. Teeth (Pis. XX, C, CI). — The canines are relatively _.,...._ robust, obtuse, and re- curved, with narrow posterior cingula. The incisors have smooth, rounded crowns of equal size, two on each side. The diastema behind the canine is 20 millimeters and forms a very charac- teristic feature of the type. The premolar cin- gula are nearly obsolete except on the inner sides of p^~*; a cingu- lum is faintly shown on the outer side of p^. The tetartocones are feebly developed throughout, espe- cially on p*, in which the deuterocone con- stitutes the chief inner portion of the crown with a ridge- like tetartocone ex- tending posteriorly. Lower jaw (PI. CII and fig. 423).— The j aw is very deep from the condyles to the bottom of the angle; there is a robust, depressed angle, prominent, tall, rectangular coronoid process; the mental foramen is very large; the symphy- sis is long (20 mm.). There is a single lower incisor on either side of the lower jaw, with the vestigial median alveolus of a second incisor on the right side. There is a deep median cleft in the incisive border. The inferior canines are heavy and laterally compressed (transverse diameter 25 mm., anteroposterior diameter 32). The anterior pre- molars, pi, are small but bifanged on either side and are ready to drop out; there is a single alveolus on the left side. The external cingulum is reduced, being marked only upon the valleys and anterior face of the premolar-molar series, with the exception of m2_3, in which the external cingulum is somewhat more developed. In all titanotheres the inner side of the molar crowns is entirely devoid of a cingulum. The hypoconulid of ms is not very broad but has a FiGUBE 424. — Sections and con- tours of skull of Brontops sp. Am. Mus. 518 (for mounted skeleton, see PI. XXXV). The horn sections and zygomata suggest those of Brontops robustus. One- eighth natural size. crenulate internal crest, which is entirely absent in Menodus. Transitional skull to B. rohustus. — In the Princeton Museum is a fine skull (No. 10061, PI. XCV) which in many respects is transitional to B. rohustus. It belongs to a large male and was used in the first published restoration of "Menodus" coloradensis bj Scott and Osborn (1887.1). Although of smaller size it agrees with B. rohustus in the presence of a pair of smooth, rounded upper incisors, decided diastemata behind the canines, and in the general conforma- tion of the zygomata, retarded development of the tetartocones on the premolars, absence of ex- ternal cingula, and large upper and lower canines. It is more primitive than B. rohustus in the presence of two rounded lower incisors, in the more rounded summits of the horns, and in the somewhat less ex- panded zj^gomatic and nasal sections. The angle of the jaw is produced backward rather than downward. Figure 425. — Hyold bones of Broniops sp. (A) (Am. Mus. 518) compared with those of the tapir (B), black rhinoceros (C), and horse (D) Oblique medial aspect of right side. sJi, Stylohyal; ch, ceratohyal; bh, basihyal; ft, thyrohyal; glh, glossohyal. Compare hyoid bones of Golichorhinus longiceps (fig. 345). The glossohyal is lacking in Dolichorhinus and possibly in all other titanotheres. Ohservations on the sTcull of the mounted skeleton referred to Brontops rohustus (Am. Mus. 518). — The generic and specific determination of this skeleton is difficult owing to the imperfection of the skull. It was originally referred by Osborn to Titanothe- rium rohustum, but later he referred it to Brontotherium 498 TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA gigas, 9 , chiefly because the carpus of this specimen is slightly different from that of Marsh's type of Brontops robustus. Unfortunately the specimen lacks all the front teeth, as far back as p' and p2. The upper pre- molars have the tetartocones well constricted, as in Brontops, and very different from the circular tetar- tocones of male brontotheres. A supposed female of Brontotherium gigas (Am. Mus. 1006) also has the resemblance to the jaws of brontotheres; from Bron- tops robustus type it differs to some extent; perhaps its nearest resemblance is to the type of Diploclonus tyleri. The available measurements of the skull and dentition, though few, are nearer to those of Brontops robustus (especially Am. Mus. 1069) than to those of large male brontotheres. The specimen is much larger than the supposed female brontotheres of B. B A C Figure 426. — Sections and contours of skulls of Diploclonus bicornulus and D. tyleri A, Diploclonus hicornuius. Am, Mus. 1476 (type); horns much as in Brontops dispar but with an accessory hornlet, basal section roundly trihedral, nasals long, zygomata little expanded. B, Am. Mus. 1081, referred to i>. tyleri: stout horns roundly trihedral in section, connecting crest high, zygomata stout. C, D. tyleri, Amherst Mus. 327 (type); horns widely oval in section, accessory hornlets more pronounced than in hicornutus, zygomata expanded. One-seventh natural size. Sections and contours of D. bicornuius and D. selwynianus (see fig. 185) indicate that these forms, with their peculiar narrow nasals, are not ancestral in type to D. ampins, with its broad and abbreviate nasals. tetartocones much restricted, but the reference of this specimen to Brontotherium is very doubtful. In Am. Mus. 518 the external cingulum of the upper premolars and molars is absent, as in both Brontops robustus and Brontotherium, so this character is not decisive. The external cingulum of the lower pre- molars is reduced, but on the whole the dentition ap- pears closer to that of B. robustus than to that of Brontotherium. The lower jaw presents no close curtum and B. gigas. The sections and contours of the horns and nasals are certainly different from those of the supposed female brontotheres and still more so from those of male brontotheres. The sections are, in fact, closer to those of Diploclonus tyleri and Bron- tops robustus (especially Am. Mus. 1083). The manus has the magnum broader, more angulate than that of B. robustus as figured by Marsh, but as a whole the manus is similar in proportions to that of Brontops EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 499 rohustus. The hind limbs mounted with this skeleton belong to other individuals. On the whole the evidence indicates that the mounted skeleton is not a female brontothere but is a member of the Brontopinae and probably Brontops rohustus. Diploclonus Marsh {"Allops" Marsh, Osborn, 1902) Plates XXXVII, CIV-CX; text figures 185, 187, 196, 200, 201, 375, 389, 391, 394, 397, 409, 426-428, 630, 639 [For original description and type references see p. 227. For skeletal characters see p. 675] Generic characters. — A phylum col- lateral to other Brontops phyla, appar- ently distinguished by the budding off of a second hornlet on the inner side of the horns and hence loiown as Diplo- clonus; also by the narrow and elon- gate nasals in the middle members of the series; hornlets rising from the inner faces of the horns. Incisors 2—1. Premolars about as in B. dispar, but with external cingula becoming obsolete. Nasals progres- sively abbreviating. Bridge over in- fraorbital foramen rather sharp and narrow. Progressive brachycephaly ; also backward prolongation of the temporal fossae. Other characters as in Brontops. A tendency to give off an internal branch, knob, or "hornlet" (fig. 427) on the inner and anterior faces of the main horns is observed in a number of titanothere skulls which otherwise show strong affinities to the Brontops phylum and especially to the species B. dispar. Marsh regarded this inter- nal hornlet as of so much importance that he assigned it generic rank in proposing the genus Diploclonus, the genotype of which is the species Diplo- clonus amplus. The presence of a small accessory horn swelling has also been observed in certain skulls of Menodus and of Brontotherium. It is doubtful, therefore, whether this hornlet is of generic value. D. amplus and D. tyleri resemble B. rohustus in many features but may represent a distinct subphylum. Subsequently the same character was observed by Osborn in two skulls in the American Museum (Nos. 1476, 1081) which were made the type and paratype respectively of the species Diploclonus {" Megacerops") hicornutus (fig. a fine skeleton with skull was obtained by the Amherst Museum and described by Lull as a third species Dip- loclonus {" Megacerops") tyleri (Amherst Mus. 327). The sum of progressive characters is as follows: (1) Hornlets on the inner sides of the horns; (2) in- creasing brachycephaly; (3) gradual or retarded evolution of premolars, p* being the most retarded tooth. The sum of retrogressive characters is as A B Figure 427. — Sections and contours of skull of Diploclonus amplus A, Nat. Mus. 4710. In this supposed female the horns are relatively short, the connecting crest well devel- oped, horn section trihedral, nasals broad, zygomata slender, B, Yale Mus. 12015a (type); a male, with horns relatively long, directed forward and placed far in front of the orhits, basal horn sections trihedral, connecting crest high, zygomata well expanded, nasals small and narrow. One seventh natural size. follows: (1) Relative abbreviation of the premolar- molar series, the index in D. hicornutus being 45, which is less than that in D. amplus; (2) apparent reduction or loss of median incisors. A doubtful genus and phylum. — There is no question whatever of the affinities of these animals to Brontops 426), the latter now referred to D. tyleri; and finally j and of their substantial proximity to this genus. 500 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA From the materials at hand, which are hmited to four or five skulls at present, it is doubtful whether they should be separated as a distinct phylum of generic value. The question turns on whether the hornlet is a sport character or a permanently progressive character, which can be settled only by more extensive material than is now available. On the whole, it seems to be wisest at present to regard these species as a subphylum collateral with the- main phyla of B. dispar and B. robustus. Affinities with Brontops dispar and Allops marsTii. — • Among the chief resemblances to Brontops dispar are (1) the marked brachycephaly, especially in the widely expanding buccal processes of the zygomata, empha- sized most distinctly in D. tyleri and D. amplus; (2) the marked backward prolongation of the occiput behind the widest portion of the zygomatic arches, which dis- tinguishes these skulls at once from those of Allops marshi and B. robustus and relates them to B. dispar; (3) the short horns with rounded summits which par- take of the general characters of those of B. dispar, although a tendency to broaden and flatten becomes marked in D. tyleri and extreme in D. amplus. The nasals show progressive abbreviation: they are elon- gate in D. hicornutus, more abbreviate in D. tyleri, and extremely abbreviate in D. amplus. Features of difference from B. dispar are not only the internal hornlets upon the horns but the somewhat more marked development of the connecting crest between the horns. Unfortunately the records as to geologic distribution are very incomplete. The geologic level of the least progressive stage, D. hicornutus, is not recorded. The level of the intermediate stage, D. tyleri, is recorded as 35 feet above the Pierre shale, 200 feet below the summit of the Titanotherium zone; this would place this animal in lower B or even in A. The level of the most progressive species, D. amplus, is not recorded. Distinctive characters of the species of Diploclonus. — This is not a monophyletic series, like Menodus or Brontotherium; it is diphyletic. The skulls at once seem to be distinguished from those of Allops marshi and Brontops rohustus by the great backward extension of the occipital region behind the zygomata. The pit in the center of the vertex relates them to other species of Brontops. The progressive broadening of the zygomata closely parallels that of the other collateral phyla. D. hicornutus (Osborn) is the most primitive form, with relatively long and narrow nasals, rounded and more erect horns, least expansion of the zygomatic arches, and least depression of the angular border of the jaw. jD. selwynianus (Cope) is a little-known animal from the Cy- press Hills, Saskatchewan, represented only by the nasal bones, which are intermediate in size between those of D. hicornutus and D. tyleri; they resemble the nasals of D. hicornutus more closely in their narrow, elongate, and laterally decurved form. D. tyleri (Lull) is an intermediate stage of evolution, char- acterized by relative abbreviation of the nasals, more anterior position of the horns, greater prominence of the hornlets, more widely arched zygomata, depression of the lower angular region of the jaw. This animal is a more pronounced development of the D. hicornutus type, the internal hornlets being larger, the horns much broader and more decidedly projecting forward. The geologic level of this animal is said to be 50 feet above the Pierre shale. It is probably from the middle levels, B. D. amplus Marsh is distinguished by very short and obtuse nasals and a prominent internal hornlet on the horns, horns broadly divergent and compressed anteroposteriorly, excessively wide zygomatic arches, stout, recurved canines. The features of this progression as seen in the superior view of the skulls are displayed in Plate CVIII, A, and in Figure 391. This robust animal shows an extreme development of the characteristics of this subphylum. Massive skuH, very broad zygomatic arches, very short horns, with a decided development of the internal hornlets. Probably belongs on the levels of Chadron of C. Standard measurements in the Diploclonus phylum, in millimeters Upper teeth Skull Jaw CM fM i o II i a Ph 675 683 s i 1 615 468 "■680 a. it o 91 68 8 |l il 665 15 '"g Is K S ■2„ o II II |i 1 D. amplus, Yale Mus. 12015a, cf (tj-pe). D. amplus, Nat. Mus. 4710, 9 D. tyleri, Amherst Mus. 327, & (type) __ ?302 307 363 133 119 136 207 205 227 40 »40 45 24 22 35 '■119 ?140 115 130 104 98 ?84 93 170 375 140 235 42 31 685 355 340 -11 127 138 -6 222 206 710 680 608 86 -690 635 192 "183 D. bicornutus, Am. Mus. 1476, & (type) 232 »35 22 500 Percentage of change from Diploclonus EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 501 Measurements of slculls and jaws associated with or referred to the Diploclonus phylum, in millimeters Skull and upper teeth Lower jaw an/i teeth pi-m3 M'-m' Symphysis to glenoid Posterior canine to hyloid of ma Pi-m3 Mi-m3 Symphysis to condyle Depth below ms D tyleri, Amherst Mus. 327 (type) 363 340 227 206 375 -343 235 232 685 500 500 »325 154 Diploclonus bicornutus (Osborn) {"Megacerops" bicornutus Osborn, 1902) Plates CIV, CVII; text figures 196, 389, 397, 409, 426, 428 [For original description and type references see p. 231] Geologic horizon. — Undetermined, probably middle beds (Chadron B). Specific characters. — Skull index indeterminable from type; total length, 680 millimeters; length of grinding series, 340; index, 50. Nasals long and narrow (104 by 86 mm.). Horns subcircular (type) to transverse- oval (Am. Mus. 1081, c? ) in basal section; with anterior and internal hornlets; connecting crest moderately developed; horns placed anteriorly to orbits. Preor- bital bridge of malar with a sharp edge. Occiput greatly produced backward. This is the least specialized stage, one which is closest to Brontops dispar except in the narrow form of the nasals. Observations on the measurements of Diploclonus bicornutus. — Diploclonus is a less conservative and less clearly distinguished phylum than Brontops, Menodus, or Brontotherium. Aside from the presence of a small secondary horn swelling, a character seen also in certain other skulls which are not referred to Diploclonus, the type of D. bicornutus presents a mingling of the characters of Allops marshi, Brontops dispar, and even in some features of Menodus. In comparison with these species the measurements of the type skull are given below: Measurements of Diploclonus bicornutus, Brontops dispar, Allops marshi, and Menodus proutii, in millimeters pi-m' pi-p< M>-m3 . Pmx to condyles- Zygomatic index. Nasal length Nasal breadth Horn length D. bicornutus, Am. Mus. U76, c? (type) 340 138 206 680 104 183 Nat. Mus. 4248, cT 340 140 207 685 78 94 102 188 A. marshi, Am. Mus. 1445 (paratype) 335 135 203 675 69 105 100 113 Carnegie Mus. 3063, 9 335 135 203 628 120 125 150 The table shows that these specimens, which are referred to four genera, agree closely in dental meas- urements but differ in the proportions of their nasals and horns. Characters of the type. — The type of D. bicornutus was long supposed to be female, but the canines and zygomata are stouter than in females of other species. The specific characters above set forth are based upon the type skull (Am. Mus. 1476). Associated with this type in the original description by Osborn as a paratype was the skull Am. Mus. 1081 (PI. CV), which is now regarded as more nearly related to an- other species, D. tyleri. The following description is thus based solely upon the type of D. bicornutus. The sex of this specimen is somewhat uncertain, but the antero- posterior diameter of the canine (506 mm.) indicates that it was a male. The lateral compression of the type skull and jaws (Am. Mus. 1476) prevents the determination of the proportions of the skull or of the zygomatic index. The wearing of the grinders indicates that the specimen represents the eighth stage of growth. Thus we conclude that all the progressive characters are very distinctly developed and indicated. Skull. — The relatively long, narrow proportions of the skull as seen from above (PI. CIV) are due partly to lateral crushing. Even with allowance for some deformation the skull is not very broad. As seen in side view (PL CIV) it combines the long nasals of A. marshi with the vertically elongate horns and backward expanded occiput of D. dispar. An anterior hornlet is plainly visible. The tuberosi- ties on the inner sides of the horns are only faintly developed, and were it not for comparison with the specimens of D. tyleri they would hardly be noticed. 502 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Similar anterior tuberosities, or hornlets, are observed in certain male skulls of M. giganteus (Am. Mus. 505). As seen ftom above (fig. 444) the parietal vertex is moderately broad. A very distinctive feature is a sharp preorbital malar bridge. There is considerable expansion behind the orbits, as in B. dispar; as in all aged individuals, the supratemporal crests are sharp and overhanging. The summits of the occipital pillars are rugose and greatly expanded. In lateral view (PI. CIV) the postglenoid process is seen to be relatively narrow and exhibits a short surface of contact with the post-tympanic. On the palatal surface of the skull the vomer apparently does not extend back to overlap the parasphenoid. There is only a slight basisphenoid rugosity. This skull shows group resemblances to Menodus trigonoceras as follows: (1) Nasals long, set very high; (2) orbit very large; (3) tooth row very long (dental index 50, 46-51 in M. giganteus); (4) zygomata not expanded and in side view pitching sharply downward and forward; (5) lower jaw resembling that of Menodus in the well-developed chin and depressed angle. Dentition. — The incisor alveoli are not well defined. The canines are much worn; they exhibit crowns of rounded form. The absence of external cingula on the premolars may be due in part to the extreme wear. The internal cingula are well developed. The tetarto- cone developments on the premolars are approxi- mately of the same stage as in B. dispar. In m^ the hypocone is an elevated part of the cingulum. Summary. — This skull may be that of an old male, affording an example of a progressive oft'shoot of one of the Brontops lower beds phyla. Diploclonus selwynianus (Cope) {Menodus s^elwynianus Cope, 1889; " Megacerops?" selwynianus Osborn, 1902) Text figure 185 [For original description and type references see p. 225] Type locality and geologic horizon. — Swift Current Creek, Saskatchewan; Titanotherium zone. Specific characters. — Nasals long (free length about 115 mm.), narrow (free width 75 mm.), abruptly rounded in front and sharply decurved at the sides, with a deep longitudinal inferior cavity. The chief ground for referring the uniquely narrow nasals of the type of this species to Diploclonus is the resemblance they bear to those of Diploclonus bicornu- tus, which is closer than to that of any other form. The resemblances in the nasal bones consist in (1) the narrowness, (2) the deep longitudinal inferior cavity, (3) the decurved sides, and (4) the massive and de- curved tips. Cope's description of this specimen from the Swift Current Creek region is cited in full elsewhere (p. 226). To this description may be added the following ob- servations. The inferior view (fig. 185) exhibits the paired cavities, or frontal antrum, at the junction of the nasals, frontals, and maxillaries; the suture be- tween the nasal bones has entirely disappeared. These nasals are very characteristic and quite dif- ferent from those of Menodus or Megacerops colo- radensis. Lambe describes the type as follows (1908.1, p. 47): The coossified nasal bones of one individual constitute the type of this species. They are long and narrow, abruptly rounded in front, and bent downward at the sides. The lower surface is deeply excavated in a longitudinal direction. Diploclonus tyleri (Lull) {Megacerops tyleri Lull, 1905) Plates XXXVII, CV-CVIII; text figures 200, 201, 397, 409, 426, 428, 630, 639 [For original description and type references see p. 234. For slteletal characters see p. 675] Geologic horizon. — Recorded as 35 feet above the Pierre Cretaceous, or 165 feet below the summit of the Titanotherium zone. Regarded by Lull as in the upper part of the lower Titanotherium zone, but more probably belonging in the level of the middle beds. Specific characters. — Brachycephalic. Zygomata widely arching. Nasals abbreviated or broad; free length, 140 millimeters. Grinding series, 363. Horns shifted forward; distinct hornlets on the inner sur- faces; horn section at the base trihedral to trans- versely oval, with sharp outer angle. Canines stout, recurved. Superior incisors 2-1. General characters. — This animal is certainly a member of a collateral branch of the B. dispar series, agreeing with the aged specunens of the typical B. dispar even more closely than the type of D. bicornutus. The species appears to represent a stage of phyletic evolution intermediate between that of the more primitive D. bicornutus and the more pro- gressive D. amplus. This conclusion, however, awaits confirmation by additional evidence. Materials. — The type of this specific stage or ascending mutation is the fine skeleton and anterior portion of the skull in the Amherst collection (No. 327) . Apparently belongmg to the same stage is the finely preserved skull and jaws (Am. Mus. 1081) originally described by Osborn as a paratype of D. bicornutus but presumably representing a more recent or pro- gressive stage of evolution. Observations on the measurements of Diploclonus tyleri. — The two skulls referred to Diploclonus tyleri differ from the type of D. bicornutus in having larger molars and more massive horns. They differ from the type of D. amplus in having larger molars. The type skull and jaw in side and top views approaches that of Brontops robustus, except that it has a well- developed pair of accessory horn swellings. The characters of the incisors, canines, postcanine diaste- mata, and premolars further suggest affinity with B. robustus; but the anteroposterior measurements of the dentition differ from those of Brontops and agree with those of Menodus trigonoceras, as shown below: EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 503 Measurements oi Di-ploclonus tyleri and Menodus irigonoceras, in millimeters D. tyleri M. trigonoce- Amherst Mus. 327, (f (type) Am. Mus. 1081, cf ras, Nat. Mus. 4291, J Pi-m5 363 355 360 Pi-p' : 136 127 136 Mi-ms 227 222 224 680 770 ! This agreement with Menodus irigonoceras, taken in connection with the relatively slender manus and pes of the type of D. tyleri as compared with B. roiustus, constitutes one of the many facts which suggest the possibility of occasional hybridization among the genera Bron- tops, Menodus, and AUops (W. K. Gregory). Description of the type. — Lull's specific defi- nition may be cited in full (Lull, 1905.1, p. 445): Horns well in front of orbits, directed somewhat forward and outward, an elongate oval in basal section with the long axes in line, rounded oval at the summit. Hornlets quite conspicuous, on the inner face of the horns midway between tlie base and summit. Connecting crest low and inconspicuous. Nasals broad, well rounded in front, and but slightly arched beneath. Zygomata expanded and deep, with a well-rounded outer face. Dentition: Superior incisors represented by the deep and well-defined median alveoli and by the lateral teeth, which remain in place and which have hemispherical crowns which show little sign of wear. The canines are lanceolate, with a well-developed postero-internal cingulum. There is a short diastema in front of, and a longer one behind, the canine. Premolars with a smooth internal cingulum, less pronounced in the middle of the tooth and with no external cingulum. The deuterocone is well developed, while the tetartocone, especially of premolar 4, is inconspicuous. The jaw is deep and robust, with the alveoli of two incisors, probably of the second and third, deep and distinct. There is no space between the lateral incisors and the canine, though be- tween the two median alveoli a considerable gap occurs. There seems to have been a small diastema behind the lower canines, which are lanceolate, though with a less prominent cingulum and not so strongly recurved as the upper ones. The same author observes that the creature most nearly resembles D. bicornutus (Osborn) and D. amplus Marsh, having certain characters suggestive of each; but there are enough important differences to render it distinct and to warrant the erection of a new species for its reception. The dental formula is If, C\, P|, M|; the two median superior and all of the lower incisors are represented by deep inclosed alveoli, as 101959— 29— VOL 1 35 if the teeth had dropped out after death. Toward the base the horns are oval to triquetrous in section; toward the summit they become rounder and rough- ened at the extremities. "It would seem," observes Lull, "from the similarity of the roughened patches to those on the rhinoceros nasals, as though the entire prominence had been clothed with skin, with two rhinoceros-like horns, a larger one at the apex and a smaller one on the summit of the hornlet." Figure 428. — Lower jaws of Diploclonus bicornutus and D. tyleri , Diploclonus bicornutus, Am. Mus. 1476 (type); animal very old and hence the angle is broad and prominent (compare the aged type of M. torvus, fig. 437, A). Canines stout and conical, external cingula not sharply separated from the eotoloph, B, D. tyleri, Amherst Mus. 327 (type); broadly resembles S. robusius. A diastema in front of pi. One-fifth natural size. Characters of referred specimen (Am. Mus. 1081). — This specimen was employed by Osborn as a paratype of D. hicornutus. Skull broad, index 85 ; tooth row elon- gate (367 mm.) ; index 50. Condyles to incisive border 710 millimeters. Horns anteriorly placed and directed obliquely forward. It is important to note that the plaster restoration of the nasals (Pis. CV, CVII) and of the anterior part of the zygomata prevents a true determination of the characters of these parts of the skull. This specimen is also of advanced age and 504 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA shows many senescent characters. The teeth are extremely worn and reveal nothing decisive; the cingula of the premolars are smooth and worn down. The horn tips are extremely rugose, and the internal hornlets are well marked (fig. 426, B). The occipital vertex is rugose, with deep paired indentations and knobs. The zygomata are greatly swollen trans- versely. As in many aged specimens of Brontops, a single large incisor is preserved in either premaxUla; the superior incisive formula was thus 2-1. Diploclonus amplus Marsh, 1890 {"Allops" amplus, Osborn, 1902) Plates CVIII-CX; text figures 187, 376, 389, 391, 394, 409, 427 [For original description and type references see p. 227] Geologic Jiorizon. — Titanotherium zone, level not recorded. Specific characters. — Skull extremely brachycephalic, zygomatic-basilar index 91. Nasals greatly abbre- viated and narrow, horns broadly oval in basal section, directed upward and outward, with conspicuous internal hornlets. Grinding series relatively reduced, length 302 millimeters, index 45. Upward flexure of premolar series anteriorly pronounced. Zygomata extremely broad with convex buccal expansions. General characters. — The nasofrontal section of the type skull first suggested comparison with a form transitional between Brontotheriwn gigas and B. curtum, but numerous other characters forbid such a phyletic reference, especially the long-pointed canines, 40 millimeters (the type was a male), the presence of only one superior incisor on each side, the retarded development of the tetartocones of the premolars, the rounded tips of the horns. All these characters remove the animal from relationship with Bronto- therium, Menodus, or Megacerops and indicate its affinity to Brontops; this is confirmed by more searching study of the character of this type, but especially by the existence of the more primitive and transitional forms D. hicornutus and D. tyleri above described. Diploclonus amplus is by far the most progressive species in this phylum, as demonstrated by (1) the extreme abbreviation of its nasals, (2) the strong development of the connecting crests between the horns, (3) the greater prominence of the internal hornlets, and (4) the transverse-oval expansion of the basal horn section, which has now reached an extreme stage, parallel to that observed in Brontops rohustus. This species, however, is readily distinguished from Brontops rohustus, in common with other members of the phylum to which it belongs, by the marked backward elongation of the occipital region behind the zygomatic arches. This specimen tends to confirm the hypothesis that the species grouped under Diploclonus formed one or more subphyla parallel with Brontops. Materials. — The type skull of D. amplus in the Yale Museum (No. 12015a) is undoubtedly a male. A supposed female skull is found in Nat. Mus. 4710. The type is the only specimen which we have yet seen that certainly belongs to this species.. Observations on the measurements of Diploclonus amplus. — The type and only known male skull of this species is very brachycephalic, although crushing may have contributed to the extremely high zygomatic index (91). The male skull approaches the paratype of Allops marshi in the anteroposterior measurements of the dentition, but in its general conformation it rather suggests B. rohustus. The nasals are reduced in size. Measurements of Diploclonus amplus and Allops marshi, in millimeters Pi-m3 Pi-p' M'-m3 Pmx to condyles Zygomatic index D. amplus, Yale Mus. 12016a (type) (?)302 133 207 675 (?)91 A. marslii. Am. Mus. 1445 (para- type) 335 135 203 675 69 Detailed description of the type. — As seen from above the skull is broad and short in proportion, the index, 91, expressing its marked brachycephaly. We are struck by the strong divergence of the horns, a feature which is probably intensified by vertical crushing. They are slightly convex on the anterior surface, with very prominent external ridges. They are united by a very deep connecting crest, as exhibited in the section. The prominent tuberosity or hornlet employed by Marsh as a generic character is 14 centimeters below the tip of the horns; it is especially well developed on the left side, measuring 37 millimeters anteroposteriorly and 35 transversely. A very distinctive feature of the horn is the sharpness of the preorbital crest. As seen from above (PI. CVIII, A) the zygomata project widely outward. A slightly distorted section of the buccal processes is shown in the diagram. The upper surfaces are somewhat flattened. In the middle of the vertex is a shght tuberosity which probably indicates the vestige of the pit so characteristic of members of the Brontops phylum. The occiput is deeply exca- vated with stout lateral pillars and rugose summits. It is considerably produced backward behind the zygo- mata. In inferior aspect the posterior nares are seen to open just opposite the third molar. The pterygoid wings of the aHsphenoid are short, rugose, flaring strongly outward; the presphenoid is keeled by the EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 505 v^omer, and there is a very prominent rugosity at the junction of the basisphenoid and basioccipital, as seen m Brontops and Menodus. Dentition. — Since the animal is of advanced age, we find just within the canine a large alveolus which cer- tainly contained a large incisor tooth. In early life there were probably two incisors, separated in the median line by a diastema. The canines are long (40 mm.) and pointed, with a postero-internal cingulum. The first premolar is a relatively stout, persistent, bifanged tooth, thrust closely against the canine, partly by crushing; the tetartocone of p^ occu- pies a very small part of the inner face of the crown, as in B. dispar. Illustrating the mechanical ineffective- ness of the internal cones of the molar teeth is the fact that whUe the ectolophs are worn out of proportion in m'~^, the internal cones of m^, m^ are still unaffected by wear. The hypocone of m' is quite prominent and still connected with the cingulum. The abbreviation of the premolar-molar series is a retrogressive character which this phylum apparently shares with the Brontotherium phylum. The molar series, measuring 302 millimeters, is actually shorter (index 45) than that in the much less specialized form D. hicornutus, in which the teeth measure 340 millimeters with an index of 50. Similarly, in the great skulls of Brontotherium the tooth row is actually shorter in the highly specialized B. platyceras than in the ancestral species B. gigas. Supposed jemale skull (Nat. Mus. 4710). — The result of searching comparison and measurements is the reference of this skull as a female of Diplodonus amplus. The morphologic difference is about as great as between the type of D. hicornutus and that of D. tyleri. The affinities to Brontops in this female skuU are shown in the foUowing points: (1) Small lateral in- cisors persistent; (2) canines of rounded form; (3) premolars with retarded tetartocones, rounded in- ternal and fainter external cingula; (4) horns of small size, with long, flat external face and rounded top section; (5) orbits with broad postorbital processes; (6) a broad zygomatic shelf, and downward extension of the occiput as in B. dispar; (7) rugosity on the basisphenoid, with the vomerine bridge carried well back. The special resemblances of this supposed female to D. amplus are (1) 'the general similarity of the horns in respect to their position, basal sections, and con- necting crests; (2) the smaller zygomatic section; (3) the sharp preorbital malar bridge. The chief points of difference between this specimen (Nat. Mus. 4710) and the Yale type of D. amplus, which is a male, are (1) absence of hornlets, perhaps a sexual distinction; (2) greater width, flatness, and slenderness of the nasals, which may be attributed in part to the nondevelopment and lack of forward advancement of the horns; (3) smaller buccal processes of the zygomata, which, however, in section suggest those of D. amplus on the inner and inferior faces especially. (See fig. 427.) The reference of this skull to D. amplus is therefore still provisional. Subfamily MENODONTINAE Titanotheres chiefly of lower Oligocene age, reaching their climax in the upper levels of the Titanotherium zone (Chadron formation). Distinguished by narrow heads (mesaticephalic) diverging into phyla with long heads (dolichocephalic) and broad heads (brachyce- phalic). Horns short, slightly shifting forward, broadly trihedral in basal section and widely divergent at the summits. Nasals typically elongate, broad and square distally, secondarily abbreviate. Incisor teeth reduced or vestigial. Grinding teeth with prominent cingula. Premolars with accelerated tetartocones. This is the second branch of the short-horned ti- tanotheres, which in many respects is closely related to the first branch, the Brontopinae; in others it appears to present original characteristics of its own. It is typified by the genus Menodus, also known as Titanotherium, and includes the related genus Allops. Between the two are transitional forms of subgeneric rank described as Menops and Anisacodon by Marsh. The resemblances of the original species of these two main phyla of Menodus and Allops, namely, Menodus heloceras and Allops walcotti, both occurring in the very base of the Titanotherium zone, are relatively close to Brontops hrachycephalus. Yet it would appear that the Menodontinae diverged from the Brontopinae before the end of Eocene time. While the Menodus phylum maintains its mesaticephalic and dolicho- cephalic form throughout, members of the Allops phylum become brachycephalic and converge toward members of the Brontops phylum. Thus Allops crassi- cornis resembles Brontops rohustus in its proportions. The ancestors of Menodus and of Allops, although not readily distinguishable in the base of the Titano- therium zone (Chadron A) , indicate that the divergence took place in late Eocene time, when the following main distinctions were established: Menodus phylum Primitively mesaticephalic, progressively dolichocephalic. Incisors §;§, extremely vestigial, buried beneath the gums. Canines very prominent, rounded, or compressed transversely. Nasals typically broad and elongate. Skeleton tall. Feet stilted, of mediportal type. Allops phylum Primitively mesaticephalic, progressively brachycephalic. Incisors reduced, with rounded summits. Canines prominent, compressed anteroposteriorly, expanded transversely. Nasals progressively abbreviating. Skeleton little known. Feet broader. 506 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES IN THE ALLOPS PHYLUM AUops Marsh Plates XX, XXI, XXXVIII, XC, CXI-CXXII, CXXXII; text figures 184, 189, 197, 207, 375, 378, 381, 387, 389, 391, 393, 394, 397, 399, 409, 413, 429-434, 608, 612, 615, 712 [For oi-iginil description and type references see p. 224. For ske:etal characters sec p. 678] Generic characters. — Mesaticephalic, progressively brachycephalic. Lateral pair of superior incisors persistent; canines compressed anteroposteriorlj^, flat- tened on posterior face; grinding teeth with moder- ately distinct cingula; premolars simpler than in Menodus but with progressive tetartocones; molars with elevated and pointed cusps, the transverse slightly exceeding the anteroposterior diameters. Nasals broad and intermediate in length, abbreviat- ing in progressively brachycephalic types; horns short, broadly trihedral in section, directed obliquely out- ward. This genus was estabhshed in 1887 by Marsh, who selected the species A. serotinus as the genotype. In 1891 this author strengthened the genus by adding the species AUops crassicornis, a more advanced form. In 1902 Osborn described the species Megacerops marshi, named in honor of Professor Marsh, which was later referred to the AUops phylum. Finally, in 1917, Osborn added the most primitive species AUops walcotti, named in honor of the former Director of the United States Geological Survey. Although these species are grouped within a single genus, they ap- parently do not constitute a direct phyletic succession. There are also puzzling affinities to Menops varians, the type of the genus Menops. Progressive brachycephaly is characteristic of these scattered and loosely related species, as shown in the following ascending series in the Titanotherium zone: Zygomatic index Upper beds: AUops crassicornis (type), brachycephalic. _ 76 Upper beds: UTenops wonons, subbrachycephalic 73 Svimmit of middle beds: AUops serotinus (type), sub- brachycephahc 72 Middle beds, lower levels: AUops marshi, mesa,ticepha.\\c- 64-69 Base of lower beds: AUops walcoUi (type), mesatice- phalic (?) Geologic distribution of AUops. — AUops walcotti Osborn: A. walcotti, from the lower levels of the lower Titanotherium zone, is a small, very primitive form, distinguished by a narrow head, long, broad nasals, and relatively long face, horns with elongate oval section, and two superior incisors. AUops marshi (Osborn): A. marshi, from the upper levels of the lower beds, is distinctly mesaticephalic. Nasals long and broad, horns subtrihedral in section, premolars more progressive than in A. walcotti. This animal is finely represented by eleven skulls in several museums, which present a series of ascending muta- tions. AUops serotinus Marsh: In A. serotinus the nasals are still elongate and the horns are slender, elongate, subtrihedral, preserving the section characteristic of A. marshi. The premolars are still in a retarded stage of evolution. This species is represented by five speci- mens, two of which appear to present transitional stages toward A. crassicornis, as follows: Skull Am. Mus. 520 appears to be in a transitional stage between A. serotinus and the more brachycephalic species A. crassicornis, for the horns are in an intermediate stage of development and the dental measurements have the degree of development of the^premolar tetartocones coincident with those in A. serotinus. Transitional skull Nat. Mus. 4938 nearly equals in some of its meas- urements the A. crassicornis type, but the premolar tetartocones are still decidedly retarded. AUops crassicornis: The type specimen of the species A. crassicornis is distinguished both by the more massive proportions of the skull, the obtuse, short and massive horns, the abbreviation of the nasals, and especially by the more advanced development of the tetartocones upon the premolars. It is by no means certain that this massive, broad-headed animal is a descendant of the types- named above. Stratigraphic position of species of AUops Stage C Level Species Upper. Middle. Lower. ?A. montanus. ?[Menops varians]. ?A. crassicornis. B Upper. Middle. Lower. A. serotinus. A. marshi. A Upper. Middle. Lower. A. marshi. A. walcotti. EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITAN OTHERES 507 Progressive pJiyletic characters. — Members of the Allops phylum are known from the whole Titanothe- rium zone. In general, the skull and teeth are inter- mediate in character between the typical Brontops and typical Menodus. The skull in males, originally mesaticephalic, shows a marked progression toward brachycephaly, the zygomatic indices rising from 64 to 76. The nasal bones in the males progressively shorten as in members of the Brontops phylum. The broadly trihedral basal section of the horns connects these elements with Menodus rather than with Bron- tops. The face is relatively longer than in Brontops and more abbreviate than in Menodus. As in Bron- tops the incisors are round topped with a formula of 2-1. One of the most distinctive features of Allops is the transversely lanceolate form of the canine teeth which enables us to connect A. vmlcotti with this series rather than with the Menodus series. The opposite grinding series are rectilinear, or nearly parallel, as in Menodus. While the grinders approach those of Menodus in having elongate or subhypsodont crowns they are less dolichocephalic in proportion — that is, the transverse diameters of the molar teeth slightly exceed the anteroposterior diameters, whereas in Menodus the reverse is the case. The dental index is high — in males 46-47, in females 45-50. The premolars exhibit pronounced external cingula as in Menodus. Briefly, these animals resemble Menodus in the trihedral shape of the horns and in numerous other characters. They differ from the true Menodus in the progressive abbreviation of the nasals, in the brachycephaly of the zygomatic arches, in the reten- tion of at least one pair of upper incisor teeth. Thus they are provisionally assigned an intermediate phyletic position. Several of the more advanced or upper-level speci- mens of ^. serotinus were discovered in the overflow de- posits of the upper Titanofherium beds rather than in the main sandstone or channel deposits. This may afi'ord some clue to the rarity of these crania. Characters of the genotype. — In describing the genotype, Allops crassicornis , in 1887 Marsh charac- terized it as a skull resembling in general that of Menodus giganteus but as differing in the possession of a single superior incisor tooth. The type possesses a pair of well-developed second incisor teeth which are always vestigial in Menodus but present in the subgenus Menops. There are other still more im- portant differences, which will be enumerated in the description of this species. Affinities of Allops. — Allops marshi has a very low zygomatic index (64-69) in comparison with Allops serotinus (72) and A. crassicornis (76), a fact which suggested the theory that A. marshi may represent the females of some other species such as B. dispar. Some of the smaller skulls referred to Allops marshi are with difficulty distinguishable from females of Brontops hrachycephalus ; others approach B. dispar, with which they agree in dental measurements (see above); many are also strongly suggestive of Menodus trigonoceras in the characters of the premolars and molars and in the horns. All known skulls of A. marshi are distinctly inferior in size to those of A. serotinus. As shown by the detailed characters of the skull and dentition and by the tables of measui'e- ment, A. serotinus and A. crassicornis combine the characteristics of Brontops and of Menodus in a remarkable manner. They exhibit the characteristic horn sections and distally squared nasals of Menodus, the sharp premolar and molar cingula of Menodus, combined with the broader skidl and broader grinding teeth, expanded zygomata, shortened free nasals, and retained incisors of Brontops. The form of the canine also is more or less intermediate between the conical canine of Brontops and the sharp-edged canine of Menodus. The type and only known specimen of Menops varians resembles Allops crassicornis in the thick outwardly divergent horns, in the wide skull (index 73), in the wide premolars and molars, in the reduplicate tetartocone of p*. On the other hand it approaches Menodus giganteus in the anteropos- terior dimensions of the dentition and skull and in the nasal and horn sections, so that it is placed in the genus Menodus. These facts suggest the possibility that some of the forms called Allops may represent a hybridization between species of Brontops and of Menodus. Measurements, in millimeters, of lower jaws and teeth correlated with and referred to Allops Posterior canine to hypoconulid of ma Pi-m3 M ,-m:i Symphysis to condyle Depth below ms A. marshi, Nat. Mus., Gidley's skull 320 335 225 375 94 ?A. walcotti, Nat. Mus., 4247 303 298-1- 200 510 108 508 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Standard measurements in tJie Allops 'phylum,"- in millimeters XJpper teeth Skull Jaw and teeth 1 "p. a a > §1 s i o T o 8 S (1, 1 ft 5 .1 1 1 a 1 1 i a, i h 3 l| SB as D "ft a >> j A. crassicornis, Nat. Mus. 4289, cf (type) . 370 150 148 142 131 220 217 213 9,10 45 44 20 30 750 ''570 76 143 146 72 210 A. serotinus, Nat. Mus. 4938,? 335 330 330 330 38 36 26 40 23 20 739 720 665 705 + 702 675 "600 565 430 525 72 78 64 74 695 710 648 690 96 68 115 81 137 106 118 133 133 206 A. serotinus, Nat. Mus. 2151,9 A. serotinus, Nat. Mus. 4251, cf (type)__ "^A (dispar) serotinus'^ Nat Mus. 1217 i-UO 140 196 195 196 203 203 198 205 200 193 179 194 183 192 191 185 186 185 169 + 30 240 230 A. marshi, Am. Mus. 1445, cf (para- 335 323 323 32C 320 31S 313 317 315 ''31C 31C 31C 30? 30C 28e 135 133 129 126 130 133 132 129 131 126 122 ?80 124 119 112 119 + 34 37 "22 466 69 615 105 100 113 35 23 24 663 660 665 427 64 620 545 625 99 93 112 105 108 140 A marshi'' Nat Mus. 4738 430 64 366 127 240 31 28 A marsW Nat. Mus. 4942 A marshi? Nat. Mus. 8314 34 22 662 655 675 i'640 673 656 643 630 640 448 420 465 67 64 69 336 113 225 35 20 375 A marshi' Nat. Mus. 1213, ? — - 98 33 34 23 25 310 115 207 33 20 520 A. marshi, Am. Mus. 501, 5 (type) A marshi, Field Mus. 6900 450 "440 ?476 450 "340 67 66 ?74 71 625 620 696 100 103 105 103 105 126 97 330 A. marshi?, Carnegie Mus. (?) 341,9 .__ A. marslii?, Nat. Mus. 1215, 9 27 34 19 20 A. walcotti, Nat. Mus. 4260, d' (type) ._ _ A walcotti Nat Mus 8753 686 105 98 100 34 Percentage of change from Allops wal- + 3C + 17 + 67 -11 + 40 + 240 ....|____ o Allops, like BiploclonuSj is a less clearly consecutive and distinguished phylum than Brontops, Menodus, or Broniothenum. For observations on the measurements see p. 507. ' Estimated. The Allops phylum as represented in the Hatcher collection of 26 sTculls and lower jaws from the Chadron formation in the United States National Museum Genus an d species Catalog No. Material A crassicornis Do Marsh 4289, 4709, 8740, 2117, 4938, 2161, 4261, 1226, 8318, 4945, 8731, 8777, 1216, 8769, 4252, 8737, & & cT 9 9 & & 9 9 & 9 9 & cf Skull. Type. Skull. Measurements agree with type. Canine not typical. Do Skull, right half of jaw. Agrees in size with type. Nasals, upper canines, and incisors Do lacking. Specific reference doubtful. Skull. A serotinus M Do Skull. Horn like that of A. crassicornis. Skull. Do Skull. Type. Do Anterior part of skull. Do - Skull. Measurements agree with A. serotinus, female. Canine not typical. Specific Do reference doubtful. Skull. Same size as large Brontops dispar. Specific reference doubtful. Do Anterior half of skull and jaws. A. A. ? serotinus Marsh marshi (Osborn) Do- Pair of jaws. Skull and teeth. Measurements agree with A. marshi. Larger than B. brachycephalus. Skull. Larger than any specimen of A. marshi; approaches A. serotinus in molars. Do_ Skull. Resembles paratype of A. marshi. Do Skull and jaws, left humerus, and pelvis. Close to type. Molars same length as those of Brontops dispar. EVOLUTION OF THE SKTJLL AND DENTITION OF OLIGOCENE TITANOTHERES 509 The Allops phylum as represented in the Hatcher collection of 26 skulls and lower jaws from the Chadron formation in the United States National Museum — Continued Genus and species Catalog No. Material A. marshi (Osborn) Do - - _- -- 1213, ? 8798 8317? 4738 4942 4778 4254, cf 4260, & 4247, ? 8753 Skull. Tooth measurements agree with A. marshi. Slender zygomatic arch. Right lower jaw and symphysis. Do -- Skull. Generic reference doubtful in absence of canines and nasals. A ? marshi? (Osborn) Do Skull. Agrees in measurements with other skulls of A. marshi. Upper teeth. Do Skull. Very young individual. Generic and specific reference doubtful. Do Skull. Vestigial lateral superior incisors. Generic and specific reference doubtful. ^ Skull. Type advanced in age; see paratype (No. 8753). Do Jaws. Do Nearly complete upper dentition (i^-m^). Paratype. Palate slightly smaller than in type. Canine measurements very characteristic of the genus. Allops walcottl Osborn Plates XX, XXI, CXI, CXII: text figures 207, 389, 391, 394, 397, 409, 413, 429, 430 [For original description and type references see p. 241) Geologic horizon. — Lower Titanotherium zone (Chad- ron A) of South Dakota. Specific characters. — Premolars with small tetarto- cones; p'-m' 285 millimeters. Incisors %. Horns elongate oval, no connecting crest. Mesaticephalic. Nasals elongate, broad. Face relatively elongate. The type skull of this species (Nat. Mus. 4260) from Chadron A is narrow and elongate, partly owing to lateral crushing. This feature conceals its resem- blance to Allops marshi, which is apparent in other features — namely, (1) primitive, long nasals, (2) horns primitively short and obliquely oval, (3) large lateral incisor (i2) and small first (ii) or median incisor, (4) premolars accelerated, tetartocones more advanced than in Brontops rohustus of level C. Observations on the measurements of Allops walcotti. — The type of this species exhibits the following measure- ments in comparison with skulls of B. hrachycephalus and Menodus heloceras, which shows that the type of Allops walcotti has relatively large premolars and small molars. Measurements of Allops walcotti, Menodus heloceras, and Bron- tops hrachycephalus, in millimeters Pi-m3 Pi-p< M"-m3 Pmx to condyles Nasal length Horn length PS ap.Xtr MS ap.Xtr A., walcotti, Nat. Mus. 4260 (type) 285 112 169 640 105 100 35X51 60X61 M. heloce- ras, Am. Mus. 14576 265 170 603 132 70 B. brachycephalus Nat. Mus. 4940, 9 265 101 160 102 32X51 62X70 Nat. Mus. 4261, cT (type) 280 »104 178 580 85 33X53 68X73 ' Estimated. The skull is crushed laterally, but probably had a low zygomatic index — that is, it was mesaticephalic. While its reference to Allops requires confirmation, its nearer affinities appear to be with this genus rather than with Brontops or Menodus. The external cin- gula of the premolars are not as sharply defined as in other primitive members of the menodontine group. Geologic and geographic distribution. — This species is represented at present by a single skull, the type (Nat. Mus. 4260), which is recorded as probably from the lower levels (A) of the lower Titanotherium zone of South Dakota. Comparison with other species. — This animal should naturally be compared with other titanotheres from the lower beds. It is readUy distinguished from B. brachycephalus by a number of characters as follows : (1 ) The skull is much more doHchocephalic, a feature that is intensified by lateral crushing (see below); (2) the nasals are long and subquadrate in form; (3) the individual measurements of the grinding teeth show that the series throughout is somewhat narrower than the grinding series in B. brachycephalus. These contrasts with the broad-skulled B. brachy- cephalus naturally suggest comparison of this animal with primitive members of the long-skulled Menodus phylum, such as M. heloceras, remains of which from the lower Titanotherium zone are sparsely known. Comparison with M. heloceras shows that A. walcotti possesses the following distinctions: (1) Horns more elongate-oval in section, less trihedral; (2) no con- necting crest between the horns; (3) alveoli for two incisor teeth of considerable size (incisors are extremely vestigial in the Menodus phylum). There remains the comparison with Allops marshi, the form to which, on the whole, this cranium seems to present the largest number of resemblances. The animal differs from A. marshi in the inferior dimensions of the dental series as well as in the detailed propor- tions of the teeth and the characters of the premolars, but it appears to be an older and more primitive form which belongs to the A. marshi phylum. In _ the horn section, in the proportions of the nasals, in the 510 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA shape of the canine teeth it strongly resembles A. marshi. At the same time Allops walcotti presents certain resemblances to Menodus Tieloceras and was long re- garded by the author as ancestral to Menodus, which it foreshadows in its apparent dolichocephaly and in the strongly developed external cingula of the pre- molars as well as in the somewhat accelerated pre- molar tetartocones. Description of the type sTcull. — The type skull, No. 4260, as figured on Plates CXI and CXII of this mono- The skull is in the seventh stage of growth. It exhibits a slender but strongly indented occiput in the center of which is a median ridge which projects from the superior border. The cranial vertex is ex- ceptionally long and narrow, with a decided lateral crest overhanging the supratemporal fossa; in the mid- region of this supratemporal crest we note (PI. CXI) two lateral projections which are also observed both in Allops serotinus (Nat. Mus. 4251) and in several skulls of the Menodus phylum. In front of these are the supraorbital projections. The horns are lateral B C D Figure 429. — Sections and contours of skulls of Allops walcotti and A. marshi A, Allops walcotti, Nat. Mus. 4260 (type); lower levels of Chadron A; a very primitive stage, approaching both Brontops hrachycephalus and Menodus Jieloceras in the character of its sections. Low horns elongate in basal section and placed not far in front of the orbits, nasals long and narrow, zygomata slender. No very definite marks of affinity with Allops are revealed by the sections. B, A. marshi, Am. Mus. 501 (type) ; horns elongate trihedral in basal section and placed considerably in front of the orbits, nasals broad distally, zygomata little expanded. C. A.marslii, Am. Mus. 1145 (paratype); the sections differ little from those of the type, although" the skull as a whole is broader. D, A. marshi. Harvard Mus.; differs somewhat from the type in the steeper profile of the horns, basal section obliquely trihedrali zygomata little expanded, parietal vertex narrow. One-eighth natural size. graph, was referred mistakenly by Marsh to Bronto- therium gigas notwithstanding its marked inferiority in size to the type of that species, which properly belongs to the Brontotherium phylum. In studying the plate and figures emphasis must be laid on the fact that the skull is extremely crushed laterally, and that its original mesaticephalic proportions are artificially increased into dolichocephaly. In other respects the plate as well as the accompanying figures and sections give an admirable idea of the formation and proportions of this primitive titanothere. in position, overhanging the sides of the face and of an elongate-oval basal section anteroposteriorly, distinct from that of the type of Allops marshi. The long axis of the section is anteroposterior, whereas in A. marshi it is oblique; the horn rises only 70 millimeters above the side of the narial aperture, 40 millimeters above the vertex of the skull. The nasals also resemble those in the type of A. marshi; they extend very far forward, contracting slightly, and cleft at the tips. The relative elongation of the face is a decided feature (PL CXI, A'), the bridge over EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 511 the infraorbital foramen being very broad, even broader than in Menodus giganfeus; there is a pre- orbital knob on the lacrimal, which is seen also in M. trigonoceras . The zygomatic arches, as shown in all three views of the skull, are deep and narrow, not expanding widely, and resembling those of the Menodus type. There is a narrow contact between the post- tympanic and postglenoid processes, and the par- occipital process is narrow in palatal view (PL CXII), wherein the dolichocephalic structure is again ap- parent but is somewhat exaggerated in this type by lateral crushing. Dentition. — There are two incisor alveoli which show that these teeth had not undergone the degeneration observed in members of the Menodus phylum. The right canine so far as preserved shows somewhat less anteroposterior compression than in the type of A. marshi. The premolars of A. walcotti exhibit pro- nounced internal and faint external cingula and feebly developed tetartocones (fig. 430). The tetartocones, however, are better developed than in the contem- porary B. hrachycephalus , since the constriction sepa- rating the tetartocone from the deuterocone in p-~^ is a little more anterior in position. Furthermore, the tetartocone of p* is not a concave spur from the cingulum as in B. h'acJiycepJialus or B. dispar. Characteristic features are the crenulation of the internal face of the deuterocones and the broad internal cingula. In regard to the proportions of the grinding teeth, comparison with the average anteroposterior and transverse diameters of the grinding teeth of five skulls of B. iracJiycepJialus shows that in this specimen the teeth are less mark- edly brachyodont, a fact in accord- ance with its supposed affiliations with the Allops phylum. The teeth are, in fact, as elongate as in members of the Menodus phylum, but this may be due partly to the lateral crushing. Detailed meas- "' urements of the tooth proportions indicate, however, that they are closer to B. hracJiyce2)halus than to those of M. trigonoceras . Characters oj lower jaw. — A lower jaw (Nat. Mus. 4247, fig. 413) agi'ees exactly in size and may be pro- visionally associated with this species. It presents the following characters: The coronoid is elevated; the angle projects downward and backward; the inen- tal foramen is below ps. ly, P^. The canines are pointed, erect, with an incomplete internal cingulum and somewhat flat inner face; pi is entirely wanting, a variable character; the cingulum on the remaining grinders is moderately developed; nis exhibits an in- cipient crenulate internal crest of hypoconulid. Measurements of jaw of Allops walcotti, Nat. Mus. 4B47 Millimeters Angle to symphysis .525 Condyle to angle 235 Canines, anterior enamel 34 Canines, anteroposterior 21 Ms, anteroposterior 88 M3, transverse 38 Pa-mj 290 This lower jaw has long, conical canines; it should be compared with the lower jaw of Am. Mus. 1495. Allops marshi (Osborn) (Megacerops marshi Osborn, 1902) Plates XXXVIII, CXII-CXVI; text figures 197, 378, 381, 389, 391, 397, 399, 409, 429, 431, 432, 615, 712 [Foi- oi-iginal description iiud type refersDces see p. 233. For slceletal characters see p. 678] Geologic horizon. — The geologic levels of the type and paratype of Allops marshi in the American Figure 430. — Upper teeth of Allops walcotti . Mus. 8753 (paratype); m^ is missing. One-half natural size. Museum are not recorded, but they are probably from upper A or lower B. A fine skull (Field Mus. P 6900) is recorded as from .50 feet above the Pierre shale and 50 feet below the level of a skull of Brontotherium hatcheri — that is, probably the upper levels of A or the lower levels of B. A female skull (Nat. Mus. 1215) slightly smaller than A. marshi is recoi'ded from middle B. Specific characters. — Skull longer than in Brontops hrachycephalus (645 to 675 mm., average 665), but shorter than in B. dispar, of mesaticephalic proportions ; zygomatic index, 64 to 69, average 66, and thus similar to females of B. hrachycephalus. Horns (105 to 140 mm.) longer than in B. hrachycephalus ; basal section 512 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA trihedral, more or less pointed or trihedral at the summits. Nasals elongate, free length 98 to 105 milli- meters. Incisors 2-1, large i^, small i^ Grinding series, length 310 to 335 millimeters, average 319; molars average 192; dental index same as in B. hrachy- cepJialus and B. dispar, namely, 47. Canines strongly compressed anteroposteriorly, length of crown 34 to 37 millimeters. Premolars with tetartocones of p^ better developed than in B. hrachycepJialus, tetarto- cones of p^ and p^ better developed than in B. IracJiycepTialus and similar to progressive members of B. dispar. Occiput not greatly prolonged back of zygomatic arches. Observations on the measurements oj Allops marshi. — From Brontops dispar the skulls of A. marsJii are dis- tinguished above all by their very low zygomatic index, 64 to 69, as compared with 77 to 87 in B. dispar. This marked narrowness, together with the small size of the canines (vertical diameter 34 mm., as compared with 40 in B. (validus) dispar), has led to the suspicion that Allops vfiarsTii may be the female of B. dispar. A comparison of the paratype of Allops marsTii with the type of B. validus is given below: Measurements of Brontops (validus) dispar and Allops marshi, in millimeters P'-mS Pi-p< Mi-m3 Canines : Vertical Anteroposterior Pnix to condyles Zygomatic index B. (validus) dispar, Nat. Mus. 4290 (type) . raarstii, Am. Mus. 1445 (paratype) 320 130 203 40 27 660 85 335 135 203 37 22 675 64 The relative widths of p* and m' also appear not to differ very clearly in the two forms: Anteroposterior and transverse measurements of p* and m^ in Brontops (validus) dispar and Allops marshi, in millimeters pi M3 Ap. Tr. Ap. Tr. B. dispar (type of validus) A. marslii (type) 40 39 60 61 72 70 75 73 A. marslii is decidedly smaller than A. serotinus and A. crassicornis, and apparently no known skulls bridge over this gap. It is larger in all measurements than B. hrachyceplialus. Materials. — This species is represented by 15 or more skulls, including the type (Am. Mus. 501, Pis. CXIV, CXV), a well-preserved skull; the paratype (Am. Mus. 1445, Pis. CXIV, CXV) ; a well-preserved skull (Brit. Mus. 4446 M, PL CXIV) whose geologic level is not recorded; an exceptionally perfect skull (Field Mus. P 6900), associated with the lower jaw and parts of the skeleton, from the upper levels of A or the lower levels of B; a fine skull in the Museum of Comparative Zoology, level not recorded, originally described by Scott and Osborn (1887.1, p. 158) as " Menodus coloradensis"; a cast of an unknown skull (Carnegie Mus. 289) ; a skull, recorded from upper A, probably a female (Nat. Mus. 1213); a skull from B (Nat. Mus. 1215). Comparison of Allops with members of the Brontops phylum. — Are these specimens females of B. dispar? There is some evidence that the specimens attributed to A. marshi are female forms of B. dispar, consisting chiefly of the following items: (1) The dental indices are the same; (2) the tetartocone development is prac- tically identical; (3) the cephalic index is mesaticepha- lic, like that of the females of B. dispar; (4) the speci- mens of A. marshi are found on lower levels of the range of B. dispar. On the other hand, the evidence against regarding A. marshi as the female form of B. dispar is somewhat stronger, as follows: (1) The skulls of A. marshi are generally recorded from lower geologic levels; (2) the skulls in the Field and Harvard museums have larger canines, indicating that they are males; (3) the occiput of A. marshi is not greatly prolonged behind the greatest width of the zygomatic arches, in contrast with B. dispar, in which the occiput is greatly produced posteriorly (see diagram); (4) the horns of A. marshi are more triquetrous or transversely oval, while the horns of B. dispar are rounded; (5) the nasals of A. marshi are much more elongate. Although the specific and phyletic distinction of A. marshi from B. dispar thus appears certain, the "group" affinity of the animals is very strong. We observe (1) the pit in the vertex of the cranium, as in B. brachycephalus , B. dispar; (2) the features in which A. marshi differs from B. dispar tend to relate A. marshi more closely to the succeeding form B. rohustus. Among the similarities between A. marshi and B. roSttsius are the following : (1) The form of the canine, which tends to obtuseness; (2) the abbreviation of the occiput behind the zygoma; (3) the broad, rugose summit of the occipital pillars; (4) the breadth and squareness of the nasals; (5) the presence of two knobs on either side of the median line of the occiput for the recti capiti muscles. The horns in their prophetic growth do not acquire the trihedral section seen in the Menodus phylum but tend to become more oval. Thus in the paratype of ^. marshi (Am. Mus. 1445) we find an approach to the transversely expanded horns of B. robustus. Among the more primitive characters of the skull distingmshing A. marshi irom B. rohustus are the smaller size and less robust structure in general, the shortness and obliqueness of the nasal section of the horns, the greater length and slenderness of the EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 513 nasals, the greater breadth of the malar bridge over the infraorbital foramen, the narrower contact between the postglenoid and post-tympanic processes. Against the theory that A . marshi is directly related to B. rohustus is, however, to be noted the important fact that in the premolar teeth the tetartocones are more advanced than in B. rohustus, in which they are singularly retarded in development. The transversely expanded canines constitute another clear distinction. Specimens referred to Allops marsM seu crassicornis. — A skull in the British Museum of Natural History collection, London (No. 5743 M), may be regarded provisionally as a very advanced or progressive stage in the evolution of this species, although it exhibits some characters which lead us to regard it as aberrant from the typical A. marshi, especially the somewhat flattened superior section of the horns, which suggests resemblance to Brontotherium leidyi; but the internally placed tetartocones of the premolars differentiate this type from any member of the genus Brontotherium, in which the tetartocones are invari- ably externally placed — that is, to- ward the buccal side of the crown of the teeth rather than toward the lingual side, as in this speci- men. The female sex of this spec- imen is apparently indicated by the small size of the horns and the slenderness of the canines and zy- gomata. The nasals are somewhat long and delicate, cleft distally; the short horns point obliquely out- ward, giving evidence of having been , subflat posteriorly. They are other- ' wise of the general type seen in A. marshi. The zygoma, although crushed, was apparently deep, with slight buccal expansion. The ex- ternal auditory meatus was a large, round opening, as in Menodus and Allops — that is, of mesaticephalic type. The measurements (see table, p. 508) agree better throughout with A. marshi than with any other type, although even in the paratype of A. marshi the tetartocones of the premolars are not so strongly developed as in this specimen. Two well-developed upper incisors are retained on the right side, i' being much smaller than i^. The lower jaw, with its flat, deep ramus, pronounced chin, slim, pointed canine (slightly flattened on the internal face), weak, noncrenulate hypoconulid of va^, resembles the primitive type of the Menodontinae in general and to a less degree that of the A. marshi type. Our conclusion is that this animal corresponds more nearly with a very progressive stage of A. marshi than with any other known species. The extremely advanced condition of the tetartocones of the pre- molars may, however, entitle it to distinct specific rank. The skeletal parts which are thought to be associated with this skull are described below. STcull in the Harvard Museum of Comparative Zool- ogy. — This skull (fig. 431) agrees with the type in the specific characters of the horns and nasals and in the possession of round-topped incisors. Progressive characters and ascending mutations in specimens referred to Allops marshi. — The type of Allops marshi (Am. Mus. 501) exhibits a zygomatic index of 67. It differs from the paratype (fig. 429) in the shape of the nasals and in the more retarded premolar tetartocones. There is a trace of the cir- cular pit in the occipital vertex. The tooth row is short (310 mm.). Figure 431. — Skull of A Hops marsh Harvard Mus. Front and side views. One-sixtli natural size. The paratype, probably a male (Am. Mus. 1445), represents a more progressive stage: (1) it is the broad- est skull referred to this species (zygomatic index 69), the skull being broadly depressed with stout zygo- mata; (2) the tetartocones are very advanced; (3) the occiput is extremely short behind the zygomata; (4) the nasals are thin and do not spread distally; (5) there are traces of a circular pit in the occipital vertex ; (6) the tooth rows are longer than in any other speci- men in the series, namely, 335 millimeters; (7) the same may be said of the molars (average 205 mm.); the grinding teeth are also slightly more elongate; (8) the tetartocones are more advanced than in B. rohustus — in fact, more advanced than in several speci- mens attributed to B. dispar. 514 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA The British Museum specimen (No. 5743 M) above described agrees well with the type and para- type in all measurements except in the zygomatic index, which is low and agrees with that of a supposed female (Nat. Mus. 1213) from the lower beds. The horns are more erect than in the paratype- The tetartocone development is about the same as in the type. The Field Museum specimen (No. P 6900) agrees well with the type in measurements but differs in the somewhat more backwardly prolonged occiput; the canines, which are large, indicate that this specimen is probably a male. It is more fully described below. Figure 432. — Lower jaws of Allops marshi and Allops? sp. A, Allops marshi, Field Mus. P 6900; ramus sweeping gently forward with lower border nearly horizontal and angle not produced downward, chin slightly convex, canine appressed to P2, external cingula not sharply defined. B, AUops'i sp., Ottawa Mus.; Cypress Hills, Saskatchewan. This specimen (one of Cope's cotypes of M, ang-ustigenis) differs from the typical Menodus in the presence of incisors (as indicated by the alveoli) and in the somewhat less hypsodont form cf the grinding teeth; the angle is not produced backward. The refer- ence to Allops is provisional. About one-si.\th natural size. The skull in the National Museum (No. 1213) agrees closely with the type of A. marshi in measure- ments and is somewhat more progressive in premolar evolution than the type of B. hrachyceplialus. Another skull (Nat. Mus. 1215) is recorded from the middle levels of B and may pertain to this species, although the occiput is more prolonged backward than in the type; the premolar tetartocones might represent either this species or B. dispar. Equally interesting is the skull Nat. Mus. 1214, referred to B. hrachycej>7iahis but possibly an ancestor oi A. marshi. Detailed characters of Allops 7narshi. ^-The. detailed characters of the type (Am. Mus. 501) are as follows: The skull, apparently belonging to a female, is in the seventh growth stage. The incisive border exhibits alveoli for a large lateral and small median incisor. The canines have short, obtuse, lanceolate crowns, a posterior and a slight lateral cingulum. The first su- perior premolar is bifanged, close behind the canine; it exhibits no tetartocone. The tetartocones are less advanced than in the contemporary 31. trigonoceras, moderately developed on p' and p^ and forming a de- pressed loop on p*; the external and internal cingula are clearly defined. The molars exhibit partial cingula on the ectoloph and rudimentary cingula on the inner side. The hypocone of m'^ is connected with the cingulum (in the paratype, however, the hypocone is separated from the cingulum as a low cusp) ; there is a rudimentary metaloph, especially on m^. The postorbital process of the frontal crest forms a rudimentary sharp angle; the orbit is large; the malar bridge is broad, with a low ridge; the buccal expansions of the zygomata are slight in top view; the edges of the frontal crest are seen to diverge widely, running to the malar ridges of the horns. The nasals are of medium length, deeply convex above and concave below, projecting beyond the premaxillaries anteriorly. The paratype skull (Am. Mus. 1445), prob- ably that of a male, is, as above noted, more progressive. In p* the tetartocone is more distinctly budded off. As in the type the canines have the short, sublanceolate section and posterior lateral cingulum quite distinct from the more pointed canines of Brontops dispar. The Field Museum skull (No. P 6900) as- sociated with a lower jaw and a manus of Allops marshi, from Phinney Springs, S. Dak., has been skillfully reconstructed by Mr. Riggs and is now one of the least distorted titano- there skulls of all that are known. It agrees closely in measurements with the other skulls referred to^. marshi and shows corresponding specific characters. Detailed measurements of the upper grinding teeth prove that the transverse measurements exceed the anteroposterior measurements throughout. This proportionate excess of transverse diameter is progressive in this brachycephalic line. Ascending mutations. — This species, therefore, in- cludes specimens which represent several "ascending mutations" in increase of size, in the development of the premolar teeth, in separation of the tetartocones, and in expansion of the zygomatic arches. Of these the geologically oldest perhaps is the National Museum specimen No. 1214, which is distinctively a B. hrachycephalus in its measui'ements. Next in order of evolution comes the female specimen (Nat. Mus. 1213), distinctly an A. marshi in its measurements. EVOLUTION OF THE SKULL AND DENTITION OB^ OLIGOCENE TiTANOTHERES 515 On a higher plane but still in an intermediate stage of development is the female type skull of A. marsJii (Am. Mus. 501). The Carnegie Museum skull No. 123, formerly referred to this species, is more probably a Brontops dispar (Am. Mus. 1445). The paratype is the largest and most progressive form known. This range of progressive evolution in the materials at hand afi'ords strong additional proof of continuity. Characters oj the lower jaw. — The characters of the lower jaw of this species are illustrated in Figure 432, A. The chin is gently convex, not angulate; the angle is but little produced downward; the lower border of the ramus is nearly horizontal. Allops serotinus Marsh ("Allops serotinus" Osborii, 1902) Plate.s XCIII, CXVII-CXX, CXXXII; text figures 184, 375, 387, 391, 393, 394, 399, 409, 433, 434 [For original description and typs reterencas see p. 225] Geologic horizon. — Titanotherium zone of South Dakota; summit (?) of middle beds (Chadron B). Specific characters. — Skull mesaticephalic. Length 705 millimeters (c?), breadth 525. Zygomatic index 74. P'-m^, 330 millimeters. In males nasals broad (133 mm.) and short (81 mm.). Horns subtriangular elongate, widely divergent. Incisors 2-1, external incisor large, median incisor reduced or wanting. Superior canines, males, 41 millimeters. Premolars with deuterocones forming main internal portion of crown. Tetartocones much smaller, especially on p''. External cingula defined on premolars, more or less continuous on molars. Measurements of Allops crassicornis and A. sero- tinus. — The skulls referred to Allops serotinus and Allops crassicornis form an ascending series, in which p'-m' rises from 330 to 370 millimeters. The three skulls referred to A. crassicornis differ from those re- ferred to A. serotinus, especially in the greater length of the premolar and of the molar series. The contrasts in measurements with Brontojjs dispar and with Meno- dus trigonoceras are shown in the accompanying table. Measurements of Allops crassicornis, A. serotinus, Brontops dis. par, and Menodus trigonoceras, in millimeters [All specimens male] P'-m3 P'-p* iVIi-ms P*, ap. by tr_ 355-370 131-150 210-220 46X68 1S43, ap. by tr 80X82 Pmx to condyles Zygomatic index Nasal length Nasal breadth 330 13.3-140 195-206 43X59 68X72 750i 705-739 76^ 72-78 72[ 68-81 146 106-137 320-345 122-145 200-215 40X60 72X75 643-665 78-87 85-94 102-120 335-360 127-136 220-225 43X51 72X70 to 82X79 670-770 66-79? 115-140 120-135 From this table it will be seen that Allops crassi- cornis and A. serotinus are distinguished by the great extent of p'-p'', by the relatively great width of p* and m^ by their high zygomatic index as compared with that of Menodus trigonoceras, and by their relatively short, wide nasals. In general the skulls of Allops serotinus and A. crassicornis combine the character- istics of Brontops and of Menodus. General characters. — This species includes crania that immediately suggest Menodus giganteus, but on close examination they are seen to be less powerful and less robust. It is noteworthy that the supraoccipital crest is less rugose and was probably provided with less powerful muscles. Materials. — The type skull (Nat. Mus. 4251) was originally recorded by Hatcher from the top of the Titanotherium zone of South Dakota. This record, however, was subsequently revised by Hatcher as possibly due to secondary erosion and deposition of the overlying Brule formation {Oreodon zone) below the true upper level. Thus we may provisionally re- gard Allops serotinus as typically of the uppermost levels of the middle beds. It is noteworthy that the matrix is a fine clay, and the skull has a brilliant yellowish-white surface. Another skull (Nat! Mus. 2151) is recorded as foimd by Hatcher is the same whitish clay matrix, about 15 feet away from the type, and entered as 80 feet above the Cretaceous Pierre shale. This fur- ther confirms the view that these animals belong in the upper portion of the middle beds, or B, and not in the upper Titanotherium zone (C). Two other skulls are known, namely, Nat. Mus. 4938 and Am. Mus. 520. Description of the type sTcull. — In addition to the general characters noted above in which this skull differs from that of the typical Menodus or the typical Brontops, it is important to record the following detailed characters: The type skull (Pis. CXVII, CXIX) is vertically crushed, and owing to this the lateral spread of the horns is unnaturally increased. It is a male speci- men. Although in the seventh stage of growth, the skull is decidedly delicate in its structure and pro- portions. It represents an animal about a fourth smaller than the majority of the specimens referred to the great animal Menodus giganteus. The inferior aspect of the skull is well represented in Plate CXVIII, Am. Mus. 520. The nasals of the type skull (Nat. Mus. 4251) are much shorter (81 mm.) than in Menodus giganteus and relatively as broad (133 mm.). The rugose expansion of the tips is an age character. The horns are long (240 mm.), ex- panding directly outward at the sides, with a low but broad connecting crest; the basal section (fig. 433) is peculiar in the incurvation of the outer border between the nasal and the malar ridges. The gen- eral form is similar to that in M. giganteus. Seen 516 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA from in front the inferior contour of the horns is straight, as in M. giganteus; the angles of the basal portion are more sharply trihedral or defined than in the type of A. crassicornis. As seen from above we observe a decided midtemporal as well as supraciliary overhang of the supratemporal crest, as in the type of A. walcotti (Nat. Mus. 4260). The supratemporal crests exhibit, in fact, a prominent flare or projection a short distance behind the orbits. The occiput is deeply excavated, but the occipital pillars are rela- than in the large type Menodus" giganteus. The distinc- tive anteroposteriorly compressed form of canine is well represented in Plate CXXXII, C. The postero- internal cusps of the premolars are much less devel- oped than in A. crassicornis or in M. giganteus — in fact, in p* the tetartocone is a feebly developed ridge, and the deuterocone forms the most internal portion of the crown; in other words, this tooth is in a primi- tive stage of evolution. The tetartocone of p^ is a mere spur. The hypocone of m' is an elevated cin- m A B Figure 433. — Sections and contours of skulls of AUops serotinus and A. crassicornis A, AUops serotinus, Nat. Mus. 4231 (type); upper levels of Chadron B; stout horns directed outward (although here represented as vertical) with a well- defined trihedral basal section (as in Menodus), nasals short and wide, zygomata relatively slender. B, A. serotinus, Nat. Mus. 4938; lower levels of Chadron C; horns directed outward and roundly trihedral in basal section, nasals short, thick, and very wide, zygomata expanded. C, A. crassicornis, Nat. Mus. 4289 (type); lower levels of Chadron 0; horns stout and very thick, basal section roundly trihedi'al, zygomata not much expanded. All one-eighth natural size. tively light and are surmounted by a simple rugose flare. As in Brontops and Menodus the occiput is decidedly produced behind the zygomata. As in Menodus the zygomatic arches are relatively deep and narrow, and the parietal vertex is also relatively more constricted than in BrontotJierium and Megacerops. Dentition. — -The dental characters of the type are highly distinctive. A large single incisor persists on each side, relatively more robust than that in A. crassicornis. The canine crowns measure 41 millime- ters anteroposteriorly and are thus slightly smaller gule, contrasting with the cone which is so distinctive a feature of Menodus. In its premolar evolution A. serotinus is more re- tarded than A. crassicornis. Female .skull Nat. Mus. 3151.— The skull (PI. CXVII, D) found not far from the' type of A. serotinus is believed to be a female of the same species. It differs from the type in several structural characters which may be attributed partly to the differences of sex, as follows: (1) It is of smaller size; (2) the horns are more sessile, resembling those of A. marshi; (3) the EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES 517 nasals are more elongate; (4) the zygomata are less widely expanded; (5) the canines are of feebler dimen- sions. In the grinding teeth the dental index, 50, is remarkably high, but the actual linear measurements of p'-m^, p'-p*, m'-m^ are the same as in the type male skull. This is in accordance with the general principle which we have found throughout titanotheres, that the females while inferior in other characters frequently present grinding teeth equal in size to those of the males. Transitional skull or ascending mutations. — The American Museum skull No. 520, as seen from above (PL CXVII, B), resembles the type skull of A. sero- tinus in many respects. As seen from below (PI. CXVIII) it has the true subgeneric characters of Allops, especially in the peculiar transverse, lanceolate form of the canine teeth and in the roimded form of the single incisor tooth. On the other hand, it differs from the type of A. serotinus and appears to be transi- tional toward a higher type in the decidedly greater Figure 434. — Coossified nasals and proximal part of horns of Allops f serotinus? Specimen from Cypress Hills, Saskatchewan, Canada, in tlie Ottawa Museum, referred by Lambe to Megacerops assiniboiensis, One- tbird natural size. abbreviation of the nasals. In top view the horns are seen to be intermediate between those of A. serotinus and A. crassicornis. Similarly, while the nasals are shorter than in A. serotinus they are nar- rower than in A. crassicornis. The linear measure- ments of the grinding teeth (330 mm.) agree precisely with those of the male and female specimens of A.' serotinus, and a still more conclusive resemblance is seen in the decidedly retarded development of the tetartocones, so far as one can judge from their worn condition. Skull Nat. Mus. 4938. — This is another skull which combines the primitive premolar structure of A. serotinus with the more abbreviated nasals and more massive form of A. crassicornis. The detailed meas- urements of the teeth in this skull agree more closely with those of A. serotinus than with those of A. crassi- cornis. On the other hand, in both size and shape of the sections of the horns the skull appears to agree more closely with A. crassicornis. Together with the skull above described (Am. Mus. 520) this skull might be cited to prove the existence of transitions between the two successive stages. Summary. — The type (Nat. Mus. 4251) and the three other skulls provisionally referred to A. serotinus — namely, the female (Nat. Mus. 2151), the transitional form (Am. Mus. 520), and the second transitional form (Nat. Mus. 4938) — all agree in the retarded state of evolution of the tetartocones, which are little further advanced than in the species Brontops dispar. The characters m which they disagree with B. dispar are found in the horns and nasals, and these either represent ascending mutations of the A. serotinus type or more probably progressive stages toward the next higher species, A. crassicornis. Allops crassicornis Marsh {"Allops crassicornis" Osborn, 1902) Plates CXIX-CXXII; text figures 189, 387, 409, 433, 608, 612 [For original description and type references see p. 228. For skeletal characters see p. 6791 Geologic horizon. — Titanoiherium zone of South Dakota, middle or upper level (B or C). Specific characters. — Skull proportions of males more robust than in A. serotinus. Skull length 750 millimeters, width 570 (estimated). P'-m^ 370 millimeters. Nasals broad and abbreviate, horns massive and obtuse. A small pair of lateral incisors. Canines as in A. serotinus. Superior premolars with distinct tetartocones and well-defined external cin- gula. Tetartocone on p' distinct but somewhat smaller than the deuterocone. Hypocone wanting on m', replaced by rudimentary metaloph. Zygo- mata widely arched outward, with buccal expansions. General characters. — The type (Nat. Mus. 4289) of this species resembles that of A. serotinus (1) in the peculiar lanceolate form of the canines, which are elon- gate, compressed anteroposteriorly, and flattened on the posterior face, perhaps a further development of a condition seen in A. marshi; (2) in the marked development of the cingulum on the premolars; (3) in the proportions of the premolars; (4) in the trihedral basal horn section. The type differs dis- tinctly from A. serotinus (1) in the far greater develop- ment of the tetartocones; (2) in the larger and more robust proportions of the skull; (3) in the short obtuse horns. Its structural character, like that of A. serotinus, is intermediate between those of the Brontops and Menodus main phyla, and in its extreme size it perhaps presents an advanced stage of evolution of the subgenus Allops — that is, of the Allops col- lateral phylum — in the same manner that Diplo- clonus amplus presents an extreme evolution of its collateral phylum. It is noteworthy that this skull differs from all the preceding male and female skulls 518 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA referred to the subgenus AUops in the following points : 1. Greater skull length, 750 millimeters; others 640 to 730. 2. Greater length of grinding series, 370; others 310 to 335. 3. Greater length of premolars, 155; others 131 to 140. 4. Greater length of molars, 220; others 192 to 215. 5. Greater length of canine; anterior face 45; others 33 to 40. This uniform increase in size in all measurements implies that A. crassicornis comes from a higher geologic level than any of the other forms and is in thorough accord with the advanced condition of the tetartocones. Geologic and geographic distribution. — The geologic level recorded for this specimen by its discoverer, Hatcher, is the upper part of the middle beds; in other words, it is the same as that assigned to the type and the female of A. serotinus. A marked progression in the evolution of the premolars is, however, posi- tive proof that this animal belongs to a very much higher geologic level than A. serotinus, though there is little doubt as to its phyletic descent from that species. Materials. — Four skulls m the National Museum (Nos. 4289 (type), 2117, 4709, 8740) are referred to this species. Description oj type. — The type skull (Nat. Mus. 4289) is undoubtedly that of an old male. The nasals are square and heavy, considerably longer than in Brontops rohustus but much shorter than in Menodus giganteus. The horns are set very wide apart, and it is a noteworthy fact that they entirely lack the con- necting crest which is so distinctly developed in speci- mens of Menodus. The basal section reveals the affinity to A. serotinus; it is distinctly trihedral, and the long axis is oblique rather than transverse as in Bron- tops rohustus. In proportions this skull is dolicho- cephalic; ratio of length(750 mm.) and width (570 mm.) gives a zygomatic index of 75; the measurements are rendered less exact, however, by the artificial spreading of the left side of the type skull. The proportions are intermediate between the mesaticephalic and the dolichocephalic types. Dentition. — We are first impressed by the reduced size of the superior lateral incisors (PI. CXXI), which are more vestigial than in any member of the true Brontops series. The canines are a very distinctive progressive development of the A. serotinus form, attaining a length of 45 millimeters. The fourth supei'ior premolar has a low but sharp and distinct tetartocone. The second and thii'd pi'emolars also have well-defined tetartocones. The third superior molar has a peculiar sharp elevation of the cingulum but no hypocone. The premolars and molars either parallel or indicate their affinity to those of Menodus in the decided development of the external cingula; P^i P') P^ also exhibit strong external cingula. THE MENODUS MONOPHYLUM (" Titanotherium phylum," Osborn, 1902; " Symborodon" torvus (jaw), Menodus, Menops, Diconodon) Like Brontotherium, the genus Menodus includes a nearly continuous series of ascending mutations from the base to the summit of the Titanotherium zone, which may be considered nearly if not quite m'ono- phyletic. The lines of separation between so-called "species" are wholly arbitrary. Briefly stated, the distinctions of the Menodus phylum as a whole are as follows: (1) Skulls dolicho- cephalic, indices 62-70; (2) premolars rapidly pro- gressive in molarization; (3) superior incisors aborted; (4) horns abbreviated, triangular, not shifted forward; (5) high dental index; (6) canines large, piercing. Phyletic characters. — Large, long-limbed or cursorial titanotheres, long skulled and short horned. Ranging from the lower to the upper Titanotherium zone. Vestigial incisor teeth and long, pointed canines. Grinding series elongate, with a high dental index. Crowns of grinders subhypsodont and with promi- nent cingula. These menodonts constitute one of the most sharply defined of all the titanothere phyla. The elongation of the feet and of the limbs indicates that of all the titanotheres Ivnown they were the most rapid travelers. While sparsely known in the lower Titanotherium zone and somewhat more abundant in the middle Titanotherium zone, they were relatively numerous in the upper zone, an indication of the more favorable conditions which this region presented toward the end of the Titanotherium period for animals of this type. They appear to have divided the honors with the great long-horned brontotheres, although their i-emains are much more rare. The short, stout, pointed horns stood in wide con- trast, however, with the great recurved horns of the brontotheres. While the canine tusks may have served as minor weapons of defense, it appears probable that the menodonts, as the swiftest members of this great group, had recourse to flight when attacked by herds of carnivores. To the anatomist the menodonts are extraordinarily interesting in presenting extremes both of dolicho- cephaly and of dolichopody — that is, of long-headed, long-limbed, and long-footed development, in contrast with the broad headed. The group affinities of these animals with members of the Brontops phylum are in- dicated by the less intermediate characters of the Allops phylum. Whereas all other titanotheres ex- hibit progressive abbreviation of the nasal bones, in these animals the nasals retain the broad, elongate form first seen in the upper Eocene Protitanotherium emarginatum. As compared with the more or less intermediate genus Allops these true titanotheres are more dolicho- EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 519 cephalic, the zygomatic index being 62 to 70. They are distinguished also by the extreme reduction within the incisive alveoli of vestiges of the upper incisor teeth, a fact which is confirmed in every speci- men, whereas in Allops we find two small but per- sistent upper incisors which pierce tha gum. In the lower jaws the incisors appear to be wanting entirely, although the evidence is less conclusive. As distin- guished from Brontops the premolar teeth are very directed, and adapted to lateral motions of the head in defense or attack; (3) persistently elongate nasals, the only phylum in which this character occurs; (4) vigorous development of the grindiag teeth, including a high dental index, a strong development of the cin- gula, and a marked vertical elongation or subhypso- donty of the crowns; (5) the elongate and piercing character of the canine tusks, which project so far above and below the line of the grinders that it is A B Figure 435. — Sections and contours of skulls of Menodus heloceras and M. trigonoceras A, M. heloceras, Am. Mus. 6360 (type) ; small horns trihedral in basal section, parietal crest narrow, zygomata very slender. B, M. trigonoceras, Am. Mus. 6355 (lectotype or type?); pointed pyramidal horns roundly trihedral in basal section, connecting crest pronounced, nasals long and wide, parietal vertex wide, and zygomata very slender. C, AT. trigonoceras (or giganieusf), Nat. Mus. 1219; horns longer with trihedral section, nasals very wide and distally expanded. One-seventh natural size. rapidly progressive in the evolution of the postero- internal cusps or tetartocones. This character is also shared by Allops. Characters oj proportion. — -The six great distinctive characters of Menodus are correlated with its length of limb, height of body, and relatively cursorial habit, as follows: (1) Dolichocephaly or length of skull con- trolling all the parts both of the bones and of the teeth; (2) short, triangular, and pointed horns, outward 101959— 29— VOL 1 36 reasonable to regard them as weapons of defense and offense; (6) the entire absence of incisor teeth, a feature in which Menodus parallels Megacerops and Brontotherium. Of these sLx characters the elonga- tion of the skull (mesaticephaly or dolichocephaly) is the most dominant in correlation with all parts of the skull and teeth. The dolichocephaly of Menodus is demonstrated in every measurement in the table below. In length 520 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA the skull almost equals the giant Brontoiherium, but in width across the zygomata it is far inferior to this animal; in the length of its grinding series it is far superior to either Brontops or Brontotherium; in the ratio of its grinding series to the entire length of the skull it is again superior, as well as in the actual measurements both of its premolar and of its molar series. Its fourth premolar is actually longer and narrower than that of Brontotherium. The sum of the transverse measurements of its grinding teeth is decidedly less than that in Brontops or BrontotJierium. The table on page 523 shows that dolichocephaly, a distinctive phyletic character of Menodus, is impressed not only upon the skull but upon the teeth. The horns preserve their triangular or trihedral shape because the connecting crest as seen in the upper view of the type of Menodus giganteus (Pi. CXXXVIII, fig. 391) is strongly developed, as well as the anterior ridge which extends downward into the sides of the nasals and the external ridge, the latter extending laterally into the antorbital bar. In the beautifully preserved male sicull from the summit of the Titanotherium zone in the Field Museum (PI. CXL) the pointed apices of the horns are clearly displayed. As observed in the side view of the same skull (PI. CXXXIX) the facial or preorbital portion of the skull is relatively elongate, and there is a broad bridge across the infraorbital foramen as well as a very deep anterior junction of the premaxillaries. The zygomatic arches are strengthened by depth rather than by breadth; it is true that a moderate expansion is observed in this and other old males, such as the type of T. "ingens," but the enormous buccal swellings characteristic of Brontops and Bron- totherium are not developed. In the auditory region a highly dolichocephalic character appears — namely, the relatively open condition of the external auditory meatus due to the noncontact or retarded contact of the postglenoid and post-tympanic processes. Similarly, the occipital condyles project widely at the back of the skull. The occiput is relatively high and ascends rapidly above the condyles. The jaw is also highly distinctive, with its long, straight lower border, its backward rather than downward project- ing angle, and its well-defined chin. It is totally different from the jaw of Brontotherium. In Menodus giganteus the dental series attains the finest proportions known in any member of the order Perissodactyla. The vigorous development of the teeth, exhibited in the large canines, in the pronounced cingula, in the length of the grinding series as a whole, in the height of the crown, in the excess of anteropos- terior over transverse measurements of the grinders, and in the progressive dental index, affords one of the most distinctive characters of this phylum as a whole. Harmonic evolution. — In progressive increase in size all parts of the skull and dentition share alike between the stages M. heloceras and M. giganteus — namely, about 50 per cent. The horns increase in length 190 per cent as compared with an increase of 250 per cent during the same period in Brontotherium — that is, they somewhat more than share the general increase in length of the skull, but they do not dominate in de- velopment to the detriment of other features as in Brontotherium. Sexual characters. — The differences between the males and females are very marked, as clearly shown in the contrast between two male and two female skulls in the American Museum. The female skulls are smaller in most of their dimensions, less rugose on the surfaces for muscular attachment. In the males the horns are more robust, more decidedly triangular rather than rounded, the triangular form being sharply defined especially at the base. In their vigorous growth they sometimes exhibit the anterior tuberous branching, as seen in skull Am. Mus. 505. In the females the horns are relatively slender, with less anteroposterior diameter at the base, and more pointed at the summit. While in both sexes the incisors are vestigial and certainly do not pierce the gum, there is marked disparity in the canines. In the males (PL CXL) the canines are formidable weapons, the anterior enamel face of one specimen (Am. Mus. 505) being estimated at 70 millimeters in length, whereas in females the canines are far more slender, the an- terior face being 45 millimeters. On the grinding teeth the cingulum is equally robust in both sexes — • in fact, the most pronounced development of this structure is observed in the female skull (Am. Mus. 1067), which is evidence that development of the cin- gulum is not a sexual character. Progressive specific stages. — Three species are defi- nitely distinguishable as characteristic of the lower, middle, and upper zones respectively. There are also two transition species and a closely connected series of "ascending mutations." (See table on p. 523.) Menodus heloceras (Cope) of Chadron A: The type of M. heloceras (Cope) was at first placed as the ances- tor of this phylum, but with some hesitation because of its imperfect preservation. So far as known it conforms in skull structure, although in a very prim- itive stage of development. Its ancestral position in the Menodus phylum was confirmed (1910) by Granger's discovery of a second skull at the base of the Titanotherium zone. Menodus {" Symborodon") torvus (Cope) is an inter- mediate stage between M. heloceras and M. trigonoceras ; it is known only from the type jaw, which is the genotype of the genus Symhorodon. Menodus {"Titanotherium") proutii Leidy: Leidy founded the genus Titanotherium on two specimens, the first was Pomel's type of Menodus giganteus; the second specimen, which Leidy fully described and measured, becomes the type of his species proutii; it belongs to an animal intermediate in size between M. torvus and M. trigonoceras. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 521 The Menodus pTiylum as represented by the Hatcher collection of 25 sTculls of species of Menodus from the Chadron formation in the United States National Museum Species M. giganteus PomeL Do Do Do Do Do Do. Do. Do. Do. Do. Do. M. (trigonooeras) giganteus Pomel. M. trigonoceras (Cope) Do Do Do Do M.? trigonoceras (Cope) M. trigonoceras (Cope) Catalog No. M. proutii (Owen, Nor- wood, and Evans) . Do Do . M. torvus (Cope) M. heloceras? (Cope) 1220, cf 1212 8745, & 8741, d' 8735, V 8761, cf 8756 8751 8765, cf 8781 8778 4745, 9 8793 1219, cf 4291, cf 4707 8760, ? 9 8768 1246 4257 4701, ? 8736 113 8799 8744 Material Skull. Old male, fuU size. Nasals typical; also horns. Posterior part of skull. Skull. Nasals and canines typical; also horns. Size medium (="B. ingens" stage). Skull. Measurements typical of smaller males; not so large as type. Skull. Slender canines. Anterior half of skull. Measurement of p'-p* agrees with that in smaller males of M. giganteus. Specific reference uncertain. Skull. Poor. Anterior half of upper milk dentition. May be large M. trigonooeras. SkuU. Complete left ramus and part of right. Last half of lower jaw and molars 1, 2, and 3 of upper right. Lower jaw. Upper milk dentition (dp'-dp^) and m', large animal. Anterior part of skull. An old male, approaching M. giganteus stage. Skull. Very fine progressive type; almost at M. giganteus stage. SkuU. Crushed skull. Inferior specimen. SkuU. Poorly preserved. Posterior parts of both rami. Skull from level B 3. Long nasals; vestigial incisor and measurements remove it from B. dispar. SkuU from level C 1 and left fore foot. SkuU. Left ramus of jaw. Type. Pair of lower jaws. Skull. Very young, m" just protruding; elongate proportions of Menodus. specific reference. Doubtful Menodus trigonoceras (Cope), of Chadron B: It is interesting to note that Cope's description of M. trigonoceras immediately followed that by Marsh of M. giganteus. M. trigonoceras, which is probably characteristic of B, or the middle Titanotherium zone, is distinguished from M. giganteus, first, by its inferior dimensions throughout; second, by the shape of the horns, the horn section being an equilateral triangle, as the distance between the malar, nasal, and con- necting ridges is practically equal. The skull of M. trigonoceras is directly intermediate in size between that of M. heloceras and M. giganteus. Menodus giganteus Pomel, of Chadron C: In Menodus giganteus, a superb titanothere, specifically equivalent to Menodus {" Brontotherium") ingens Marsh, absolutely determined as characteristic of the upper Titanotherium zone, and even of the uppermost levels (Chadron C 3), we reach the climax of this monophyletic series, which is distinguished by the extreme development of all the distinctively phyletic characters as compared with the inferior stages. Its relative abundance indicates that it was capable of holding its own in the struggle for existence between the numerous phyla of Brontops and Brontotherium. Remains of species of Menodus have been found at the stratigraphic levels indicated below: Upper Titanotherium zone: M. giganteus Pomel. Skulls large (type 755 by 553 mm.). Buccal processes of zygomata | strongly developed. Tetartocones of p'-p* distinct. Hypo- cone of m' usuaUy separated from cingulum. Horns large, directed obliquely outward and upward. Connecting ridge prominent. Middle Titanotherium zone: M. trigonoceras (Cope). Skulls of medium size (type 678 by 490 mm.). Buccal processes of zygomata moderately developed. Tetartocones of p'"^ distinct. Hypocone of m^ sharp and distinct or a sharp cingule. Horns of medium size; basal section equUateral; connecting ridge. Lower Titanotherium zone: M. heloceras (Cope). Skulls small (width of type across zygomata 392 mm.; nasals to occiput (estimated) 545 mm.). Horns smaU, subtrihedral; basal section subtriangular; internal angle rounded; no con- necting ridge. Ohservations on the measurements of the Menodus series. — The species of this genus constitute an as- cending series extending from the lowest to the highest levels of the Titanotherium zone. The range in the chief measurements of the successive stages may be epitomized as follows: Measurements of species of Menodus, in millimeters Pi-m3 Pi-p* Mi-mS Pmx to condyles. Zygomatic index. Horn length 385-465 141-176 246-285 760-825 62-70 150-290 333-360 127-136 203-224 628-770 66-?79 132-214 300-313 265 115-119 190-192 170 ?655 603 ?74 ?79 70 522 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA From this it will be seen that in our present collec- tions there are considerable gaps between the suc- cessive stages so far as regards the measurements p'-m^, p'-p^, and m'-m', but that the other measure- ments overlap. The nasals in this phylum remain very long, whereas in other phyla they are reduced in length. This is a remarkably consecutive and distinct phylum. Additions to the already large number of skulls may give a continuous series of measurements in each column. Geologic level. — Unfortunately, the records of the geologic level of members of this phylum are by no means so exact as those of members of other phyla. No member of this phylum is certainly recorded from the lower Titanotherium zone of South Dakota, although the primitive characters of the oldest known species, M. heloceras, indicate that it belongs on this lower level. In Wyoming one skull of M. Jieloceras (Am. Mus. 14576) has been recorded from the base of the Titanotherium zone. Only one skull (Nat. Mus. 4257), belonging to the species M. trigonoceras, is definitely recorded as from the middle TitanotTierium zone (level B). Another skull, referred to M. proutii (Nat. Mus. 4701), is tabulated from the lower level of C. This uncertainty ceases, however, in the upper beds, in which we have two positive records — namely, Nat. Mus. 4291, entered as from the upper Titano- tTierium zone (C), and a skull in the Field Museum (P 5927), recorded as found near the top of the upper Titanotherium zone, or upper C. It is thus probable that Menodus persisted to the very end of the great titanothere epoch. SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES IN THE MENODUS PHYLUM Menodus Pomel, 1849 (Tiianoiherium Leidy , 1852; Symborodon Cope (-m3 Dental index Pi-p« M'-m3 P', ap. by tr M3, ap. by tr Brachy- cepbalic: Brontops robustus, Yale Mus. 12048 (type) 766 667 350 46 137 220 40X65 81X92 Dolicbo- cephabc: M. giganteus, Field Mus. P6927 (typical) Hyperbracby- cepbalic: Brontotherium gigas elatum, Am. Mus. 492 (typical) 825 515 425 51 150 270 - 58X73 "■100X78 830 740 353 42 126 241 47X72 91X99 1 Univ. Wyoming Mus. " A surprising fact is that the horns in the unerushed M. gigantetis (Field Mus. P 5927) are almost as far forward as in the unerushed B. gigas. In the unerushed B. platyceras and B. ramosum the forward displacement appears extreme. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES The dolichocephalic character of Menodus in its typical species M. giganteus is demonstrated by every measurement of the accompanying table, in contrast with the brachycephalic Brontops rohustus and hyperbrachycephalic Brontotherium gigas. 523 the Standard measurements of slcuU, jaw, and teeth in the Menodus phylum, in millimeters Upper teeth Skull Jaw and teeth ! & 1 S o 1 l| i° o 1 i g S 1 t o 1 1 1 1 1 1 a P4 a f S > s .g § o II g| ■-3 1 ■a 8 1 1 M. giganteus group M. giganteus, Univ. Wyo., cf 465 176 285 60 35 815 M. giganteus Pomel (type) "280 M. giganteus ("ingens"), Yale Mus. 12010, cT . ..... 428 162 266 553 755 755 750 695 712 125 150 175 105 120 125 140 140 145 M. giganteus, Nat. Mus. 1220, cf 798 825 760 777 800 !'805 770 178 290 M. giganteus, Field Mus. P 5927, cf .._ "Menops" varians, Yale Mus. 12060, cf 425 410 393 390 "■385 360 150 155 153 145 '■141 135 270 255 250 245 "246 223 52 "■70 33 35 31 515 555 545 590 62 73 70 400 125 280 47 690 M. giganteus, Am. Mus. 505, cf (neotype) 195 M. giganteus, Nat. Mus. 8741, cf - M. giganteus. Am. Mus. 506, 9 »40 45 20 27 "770 738 122 "115 404 347 364 109 115 "260 238 242 "651 M. proutii-trigonoceras group M. giganteus (trigonoceras) , Am. Mus. 1066.- 546 70 592 M. giganteus (trigonoceras), Am. Mus. 1007 - 590 M. giganteus (trigonoceras), Nat. Mus. 4291, c? 360 355 136 133 224 225 47 44 26 25 770 725 "510 '440 490 66 60 "720 "670 130 127 137 "125 '135 135 145 132 150 214 132 M. giganteus (trigonoceras), Am. Mus. 1067, 9 360 126 237 39 — - 610 M. trigonoceras, Am. Mus. 6355 (lecto- M. trigonoceras, Munich Mus. (mounted "355 345 "690 M. trigonoceras. Am. Mus. 6356 (cotype)_ 129 220 — - 26 140 356 "120 245 32 M. trigonoceras, Nat. Mus. 1219 M. trigonoceras, Nat. Mus. 4257 M. trigonoceras, Carnegie Mus. 3068, 9 _, M. proutii, Am. Mus. 9335 345 335 333 136 127 221 220 203 192 42 33 39 '■21 22 115 120 120 190 150 670 628 535? 79? 655 313 119 M. proutii, Nat. Mus. 8736 680 335 234 M. proutii, Nat. Mus. 4701, 9 300 115 190 32 ---- "655 485 »74 .... 120 125 M. ("Symborodon") torvus, Am. Mus. 6365 (type). . 310 100 210 "535 M. heloceras M. heloceras. Am. Mus. 6360 (type) 392 "480 7 "■79 "545 "620 M. heloceras, Am. Mus. 14576 "265 75 170 67 "603 37 132 33 — - 70 30 Percentage of increment from M. helo- ceras to M. giganteus 1 524 TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements of sTcuUs and jaws associated with and referred to the Menodus phylum, in millimeters Skulls and upper teeth 'Lower jaws and teeth Interior canine to hypoconu- lid of ma pi-ni3 Mi-ms Symphy- sis to glenoid Posterior canine to hypoconu- lid of ms Pi-ma Mi-in3 Symphy- sis to condyles Depth below ma M . giganteus Pomel (type) _ »280 -260 262 237 242 245 238 -651 625 610 590 172 M. giganteus. Am. Mus. 506, ?_ "404 "385 "246 "■ess '387 385 369 -404 393 360 364 356 347 176 M. giganteus, Nat. Mus. 4735 (?4745) 137 M. giganteus (trigonoceras), Am. Mus. 1067, 9 M. giganteus (trigonoceras). Am. Mus. 1007 370 355 225 543 144 160 M. giganteus (trigonoceras). Am. Mus. 1066, 9 . --_ ._- - - 360 345 223 220 592 M. trigonoceras, Am. Mus. 6356 (cotype).. M proutii, Nat. Mus. 113 (type) 370 ■■514 234 "227 «214 ''208 »515 °«535 i 522 163 M. trigonoceras. Am. Mus. 6345b, ? 9 "323 = 312 . The canines were evidently small; the fangs measure 23 millimeters anteroposteriorly, 20 transversely; the 526 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA crowns are at present broken away but could not have projected much above the level of the other teeth. There is absolutely no space for the first premolar; the second is placed directly behind the canine and has an elevated anterior, noncrescentic lobe and a posterior crescent; the third and fourth premolars exhibit both anterior and posterior cres- Menodus proutii (Leidy) {Titanotherium proutii Leidy, 1852) Plates LXXXII, CXXV, CXXVI; text figures 160, 409, 438 (For original description and type references see p. 205] Geologic Tiorizon. — Precise geologic level unknown, probably middle level of the Titanotherium zone, Chadron A 3 or B 1. Figure 436. — Skull of Menodus heloceras , Side view; As, top view; Aj, palatal view. One-sixth natural size. Am. Mus. 14576; found at the very base of the Titanotherium zone immediately overlying beds probably equivalent to Uinta C, Beaver Divide, near Hailey, Wyo. This very primitive skull agrees with the type of Menodus heloceras in the trihedral basal horn section, elongate proportions, and slender zygomata. It is also structurally allied to Allops walcotti and Brontops brackycephalus. The upper molars, though poorly preserved, resemble those of Brontops brackycephalus rather than those of Menodus proutii. cents, no internal cingula, faint external cingula. The vertical arching of the premolar series is very pro- nounced. The molars also are devoid of internal cingula but present more or less complete external cingula. Specific cJiaracters. — Size of type jaw slightly smaller (mi-nis 234 mm.) than that of jaws referred to M. trigonoceras. Referred upper teeth (p'-m^ 313 mm.) also smaller than in M. trigonoceras. Upper and lower premolar-molar series with cingula. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 527 Characters of the type (Jedotype) lower jaw. — The reasons for regarding this lower jaw fragment (Nat. Mus. 113) as the type, or lectotype, of this species are given on page 210 of this monograph. The speci- men consists of part of the left ramus of the lower jaw containing the posterior alveolus of ps, both alveoli of p4, and the three molars, which are much worn and somewhat damaged in certain places. The molars exhibit the strongly de- veloped external cingula that are characteristic of the true Menodus, and the measurements show that we have here a small member of the Menodus phylum, distinctly larger than the very primitive M. heloceras but somewhat smaller than the typical M. trigonoceras of the middle Titanotherium zone. Measurements oj Menodus proutii. — In the type lower jaw of this species the true molar series (234 mm.) is about 5 to 10 milli- meters shorter than in jaws re- ferred to M. trigonoceras. The specific name proutii is accordingly applied to the smaller members of the "trigonoceras" group. If we adopt the trinomial system of nomenclature the specific name proutii might include M. proutii proutii for the smaller skulls, and M. proutii trigonoceras for the larger skulls. The molars of the type are 20 millimeters longer than in the type of Symhorodon torvus Cope, which is also a menodont, and the jaw as a whole was considerably larger than those referred to AUops marshi. In a National Museum skull (No. 4701, ? ) referred to M. proutii the premolar-molar series is 45 millimeters shorter than in the typical M. trigonoceras, the basilar length is also much shorter, and the zygomatic index (74, estimated) is relatively high. Description of the neotype of M. proutii. — The upper jaw selected by Osborn as a neotype is a specimen (Am. Mus. 9335) from Lance Creek, Wyo., recorded from the middle Titanotherium zone. The upper molars of the neotype are of conformable size with the lower molars of the type. In fact, the well-worn lower premolar-molar series of the type jaw has been found to articulate fairly closely with the less worn upper premolar-molar series of the neotype maxilla (fig. 438). The neotype upper grind- ers exhibit the characteristic internal and external cingula of Menodus, lofty ectolophs, tetartocones strongly developed on p^, p^, and p*, a prominent tri- angular hypocone on m', and molar crowns distinctly elongated anteroposteriorly; they also exhibit rudi- ments of the crochet and antecrochet. Figure 437. — Lower jaws of Menodus {Symhorodon) torvus and M. trigonoceras Menodus torvus, Am. Mus. 6366 (type) ; a very aged animal with angle very prominent and truncate, cheek teeth with sharply defined external cingula, and incisive border edentulous as in Menodus. B, M. tngonoceras, Nat. Mus. 4745; a very characteristic jaw, showing rather full symphyseal region, angle produced downward and backward, canines and cheek teeth sharply cingulate and subhypsodont. One-fifth natural size. Additional measurements of Am. Mus. 9335 (neo- type of M. proutii) are given below. Millimeters P2, ap. bytr 26X28 Millimeters Canine, vertical (esti- mated) 39 Canine, anteroposterior.- 24 P'-m3 313 pi-p4 119 M'-m' 192 P^ap. bytr 34X41 F\ ap. by tr 38X49 Ml, ap. by tr 59X04 M2, ap. bytr 69X62 M3, ap. bytr 69X67 Dolichocephaly is strongly marked in m^~'. The measurements show that this specimen is smaller than Carnegie Mus. 3068. 528 TITANOTHERES OF AJSTCIENT WYOMING, DAKOTA, AND NEBRASKA Transitional skulls jrom M. proutii to M. trigono- ceras. — A skull (Carnegie Mus. 3068 or 558?) assigned to M. trigonoceras proutii appears to be distinguished subspecifically from the more progressive stage M. trigonoceras by the retarded development of the tetartocones of p*, by the more primitive size and section of the horns, by the hypsodont characters of the external cusps of the molars, and by the correspond- ingly deep fossettes. This apparently occupies an intermediate position between the M. Tieloceras of the lower beds, M. torvus, and the M. trigonoceras of the middle beds. The sex is apparently female. To this may be attributed the fact that the horns are somewhat more primitive, elongate-oval in section. The skull is that of a young animal in the sixth stage of growth, the protocone of m' being barely worn. As shown in the comparative measurements, the superior grinding teeth are of strikingly dolichocephalic type, the measurement p'-m^, inclusive, being 320 millimeters. Figure 438. — Upper teeth of Menodus proutii Am. Mus. 9335 (neotype); Chadron B, Lance Creek, Wyo.; exhibits the generic characters of Menodus. Canines conical with sharply defined cingulum, both external and internal cingula of the premolars and external cingulum of the molars sharply defined, molars elongate anteroposteriorly. Premolar pattern the same as in Brorttops hrachycepJialus. The measurements of these teeth indicate reference to Menodus prfmtii. One-third natural size. In this skull, as in dolichocephalic skulls generally, the bridge over the infraorbital foramen is broad and flat, and the external auditory meatus is widely open below. A wide space separates the postglenoid and paroccipital processes. The dolichocephaly of the dental series is also shown in the spacing of the teeth, which is so considerable that there is a slight diastema between the first superior premolar and the canine; cingula appear between the grinding teeth. P' is a large, actively functional tooth, elongate antero- posteriorly. The canines have the highly charac- teristic form of Menodus (Titanotherium) , being long and pointed, with anterior and posterior cingula; at the same time, their slender section indicates that they belong to a female. Other distinctively Menodus characters are the crenulate internal faces of the deuterocones and the crenulate and strongly developed external and internal cingula with sharp edges. In the grinding teeth the hypocones are very prominent, and both the internal cones and the external crescents exhibit deep slopes; this elongate or subhypsodont character of the grinders, distinctive of all true Menodus teeth, is very striking. Menodus trigonoceras (Cope) {Symborodon trigonoceras Cope, 1873; "Titanotherium trigono- ceras" Osborn, 1902) Plates XX, XXII, CXXVIII-CXXXII, CXXXIV, CXXXV, CLVII; text figures 172, 378, 382, 396, 399, 409, 435, 437, 439, 440, 613-616, 630, 639 [For original description and type references see p. 213. For skeletal characters see p. 683] Type locality and geologic Tiorizon. — Titanotherium zone, level Chadron B. Specific characters. — Skull larger than in M. Tielo- ceras. Premaxillaries to condyles 670 to 725 milli- meters, tip of nasals to top of occiput 670 to 738. Mesaticephalic, width across zygomata 485 to 535 (?) millimeters. Index 74 (?)'. Horns, out- side length 132 to 190 millimeters, basal section triangular, tips pyramidal; horns a little in front of preorbital malar ridge. Nasals squared distally, free length 115 to 140 millimeters, breadth 125 to 135 (?). Buccal swelling of zygoma more decided. Premolar-molar series 333 to 360 millimeters, premolars 127 to 136, molars 203 to 225, canines, anterior d', 42 to 47. Premolar tetartocones more progressive, tetartocone of p"* more pronounced. This animal is directly successive to Menodus proutii and connects this species by a series of "ascending mutations" with M. giganteus. on the measurements of Menodus trigonoceras. — In the typical skulls the premolar- molar series is 83 millimeters shorter than in the type of M. {" Brontotherium") ingens. Several skulls (Am. Mus. 1066, 1067, 1007; Nat. Mus. 4291) which were formerly referred to M. giganteus have the premolar- molar series over 60 millimeters shorter than in the type of M. giganteus and appear to belong rather with M. trigonoceras. Between the largest skull now referred to M. trigonoceras (Am. Mus. 1066) and the smallest referred to 31. giganteus there is a marked difference in the length of the true molar series (23 mm.). Hence in our collections at present, with reference to the longitudinal dimensions of the grinding teeth, M. trigonoceras does not quite overlap M. giganteus but is separated by a small but distinct interval. M. trigonoceras may be distinguished from Allops serotinus by the greater relative anteroposterior measurements of the true molars, by the lesser width of p*, and by the great length of the nasals, as follows : Observations EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 529 Measurements of Menodus trigonoceras and Allops serotinus, in millimeters Pi-m3 Pi-p< Mi-m^ Pmx to condyles Zygomatic index Nasal length P*, ap. by tr M^ ap. by tr M. trigono- ceras, Nat. Mus. 4291, d A. serotinus. Am. Mus. 620 360 330 136 133 224 206 770 720 66 78 130 68 "41X55 43X65 <■ 79X73 78X82 " Am. Mus. 6356. Materials. — This important intermediate stage is represented by the type and paratype skulls in the American Museum (Nos. 6355 and 6356, Cope collec- tion) ; also by two skulls in the National Museum (Nos. 4257, 1219). Nat. Mus. 4257 is definitely recorded by Hatcher as from the upper level of B, the middle Titanotherium zone, a fact of extreme importance, as helping to determine the geo- logic level of this spe- cies. Another skull (Nat. Mus. 4701, ? ) agrees in measurement with M. proutii but is recorded from the base of C; it thus may be a female of M. trigono- ceras. There is no jaw positively associated with M. trigonoceras; a referred jaw is Am. Mus. 1007, which is transitional to M. giganteus in its measurements. If measurements are reliable as indications of specific affinity we should have to add to this species the skulls Am. Mus. 1066, cr', 1067, ? , and the jaw Am. Mus. 1007, which were formerly included under M. giganteus.^ Characters oj the type and paratype of M. trigono- ceras. — Cope's type (Am. Mus. 6355) fortunately is a skull in which all the superior portions are fairly pre- served, as shown in Plates CXXVIII and CXXIX; the principal missing parts are the premaxdlaries, maxil- laries, and occipital condyles. A single molar tooth, m^, shows that this type skull is young or in an early stage of growth, while the paratype skull (Am. Mus. 6356) is entering the seventh stage of growth. This disparity of age explains some differences be- tween these two specimens, especially in the zygomatic arches, which are wholly attributable to growth or age. The type skull (No. 6355) belongs to a young indi- vidual; the nasals are long and rather thin, with nearly parallel sides, truncate distally. The horns are short, widely divergent, with trihedral section and a well-defined intermediate connecting crest, which is placed at the back part of the horn. The cranial vertex is of moderate width and extends backward into a slightly expanded occiput with lateral pillars, extending into rugose summits; the zygomatic arches are slender in vertical section, but they show marked expansion. The third superior molar bears a small, very sharply defined hypocone, a distinctive phyletic character of this series. The paratype skull consists of the anterior portion (Am. Mus. 6356) of a fully adult individual in the seventh stage of growth. The more advanced age presents us with the adult characters of this species, especially the thickening and rugose expansion of the nasals distally, the thickening and moderate expansion of the zygomatic arches, the well-developed preorbital and postorbital processes. Dentition. — The specific identification of the para- type with the type is rendered more positive by the Figure 439. — Skull of Menodus trigonoceras Side view. Tiiis skull (now in the Munich Museum, formerly Am. Mus. 1066) is vertically crushed, so that the nasals are tilted upward, the horns crushed downward, and the vertical diameters of the orbit and parietal region lessened. One-sixth natural size. close similarity in the structure" of the third superior molar in both skulls. This tooth has a sharp and dis- tinct hypocone. Other dental characters have weighty specific value (Pis. CXXXI, CXXXII). The techni- cal formula, I^, C^, P^, M^, fails to convey an idea of the vestigial character of the incisor teeth, the crowns of which barely break through the bone of the incisive border and certainly did not cut through the gum; thus the premaxilla of this animal should be described as functionally edentulous. The sex of the paratype is apparently female, the canine being rather slender and elongate. The maxillary teeth are rather progressive in development; the first premolar exhibits a rudi- mentary tetartocone, or four cusps in all. The tetarto- cones on p^, p'' are well developed but still much inferior in size to the deuterocones; the premolars are further distinguished by pronounced internal and external cingula. Throughout the premolar-molar series we observe a deep pit in the midvalley, just internal to the ectoloph, which is homologous with the 530 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA medifossette of the rhinoceros and horse molar. Just internal to this pit two ridges enter the median valley. These are apparently homologous with the "crochet" and " antecrochet " of the rhinoceros and horse molars. The molar teeth also exhibit well-developed external cingula; the internal cingula, as in all other titano- theres, are incomplete opposite the protocones. Skulls of M. trigonoceras. — Of the six skulls in the National Museum collection referred to Menodus ™ ri .-^-^ Menodus Mffonoceras ^ Col. /fus., Denver- fl k jg^' ^^^B ', d HM^^^gtt^^c^ SlxJ p f Vlv^ lj . ,|j! ,■ l^s r% fi^ Figure 440. — Skull of Menodus trigonoceras In the Colorado Museum, Denver. The nasals and horns were detached when the skull was found, but there is no question as to the association. trigonoceras one (Nat. Mus. 4257) is recorded from the upper levels of the middle Titanotherium zone. It belongs to a small male in the seventh stage of growth. The anterior measurement of the canines is 42 milli- meters (estimated). Two extremely small vestigial incisors persist upon one premaxUla only. In the first superior premolar the tetartocone is connected by a confluent crest with the deuterocone, as in other species of this genus. The tetartocone cusp on p^, p^ is very distinct, especially in p''. From the vertex to the tips of the nasals the skull measures 665 millimeters; the free length and the free width of the nasals are nearly equal, namely, 120 by 125 millimeters. The horns measure 150 millimeters from the edge of the anterior nares to the tips. On the inferior surface of the skull the vomer extends back from the presphenoid as a distinct narrow keel. In another skull, an old male (Nat. Mus. 1219), as in the more advanced or eighth stage of growth, the horns measure 195 millimeters as compared with 150 in the specimen just described. This is a natural growth increase. It is important to note that the nasals are practically the same length as in the younger speci- mens, namely, 115 millimeters; but they are greatly increased in breadth (160 mm.), also in the rugose expansion of their tips, which support a pair of lateral tuberosities. This animal is evidently a very old male, and its senescent character is emphasized by the entire disappearance of the vestigial upper incisors. It is also a very large individual. The premolar-molar series measures 345 millimeters. In contrast with the two skulls described above there is a third skull (Nat. Mus. 4701) belonging to a young female which is recorded by Hatcher from level C 1 of the upper Titanotherium zone. The premolar-molar series measures 300 millimeters. The female sex is indicated by the small, pointed ca- nines (34 mm.). Inconsistently with its high geo- logic level as recorded, the skull exhibits a relatively low stage of development in its premolar teeth, the tetartocones being less clearly separated off than in the old male above described. Although the animal is still young, the incisors have entirely disappeared, as well as all traces of their alveoli. The internal cingulum of the premolars is quite as prominent in this female as in the males, again proving that the cingulum is not a sexual character. There is a large hypocone on the last superior molar, which thus appears to be a specific if not a generic character. The Ottawa Museum skull jrom the lower Oligocene of Swift Current River, Assinihoia, Canada. — The frag- mentary cranium described and figured by Cope (1891.2, p. 10, pi. 6) as "Menodus americanus" very probably belongs to M. trigonoceras. Menodus giganteus Pomel (" Brontotherium" ingens Marsh, 1873; "Titanotherium ingens" Osborn, 1902) Plates XVIII, XX, XXIV, XLVII, XLIX, CXXXIII, CXXXV-CXLI; text figures 24, 159, 227, 228, 375, 381, 387-389, 391, 393, 394, 396, 399, 400, 406, 409, 441-446, 617- 619, 630, 640, 642, 701, 713, 715-719, 744 [For original description and type references see p. 204. For skeletal characters see p. 687] Geologic horizon. — Upper Titanotherium zone. Specific characters. — Skull very large, premaxillaries to condyles 770-825 millimeters, tip of nasals to top of occiput 712-755+ . Dolichocephalic, width across EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES 531 zygomata 515-553 millimeters, index 62-70. Horns, outside length 150-290 millimeters; basal section tri- angular, tips pointed; horns much in front of antorbital malar ridge. Buccal swelling of zygoma about as in M. trigonoceras (that is, moderate). Premolar-molar series, 360-428 millimeters; premolars 133-162, molars 223-270; dental index 51 (typical). Premolar tetar- tocones very progressive but still connected with deuterocones by a narrow strip, tetartocone of p* very pronounced. Canines c? very long (70 mm.). Menodus giganteus was certainly a contemporary of its long-horned rival Brontotherium gigas and probably extended up to the period of the still more specialized Brontotherium platyceras. While far inferior in length of horn to those brontotheres, it was certainly superior to them in the possession of a superb series of grinding teeth with connecting crowns and more elevated crests and cones. The sexual characters of M. giganteus are very clearly brought out by comparison of two male skulls, Nos. 1066 and 505, and two fe- male skulls with jaws, Nos. 1067 and 506, in the American Mu- seum collection. A fine skull in the National Museum (No. 1220) also belongs to a male. The skull Nat. Mus. 4291 is recorded from the highest level of the upper TitanotJierium zone. The ' male skulls exceed the female in the width of the zygomatic arches, thus proving that, as in the brontotheres, the buccal proc- esses are partly secondary sexual characters. The comparatively slender, pointed horns, and pointed canine teeth of the fe- males are well exhibited in Plate CXXXV, B. This figure shows incidentally the extreme effects of lateral crushing as con- trasted with the extreme effects of vertical crushing and deformation of the skull. The type specimen of the species "B. ingens" was erroneously employed by Marsh to complete the characters of the genus Brontotherium, but we now know that the genotype of Brontotherium — namely, B. gigas — is a very different animal in many ways. The type specimen oi"B. ingens " in the Yale Museum is a fine representative of the species, as described in detail below. The absence of the premaxillary bones in this specimen led to many errors regarding the condition of the incisor teeth; we now have conclusive evidence that in this species, as in M. trigonoceras, the incisors are atrophied, vestigial, or entirely wanting. The evidence as to the character of this species afforded by the type specimen is now supplemented by that of an exceptionally fine series of skulls in the American, National, and Field Museums. Observations on the measurements oj Menodus gigan- teus. — The true molars of the fragmentary type lower jaw agree in measurement and other characters with jaws referred to this species. In the referred skulls the grinding teeth range from 385 to 465 millimeters in length, a difference of 80 millimeters. The largest skull (in the University of Wyoming) has a longer dentition than any other known titanothere (p'-m^ 465 mm.). The extreme contrasts between the narrow mesati- cephalic skull and dentition of Menodus giganteus and the broad (brachycephalic) skull of Brontotherium platyceras are shown in the table below. Figure 441. — Restoration of Menodus giganteus By Charles R. Knight. About one-ninth natural size. Measurements of Menodus giganteus, Brontotherium platyceras, and B. gigas, in millimeters P'-m3 Pi-p* M'-m3 Molar index P'', ap. by tr M', ap. by tr Pmx to condyles- Zygomatic index. Nasal length Horn length 425 150 270 32 825 62 175 290 58X73 100X78 B. platy- ceras, Field Mus. 12161 340 120 223 25 80 38 ■390 47X72 91X99 ' Estimated. 532 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA From Allops crassicornis we observe that M. gigan- teus is distinguished by the greater length of p'-m^ and especially of the true molars {M. giganteus 246-285 mm., A. crassicornis 220), by the greater length of the nasals, and by the much lower zygomatic index. Figure 442. — Skull of Menodus giganteus Yale Mus. 12010, type of Broniotherium ingens Marsh. Front view. One-eighth natural size. This view shows well certain features of Menodus, as follows: Horns trihedral and pointed, nasals dis- tally broad, zygomata deep with small buccal expansions, canines large, cheek teeth with sharp external cingula. From Brontops robustus M. giganteus is readily distinguished as follows: Measurements of Menodus giganteus and Brontops robustus, in millimeters M. giganteus B. robustus Pi-m3 .. _ __ -- 385-465 141-176 246-285 50X68 to 58X73 93X89 to 100X78 777-825 62-73 105-175 340-376 pi-p«. .. 139-151 Mi-m'. . - 220-236 P*, ap. by tr _ 43X55 to 46X69 M', ap. by tr Pmx to condyles Zygomatic index Nasal length 73X84 to 90X89 743-813 76-83 52-90 Geologic and geographic distribution. — The geologic range of this species as recorded by Hatcher is from the summit of B, the middle zone, to the summit of C, the upper Titanotherium zone. Materials. — The species is represented by Pomel's type jaw, which has been destroyed and is known only from Leidy's figure and measurements; also by Marsh's excellent type skull of "B. ingens" in the Yale Museum (No. 12010), found in Colorado; by three skulls in the American Museum (Nos. 505, c? ; 1066, c? ; 1067, ? ); by six fine skulls in the National Museum (No. 1220, etc.) ; and by the superb skull asso- ciated with cervical and anterior dorsal vertebrae (Pis. CXXXIX, CXL) in the Field Museum of Chicago (No. P 5927). The largest skull of all is that in the University of Wyoming Museum. According to the measurements given in the table above, the skulls Am. Mus. 1066 and 1067 and Nat. Mus. 4291, which are all referred here to M. gigan- teus, are much closer to M. trigonoceras and are sepa- rated from the true M. giganteus skulls by a consider- able gap. Characters of Marsh's type of "B. ingens." — The principal characters of the type skull as given by Marsh with measurements are entered in the table above. Additional characters are as follows: This animal is in the eighth stage of growth, advancing toward the ninth. The tips of the internal cusps of m'^ are slightly worn, barely showing the dentine. The top of the skull is uncrushed but somewhat depressed just behind the connecting horn crest. The right horn and right half of the nasals are partly restored, although figured as complete in Plates CXXXVII and CXXXVIII. In the left horn, which is more perfect, the triangular shape is very marked. Figure 443. — SkuU of Menodus giganteus Yale Mus. 12010, type of Broniotherium ingens Marsh. Palatal view. One-eighth natural size. Drawing made by Berger under the direction of Marsh. This view shows Menodus characters as follows: Skull dolichocephalic, nasals long and distally broad or squarish, horns trihedral in section and pointed, tooth rows rec- tilinear, cheek teeth with sharp internal and external cingula, premolars with large postero-lnternal cusps, molars elongate an- teroposteriorly, zygomatic expansion moderate. The anterior angle of the horn runs directly into the nasals. The premolars exhibit very broad internal cingula and pronounced external cingula; the first premolar displays an internal crest formed of the deuterocone and tetartocone. The second and third premolars have the tetartocones more distinct, ele- vated, and sharply separated off from the cingula; the deuterocones and tetartocones are subequal, EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 533 thus being a progressive advance beyond the M. tri- gonoceras condition. The hypocone of m' is tubercu- late, with a narrow crest extending off toward the metacone; this tooth exhibits a sessile "crochet" and "antecrochet." Additional measurements of the type of " B. ingens" Millimeters Summit of occiput to tip of nasals 755 Extreme spread of horns 490 Free length of nasals 125 Width of nasals 125 Molar-premolar series, estimated 428 True molar series 265 Dentition. — The incisors either consist of extremely reduced crowns in depressed alveoli, resembling the vestiges seen in M. trigonoceras, or are entirely obsolete. Of the male skulls in the American Mu- seum No. 1066 shows two vestigial alveoli and No. 505 shows one small alveolus; similarly, in the National Museum No. 4291 shows two alveoli partly closed. In one of the female skulls (Am. Mus. 1066) there is a single vestigial incisor on one side, lying in its alveolus below the surface of the jaw. Canines: Contrasting with this atrophy is the hypertrophy of the canines, which are readily dis- tinguished from those in members of the Bronto- therium phylum by their long and pointed crowns, circular to suboval in section. Almost completely encircling the base of the crown is a prominent cingulum; this is a very exceptional and distinctive character; in the females the cingulum is perhaps somewhat less prominent. In the males the largest canines measure about 70 millimeters; in the females the canines measure about 40 millimeters. The chief distinction of the female tusks is their much more slender and pointed character. Premolars: The persistence of the first premolars in both jaws is a general characteristic of this species, although not observed in the specimen in the Field Museum. A peculiarity of p' is the presence of a distinct tetartocone, which is even stronger in "B. ingens " than in M. trigonoceras. In the succeeding premolars, p^-p*, the tetartocones are more or less well defined and distinct from the deuterocones, either connected by a low ridge or altogether separate, the proportion between the size of the deuterocone and tetartocone being as 5 to 3. This acceleration of the tetartocone is a progressive character; it is less ad- vanced in the American Museum skulls than in the Yale Museum type of "B. ingens," which was prob- ably found on a very high geologic level. As a rule the tetartocones increase in distinctness as we pass backward from p' to p^, but as in all other titano- theres the tetartocone in p* is generally less distinct. A mesostyle ridge is faintly developed on p* in some specimens of Menodus. It is best developed in cer- tain specimens of Brontofherium and Megacerops in which p* is more molariform than in Menodus. Both molars and premolars have deep pits or "medifossettes" in the midvalley, just internal to the ectoloph; this "fossette" is bounded internally by folds homologous with the "antecrochet" and "crochet" of rhinoceros molars. The molars and to a less extent the premolars are proportionately nar- rower and longer than in the brachycephalic phyla. Molars: The "fossette" just mentioned is correlated with the vertical elongation of the ectoloph, which now measures 80 millimeters in height, while the pro- tocone measures only 28 millimeters; the outer wall of the tooth is thus between two and three times as high as the inner wall. As in the preceding species of this phylum, the internal cusps also of the grinding teeth have steep slopes. The hypocone of m^ shows individual variability: it is either large and distinct (Am. Mus. 1067), or small and distinct (Nat. Mus. 4291), or confluent with the cingulum (Am. Mus. 1066). The cingulum is certainly the most conspicuous and distinctive character of the grinding teeth; not only does it encircle the canine, but it is sharply defined upon the outer faces of the entire superior and inferior premolar-molar series (Pis. CXXXVI, CXXXVII, CXXXIX). The superior premolars are readily distinguished by the broad shelf-like internal cingulum, unlike that in any other phylum. The superior molars are also cingulate upon the inner sides. The only part of the entire dental series which entirely lacks the cingulum is the inner side of the inferior premolar-molar series, where no titanothere displays a cingulum. The grinding series as a whole, therefore, is distin- guished by its great length, by the proportionate length of the individual teeth, the anteroposterior slightly exceeding the transverse diameter (in the molars), and by the pronounced development of the cingulum. Skull. — The chief distinction of the skull Ues in its proportions; it is relatively long and narrow, the average ratio of two male skulls being, length 773 millimeters, breadth 545 millimeters, and consequent index 69, whUe in the type of Brontops rohustus the ratio is 765 to 667, and the index is 87. In all the skulls the nasals are large and quadrate, measuring from 127 to 175 millimeters in length and from 125 to 145 and more in breadth. As in M. trigonoceras, advancing age is accompanied by a rugose develop- ment of the extremities, with a lateral expansion of the tips and a deepening of the median cleft. Other growth characters of the skull are the increasing length of the horns from 150 to 290 millimeters, the increasing rugosity and breadth of the occiput, the widening of the buccal expansions of the zygomata. These buccal expansions, however, remain limited in extent and do not involve a large part of the zygomatic arch as in Brontotherium; immediately below and slightly anterior to the buccal expansion is a very characteristic, 534 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA slightly downward projection. Other conspicuous characters are the relatively large orbits (as compared with those of Brontotherium) , the bridge over the infraorbital foramen, the steep slope of the occipital vertex (contrast Brontotherium). Additional features of advancing age are the development of the marked supraciliary expansion of the lateral frontal parietal crest (Nat. Mus. 1220), the obliteration of the suture between squamosal and malar. Horns. — The distinctive characters of the horns are not only the connecting crest, which, as shown in the section, is situated very far back, but the normal triangle with the external angle at the vertex. (See figs. 399, 444, 445.) The skull has other distinctive features: First, we observe the width of the bridge over the infraorbital foramen; as in the Palaeosyopinae and also in the related Brontops phylum this bridge extends into a thin plate anteriorly, so that the fora- men is distinctly seen on the side of the face, whereas in Brontotherium and Megacerops the malar bridge is narrow and convex and the foramen is not seen in side view. This elongate condition of the infraorbital canal and breadth of the malar bridge is correlated with the dolichocephaly existing throughout the A B Figure 444. — Sections and contours of skulls of Menodus giganteus and M. varians M. giganteus: A, Am. Mus. 505, cf (neotype), and B, Yale Mus. 12010 (type of Brontotherium ingens); relatively long-pointed horns witli a prominent anterior swelling suggestive of the accessory hornlet in Diploclonus, horns trihedral in hasal section, connecting crest near the plane ot the posterior face of the horns, nasals long and wide, zygomata little expanded. C, M. varians, Yale Mus. 12060 (type); horns thiclser (especially at the top), basal section widely trihedral, nasals broad and thicli, zygomata somewhat expanded. One-ninth natural size. direction, which is chiefly outward and slightly up- ward. A very distinctive character in front view is the straight inferior contour of the horn (see fig. 442); all members of the Brontotherium and Megacerops series have a curved or convex lateral inferior contour as seen from in front. Correlated with the lesser strain of the horns and the great breadth of the anterior nares the maxillary pillars on either side of the anterior nares are much thinner than in the stout- horned species. The basal horn section as compared with that of M. trigonoceras has already been described and is very distinctive; briefly it consists of an isosceles skull. In the midline of the parietal crest there is observed a median ridge clearly shown in the section, there is also occasionally a conspicuous knob in the midparietal region. In the occipital region we observe a mastoid foramen; the postglenoid and paroccipital processes are proportionately narrower and deeper than in the Brontotherium series, another fact in keep- ing with the dolichocephalic structure of the skull. Again there is narrower contact between the post- glenoid and post-tympanic, a more open auditory meatus, and a wider space behind the ear than in the brachycephalic types. EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 535 Lower jaw. — Dolichocephaly is also influential in the jaw structure, as among the long-skuUed rhinoc- eroses. The rami of the jaw are long and deep, the lower border being carried horizontally forward until it terminates in a well-marked angulation of the chin; from this point it ascends sharply to the incisive border; the angle is broad, with a marked backward extension. This decided angulation of the chin is quite distinct from the convex lower border and shallow, sloping chin in the Brontotherium series. Another distinctive feature is the greater size of the mental foramen (Am. Mus. 1067) and its more pos- terior position beneath the fourth premolar. In the Brontotherium series the foramen is usually smaller and more anteriorly placed. The angle has a smooth border, is rather delicate and narrow but none the less prominent, flaring obliquely backward with an even curve at the postero-inferior border, in contrast with the Brontofherium angle, which is broader, with a rugose border and truncate apex. The coronoid is broad, has a smooth, rounded anterior edge, and is not strongly curved backward. The smooth backward extension of the angle, the straight lower border, and the definition of the chin are characters exactly paralleled in the dolichocephalic rhinoceroses. Variation in size. — This is not only distinctive of sex, the females being more slender in all their pro[)or- tions, but of the stages of growth which are recorded in the measurements of the skulls, and also of the stages of vertical succession or evolution which are recorded in the measurements chiefly of the premolar- molar series, as shown in the tables on page 523. Final stage of evolution. — In the Field Museum, Chicago, there is a superb skull (No. P 5927), found near the top of the upper TitanotJierium zone at Phinney Springs, S. Dak. With it were discovered the lower jaws, cervical and anterior dorsal vertebrae. (See p. 686.) The skull is in a beautiful state of pres- ervation and has been very accurately reconstructed by Mr. E. S. Riggs. It belongs to a fully adult animal, with well-worn teeth, and strongly accentuates all the characteristic features of the type of M. {"Bron- totherium") ingens. The lateral and anterior views (Pis. CXXXIX, CXL) accordingly give the most per- fect idea of the extreme development of this phylum. Noteworthy peculiarities in the dentition are these: (1) PMs present on the left side of the skull but absent on the right. In most other specimens p' is constant; this, however, proves that in Menodus as in Brontops p' is a variable tooth. (2) Pi is also absent in this specimen, the canine being closely crowded against P2. (3) The third inferior molar exhibits a very small hypoconulid which lacks the internal cup or crescent. Noteworthy progressive features are the presence of an anterior swelling or incipient hornlet on both the horns, somewhat similar to that observed in the type of Diploclonus hicornutus, and the very anterior position of the bases of the horns, which are almost 101959— 29— VOL 1 37 as far in advance of the line of the orbits as in the type of Brontotherium gigas. This feature does not, however, interfere with the great length of the nasals, which protrude far beyond the vertical line of the pre- maxillaries. Notable, also, are the relatively great length of the horns (outside measurement 290 mm.). The detailed measurements of this important skull are presented in the table on page 523. Observations on the Menodus giganteus of the Uni- versity of Wyoming. — The largest dentition among known titanotheres is exhibited in a specimen of Menodus giganteus in the University of Wyoming, from the upper Titano- therium zone of Bates Hole, Carbon County, Wyo. The premolar se- ries (p'-m^) measures 465 millimeters, as compared with 410 in the type. The premolars are very large (176 mm.) and highly progressive They con- form in all details to the Menodus type. They show very clearly the enamel folds and ridges which are called "crista," "protoloph," and "meta- loph." Their tetarto- „ aa^ a ^- j ^ Figure 445. — Sections and con- cones, although large, are tours of skull of Menodus gi- still connected by a bridge ganteus with the deuterOCOneS, as Am. Mus.SOG, 9 (of. fig. 446). in females in the Menodontinae ^en- °^ '^'^ species the horns are slender but 11 rpu J" '^ ha.ve a relatively high connecting crest, erally. Ihe dimensions the tasal section is obUquely and roundly of the premolars and mo- t"hedral, the nasals are long and are de- . ,, curved at the tip. One-eighth natural lars are as loilows: size. Millimeters Pi, ap. by tr 25X26 P2, ap. by tr 40X47 ps 51X59 P^ (estimated) 58X73 Ml (estimated) 81X77 M2 (estimated) 100X80 M^, ectoloph, anteroposterior 114 M-, transverse (across mesostyle) (estimated) 95 M^, height of ectoloph (paracone) 88 MS, ap. by tr. (estimated) 100X78 M^, transverse (across niesost3'le) 90 Menodus varians (Marsh) {Menoys varians Marsh, 1887; " Titanotheriiim ingens Marsh," Osborn, 1902) Plate CXLII; te.xt figures 181, 409, 444 [For original description and type references see p. 223] Geologic horizon. — Geologic level not known, prob- ably upper Titanotherium zone. Specific characters. — The skull is relatively shorter or less extremely dolichocephalic than that of M. giganteus. Skull length, premaxillaries to condyles 536 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 760 millimeters, nasals to top of occiput 695, width across zygomata 555, zygomatic index 73 (that is, sub- brachycephalic). Nasals decidedly shorter than in M. giganteus (free length 105 mm., breadth 140). The horns are outwardly directed but of similar section to those in M. giganteus. Two vestigial upper inci- sors retained on each side. Premolar-molar series (410 mm.) relatively very long (dental index 54). Premolar tetartocones very distinct. P* with redupli- cate tetartocone. Hypocone on m' large and distinct from the cingulum. This animal appears to be specifically distinct from M. giganteus and to present a form which tends to bridge over the structural gap between Menodus and Allops, since while exhibiting all other characters of the protocone and the hypocone of m^ being well worn. In its superior aspect the skull is very similar to Marsh's type of ingens, although less dolichocephalic; the horns are connected by the characteristic low trans- verse crest. In the plane of the posterior faces they are crushed downward and outward in such a manner as not only to change their normal elevation but to decrease the acuteness of the trihedral section and Figure 446. — Lower Jaws of Menodus giganteus A, Am. Mus. 500, ? (cf. flg. 445); B, Field Mus. P 5927, a large male. Both jaws have the ramus elongate, the into a rounded elbow, and the teeth sharply cingulate. One-fifth natural size. ngle produced posteriorly M. giganteus, it possesses the short nasals character- istic of Allops. That this skull may be an Allops is further indicated by its sub-brachycephaly (index 73) as in Allops serotinus, also by the proportions of its horns and nasals. Materials. — The species is known only from the type skull in the Yale Museum (No. 12060), which is represented in Plate CXLII. Description oj the type. — The type skull is that of an old animal in the eighth to ninth stage of growth, both make it difficult to express in cross section (fig. 444, C) their actual form. The nasals had the spread but not the length characteristic of Menodus. In inferior aspect of the skull the nares open immediately behind the second molar, as in M. ingens. There are traces of a median vomerine keel and a very prominent rugose projection at the junction of the basisphenoid and basioccipital, which is apparently broken away in the type of M. ingens. As in M. ingens the orbit is directly above the posterior portion of the first molar. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 537 Measurements of Menodus varians Millimeters Length of skull, vertex to tip of nasals 695 Basilar length, occipital condyles to premaxillary 760 Transverse width across zygomata 555 Molar-premolar series 410 True molar series 255 Premolar series 155 Canine crowns, anteroposterior diameter 35 Free length of nasals 105 Free width of nasals 140 Dentition. — The alveoli for two vestigial iacisors upon each side persist, with a narrow median diastema between them. There is a vestige of a third alveolus on one side. The alveoli are exceptionally shallow, and these teeth were undoubtedly vestigial. The canines are so robust as to indicate that this was a male animal. The cingulum is continuous around the anterior face; the crowns are broken off. Close behind the canines are the bifanged, well-developed first premolars; these teeth are distinguished as in M. giganteus by very broad, crenulate internal cingula and prominent external cingula; in p^ and p^ the tetartocones are very prominent. P* exhibits an interesting example of correlated bilateral variation in the double conical summits of the tetartocones; this, however, is probably due to the advanced con- dition of the cingulum. The hypocone is strongly developed and entirely distinct from the cingulum upon m^, as in Marsh's type Diconodon montanus and in several specimens of M. trigonoceras. Relationship to Menodus. — In describing this sup- posed genus, Menops, Marsh observed: "The present genus is most nearly related to Diconodon and in its molar teeth agrees with that form. It differs in the presence of two upper incisors on each side." The entirely vestigial character of the incisor teeth natu- rally forbids our assigning them generic value. This animal presents so many points of close similarity with the type of M. ingens that the single decisive specific character which can be selected is the abbre- viation of the nasals and the somewhat less extreme dolichocephaly. The nasals are less quadrate than in M. ingens, the free length (105 mm.) being less than the free width (140 mm.). Even this character is possibly attributable to individual variation, since some of the typical specimens of M. giganteus and M. trigonoceras show a relative abbreviation of the nasals. A similar broadening and abbreviation of the nasals occurs in the subgenus Allops. The inferior contour of the horns is less straight, and the horns apparently diverge more widely than in M. ingens, a condition partly attributable to the downward crushing of the skull or perhaps indicating affinity with the fiat, outwardly directed horns of Allops. The reduplication of the tetartocone on p'* is another feature observed in Allops serotinus. A more impor- tant difference is that the skull is proportionately shorter and less dolichocephalic than in M. ingens^ the proportions being, length 760 millimeters, breadth 545, as compared with length 825, breadth 550. The hypocone of ni'' is even larger and more distinct than in most specimens of M. giganteus, resembling that in the type of Diconodon montanus. This may be a specific or progressive character. We observe the similarity to M. giganteus in many other details, such as the sections of the hdrns, the shape of the zygomatic arches, the presence of small alveoli for the incisors, the antorbital knob on the molars, the comparative length of the face, the ap- parent exposure of the mastoid bone, the sharp and horizontal shelf of the top of the occipital pillars, the bifanged, well-developed first premolars, the ex- treme cingulate development of the canines and grind- ing teeth, the pi-ominence of the tetartocones and of a hypocone on m^ Our conclusion is that the genus Menops is probably equivalent to Menodus, while the species M. varians may be regarded provisionally as valid and distinct. Menodus montanus (Marsh) {Anisacodon montanus Marsh, lS75; Diconodon montanus Marsh, 1876; " Symborodon montanus" Osborn, 1902) Text figures 175, 447 [For original description and type references see p. 217] Type locality and geologic horizon. — Recorded as from "northern Nebraska"; Titanotherium zone. Specific characters. — Nasals narrow, relatively ab- breviate, so far as preserved, resembling those of Allops serotinus, grinding teeth with transverse diam- eter exceeding the anteroposterior, thus resembling Allops. Incisors reduced or vestigial, as in Menodus, m^ with a distinct hypocone, prominent external cingula on all grinding teeth, and a strong internal cingulum on the premolars. M'-m^ 218 millimeters. General characters. — The genotype (Yale Mus. 10022) of Anisacodon montanus is an incomplete fragment of a skull, together with the complete molar series of both sides, portions of the left maxillary, of the left zygomatic arch, the ex- tremities of the nasals, and the left superior pre- molars. There are paired alveoli for p^ As in Menodus the incisive border is narrow, sharp, and functionally edentulous, but it exhibits two vestigial alveoli. The premaxillary contains two caniae al- veoli. The internal cingulum of the premolars is prominent and rounded; the external faces of both premolars and molars exhibit a sharply serrate cin- gulum. The third superior molar presents a conical hypocone quite distinct from the cingulum. To this the generic name Diconodon, originally applied by Marsh, refers. The grinding teeth are of about the same size as those of Menops varians, the molar fossettes are very deep. 538 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements of Menodus monianus Millimeters Canine alveoli, anteroposterior diameter 31 Canine, transverse diameter 19 Nasals, transverse 96 Nasals, longitudinal (so far as preserved) 87 Second superior molar, transverse 88 Second superior molar, anteroposterior 76 Molar series, superior 218 Affinities to Menodus. — As shown in the above de- tailed description this animal agrees with M. "ingens," and especially with M. varians, in the vestigial char- FiGURE 447. — Teeth and nasals of Menodus montanus valid species, although more perfect material may relate it still more closely or even specifically to M. giganteus. SECTION 6. THE BRONTOTHERIINE GROUP We now consider the brontotheriine group as dis- tinguished from the menodontine group. It contains two phyla, closely affiliated in their Eocene origin but widely diverging in their Oligocene evolution — ■ namely, the Megaceropinae and Brontotheriinae. GROUP CHARACTERS Lower Oligocene titanotheres of me- dium to larger size. Horns progres- sively elongating in the males until they attain great size at the expense of the nasals, which are atrophied. Face abbreviated, or brachyopic. Skulls with broadly spreading zygomatic arches and brachycephalic characters in the audi- tory region, in the occiput, in the broad proportions of the upper grinding teeth, and in the arching of the opposite series of grinding teeth. All grinding teeth devoid of external cingula in males. Premolar grinding teeth precociously becoming more molariform than in other groups. Orbits small. This group includes all the known long-horned titanotheres, both the smaller megaceropines and the larger brontotheres. Of the two the mega- ceropines are less formidable animals, without incisor teeth, and with a less powerful action of the horns. The brontotheres are hj far the most formid- able of all the titanotheres known, ani- mals of gigantic size, with powerful horn action, hence appropriately termed by Marsh "thunder beasts." The ana- tomical resemblances between these animals are by no means confined to the superficial similarities but extend to all parts of the skull and teeth, as we have seen in the introduction of this Yale Mus. 10022 (type of Diconodon monianus Marsh). A, Third right upper molar. The elongate pro- portions and hypocone surrounded by a cingulum are seen also in certain other specimens of Menodus (of. Am. Mus. 1067). B, Fourth upper premolar and first and second true molars of the left side. The chapter, and pomt tO a COmmOU anCCStry elongate proportions and sharp internal and external cingula are seen as in J/enoii«5. C, Alveoli of the . -p„„p„„ time upper incisors and canines. The upper incisors were vestigial, as in Menodus; the canines were large. -^^ -^"*- . ' t .-i u implying male se.x. Di, Top view of distal portion of nasals. The paired projections are seen as in The phylctlC characters 01 the brOUtO- Menodus. Di, Front view of nasals. One-half natural size. acter of the incisors, the bifanged premolar, the pres- ence of a distinct hypocone on the last superior molar, the marked external cingula on both premolars and molars, and the strong internal cingula on the pre- molars. It apparently differs from Menodus in the narrow nasals and the more transversely extended proportions of the molar teeth. It seems probable that this fragmentary specimen should be considered provisionally as the type of a theriine group common to Megacerops and Brontotherium are indicated below: 1 . There is a general increase in size of the skull and skeleton. 2. The postorbital region of the skull shows rapid elongation, the preorbital region rapid abbreviation, occiput widely prolonged behind the zygomata, or- bits small, and never a prominent postorbital process. In uncrushed skulls the orbits do not appear as if closed posteriorly. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 539 3. The zygomatic arches spread greatly in males, less in females; the zygomatic index becomes very high in extreme forms; the postglenoid unites with the post-tympanic process to inclose the external audi- tory meatus; the transverse measurements of the su- perior grinding teeth exceed the anteroposterior measurements throughout; the occiput greatly in- creases in breadth and slowly in height; the opposite grinding series become arched and strongly bent up- ward, the opposite canines thus being brought near together. One of the most distinctive and constant features of the skull is the anteroposterior convexity on top of the parietals and the general flatness of the skull top in contrast to the deep concavity of this region in the Menodontinae. 4. The dental index appears to be on the average somewhat lower than in the menodontine group. The variability of the dental index is probably due to crushing. The grinding series does not increase in length so rapidly as the skull but increases greatly in width. The internal cingula tend to disappear or degenerate. 5. The canines are obtuse or recurved, massive in males, small in females, never elongate and pointed. 6. The premolars are distinguished by the acceler- ated development of the tetartocones, which are placed farther in toward the center of the crown — that is, away from the internal or lingual border, a common distinctive feature; the anterior premolars, p-^, are very progressive, with distinct tetartocones, but nevertheless are thrust inward toward the lingual line and tend to drop out in old age. 7. The internal cingula of the premolars are massive and bluntly crenulate, gradually becoming more or less confluent with the base of the crowns. 8. The dominant feature of the skull is the hypertro- phy or elongation of the horns and the corresponding atrophy or abbreviation of the nasals, a compensa- tory character. The horns are arrested in develop- ment in the females, but the nasals are abbreviated in both sexes, though less abbreviated in females than in males. The brontotheriine group possessing these common characters early subdivided into two very distinct phyla which we term respectively the genera Mega- cerops and BrontotJierium, animals which enjoyed an independent simultaneous development from the base to the summit of the Titanotherium zone. Members of these two phyla inherited a number of ancestral characters and also a number of predisposi- tions to a similar evolution, which are enumerated in the phyletic and family definitions above. Thus in both phyla the horns progressively increase in size, the teeth undergo similar changes. Megacerops, however, is readily distinguished from Bi'ontotherium in many parts of the skull and teeth and probably also in the skeleton, as we shall un- doubtedly demonstrate when the skeleton becomes fully known. The most conspicuous points of differ- ence are shown in the accompanying table. Comparison of features of memhers of the Megacerops and Brontotherium pJiyla Megacerops phylum (subfamily Megaeeropinae) Brontotherium phylum (subfamily Brontotheriinae) 1. Animals of small to medium size, either slowly increas- ing or arrested in size. 2. Skulls mesaticephalic to brachyeephalic. 3. Horns rounded in section, vertical in position; placed rather above the orbits and not greatly shifting for- ward, with the connecting crest small or absent. 4. Narial aperture high and narrow. 5. Nasals thin and progressively reduced in lengtli. 6. Incisor teeth usually vestigial; incisive borders edentu- lous in males and females. 7. Canine teeth small and closely approximating to one another. 1. Animals of small (B. leidyi) to extremely large size, in- creasing to the largest size. 2. Skulls dolichocephalic to brachyeephalic, finally attaining a high zygomatic index. 3. Horns transversely oval to flattened in section, widely divergent, shifting forward and progressively developing a high connecting crest. 4. Narial aperture constricted by the heavy buttresses sup- porting the horns. 5. Nasals thick and rapidly reduced to short, obtuse knobs. 6. Superior incisor teeth invariably persistent, 2-1 in males. 7. Canines large, obtuse, arrested in growth, separated from one another. So far as is indicated by the breadth of the cheek teeth, the curvilinear premolars, the upward flexure of the premolars, and the broad zygomatic arches (for example, M. hucco), Megacerops appears to be even more brachyopic than Brontotherium. Megacerops is less extreme in the horns than Brontotherium but more extreme in the somewhat closer approximation of the canines and stronger arching of the premolar series. The grinding teeth of Megacerops are of the same type as those of Brontotherium; the molars are not readily distinguishable; the premolars of Megacerops are smaller with reduced internal cingula. While the incisors are usually absent in the adults there is evidence that they were present in young animals. A skull in the National Museum which resembles Megacerops in its horn and nasal structure exhibits large superior incisors. 540 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA SEXUAL CHAEACTERS In the females of both series we observe a far more marked distmction from the males than that observed in the menodontine group. Not only are the canine teeth smaller, but there is a pronounced diEference in the proportions of the horns. There are certain other detailed characters which so far as our observations are valid seem to separate the females from the males. A summary of these contrasting sexual characters in Brontotherium is given below. Males Horns long; connecting crests large and prominent; nasals decidedly short. Canines larger; incisors more con- stant, formula f;! or f;-?-. Buccal processes of zj'go- mata greatly produced; occi- put greatly extended behind the line of the zygomata. Females Horns relatively short; con- necting crests less promi- nent. Nasals relatively long. Canines smaller; incisors less . persistent, sometimes want- ing. Buccal processes of zygomata less prominent; occiput not so widely ex- tended behind zygomata. From some specimens it would appear also as if the premolars of brontotheres were somewhat less pro- gressive in evolution in the females than in the males. This would controvert the general principle observed in Menodus that the grinding series of the teeth, which are so essential to the nutrition of the females, is the one character in which the sexes do not differ. Specimens of the female sex are smaller in size through- out, as seen in the detailed table of measurements. SUBFAMIIY MEGACEEOPINAE Relatively small, long-horned titanotheres, known chiefly from the middle Titanotherium zone. Horns precociously evolved, vertical in position, placed above the orbits, with little or no connecting crest. Incisor teeth much reduced or actually vestigial, canines very small, placed close together, thus tend- ing to contract the premaxUlaries. Geologic Jiorizon and geographic distribution. — So far as known these animals are of medium size or rela- tively small and are recorded chiefly from the middle Titanotherium zone of Cedar Creek, Colorado, and from the lower portion of the upper Titanotherium zone of South Dakota, possibly also from Assiniboia, Canada. Four skulls of Megacerops were found in lower Oligocene (Chadron) deposits at the levels indicated below, chiefly according to the records of J. B. Hatcher: Level C: ?M. acer (type). M. copei, Nat. Mus. 4711 (type;. M. acer, Univ. Wyo. Mus. Level B: M. bucco, Nat. Mus. 4705, 5 . (Level rather doubtful.— J. B. Hatcher.) Distinguishing features. — The males develop long horns, which, imlike those of the typical Brontotherium., are placed only slightly in front of the orbits, so that, as shown in Charles R. Knight's models and restora- tions (PI. XVIII, C; fig. 454), the eyes appear almost directly below the horns. As in the brontotheres the eyes were small. The horns are directed almost vertically upward, with the long basal axis placed obliquely, the basal section never tending to become transversely oval, as ia Brontops roiustus and Bronto- therium, or triangular, as in Menodus. The reason for this is that the horns are supported or braced prin- cipally in a fore and aft direction, instead of across the skull by means of the connecting crest, indicating that they were used largely in a vertical or tossing motion of the head rather than in the lateral motion characteristic of the brontotheres. The narrowness of the chin and premaxillary region and the entire absence of incisor teeth indi- cates that these animals were provided with a narrow and pointed prehensile upper lip, contrasting with the somewhat broader lips in the brontotheres. A decided and highly characteristic feature is the marked abbreviation (brachyopy) of the facial por- tion of the skull, correlated with the cyptocephaly or upward flexure of the anterior grinding teeth. Materials. — Remains of Megacerops are compara- tively rare. Their existence was first made known by Leidy in 1871 (see p. 210) through the type spe- cies Megacerops coloradensis ; secondly, by Cope in 1873 from the skulls from Colorado described as Symborodon bucco, S. acer, S. altirostris. Cope's specimens are in the American Museum of Natural History. A lower jaw ia the American Museum (No. 6364) appears to represent a very small new species of this genus, known as M. riggsi. Two ex- ceptionally complete skulls from South Dakota are in the National Museum, one of which is referred to a third species, M. copei, related to M. bucco. The first (Nat. Mus. 4705, skull O') is small horned, ap- parently belonging to a female of M. bucco, and is recorded by Hatcher from the top of Chadron B ; the second (Nat. Mus. 4711, skull V'), the type of M. copei, apparently a male, is recorded by Hatcher from the middle beds and was known by the collectors as the "rabbit skull," because of the resemblance of the flaring horns to the ears of the jack rabbit. The Megacerops phylum as represented in the Hatcher collection of six skulls and lower jaws from the Chadron formation, in the United States National Museum Genus and species Catalog No. Materia] M bucco (Cope)._ 4705, ? Fine skull. Typical. Do 4700, ^ Skull. Close to M. acer Cope. Differs from typical Mega- cerops in having single up- per incisors. Horns and ca- nine of Megacerops type. M copei (Osborn). 4711, & Skull. Type. M riggsi? Osborn^ 1236, cf Lower jaw. M riggsi Osborn _ . 5412, cT Right jaw, with symphysis; contains molars. M sp S786, ? Lower jaw. Poor. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TXTANOTHERSE 541 Our knowledge thus rests chiefly on six skulls and two lower jaws as representing this genus. There are also isolated horns and the top of a cranium in the American Museum collection and fragmentary skeletal material (figs. 625, 629, 638). In the IJni- versity of Wyoming there is an excellent skull of Megacerops acer. General characters. — These specimens agree in the exceptionally small size of the canines, in both the males and the females. The nasals are thin in two of the species, M. hucco and M. copei. M. acer is some- what divergent in the thickening of the nasals and in other characters. It is thus apparent that Megacerops is a separate collateral phylum, resembling Brontotherium in the elongation of the horns and paralleling Menodus in the degeneration of the incisors; but it differs from both phyla in the shape and position of the horns and in the approximation of the canines toward the median line. STceleton. — The skeleton is known only from a few portions associated with M. acer in the American Museum, which indicate that the animals of that species were rather small. While generally of smaller size and differing widely from Brontotherium in the entire absence or vestigial character of the incisor teeth, the position and basal section of the horns, and some other characters, these animals present many resemblances to Brontotherium, especially in the elongate horns — in some cases oval in top section — in the backward prolongation of the occiput, the broad contact between the postglenoid and post-tympanic, the roundness of the malar bones, the suddenly projecting buccal expansions of the zygomatic arches, bluntness of the canines, round- ness and bluntness of the internal cusps of the pre- molars; also the abbreviation of the premolar series and reduction of the cingula, the strongly recurved coronoid, and the general contour of the jaw. The common characters of these Megacerops species are clearly perceived in a comparison of the transverse sections of the horns and of the nasals, which are highly characteristic. The main features of corre- spondence between Megacerops copei and Menodus torvus are, first, that the nasals are very thin, even in the portion between the horns; second, that the horns are set widely apart at the base. (This char- acter is obscured by lateral crushing in the male type skull of M. copei.) Reference to the detailed descriptions of the skulls in these two species shows that they are closely related but that M. copei is more primitive, especially in the retarded con- dition of the tetartocones, the section of the buccal processes, and the persistence of the reduced in- cisors. SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES IN THE MEGACEROPS PHYLUM Megacerops Leidy, 1870 (Megaceratops Cope, 1873, in part (M. acer); Symborodon Cope, 1873, in part (5. hucco, S. altirostris) ; "Symborodon" Osborn, 1902) Plates XVIII, CXLIII-CLX, CLXXXVI; text figures 24, 164, 167, 169, 375, 378, 390, 392-394, 398-400, 434, 448-456, 625, 629, 638, 640, 719, 744, 746 [For original description and type references see p. 208. For skeletal characters see p. 691] Generic characters.— Incisors, typically vestigial. Ca- nines small, obtuse. Grinding teeth without cingula, deflected upward. Premolars with progressive tetar- tocones. Skull brachy cephalic to hyperbrachyce- phalic (zygomatic index 84), brachyopic. Nasals slender, narrow, decurved, abbreviated progressively. Horns set vertically, typically without connecting crest; placed above orbits; rounded in section. The genotype of Megacerops (1870) is the species M. coloradensis Leidy, represented by nasals and horns (fig. 448). The genotype of "Symborodon" (1873) is the species S. torvus Cope, which is repre- sented by a lower jaw. (See p. 211.) This jaw does not belong to the same phylum as Megacerops because it proves to be that of a Menodus. General characters. — Dentition: I^°. Incisors re- duced, vestigial, or wanting; canines reduced, obtuse; opposite grinding series arched to strongly arched; upward flexure of face and premolar series as seen in side view extreme; length of premolar-molar series generally less than two-fifths that of the skull, from premaxillary tips to occipital condyles; premolar series very short; internal cusps of grinding teeth low, robust, well rounded, ectolophs sharply depressed to the crowns of the teeth; anteroposterior diameter of m- and m' less than transverse diameter; no cingula be- tween grinders; P||; p' subc[uadrangular, outer wall not overlapped posteriorly by ectoloph of p^; premolar tetartocones exhibiting early and pronounced develop- ment; premolars with internal cingula blunt, reduced or absent, external cingula variable; molars without internal cingula, external cingula faint or absent; hypocone of m' prominent, triradiate. Skull: Skull proportions mesaticephalic to brachy- cephalic; facial portion of skull much abbreviated; premaxillaries contracted; cranial portion of skull elongate; anterior narial aperture high and narrow; preorbital malar bridge very narrow, mainly com- posed of the median ridge, which is very prominent, subcylindrical, in side view concealing the infraorbital foramen almost entirely; anterior portion of malar stout, rounded; malar below postorbital process strongly convex; free nasals tapering, progressively abbreviated; horns of medium to large size, forward shifting slight or wanting, basal section deep antero- posteriorly, with antero-external or maxillary face flat. 542 TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA oblique, and strongly convex inner face; summit rounded to oval in section; zygomata strongly arched, buccal expansions finally extreme, in section broad rather than deep; occiput moderately produced back- ward behind zygomata; basisphenoidal rugosity absent, vomerine septum (?) absent. Jaw: Jaw robust rather than deep, with shallow or concave chin, coronoid strongly recurved. Angle deep, rugose. Incisors : In most adult specimens the incisors have been shed and are represented only by alveoli. Thus the type of M. acer has medium-sized superior incisor alveoli. A specimen (Nat. Mus. 4700) closely re- sembling M. coloradensis in horn structure has per- sistent superior incisors. Oiservations on the measurements of the Megacerops series. — The members of this series are distinguished from Brontotherium chiefly by the cylindrical horns and by the feeble development of the connecting crest. The basilar length (pmx to condyles) is shorter than in any species of Brontotherium except B. hypoceras and B. leidyi. The premolars as a rule are shorter and smaller than in Brontotherium, as shown below. The molars, however, in proportion to the basilar length of the skull, are sometimes relatively larger than in Brontotherium, as shown in the following tables: Molar index in species of Megacerops and Brontotherium M. acer, Am. Mus. 6350, 9 (^) M. acer, Univ. Wyoming (|f|) M. bucoo, Am. Mus. 6345a (type) (Iff) M. copei, Nat. Mus. 4711 (type) (H^) B. ?tichoceras, Nat. Mus. 8313 (Mi) B. platyceras, Field Mus. 12161 B. curtum, Yale Mus. 12013 (type) B. gigas, Am. Mus. 492 B. leidyi, Nat. Mus. 4249 (type) B. leidyi, Carnegie Mus. 93 Anteroposterior and transverse dimensions of p* and w? in species of Megacerops and Brontotherium, in millimeters M. acer?, Am. Mus. 6350 (type of M. altirostris) M. acer, Univ. Wyoming , M. bucco. Am. Mus. 6345a (type) M. bucco, Am. Mus. 6353 M. bucco, Nat. Mus. (skull A) B. gigas hatcheri, Nat. Mus. 4262 B. gigas, Am. Mus. 492 B. gigas (hatcheri), Carnegie Mus. 341 83 80 ■74 99 84 o Estimated. The internal cingula of the premolars are usually more reduced than in Brontotherium. The close kinship of Megacerops to Brontotherium is revealed in many details of the incisors, canines, premolars, and molars, in the great expansion of the zygomata, and in the possession of a midparietal protuberance. Measurements of species of Megacerops and Brontotherium, in millimeters Stage 1: B. leidyi Stage 2: B.?ticho- ceras, Nat. Mus. 8313 (A) Stage 3 Car- negie Mus. 93 Nat. Mus. 4249 (type) M. acer M. Type (c?) Univ. Wyo. rostris" (type) Pi-m3 300 190 665 66 114 104 290 186 665 118 107 313 196 695 76 90 111 330 Mi-m3 60 290 200 646 84 70 242 215 Pmx to condyles — Zygomatic index — Nasal length Horn length 630 44 165,?? Standard measurements in the Megacerops phylum, in millimeters Upper teeth Skull Jaw and lower teeth ? a 313 ft ft & "126 1 "196 > 1 D 34 0.t-. |i 8.2 D 33 8 a 695 h N 538 'c' g l| i o 76 1 '750 1 i f W 111 '175 165 290 242 '185 190 349 S 122 s 232 11 o 38 S jl ■a- o 33 g ft a Brontotherium ?tichoceras, Nat. Mus. 8313, & 90[ 126 '110 '127 593 M. coloradensis (type) _. M. ("Symborodon") acer. Am. Mus. 6350, 9 (typeofM. "altirostris") --- 330 127 215 '630 610 '640 '690 750 '640 44 60 70 =90 117 114 115 130 M. ("Symborodon") acer. Am. Mus. 6348, cf (type).. M. acer, Univ. Wyoming 2, cf (?) 300 '300 315 317 «294 99 noo 116 119 106 200 200 204 198 195 646 '665 645 543 «665 '520 84 '81 M. ("Symborodon") bucco. Am. Mus. 6345a, d' (lectotype) M. bucco, Nat. Mus. 4705, 9 .. .. _._ 22 29 28 20 27 80 135 M. bucco. Am. Mus. 6353, cf - M copei, Nat. Mus. 4711 (type) '620 =601 80 125 =300 M. riggsi. Am. Mus. 6364 (type) 282 85 194 '465 1 EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 543 Sequence qf species. — The following summary affords the principal characters by which the chief species so far recognized may be distinguished from one another: 1. Megacerops coZoradensis Leidy is the genotype. The type is lost and is known only from the figure of the coossified nasals and horns, which indicates an animal about the size of M. bucco. Titanotherium zone, Colorado. 2. Megacerops (" Symborodon") bucco (Cope) is a more pro- gressive species. It is decidedly brachycephalic. It still retains, however, the long thin nasals. The horns have shifted to a more anterior position. The buccal processes of the zygomata now become very prominent, as in the brontotheres. The occipital pillars begin to expand, the parietal crest is broader, and as a distinctive, progressive character, the tetartocones of the pre- molars are more developed. It is important to note that this extreme type probably belongs to the upper beds. Colorado and South Dakota. Titanotherium zone, level Chadron B. 3. Megacerops acer Cope is represented by the male type skull and by a female skull which Cope made the type of his species M. altirostris. It is also mesaticephalic and differs from M. copei in the thick, short nasals, in the divergence of the horns, in the somewhat more anterior position of the horns, while it resembles M. copei in the small size of the tetartocones of the premolars. The occiput (PI. CLVII, C; fig. 451) is readily distinguished from that of any brontothere by its slender characters, indicating that the muscles of the neck were not so robustly developed in these animals. Colorado and South Dakota. Titanotherium zone, upper beds. 4. Megacerops copei (Osborn), named in honor of Professor Cope, appears to belong to the middle Titanotherium zone (Chad- ron B). In proportions the skull is mesaticephalic; the horns are vertical, elongate as seen from the front, and their form suggested the unique name "rabbit's ears," which is applied to this skull. They are placed typically directly above the orbits, yet the nasals are very thin, and, as shown in the section (fig. 450), there is no connecting crest. Colorado and South Dakota. Titanotherium zone, upper beds (Chadron C). 5. Megacerops assiniboiensis Lambe is an animal of small size, known only by the jaw. Saskatchewan, Swift Current Creek. Titanotherium zone. 6. Megacerops riggsi Osborn is distinguished by its especially short massive jaw. Colorado. Titanotherium zone. Synopsis of specific cJiaracters qf Jour species of Megacerops M. copei, Nat. Mus. 4711 M. altirostris, Am. Mus. 6350 ' (type) Skull Nasals Horns Horns, malar ridge Horns, top section Horns, connecting crest Horns, position Internal flange of buccal process. Occiput and pillars Occiput pits Occiput parietal crest Premolars Mesostyle p' Mesaticephalic Thin Vertical Prominent Strongly oval None Over orbits Not prominent Narrow Absent? Narrow Tetartocones smaller. Absent Extremely brachy- cephalic. Thin, cf long, 9 medium Divergent Absent Rounded to oval None c? anterior to orbits 9 ?intermediate Prominent Expanding Absent? Broad Tetartocones more de- veloped. Present Mesaticephalic? Thick, short Divergent Intermediate Oval Well-defined [Anterior to orbits (cT? ( 9 not prominent Narrow Pronounced Narrow Tetartocones smaller. Mesaticephalic? Thick, short. Divergent. Faint or absent. Rounded to oval. None. Intermediate. Narrow. Absent?. Intermediate. Tetartocones smaller. Present. o A small female. Jaw cTiaracters. — The jaws of Megacerops are read- ily distinguished by the abbreviation of the anterior portion corresponding with the extreme abbreviation or brachyopy of the facial region, with which the ab- breviation of the lower premolar series is also corre- lated. A second distinction is the narrowness of the chin and the small size and approximation of the *• Regarded as the female of M. acer. canines, correlated with the reduction of the lower incisor teeth. The anterior part of the face in Mega- cerops is contracted and the lips were probably narrow and pointed in contrast with the broad, massive lips of Brontotherium. Details of the contrasts in the character of the jaw are as follows: Jaw characters in Megacerops and Brontotherium Megacerops Brontotherium Short, massive Shallow Broad, posterior border vertical ?It Absent? Symphyseal region in side view Very shallow. Broad, posterior border often vertical. Pi in fully adult jaws Diastema in front of pi Absent. ? Swollen at base with massive posterior cin- gulum. Typically pronounced. Upward flexure of premolar series 544 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Megacerops coloradensis Leidy Text figures 164, 448 [For original description and type references see p. 20S| Tyfe locality and geologic Tiorizon. — Colorado; Ti- tanotlierium zone. Exact locality and level unknown. Specific characters. — Nasals long (110 mm., esti- mated), sharply decurved and tapering distally; horns cylindrical with slightly flattened tips, external length 175 millimeters (estimated). Connecting crest very low. Materials. — The type specimen, consisting of the coossified nasals and horns, has been lost, and the characters of the genus rest upon Leidy's original de- scription and figures and on the model (Am. Mus. 9018, fig. 448) made from them. Transitional referred specimen, Brontotherium ticho- ceras. — In the National Museum there is a beautifully preserved skull (No. 8313, with associated lower j aw) which in horn structure recalls the type of M. coloradensis, except that the nasals and horns are somewhat shorter. This skuU is of extraordinary interest inasmuch as it combines the nasals, horns, and absence of connec- ting crest which are char- acteristic of Megacerops with the large incisors and canines which had T? AAo a„+;„„„.,„^ hitherto been regarded Figure 448. — Sections and _ *= contours of nasals and horns as characteristic only of of Megacerops coloradensis. BrontotJierium. The pre- Ara. Mus. 9018, a model made trom molars are relatively Small, Leidy's figures and from measurements . " . of iiis type, wiiich lias been lost. One- and the internal cmgula seventh natural size. j^^^^ ^^^^^^^ completely disappeared, much more than in BrontotJierium and even more than in most Megacerops skulls. The measure- ments of this skuU approach those of the type of Megacerops hucco except that the basilar length is greater, the zygomatic width and horn length are less. It should also be compared with Brontotherium hatcheri, but it differs from that type in its shorter horns, absence of connecting crest, and absence of in- ternal cingulum on the premolars. Megacerops bucco (Cope) {Symhorodon bucco Cope, 1873; "Symhorodon iorviis" Osborn, 1902) Plates CXLIV, CXLV, CLIII-CLVI; text figures 169, 170, .378, 392, 393, 399, 449, 719, 744 [For original description and type references see p. 2121 Type locality and geologic horizon. — Cedar Creek, Logan County, Colo.; Titanotherium zone. Specific and generic characters. — I^l^j P^. Superior incisors probably absent; premolars with cingula vestigial or wanting, tetartocones nearly as large as deuterocones and more distinct then in M. copei, a faint mesostyle upon p*. Skull shows progressive increase in size over M. copei, length nasals to occiput 750 millimeters, premaxillaries to condyles 665 (esti- mated); extremely brachycephalic, index 100 (esti- mated); nasals thin, medium in length, broad (90 by 130 mm.); no connecting crest between horns; horns c? short (185 mm., estimated); zygomata broadly ex- panded; occipital pillars expanding, with wide superior rugosities. SlcuU and jaw selected hy Cope as types of Symhorodon hucco. — The type skull of S. hucco Cope includes a fairly preserved skull (Am. Mus. 6345a) with enor- mous zygomatic expansions. The jaw placed with this skull by Cope appears to belong to another phylum. The premaxillaries are wanting. All the maxillary teeth on both sides are preserved with the exception of p^ The horns and the anterior portions of the orbits have required much restoration but serve to afford some very distinctive characters. The skull is readily distinguished as a Megacerops by the entire absence of the connecting crests between the horns, which are set widely apart but so damaged that a perfect basal section can not be made. The nasals are very thin and of medium length. The zygomata arch very widely and exhibit a flattened section which is even more extreme than that of Brontotherium gigas and quite distinct from the con- vex section of the skull Am. Mus. 6346, also referred by Cope to S. hucco but regarded by us as pertaining to a species of Brontotherium, probably B. curium. Posteriorly the cranium broadens out into the occiput, but it is important to note that this breadth and the peculiar zj'gomatic section are altered by crushing. Dentition. — The animal is in the eighth stage of growth, all the internal cones of the teeth being worn except upon m^. The external cingulum is obsolete throughout the grinding series, as in M. acer. The internal cingulum is entirely lacking on p^ and is very slightly marked on p^. It is also wanting on the inner sides of the molars. As a marked progressive character, the internal cusps of the premolars are robust and well roimded. The hypocones on m^ m- are prominent and bidge on the lingual side. The hypocone is well developed on m^, triradiate in form, and connected with the cingulum by only a slight posterior ridge. Female slcull of M. hucco in the National Museum. — The skull Nat. Mus. 4705 is that of an aged female in the ninth stage of growth, the horns of which are set very wide apart (PI. CXLV, B; figs. 392, E, 449, B). The well-worn canines measure only 25 millimeters anteriorly and are proportionately the smallest we have observed in any species of titano there. The EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 545 evidence regarding the incisors is not positive, but there were apparently two vestigial teeth, which certainly could not have been functional. The premolar-molar series measures 315 millimeters. The internal cingula on the premolars is vestigial or en- tirely wanting. The skull appears to be brachy- cephalic, the estimated index being 81; the measure- ment from condyles to symphysis is 645 millimeters, while the width across the zygomata is estimated at 525 millimeters, an excess of length over breadth of only 120 millimeters. The nasals are very broad but at the same time short, the free length being only 80 millimeters. The horns are short, measuring 190 millimeters on the outer side, and exhibit at the base the section so characteristic of this genus, which is due to the flatness of the anterior or maxillary face and the convexity of the posterior buttress. Megacerops acer Cope (Megaceralops acer Cope, 1873; Symborodon altirostris Cope, 1873; "Symborodon acer Cope," Osborn, 1902) Plates XVIII, CXLVI-CLII, CLVII; text figures 167, 170, 375, 378, 390, 392, 399, 400, 450-453, 625, 638 [For original description and type references see p. 211] Type locality and geologic Tiorizon. — Cedar Creek, Logan County, Colo.; Titanotherium zone, level un- known. Specific and generic characters. — I^^, P^^. Incisors greatly reduced but more persistent than in M. hucco; canines small; p^ small, rounded, p^-p^ with tetarto- cones distinct but smaller than in Menodus torvus, a prominent mesostyle upon p^ (No. 6350), narrow internal cingula; hypocones of m^-m^ large, projecting on lingual side, hypocone of m' triradiate. Nasals thick, short, and moderately broad (60 by 114 mm.); horns c? 290 millimeters, 9 165, basal section typical, summits transverse oval; cranial vertex dolichocephalic; occiput high, narrow, not deeply indented superiorly. Zygomata 9 with buccal proc- esses flattened or slightly concave above, size mod- erate, nasals to occiput 640 millimeters. This differs from M. copei in the form and direction of the horns and presence of a connecting crest in the male, in the long, narrow parietal vertex, and in the thiclaiess of the nasals. Materials. — This was the second species of "Sym- horodon" described by Cope from Colorado in 1870. The type specimen (Am. Mus. 6348) is the skull of a male titanothere of medium size but with long horns, from the Titanotherium zone of Cedar Creek, Logan County, Colo. The exact geologic level is not known. Subsequently Cope proposed the name "Symborodon" altirostris for a skull of the same species (Am. Mus. 6350), a female, as first pointed out by Osborn in 1896. In addition to these two skulls there is for- tunately a third (Am. Mus. 6349), probably also a female, in which the right zygoma is preserved. Relationships to other species of Megacerops. — There can be no question as to the generic relationship of this animal to Megacerops hucco and M. copei. It presents a number of interesting and significant points of con- trast and agreement. The affinity is seen especially in the form and position of the horns in the type, which indicate that they were used in the manner character- istic of other members of the genus. They point up- m^. B Figure 449.- -Sections and contours of skull of Megacerops bucco A, Am. Mus. 6345a (type); horns directed upward and forward, their basal section rounded with a flattened external face, the section of the upper part of the horn transversely oval, nasals wide, zygomata widely expanded. B, Nat. Mus. 4705. In this supposed female the horfls, as in all other members of this genus, are sub- cylindrical, their basal section has a flat external face, and the remainder of the section is well rounded; nasals rather short; zygomata moderately expanded. One-seventh natural size. ward and slightly outward, and as they are strength- ened posteriorly their bases do not lie very far in front of the orbits. This relatively long-horned titano- there was, therefore, a true Megacerops, a conclusion which is further supported by the resemblance of the basal horn section to that of "Menodus torvus " and by its wide contrast to that of any species of Menodus, such as M. trigonoceras, or of Brontotherium, such as B. gigas. 546 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Seen from in front the horns are placed close together at the base and diverge more at the summits than in the other species, although the divergence is less than in B. gigas, for example. That this divergence was correlated with the development of a shallow trans- verse connecting crest in the males, which was also present in certain females (Am. Mus. 6349) but nearly wanting in the supposed female Am. Mus. 6350 (PI. millimeters and are relatively much shorter than in M. bucco and M. copei. This stage is therefore parallel to that of B. curium among brontotheres. The crania of these small animals are rather mesatice- phalic than brachycephalic. The name altirostris, given to the female skull by Cope, referred to the elevation of the nasals above the premaxillae when seen from in front. B A Figure 450.- c -Sections and contours of skulls of Megacerops copei and M. acer A, Megacerops copei, Nat. Mus, 4711 (typo) ; horns long and subvertica], their basa] section thiclc, the external face flat, and the remainder of the section well rounded; the section of the upper part of the horn widely oval, with a recurved external angle; no connecting crest; nasals larger and broader than in ^f. acer and narrow at the end. B, M. acer, Am. Mus. 6348 (type); horns long and slightly inclined forward, their basal section roundly quadrate with a concave external anterior face; the section of the upper part of the horn transversely oval; a connecting crest; nasals short. C, M. acer, Am. Mus. 6350, 2 (type of Symborodon altirostris Cope, now referred to M. acer): horns relatively short and directed forward, basal section roundly quadrate, with a long concave external face, section of the upper part of the horn transversely oval, nasals short and thick. All one-seventh natural size. CL), proves that it was adapted to resist the lateral strains to which the horns were subjected. Yet in M. copei and M. hucco the horns are divergent and the nasals are as thin between the horns as toward the tips. Nasals. — Correlated with the somewhat more an- terior position of the horns and partly by compen- sation of growth there is a very marked abbreviation of the nasals, which now measure only 60 by 114 Occiput. — The occiput resembles M. copei also in the form of the top of the occiput (fig. 451), which is not expanded and, in fact, is peculiarly narrow and simple. Instead of the knobs (fig. 378, F) seen in Brontotherium there is a pair of pits (fig. 451) on either side of the center of the occipital crest. The occiput lacks the broad superior flare, which is correlated with brachycephaly and the lateral motions of the skull. EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 547 A very distinctive feature is the fact that it is not incurved in the median line superiorly. The buccal expansion of the zygoma is preserved only in one of the female skulls (Am. Mus. 6349), in which it is seen to be broad and flattened above, as in M. hucco, paralleling that of Brontotherium, gigas. The portion of the malar preserved in No. 6350 is stout and rounded. Teeth. — That the superior incisors were greatly reduced is demonstrated by the small alveoli in the female skull Am. Mus. 6350, which, however, are not so small as in Menodus. The rounded first pre- molar resembles that of M. copei; the slightly defined cingulum on the inner side of the premolars and the rather retarded development of the tetartocones dis- tinguish these teeth from those of M. hucco. There is a well-marked mesostyle on p''. As in M. hucco and M. copei, the distance from the anterior edge of m' to the premaxillary border and the proportionate length of the premolar series arc excessively short, com- pared with Menodus, Allops and Brontops, and even compared with Brontotherium (Pis. CLI, CLII, and CLXV). The upward flexure of the premolar series and elevation of the canines and incisors is very marked and in connection with the elevated position of the nasals and perpendicularity of the horns must have imparted a peculiar retrousse character to the face (PI. CXLVI). Sexual characters. — The males are well distinguished from the females by the length of the horns, which is 290 millimeters in the male (Am. Mus. 6348) as com- pared with 177 in one female (Am. Mus. 6350) and 138 in the second female (Am. Mus. 6349). The basal section of the horns is substantially similar, and the summits are transversely oval in section; but in one of the female skulls (No. 6350) the horns project forward without recurvature, while in the other they are recurved as in the male skull. There is a strong convexity of the cranial vertex in the frontoparietal region (a family character) in the male No. 6348 and the female No. 6350, which is feebly developed in the female No. 6349. The horn of the latter has a malar ridge. Additional ohservations on skulls of Megacerops acer. — The type of M. acer is a relatively long and slender male skull (Am. Mus. 6348) lacking all the teeth, the zygomatic arches, the maxUlaries and premaxil- laries. The skull is sharply distinguished from an}^ previously described by its obtuse and short but tapering nasals, long, recurved horns with deep an- teroposterior basal section and oval tips, a narrow vertex, and narrow, simple occiput. In addition to these characters there appears to be a strong convexity in the central portion of the top of the cranium not altogether due to crushing, which is characteristic of Brontotheidum and Megacerops. The basal section of the horns indicates that they are strengthened not only by a connecting crest but by a posterior ridge passing backward above the orbits, which is separated by a flat surface from the malar ridge. The skull Am. Mus. 6350 resembles that of M. acer in the abbreviation of the nasals and narrowness of the vertex and the occiput and differs from M. acer in characters which Cope thought to be specific but which are now found to be sexual. These female characteristics are the relatively short horns and com- paratively low connecting crest, as shown in the section (fig. 450); the only difference of note is the convexity between the malar and posterior ridges of the horns. The anterior teeth of this type have been broken away and lost since the original description. The superior incisors are represented by two small alveoli, placed upon a nari'ow border, indicating that these teeth were small and disappeared in old age. The formula was, therefore, I^"^, P-^. The crowns of the canines have been destroyed. These teeth were of small size, apparently as in M. hucco. The first pre- molar is a small tooth with three cusps (protocone, deuterocone, and tritocone); the second, third, and fourth premolars e.xhibit four well-developed and dis- FiGURE 451. — Upper part of occiput of Megacerops acer Am. Mus. 6351, showing pits tor the ligamentum nuchae and rugosities for the recti capitis laterales. One-fourth natural size. tinct cusps, including a convex tetartocone. A fea- ture which may be specifically characteristic is the distinct mesostyle upon p*. A crenulate and not sharply defined cingulum is observed on the inner side but is entirely wanting on the outer side of the premolars. This more or less vestigial condition of the cingulum is also a distinctive character of the series to which this animal belongs, as well as of the old males of the species of Brontotherium of the upper beds. The molars are very broad. On m' the hypo- cone is triradiate, well developed, but not distinct from the cingulum. Another well-marked feature is the prominence and roundly blunt character of the hypocones of m'-m'. A female skull (Am. Mus. 6349) fortunately pre- serves the right zygomatic arch, indicating that this is broad and somewhat flat superiorly, having a sec- tion similar to that of Brontotherium gigas but less robust. A distinctive character is the breadth of the postglenoid process. The occiput has the relatively slender proportions seen in the type and cotype. The rugosities on top of the occipital pillars are much less extensive than in Megacerops rohustus. 548 TITANOTHERES OF ANClfiNT WYOMIMG, DAKOTA, AND NEBRASKA Referred, skull, Megacerops acer, University of Wyo- ming Museum. — ^A very fine skull (figs. 452, 453) in the University of Wyoming, collected by Mr. W. H. Figure 452. — Skull of Megacerops acer Dniv. Wyoming Mus. 2. Side (Ai) and top (Az) views. One-sixth natural size. This sliiill combines the characters of several supposed species of Megacerops. In the form and length of the horns it approaches the type of M. acer; in the character of the nasals it resembles the type of Symborodon attirostris; in the general form of the skull top it resembles especially the supposed female of M. biicco (Nat. Mus. 4705); and in the dimensions of the grinding teeth it agrees with the lectotype of M. bucco (Am. Mus. 6345a). Eeed in the upper Titanoiherium zone near Alcova, Natrona County, Wyo., has somewhat smaller horns than the type. Its nasals recall those of the type of S. altirostris. The premolars, as in other members of the genus, have the internal cingula nearly obsolete and the tetartocones subequal to the deuterocones; the width of the molars exceeds the length. The zygo- matic index (84) is high. In addition to the measurements of this skull given above (p. 542), we record also the following: MilUmeters Diastema behind canine 20 PS ap. by tr 34X45 M2, ap. by tr 73X84 M3, ap. by tr 78X83 Molar inde.x 29 Megacerops copei (Osborn) {Symborodon copei Osborn, 1908) Plates CXLIII-CXLV; text figures 24, 203, 390, 394, 399, 450, 454, 640 [For original description and type references see p. 235] Geologic horizon — Middle Titano- iherium zone. South Dakota. Specific and generic characters. — JZiO.^ p4 Incisors (type) persistent but greatly reduced; canines very small, reduced, c? 28 millimeters; premolars with cingula reduced or absent; tetartocones connected with deuterocones by a longitudinal ridge. Skull: Nasals thin, short and broad in proportion, 80 by 125 millimeters; horns S 300, no connecting crest, transverse oval near summit; buccal processes of zygomata c? stout and convex; malar in front of buccal process very deep, beneath postorbital process stout, convex; occipital pillars medium, not greatly expanded at the summits. Size small, premaxillaries to condyles 620 millimeters. Materials. — This species is known only from the type skull (Nat. Mus. 4711, skuU V), which is finely illus- trated in Plates CXLIV, CXLV. It is from the middle Titanotherium zone, probably from the upper levels or even lower, and impresses one with the excessive thinness of the nasals, which in these animals are very broad posteriorly (120 mm.), while not of very great free length. The animal is in the eighth stage of growth and has thus attained its fully adult characters. The small size of the canines (28 mm.) might lead us to regard it as a female, but all the specimens belonging to this genus are characterized by small canines, and the great length of the horns EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 549 (301 mm.) proves that it is a male. The incisors were apparently very small and shed early, being indicated solely by a single alveolus on each side. Figure 453. — Skull of Megacerops acer CJniv. Wyoming Mus. 2. Palatal view. One-sixth natural size. Shows striking resemblance to the supposed female of M. bucco (Nat. Mus. 4705) and to the type of Symborodon aUirosins, Affinity with Broittotlierium is also indicated in the form of the canines, the highly progressive submolariform premolars, and the broad molars. The canines are similar to Brontotherium canines but of much smaller size. This degeneration of the anterior teeth was shared by the first premo- lar, which is a small tooth placed close to the canines; in spite of its small size, however, it very probably had a well-developed tetartocone as in all other Brontotheriinae, indicated by the angu- late postero-internal border of the much worn tooth. The succeeding premolars, p^-p"*, exhibit progressive development of the tetartocones (Pis. CXLIII, CXLV); these cusps are still connected with the deuterocones by a low crest, however, and are not clearly distinct; the internal cingula are feebly developed and obsolete opposite the deuterocones. M^ has a heavy triradiate hypo- cone. The grinding series as a whole measures 300 millimeters, exactly as in M. hucco, but the skull of this specimen is 130 millimeters shorter than in M. hucco. It would appear that in this species, as in Brontotherium, the dental series does not increase proportionately with the skull. Skull. — The lateral view of the type male skull (PL CXLIII) at first sight strongly suggests that of B. gigas; but a close comparison reveals the unpor- tant difference that the horns are placed very much farther back; they are almost directly above the orbits with buttresses extending backward at the base over the orbits. The horns thus obtained a firm support posteriorly, a mechanical adaptation that compensates for the entire deficiency of a trans- verse connecting crest. When seen from in front (PI. CXLIV) the horns are chiefly vertical and parallel, although this is somewhat exaggerated by lateral crushing; they certainly do not flare outward as in B. gigas. This unique position correlated with the narrowness of the occiput indicates chiefly a vertical motion of the skull in using the horns and explains the absence of a connecting crest, which would be of great service in protecting the nasofrontal region from fractures occasioned by side blows. The horns differ from those of M. acer in the stronger develop- ment of the transverse oval section near the summits caused by the sharp development of the malar ridge. Megacerops assiniboiensis Lambe Text figures 205, 434, 455 All that is known of this animal is fully stated on pages 239-240. The lower jaw is shown in Figure 455. Megacerops syceras (Cope) (Menodus syceras Cope, 1889) Text figures 186, 456 [For original description and type references see p. 226] This species is known only from the coossified nasals and horn cores. It is probably a member of this phylum. Figure 454. — Restoration of Megacerops copei By Charles R. Knight. About one-ninth natural size. 550 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Megacerops riggsi (Osborn) Plates CLVIII-CLX; text figures 208, 455_ I For original description and type references see p. 242] Type locality and geologic horizon. — Horsetail Creek, northeastern Colorado ; Titanotherium zone. Specific characters. — Of small size, smaller than any Imown individual of Megacerops or Brontotherium . A Figure 455. — Lower jaws of Megacerops assiniboiensis and M. riggsi A, Megacerops riggsi, Am. Mus. 6364 (type). A small titanothere having a massive short jaw, a short chin, and a swollen ramus. (See PI. CLVIII, A.) The cheek teeth lack cingula. B, if. assiniboiensis, Ottawa Mus. (type). A small short jaw provisionally referred to Megacerops. The grinding teeth lack e.xtemal cingula. One-flfth natural size. very massive jaw with a small coronoid process and a very short symphysis. Premolar series greatly abbreviated (85 mm.). Premolars and molars with reduced external cingula. The type of this species, named in honor of Mr. E. S. Riggs, of the Field Museum of Natural History, is a jaw in the Cope collection (Am. Mus. 6364) which was wrongly referred by Cope to his species "Symhoro- don" acer. It represents a highly specialized and small form of Megacerops. SUBFAMILY BRONTOTHERIINAE Titanotheres attaining the largest size, chiefly of the upper Titanotherium zone, although known from the lower beds upward. The horns progressively longest, most broadly oval, and flattened, shifting forward with the extreme reduction of the free portion of the nasals; very prominent connecting crest, pro- gressively increasing size. Two pairs of persistent superior incisor teeth in males; canines large, obtuse. Brachycephaly expressed in the great horizontal expansion of the buccal processes, in the proportions and arching of the grinding teeth, and in the inclosure of the auditory meatus in the males. Occiput much produced behind the zygomatic arches. Vertex of skull in superior view elongate. The ancestry of this great Oligocene phylum may possibly be found in the upper Eocene, perhaps in species of Diplacodon or of Eotitanotherium, described on pages 439, 441. In these huge animals the titanothere family reached a climax. The generic name "thunder beast," based on the genotype species Brontotherium gigas, is highly appropriate because it applies to the most robust and most massively horned not only of the titanotheres but of all the known Perissodactyla. Marsh mistakenly associ- ated with the type jaw of B. gigas the skull of Menodus, "B. ingens," to define the generic characters of Brontotherium, and he assigned the actual skuU of B. gigas to a different genus, naming it Titanops elatus; but we have found that the lower jaw of B. gigas is in a stage slightly antecedent to that of Titanops elatus Marsh. We have also discovered that this great animal Brontothe- rium gigas {elatum) is a central form, whose ancestors {B. leidyi) extend down to the base of the Titanotherium zone and whose successors {B. platyceras) extend up to the very summit of Chadron C (the upper Titanotherium zone). Thus in the present memoir Brontotherium is shown to embrace a most remarkable and nearly monophyletic series or succession of species, eight or possibly nine of which are now known from the lower to the higher levels, present- ing certain common generic char- acters throughout. Modified by a progressive increase in size and by a series of remarkable stages in the evolution of the horns, in the recession of the nasals, and in the expansion of the buccal processes of the zygomata, they culminate in a unique type of perissodactyl skull, that of B. platyceras, which appears to be the last of its great race. This species is cer- tainly in the last stage of evolution along its line. Affinities. — The nearest allies of Brontotherium are members of the genus Megacerops, but the true brontotheres (the males at least) are readily dis- tinguished by the presence of two pairs of upper and lower incisor teeth, by their robust canines, the Figure 456. — Sec- tions and contours of skull of Mega- cerops? syceras Ottawa Mus. (type), provi- sionally referred to Mega- cerops partly because the basal section of the horns is roundly quadrate and has a flat external face. Nasals of moderate length, connecting crest low. One-seventh natural size. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 551 flattened oval section of the horns, the progressive connectuig crests between the horns, and ia general by their superior if not dominant size. The jaw of B. gigas (PL CLXXI) is very characteristic of the genus, with its recm-ved coronoid process, shallow chin, robust mandibular section, two pairs of incisor teeth, and obtuse rounded canines. The finely pre- Horns. — The males of all these animals exhibit progressively broad and flattened horns, transversely oval from base to summit, diverging widely at the summits, with the great connecting crest rising be- tween them (unlike the typical Megacerops), to resist lateral strains, and evolving at the expense of the constantly diminishing free portion of the nasals (fig. 457). Skull. — The top view of the cranium is very long (fig. 459), owing to the extreme forward position of the horns and the great backward extension of the occiput behind the zygomatic arches. The cranium proper is dolichocephalic; the expansion of the buccal processes of these arches is, however, so broad that the total breadth of the skull finally equals the total Figure 457. — Composite sections showing tlie evolution of the horns and reduction of the free nasals in the Brontotherium phylum a, Broniothenum leidyi, Nat. Mus. 4249 (type), Chadron A 2; &, Brontoihenum hypoceras, Nat. Mus. 4273, Chadron A 2; c, BTonlotherium hypoceras, Nat. Mus. 4702, Chadron A 37; d, Brontotherium hatcheri. Am. Mus. 1070, Chadron; e, Brontotherium gigas, Yale Mus. 12061 (type of B. elatm), upper (?) Chadron; f, Brontotherium gigas, Am. Mus. 492, Chadron C; g, Brontotherium curtum, Yale Mus. 12013 (type), Chadron C; h, Brontothe- rium curtum, Nat. Mus. 4946, Chadron C 3; i, Brontotherium ramosum. Am. Mus. 1447 (type), Chadron C; j, Brontotherium platyceras. Am. Mus. 1448, Chadron C. All one-fourth natural size. In the earliest stage, B. leidyi, the horns are small and placed near the orbits, there is no connecting crest, and the nasals are long and slender; in the latest and most specialized stage, B. platyceras, the horns are extremely long and placed far in front of the orbits, the coimecting crest is very high, and the free portion of the nasals is practically vestigial. Between these extremes lie a con- siderable number of intermediate stages. served type skull of B. (Titanops) elatus (PL CLXXV) enables us fully to characterize the genus. The suc- ceeding and stni more advanced stages in the evolu- tion of this phylum were originally named as foUows: Menodus dolichoceras Scott and Osborn, Titanops curtus Marsh, Titanops medius Marsh, TitanotJierium ramosum Osborn. 101959— 29— VOL 1 38 length, the zygomatic index in B. platyceras being 110, or hyperbrachycephalic. This condition is best imderstood when the skuU is viewed from below (fig. 393). The whole structure of the skull, including the broad and spreading occiput (fig. 378) and the excessive buccal expansion, is adaptively adjusted to the development of the horns, which from their feeble 552 TITANOTHERBS OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA development in the females (as in the type of Menodus peltoceras Cope {=lBrontoiherium curtum, figs. 459, 477, 478) are judged to have been fighting weapons in the males. FHarv.Mu5..type EYaleMu5.l20l3,type D Yale Mu5. 12061 CAM. 1070 B Nat.Mus.427Jn«)type ANat.Mu5.4249,t^pe I -^ ^y nhypoceras AZJi.n^^otyf^ lleidyi N.M. 4249, type Figure 458. — Basal sections of the horns in the Bi-ontotherium phylum A, Brmlotherium Icidyi, Chadron A 2; B, £. hypoceras, Chadron A 2; C, B. Jiatcheri, Chadron; D, B. gigas, Chadron ?C; E, B. curtum, Chadron C; F, B. platyceras, Chadron. These sections show the progressive change from the small anteroposterior oval of B. Uidyi to the very wide transverse oval of B. platyceras. increase in size of the skull as a whole (24 per cent). The total length of the premolar series diminished, but in B. gigas the individual molars are enormous — for example, m^ (ap. by tr.) 91 by 94 millimeters, as Figure 460.- — Contrast in contour of horns and nasals between male and female brontotheres The female (a), Am. Mus. 1006 (referred to B. gigas), differs from the male (b) , Am. Mus. 492 (referred to B. gigas), chiefly in having short horns and nasals, combined with a high connecting crest. Figure 459. — Skulls cf male and female brontotheres Top view. About one-ninth natural size. A, Broniotherium platyceras. Am. Mus. 1448 (cotype) . In this male skull vertical crushing has emphasized the width of the zygomata. B, B. curtum (peltoceras). Am. Mus. 1006. This female skull is small, the horns are short, and the zygomata are unexpanded. Grinding teeth. — The grinding teeth suffered prob- ably from the highly "selective value" placed upon the horns and were retrogressive in many respects; the total increase of length in the grinding series (21 per cent) thus did not fully share in the progressive compared with M. giganteus (Am. Mus. 505), m^ 87 by 84. The breadth of p* was also exceptionally great, and nowhere else do the tetartocones attain such vigorous development as in B. gigas, not excepting Menodus giganteus. EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 553 Sexual characters of brontotheres Horns elongate. Connecting crest very high. Occiput very robust and back- wardly produced. Zygomatic arches very broad. Incisors persistent. Canines larger. Horns abbreviate. Crest less developed. Occiput less robust and less produced backwardly. Zygomatic arches less broad. ?Incisors less persistent. Canines two-thirds size of male canines. The contrast between the males and females gener- ally is well brought out in the accompanying figures of the male and female skulls of B. gigas. It is obvi- ous that a number of correlated characters disappear in the nondevelopment of the horns in the females, especially the marked width of the zygomatic arches, which may have been partly defensive structures, and the nonextension of the occiput posteriorly, which was probably designed in the males for the support of the great muscles of the neck. Standard measurements in the BrontotJierium 'phylum, in millimeters Upper teeth Skull Jaw and lower teeth 1 ft 1 1 S I 1 a a § s| 1 o i 3 e 1 tsi 13 1 D 3 .ft 1 ft 1 i £ 1 1 ■i S w ? S ■3 1 ■a S 3. .1 ft s| 1 s 1 1 B. platyceras, Am. Mus. 1448, cf 337 »123 221 40 30 '■728 ^815 .... '■693 20 81 "433 MOO -390 '399 380 365 365 355 160 B. platyceras, Field Mus. 12161, cf B. ramosum. Am. Mus. 1447, cf (type).- B. curtum, Yale Mus. 2013, c? (type). 340 350 350 120 °137 223 216 '■228 36 29 880 741 "790 710 '775 -610 80 ?78 895 665 795 38 40 52 167 95 °105 34 — - B. curtum, Nat. Mus. 4946, & 348 128 224 218 222 228 211 235 197 197 ■" <'840 '■780 '■673 620 »670 545 74 — - 65 105 B. curtum Nat. Mus 1211, cf B. curtum, Am. Mus 1005, ? 345 345 130 129 27 20 -80 635 44 101 B. curtum, Nat. Mus. 1232 cf R. ciirtiim?, Rrit. Mns. iifi2f) 34 38 31 700 825 563 80 "620 34 45 85 110 163 320 B. medium, Nat. Mus. 4256, cf (type).. B. medium?, Nat. Mus. 4699 365 293 305 138 117 106 B. medium?, Nat. Mus. 4716, 9 B. dolichoceras, Harvard Mus., cf (type) B. gigas. Am. Mus. 492, cf 49 87 90 115 310 365 353 356 343 348 126 132 133 "■133 241 218 218 227 35 35 33 32 23 24 830 773 770 775 "SIO 740 640 87 82 793 B. gigas (hatcheri), Carnegie Mus. 341.. B. gigas (hatcheri), Nat. Mus. 4262, cf -- B. gigas, Nat. Mus. 4244 70 67 90 110 107 105 275 170 350 350 »121 235 33 28 625 657 84 728 805 B. gigas, Yale Mus. 12061 (type 260 258 365 117 34 30 600 B. gigas?, Am. Mus. 1006, $ 335 330 127 217 »37 22 715 660 800? 79 65 85 90 100 105 72 74 114 118 56 88 130? 102 126 120 107 118 105 180 B.? tichoceras. Harvard Mus. (type) B. hatcheri. Am. Mus 1070 cf 132 118 118 192 198? 35 31 265 200 220 250 143 '■165 104 107 260 375 -120 263 35 29 fiOO B. hatcheri, Field Mus. P 5926, cT »310 320 -710 530 646 620 540 — - 760 B. hatcheri?, Univ. Wyoming 1 cf B. hatcheri, Nat. Mus. 1216, c? (type) -330 »116 219 37 28 630? 75 700 B. hypoceras?, Nat. Mus. 4702 B. hypoceras, Nat. Mus. 4273 (neotype). B. leidyi, Carnegie Mus. 93, c? (paratype) B. leidyi, Nat. Mus. 4249 (type) Percentage of increment, B. leidyi to B. 350 305 300 290 20 133 -123 120 114 12 212 189 190 186 22 715 34 30 665 665 26 440 66 615 625 27 335 127 213 33 31 565 40 .... 554 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements, in millimeters, of sTculls and jaws associated with and referred to members of the Brontotherium phylum Skull and upper teeth Jaw and lower teeth Anterior canine to hypoconu- lid of m3 pi-ms M'-ms Premaxil- lary to glenoid Posterior canine to hypoconu- lid of m3 Pi-ms Mi-ms Symphy- sis to condyle Depth below ma B. platyceras. Am. Mus. 1448, cT 362 »372 337 350 350 365 221 216 240 232 »483 B. ramosum, Am. Mus. 1447 (type). B. curtum, Yale Mus. 12013, cf (type). B. medium, Nat. Mus. 4256, c?.. . .. ._ 450 400 393 365 285 272 730 632 ? B. medium, Am. Mus. 1061 181 R gigas, Am. Mus. 492, r? 380 353 241 595 B. (Titanops) elatum, Yale Mus. 12061 260 263 233 219 220 222 213 195 231 B. hatcheri, Am. Mus. 1070, cT 360 360 375 358 "330 332 338 335 284 "352 600 633 630 538 522 565 "465 690 179 B. hatcheri, Nat. Mus. 4262 125 B. hatcheri, Univ. Wyo., c? 320 198 R. hati^hfiri, Am. Mus. 1068, (f 345 330 166 R. leirlyi, Am. Miis. 516 133 300 190 "275 353 100 M. coloradensis, Nat. Mus. . . 351 '313 !'192 517 116 Observations on the measurements of the Brontotherium series.- — This phylum is as consecutive and as distinct from other phyla as that of Menodus. The separation of certain "ascending mutations" as "species" is very arbitrary. Yet B. leidyi is very far removed from B. platyceras. The generic contrasts in measurements which have been given above indicate that in comparison with members of the menodontine group {Brontops, Allops, Menodus) the male individuals of the typical spe cies of Brontotherium from the upper beds are distin- guished by very short tapering nasals, extremely long flattened horns, long skull top, widely expanded zygo- mata, very short wide premolars, and massive broad molars. The progressive allometric evolution from Bronto- therium leidyi to B. curtum and B. platyceras may be epitomized as shown in the accompanying table: Allometric evolution from Brontotherium leidyi to B. curtum and B. platyceras [Measurements in millimeters] Pi-m3 Pi-p< M'-mS Pi, ap. by tr. M', ap. by tr. Pmx to condyles Zygomatic index Nasal length Horn length B. platyceras: Field Mus. 12161, S 340 120 223 880 80 38 390 Am. Mus. 1448 42X67 84X93 B. curtum: Yale Mus. 12013 (type) . 350 "228 790 78? 52 380 45X63 72X84 Nat. Mus. 4946 348 365 128 138 224 235 '■840 825 °810 830 770 710 74 87 65 45 90 87 70 90 105 74 114 365 B. medium, Nat. Mus. 4256, itothe>-iu7rt Sigas^ male (Am. Mus. 492), right superior canine, external view. C^, Brontotherium leidyi (Carnegie Mus. 93), right superior canine, external view^; C^ the same, left upper canine, internal view. D, Brontops robuiiws, male (Am. Mus. 1083), right superior canine, external view. £>, Menodus giganteus, male (Am. Mus. 505), right superior canine, external view; E^, the same, left superior canine, internal view. F, Allops walcottit type (Nat. Mus. 4260), right inferior canine, external view. G, Menodus heloceras (Carnegie Mus.), right superior canine, external view. H, Bronto^ therium medium. (Am. Mus. 1051), right inferior canine, external ^/ie.vv. Ii, Brontotherium. leidyi (Am. Mus. 516), right inferior canine, external view; I^, the same, left inferior canine, internal view. Ji, Menodus trigonoceras (Nat. Mus. 4745), left infe;-ior canine, external view; J^, the sam.e, right inferior canine, internal view. All natural siz;e U. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE XXI A, Brontops hrachycephalu LEFT UPPER PREMOLARS OF OUGOCENE TITANOTHERES male (Nat. Mus. 4258). B, AUops walcotti, male, type (Nat. Mus. 4250). leidyi, type (Nat. Mus. 4248). All natural sise C, Hrontotherium ^'3 < £ o a i U. S. GEOLOGICAL STJHVET tif}^ dps dp4 MONOGRAPH 65 PLATE XXIII cd. As ''^> Ai JUVEiNiLb JAW Kbi-bRRED BY MARSH TO BRONTOPS One-half natural size. (See p. 455.) Important because it ikeleton. Ai, External view of the left ramus; A;, internal view; A^, superior view of the left ramus. cd. Condyle; di, , alveolus for second deciduous incisor; dij, alveolus for third ning in with deciduous series and probably belonging with it); Generic and specific refereni exhibits the chara(5ters of the juvenile ; view of the right ramus; A3, anterior alveolus for fir^ deciduous incisor; di2 deciduous incisor; p\^ fir^ premolar (coi dp2, second deciduous premolar; dps, third deciduous premolar; dpi, fourth deciduous premolar U. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE XXIV ?^Z JUVENILE JAWS AND TEETH OF OLIGOCENE TITANOTHERES A, Menodus giganteus (Am. Mus. 510), superior vie-wr. B, Menodus giganteiis (Am. Mus. 509), superior view. Ci, Brontops dispar Carnegie Mus. 124), external view of right ramus; C2, the same, superior view of dentition. All one'half natural sise V. S. GEOLOGICAL SURVEY MONOGRAPH 55 PLATE XXV SUPERIOR DECIDUOUS AND PERMANENT GRINDING TEETH OF MENODUS GIGANTEUS Ai, Am. Mus. 497, deciduous premolars and firA permanent molars of left side, crown view^; A2, the same, showing permanent teeth embedded in the alveolar region. Both t^vo'thirds natural si^e CHAPTER VII EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES SECTION 1. METHODS BY WHICH THE TITANOTHERE SKELETON HAS BEEN STUDIED PRINCIPIES OF THE EVOLUTION OF THE IIMBS OF HOOFED ANIMALS The postcranial skeleton of the titanotheres, al- though less fully known than the skull, is no less significant in its bearing on our knowledge of the evolution of these animals and of that ancient West which was their habitation or the scene of their migra- tions. To understand that ancient West we must try to bring its animals and plants back to life. The attempt to restore the titanotheres as living and migrating animals has led to the establishment of four new principles in the evolution of the limbs of the hoofed mammals generally, principles that have been worked out by the author in cooperation with Dr. William K. Gregory, who has published (Gregory, 1912.1, pp. 267-294) a preliminary study which includes many independent observations of his own on the mechanics and adaptations of limb movement. These principles were discovered through comparison of the skeleton and musculature of all the perissodac- tyls and of the proportions of the upper and lower limb segments in a large number of ungulates, in- cluding perissodactyls, artiodactyls, amblypods, and proboscideans. These four principles of limb evolu- tion, which had been only partly or incompletely recognized previously and which are fully described in Chapter IX, are briefly as follows: 1. The relative length and the angulation of the upper and lower segments of the limbs and the planes of the articular facets furnish a means of elucidating the adaptations to speed and to weight in all the hoofed mammals, living and extinct. Thus by de- termining the relative lengths and proportions of the limb segments among living forms in which the speed, weight, and general limb movements are loiown, we may estimate the adaptations to similar functions and habits in the titanotheres and other extinct forms. 2. Apart from their ancestral paleotelic adapta- tions, all ungulates, in their bony and muscular systems, show secondary cenotelic adaptations to similar mechanics of speed and weight, which form closely analogous or convergent groups and are exhib- ited in the form and the proportions of the limbs and of the shoulder and pelvic girdles. 3. Within each of the nine families of perissodactyls that are more or less closely related to the titanotheres analogous or convergent adaptation produces closely similar limb and shoulder-girdle forms from more or less dissimilar ancestral forms. 4. Between the primitive, light-limbed, subcursorial Lambdoiherium type and the ponderous Brontotherium type, the titanotheres pass through four stages of limb types (figs. 685, 686). From a light body and limb type {Lambdotherium) , analogous to that of the primi- tive cursorial horses, they enter a medium limb stage (Eotitanops) like that of the tapirs, then pass through something near a primitive rhinoceros stage {Mesa- tirJiinus), and ultimately attain the final titanothere stage {Brontotherium), which is in some respects similar to that seen in the elephants. The titanotheres and other hoofed mammals that exhibit these four stages in the development of the limbs are broadly designated as follows: 1. Subcursorial digitigrades, partly perfected in swift limb movements, including PJienacodus (condy- larth), EoMppus (horse) , Lamhdotherium (titanothere), primitive types of ungulates of lower Eocene time, in which limb proportions are inherited from ancestral unguiculates and show evidence of remote ambulatory and even of still more remote arboreal adaptation (Matthew, Gregory). In these animals the radius and tibia are relatively long; the metapodials, typified by Mts III and Mtc III, are relatively short. 2. Mediportal digitigrades, of medium weight and speed, with moderately heavy body and limbs and clumsy motion (digitigrade), such as Tapirus (tapir), MesatirMnus (titanothere). Most middle-sized quad- rupeds of middle Eocene time have limb proportions intermediate between the cursorial and graviportal extremes. These proportion ratios survive in the existing Tapirus, which, however, in its body and limb proportions is more cursorial than the Eocene titanotheres. 3. Subgraviportal digitigrades, partly transformed into or prophetic of the weight-bearing (graviportal) type, such as Palaeosyops (titanothere). 4. Graviportal digitigrades, of perfected weight- bearing type, with angulate limbs capable of more or less rapid movement according to the length and the angulation of the limbs, such as Opsiceros (black rhinoceros), Brontotherium (titanothere). These ani- mals are all digitigrades — that is, the feet rest partly on terminal hoofs, partly on pads beneath the phal- anges. They are heavy or bulky forms, such as the rhinoceroses and titanotheres, which retain the digitigrade type of foot, although some (Menodus) have straight hind limbs. Besides the four types of limbs or limb movement that are exemplified in the titanotheres there are three other types, which are not yet known in animals of the titanothere group, as follows: 583 584 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 1. Primitive ambulatory (plantigrade), such as Pantolamhda (amblypod) ; also the opposite extreme. 2. Cursorial unguligrade, with swift type of limb and secondarily elongated feet, such as NeoJiipparion, Equus (horse), Gazella (gazelle), all light, long-footed Figure 482. — Evolution of the skeleton in titanotheres A, First stage (subcursorial): EotitaTiops borealis^ lower Eocene. B, Middle stage (mediportal): Palaeosyops Uidyi; middle Eocene. C, Final stage (graviportal); iJroniops ro6«siw5; lower Oligocene. Scales approximate; A too large. CSeefig.483. types, in which the limbs terminate in single hoofs and the pads beneath the phalanges are reduced or wanting. 3. Rectigrade-graviportal, straight-limbed, weight- bearing type, with special pillar-like or vertical weight-bearing disposition of the limbs and short gravigrade feet, such as ElejpJias, Mastodon (elephants) ; also such clumsy, slow-moving forms as Coryphodon and Uintatherium (amblypods), with gigantic or heavy bodies supported on straight or column-like limbs in which the terminal phalanges and hoofs are reduced and the limb is supported on a heavy pad. In general, the stages represented by these types indicate that two main divergent direc- tions were taken in limb adaptation — namely, into cursorial or speed types or into graviportal or weight types. Bridging over these extremes are types that combine speed and weight. In the titanotheres there is an evolution of constantly changing proportion both in the mass or weight of the muscles and bones and in the length of the limb segments, adapted to con- stantly changing habit of speed and weight, passing through the subcursorial, mediportal, subgraviportal, and graviportal types and ap- proaching a rectigrade-graviportal type in the huid limbs. Thus, while the bones of the head are constantly changing, every muscle and bone in the skeleton is also constantly changing. SIZE AND PROPORTIONS OF EOCENE TITANOTHERES As shown in Figures 483 and 661, the members of the six or seven phyla of middle Eocene titanotheres differ much less in the height of the shoulder and of the hip than in weight and speed. The height of a quadruped depends upon the total combined length of the segments — the scapula, hu- merus, radius, manus — modified, by the flexure or angidation at the four joints — shoulder, elbow, wrist, and phalangeal joints. Quadrupeds differ widely in respect to angulation : heavy animals have straighter limbs than light, swift-moving animals. The normal angles at the shoulder, the elbow, and the phalangeal joints may be determined precisely by a study of the planes of the articular facets (figs. 518, 664, 666, 667). Naturally the best method of ascertaining the height of the animal is to make a restoration (Palaeosyops, fig. 536), if a sufficient number of parts are available, laying out the limbs in their proper angulation, as indicated by the planes of the articular facets and measuring the net height directly. It is seldom that the material is so complete that all the limb segments and articular facets can be measured and determined for the purpose of estimat- ing height, so that we may be forced to make estimates based on the ratios of the length of the parts preserved to those of similar animals of loiown height. EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 585 Broniups .^rontops robzistzics'i ¥ maepsyops-^yogruiffmsYi-''^'^^ EoUtanops KesatirMnus Manteoceras - Jrianteoceras ? SoHtanops — princeps A E F H J K =10-m.. iS.Ocm. 66c7n. 80.7cm. 89c7rv. 90cm,. 92cTrv. 201cm. I03cnv. JOicm. JBOcnv. 250.2om, 'ISin,. ^2ft.2m,. =Zfb.7\m. =2fi.Min. '2ft.ll^m. =3ft. ^3ft.3%m. ^Sft-Hiin. =3ft.5in. =6ft.3m.^8ft.2\iiu FiGTJRE 483. — Estimated height at the shoulder (to top of scapula) of Eocene and Oligocene titano- theres (see below), compared with that of the tapir {Tapirus indicus) A, Lamhdotherium popoagkum: B, Eoiitanops gTegoryi; C, Eotitanops pTinceps: D, Eotitanops major; E, Mesatirhinus petersoni; F, Manteoceras manteoceras (female); Q, Tapirus indicus; H, Palaeosyops leidyi; I, DolichorUinus hyognathus; J, Manteoceras manteoceras (male); K, Brontops rohustus? (female, Am. Mus. 518); L, Brontops robustus (type, male). One twentieth natural size. Estimated Jieights of Eocene titanotheres Length of humerus (centimeters) Height to top of scapula Designation in Figure 483 Lower Eocene:- Lambdotherium popoagicum Eotitanops gregoryi Eotitanops princeps Eotitanops major Middle Eocene: Mesatirhinus petersoni Manteoceras manteoceras, ? Tapirus indicus (modern tapir) . Limnohyops monoconus? Palaeosyops leidyi Manteoceras manteoceras, cf Upper Eocene: Dolichorhinus hyognathus Lower Oligocene: Brontops robustus Brontops robustus ' 20.3 26+ '^29 25 29.3 32.5 35 53 6L 5 32 35.6 38 39.3 38.6 78 82 -38 45.6 ''66 80.7 "^90 92 •92 101 /104 "103 M90 250.2 15 A 18 B 26 C 31.8 D 35 E 35K F 36 G 36 39M H 41 J 40J^ 75 983^ « Scapula and humerus conjectural. t Humerus known; scapula and radius conjectural. Restored from known relative length of humerus, scapula, and radius in allied forms. <: Forearm and manus Princeton Mus. 10013, humerus and scapula supplied from other individuals. '* Femur known; other segments unknown. Height computed by assuming same relative length of hmb segments as in Palaeosyops leidyi. ' Manus, forearm, and humerus known, scapula computed. f Femur known, other segments unknown. Height computed by assuming same relative length of limb segment as in Palaeosyops leidyi. Composite restoration (fig. 579). '' Measured from mounted skeleton (Am. Mus. 518). • Type skeleton (Yale Mus. 12048). 586 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA DIVERGENCE AND CONVERGENCE IN THE SKELETON OF POLYPHYIETIC SERIES DIVERSE ADAPTIVE TYPES OF LIMB STRUCTURE Our study of the locomotor skeleton of the titano- theres strongly supports the polyphyletic theory that we derived from the study of the skull and the teeth — namely, that there were two larger branches or groups of Eocene titanotheres, the palaeosyopine and the Manteoceras-DoUchorhinus, each of which was divided into two or more smaller branches or subfamilies. The interpretation of the structure of the feet and limbs would be simple if the feet and limbs of all these six or seven genera were invariably divergent, but the law of convergence again comes into play, and long after one subfamily has diverged from another there remain or arise within each subfamily forms which, through convergent adaptation and inheritance, par- allel in foot and limb adaptation forms in the other subfamilies. To make this double convergence clear we recall the existence in Eocene time of five distinct groups or sub- families of titanotheres with respect not only to geo- logic level but to divergence as to speed and to weight, namely: Palaeosyopine group: Lower Eocene: Lambdotheriinae (cursorial types). Eotitanopinae (subcursorial types). Middle Eocene: Palaeosyopinae (mediportal and subgraviportal types) . Manteoceras-DoUchorhinus group : Middle and upper Eocene: Manteoceratinae (subgraviportal types). Dolichorhininae (mediportal types). The Lambdotheriinae (fig. 486) are primitively cursorial, like the early Eocene Equidae and Lophio- dontidae. How far the Eotitanopinae may be directly ancestral to the Palaeosyopinae or Manteoceratinae we do not know. The known Eotitanopinae (figs. 484, 492) are light- limbed, or subcursorial — that is, the feet are relatively shorter (more mediportal) than those of the tapir and less light in structure than those of the primitive Equidae. In the various Palaeosyopinae we find a condition somewhat transitional between the tapir and the heavier types, like the rhinoceros. These animals include more heavy-limbed (Palaeosyops) and light- limbed (Limnohyops) types. Similarly within the Manfeoceras-DolicTiorJiinus group there are two series of forms, mediportal {Mesa- tirhinus) and subgraviportal (Manteoceras). Thus we discover mediportal Palaeosyopinae (such as Limno- hyops) and mediportal Dolichorhininae (such as Mesa- tirMnus), which are somewhat alike in their limb adaptations and can be separated only by careful scrutiny of certain less conspicuous features of ances- tral separation, which bring out the subfamily char- acters. There are also graviportal Palaeosyopinae (such as Palaeosyops), which resemble graviportal manteoceratines (such as Manteoceras) in some respects but differ in others. It has taken a vast amount of study of the scattered and often unassociated limb materials to determine the I'eal subfamily relationships in the limbs of these different forms, which are concealed by the veneer of similar adaptation or ceno telle resemblance; but it may now be demonstrated positively that in each subfamily of the middle Eocene titanotheres there were relatively light-limbed and relatively heavy- limbed forms. The differences between these adaptive extremes of relatively rapid-moving and slow-moving forms may be observed and measured in every single bone of the limbs, and especially in the small bones of the carpus and tarsus. To illustrate how a single bone may be highly distinctive, an outline of the central bone of the carpus, the magnum, as it appears in different forms, is given herewith (fig. 485). On the left is the magnum of the subcursorial Eotitanops, relatively high and narrow; on the right that of the subgraviportal Palaeosyops rohustus, relatively broad and low. In general, high and narrow proportions of the magnum characterize cursorial forms and low and broad proportions char- acterize graviportal forms. Thus the transformation of this single bone reflects the kind of allometric change which prevailed in all parts of the skeleton. Or, to take the foot as a whole (fig. 484), the manus of MesatirJiinus represents the mediportal extreme among the titanotheres analogous to that of Tapirus, while the manus of Palaeosyops represents the sub- graviportal extreme analogous to that oi Hippopotamus. Both types of feet occur at the same geologic levels and in the same geographic regions; they belong to con- temporaneous titanotheres, one perhaps seeking forest ground like the habitat of Tapirus, the other the bor- ders and possibly the waters of rivers and lakes like the habitat of Hippopotamus. There are thus, as shown in Figures 502, 503, 510, 515, 620, wide adaptive radiations among the Eocene titanotheres in modes of locomotion on different kinds of soil. All these adaptively diverse types of limb structure appear to have been derived from a small and primitive type, which was a subcursorial, light-limbed, slender- footed, and relatively speedy animal, well adapted for escape rather than combat. The adaptive transformation of the limbs of titano- theres has not yet been traced so continuously as that of the skull or of the teeth; the scattering of parts of limbs has rendered the association and identification of many separate bones exceptionally difficult. We are still in doubt as to the limb structure in certain phyla, especially in Telmatherium. Much remains to be discovered through further exploration in the field. EVOLUTION OP THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHEEES 587 This deficiency of material renders all the more valuable the broader investigation and comparison with other Perissodactyla which will be presented in Chapter X. The new and precise system of compara- tive measurements of the limbs establishes certain subphyla (genera). Members of these six or more phyla were separated by the weight, shape, and pro- portions of the body, feet, and limbs, such separation being indicative of a considerable range of size, speed, and migrating ability in search of food. This demon- FiGUEE 484. — The phyla of Eocene titanotheres, as represented by the manus A, Lambdotheriinae (,Lambdotherium); B, Eotitanopinae (Eotitanops); C, Palaeosyopinae (Limnohyops); D, Palaeosyopinae iPalaeosyops); E, Manteoceratinae (.Manteoceras)] F, T>o\iehorhininae (Mesatirhinu^). One-third natural size. laws of limb adaptation and enables us to estimate fairly closely the speed and weight-bearing powers of the various kinds of titanotheres. 1. Existence of many pliyla. — The chief conclusions drawn from the limb bones of titanotheres, moreover, confirm those drawn from the skull — namely, the exist- ence of six or more distinct phyla in the lower Oligo- cene. That there were six or more contemporaneous but more or less independent phyla, characterized by differences of limb structure and modes of locomotion, is clearly shown, as well as the fact that these include two main phyla each of which embraces at least three strates the existence of two early radiations (sub- families) and of subsequent branch radiations (genera). FiQUKE 485. — Progressive broadening of the magnum in Eocene titanotheres A, Eotitanops (subcursorial) ; B, Mesaiirkinus (mediportal) ; C, Lim- nohyops (mediportal); D, Palaeosyops (subgraviportal) . One-balf natural size. 2. Adaptive characters superposed on ancestral. — • Similar limb and foot proportions, like similar skull 588 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA proportions, were independently developed in unre- lated phyla and are often misleading as to real rela- tionships. Thus the subfamilies duplicate each other in adaptation; there arose in both subfamilies short- footed and long-footed forms, as summarized in vari- ous tables. TERMS USED IN DESCRIBING THE SKELETON OF THE TITANOTHERES The following list includes most of the terms used in this monograph in describing the postcranial skeleton of the titanotheres: Terms used in describing the 'postcranial sTceleton of the titanotheres English terms Shoulder girdle and fore limb Scapula Superior or suprascapular border Anterior or prescapular border Posterior, postscapular, axillary border Spine Tuberosity of spine Supraspinous or prescapular fossa Infraspinous or postscapular fossa Internal subscapular surface Neck of scapula Coracoid process Coracoid border Glenoid cavity Humerus Head Greater tuberosity Lesser tuberosity Bicipital groove Deltopectoral crest Deltoid tuberosity Tuberosity for the teres major or latissimus dorsi- Winding (brachialis) surface Ectocondyle Extensor (supinator) crest Entocondyle Distal articular surface "Capitellum" for radius "Trochlea" for ulna Olecranal, anconeal (posterior) fossa Supratrochlear (anterior) fossa Radius Head Internal process External process Radiohumeral or trochlear facet Biceps tubercle Extensor groove Brachialis rugosity Styloid process Radiocarpal facet Ulna Olecranon Olecranal fossa Ulnohumeral trochlea External facet of the ulnohumeral trochlea Coronoid process External process Lesser sigmoid fossa Ulnoradial facets Ulnocarpal facet Styloid process Carpals Proximal row, scaphoid, lunar, cuneiform, pisiform Posterior tuberosity of lunar Scapula Margo suprascapularis Margo prescapularis Margo axillaris Spina scapulae Tuberositas spinae scapulae Fossa prescapularis (supraspinata) Fossa postscapularis (inf raspinata) Facies subscapularis Collum scapulae Processus coracoideus Margo coracoideus Ca vitas glenoidalis Humerus Caput humeri Tuberculum maj us Tuberculum minus Sulcus bicipitalis Crista deltopectoraUs Tuberositas deltoidea Tuberositas teres Facies spiralis Ectepicondylus Crista " supinatoria " (extensoria) Entepicondylus Facies distalis capitellum trochlea humeri Fossa olecrani Fossa supratrochlearis Radius Caput radii Processus internus capitis radii Processus externus capitis radii Facies proximalis (radiohumeralis) radii Tuberositas bicipitalis radii Sulcus extensoria Rugositas brachialis Processus styloideus radii Facies distalis (radiocarpalis) radii Ulna Olecranon Fossa olecrani Incisura semilunaris (facies ulnohumeralis) Facies externa incisurae semilunaris Processus coronoideus (anconaeus) ulnae Processus externus ulnae Fossa sigmoidea minus Facies ulnoradialis Facies distaUs (ulnocarpalis) Processus styloideus ulnae Carpalia Os scaphoideum, os lunare, os cuneiforme, os pisiforme. Tuberositas posterior ossis lunaris cb. t. sp. fas. sup. sp. fos. inf. sp. glen, h. cap. t. maj. t. min. s. bic. t. del. t. te. brack, ant. ectep. cr. "sup." eniep. cptl. trch. fos. ol. pr. in. pr. ex. cptl. h. tbclm. rad. s. ex. br. ant. pr. sty. ol. fos. ol. inc. sml. fac. ex. pr. anc. pr. ex. u. fos. sig. min. pr. sty. ul. sc, lu., cu., EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES Terms used in describing the postcranial skeleton of the titanotheres — Continued 589 English terms Shoulder girdle and fore limb — Continued Carpals — Continued. Distal row, trapezium, trapezoid, magnum, unci- form. Posterior tuberosity of magnum Metacarpals II-V Phalanges, 1st, 2d, 3d, or ungual Sesamoids Sacrum, pelvic girdle, hind limb Sacrum Sacrals 1-4 Sacro-iliac rugosity Pelvis Right and left innominate bones Ilium Superior or supra-iliac border Sacral process Lateral process Iliosacral rugosity Dorsum or dorsal surface (gluteal fossa) _ Ventral surface (iliac fossa) Neck of ilium Rectus tuberosity Pubis Symphysis pubis Pubi-ischiadic symphysis Pectineal tuberosity, pubic spine Obturator (thyroid) fenestra Ischium Obturator (thyroid) fenestra Tuberosity Ischial spine Femur Head Ligamentum teres pit Digital fossa Great trochanter Second or lesser trochanter Third trochanter Linea aspera Plantaris fossa Internal tuberosity External tuberosity Entocondyle, inner femorotibial trochlea. Ectocondyle, outer femorotibial trochlea- Patellar trochlea, surface Inner keel, outer keel Patella Tuberosity for the quadriceps femoris Tuberosity for the patellar ligament Tibia Tibiofemoral trochlea Entocondylar surface Ectocondylar surface Spine Cnemial crest, tubercle Supero-external tuberosity Supero-internal tuberosity Popliteus fossa Distal posterior process Tibiocalcaneal trochlea Internal malleolus Carpalia — Continued. Os trapezium, os trapezoideum, os magnum, os unciforme. Tuberositas posterior ossis magni Metacarpalia II-V Phalanges Ossa sesamoidea Sacrum Vertebrae sacrales 1-4 Rugositas iliosacralis Pelvis Os innominatum, dextrum et sinistrum Ilium Margo superior Tuber sacrale Tuber coxae Rugositas iliosacralis Facies dorsalis Facies ventralis CoUum ilii Processus antero-inferior Pubis Ramus superior Symphysis pubi-ischiadica Tuberositas pectinea Fenestra thyroidea Ischium Fenestra thyroidea Tuberositas ischii Spina ischii Femur Caput femoris Fovea capitis Fossa trochanterica (digitalis) Trochanter major Trochanter minor Trochanter tertius Linea aspera Fossa plantaris Tuberositas interna Tuberositas externa Entocondylus Ectocondylus Trochlea patellaris Carina interna, carina externa Patella Tuberositas tibiae Tuberositas ligamenti patellae Tibia Facies proximalis Facies entocondylaris Facies ectocondylaris Spina Crista cnemialis (tuberculum) Tuberositas supero-externa Tuberositas supero-interna Fossa poplitealis Processus distalis posterior Facies articularis inferior Malleolus internus tz., id., mg., unc. II, III, IV, V. tu. sac. tu. cox. pr. ant. inf. pu. ram. sup. pb. f. thyr. tu. is. sp. is. /. cap. lig. ter. tr.\ t.' tr.'^, t." tr.^, t." t. in. t. ex. con. in. con. ex. trch. cr. en. t. ex. t. in. mal. int. 590 TITANOTHEBES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Terms used in describing the postcranial sTceleton oj the titanotheres — Continued Sacrum, pelvic girdle, hind limb — Continued Fibula H ead Fibulotibial facet Shaft Fibulocalcaneal facet External malleolus Calcaneum Tuber calcis Sustentaculum Sustentacular facet Ectal facet Inferior facet Cuboid facet Astragalus Astragalotibial trochlea H ead Neck Navicular facet Cuboidal facet Tarsals Navicular Entocuneiform Mesocuneiform Ectocuneiform Cuboid Metatarsals II-IV Phalanges Fibula Caput fibulae Facies articularis superior Corpus fibulae Facies articularis inferior- Malleolus externus Calcaneum Tuber calcis Sustentaculum tali Facies sustentacularis Facies ectalis Facies inferior Facies cuboidea Astragalus Trochlea astragali Caput astragali Collum astragali Facies navicularis Facies cuboidalis Tarsalia Os naviculare Os entocuneiforme Os mesocuneiforme Os ectocuneiforme Os cuboideum Metatarsalia II-IV Phalanges fb. mal. exi. cal. tu. cal. sus. ect. inf. ch. as. cr. ex., ci (nav.) icb.) ch. ml. II-IV. ph. SECTION 2. THE POSTCRANIAL SKELETON OF LOWER EOCENE TITANOTHERES SUBFAMILY LAMBDOTHEEIINAE , Lambdotherium Lambdotherium is a small, swift, slender-limbed, relatively abundant animal, less abundant than the contemporary horses but more abundant than the small lophiodonts (Heptodon). The largest Lamb- dotherium measures about 14 inches (350 mm.) at the shoulders, as compared with the smallest Eotitanops (E. gregoryi), which measures about ISJ^ inches (456 mm.) at the shoulders. It exceeds in size most of the contemporary species of Eohippus and equals the large EoMppus robusius of the lower Wasatch. Lambdotherium popoagicum The materials of L. popoagicum are extremely scanty; in only two specimens (Am. Mus. 4880, 14903) are parts of the skeleton associated with the teeth, and these parts are very fragmentary. (See figs. 486- 489.) In the atlas (fig. 487) the vertebrarterial canal pierces the anterior portion of the base of the trans- verse process; that is the primitive condition in tita- notheres and m Perissodactyla generally. The restoration of the fore limb (fig. 488) is highly conjectural, because the lengths of the bones are un- known. The proportions are heavier than in Eohippus. The ratio of the radius to the humerus is estimated at 90 per cent, the same as in Eohippus. The scapula (fig. 487) presents an elongate neck (tr. 19 mm.) and the lower part of the postspinous fossa is very narrow; the lower part of the prespinous fossa, which is not all preserved, is slightly broader; the spine descends rather low. The distal end of the humerus indicates the presence of a small entocondyle, and its narrow rotula accords with the deep, laterally compressed proximal articular surfaces of the ulna and radius. The length of the radius is estimated at 103 millimeters; the shaft is rather narrow and sharply convex. The ulna is elongate and has a high, deep, laterally compressed olecranon process; the proximal portion of the shaft behind the radius is deep. The carpus (fig. 489) is correlated with this slender structure, being relatively high and narrow but broader than in Eohippus; the scaphoid rests chiefly on the magnum anteriorly, and the high and narrow lunar exhibits a broad supporting unciform facet and an oblique nonsupporting magnum facet. The magnum, wanting in this specimen, was probably small; it is relatively broad in Eohippus. The cimei- form is rather high and narrow. The manus exhibits four digits; the ends of the metapodials show a tendency to functional tridac- 1 tylism, because the median digit (III) is larger than EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENB TITANOTHERES 591 the adjoining digits. This enlargement of D. Ill is indicated not only by its diameter but by the enlargement of its proximal phalanx, which is much broader than the proximal phalanges of D. 11 or D. IV. This indicates a rather pronounced tendency to mesaxonic structure, whereas the middle Eocene titanotheres exhibit either a primary or secondry paraxonic and finally isotetradactyl condition. D. V is somewhat shorter and more slender. The structure of the astragalus of this specimen (Am. Mus. 4880), as inferred from the distal end of the tibia which is preserved, indicates that the keels of the astragalus were quite sharp. The tarsus is compared with that of the less cur- sorial Eotitanops and the more cursorial Eohippus in Figure 502. An astragalus associated with Am. Mus. 2991 exhibits a high, long neck and a high, narrow sustentacular facet, showing that the astragalo- cuboidal facet was very narrow (figs. 491, 503). Figure 486. — Reconstructed skeleton and restoration of Lambdotherium popoagicum Cope Made by E. S. Christman under the direction of W. K. Gregory. One-tenth natural size. This provisional reconstruction is based on the following American Museum specimens from the Wind River Basin, Wyo.: 14007 (Alkali Creek, Wolton), maxilla, malar, top of cranium; 14903 (Alkali Creek, Buck Spring), squamosal, basicranial region; 14899 (Alkali Creek, Buck Spring), lower jaw; 4880, distal end of scapula, fragments of humerus, radius, ulna, and manus; 14921 (Wolton), astragalus and calcaneum. Hemaining parts conjecturally modified from Eohippus and other contemporary perissodactyls. SUBFAMILY EOTITANOPINAE Eotitanops Our knowledge of the skeleton of the species of Wind River Eotitanops is relatively small; we know that there were gradations of size from smaller and 101959— 29— VOL 1 41 fac.ar±.post lighter to larger and heavier, subcursorial forms, the general evolution being in the direction of the laiown middle Eocene titanotheres. The pes especially gives us valuable records of this progressive increase in size and weight. As compared with the middle Eocene titanotheres of the Bridger formation these lower Eocene forms were small and light limbed and had high, narrow ankle and wrist joints, resembling B most nearly the light-footed -' members of middle Eocene age, such as Mesatirhinus . In the skeleton, as in the skull, there are general primitive features that accord with the other lower Eocene perissodactyls, such as the lophiodonts (Hep- todon) and horses (EoMppus). The details of the carpus and tarsus remind us also of these lower Eocene perissodactyls, although Eotitanops is decidedly larger and less distinctively cursorial than either Heptodon or Eohippus. The abbrevia- tion of the fifth or outer digit (Mtc V) of the manus forbids our describing the animal as functionally tetradactyl; taken with the extreme dis- placement of the lunar on the unciform (a ti-idactyl specialization) this reduction tends to support the idea of Gregory that these animals, after having entered an evolution toward tridactylism, reversed the process and reentered tetradactylism in middle Eocene time. Measurements, in millimeters, of teeth and limb bones referred to Eotitanops, compared with Lambdotherium and Tapirus Figure 487. — Atlas and scapula of Lambdothe- rium, popoagicum \m. Mus. 4880, Wind Eiver Basin, Wyo., Wind Eiver for- mation. A, Distal portion of left scapula, outer side view; B, rear view of incomplete atlas. Two-thirds natural size. Lower grinding teeth (p^-ms). Median metatarsal, length Median metacarpal, length Humerus, length Femur, length 64 78. 4 98 85 Lower grinding teeth (pj-ma)- Median metatarsal, length Median metacarpal, length Humerus, length Femur, length E. princeps E. major 105 87 205 ■250 118 114 113 ° Estimated. 592 TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA tb.rn/j Figure 489. — Forearm and man us of Lambdotherium popoagicum Am. Mus. 4880, Wind River Basin, Wyo., Wind River formation, level B. Incom- plete left forearm and manus. The length of the radius and of the metacarpals is conjectural. Ai, Front view; Aj, outer side view of forearm. One-half natural size. Figure 488. — Fore iimb of Lambdotherium popoagicum Am. Mus. 4880, Wind River Basin, Wyo., Wind River formation, level B. Incom- plete forearm, external view. The length of the elements is conjectural. One- half natural size. Figure 490. — Left manus of Lambdotherium Eotitanops A, Lambdotherium (Am. Mus. 4880); B, Eotitanops (Am. Mus, 296), showing the ancestral type of titanothere. The length of the metacarpals in Lambdotherium is conjectural. One-half natural size. EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 593 The association of limb bones with teeth of the same individual is a very rare occurrence. Thus the reference (see table on p. 585) of several of the hind feet is inferred only from the size and degree of slender- ness of the limbs. Eotitanops gregoryi Osborn There is reason to believe that Eotitanops gregoryi was both small and slenderly proportioned; that E. Figure 491. — Astragalus of Lambdotherium popoagicum Am. Mus. 2991, Wind River Basin, V^yo., Wind River formation, level B. Left astragalus, front and rear views. Two- thirds natural size. horealis and E. princeps were still of light construc- tion, corresponding with the delicately constructed skull of E. horealis ; and that the littl e-known E. major was somewhat more massive in its proportions. The whole range of adaptive radiation of the limbs is parts may be conveniently described under a single heading. Atlas of Eotitanops horealis The atlas of E. horealis is preserved in the neotype skeleton (Am. Mus. 14887, fig. 495). In that speci- men, which is the earliest known, we observe a broad resemblance to the atlas of the middle Eocene Mesa- tirhinus and of the existing Tapirus, both probably representing the primitive perissodactyl type. The characters are (1) centrum relatively elongate, (2) facets for axis in obliquely convergent planes, (3) vertebrarterial canal entering slightly above posterior rim of transverse process and issuing on lower median face of same. Comparison with the atlas of Mesa- tirhinus (fig. 560) indicates that we have here the ancestral titanothere type, which is highly modified in some of the progressive forms. Cervical and dorsal vertebrae of Eotitanops princeps (type) The two posterior cervicals (C. 6, C. 7), three scat- tered dorsals, and one caudal of the type (Am. Mus. 296, fig. 496) show the following characters: (1) Neck relatively short as in Palaeosyops; C. 1 to C. 7 estimated at 180 millimeters as compared with 320, Figure 492. — Restorations of the lower Eocene titanotheres of the Wind River formation A, Lambdotherium popoagicum: B, Eotitanops princeps; C, Eotitanops gregoryi. One-thirtieth natural size. subcursorial or less truly cursorial than either Lamh- dotherium, Heptodon, or Eohippus. Estimates of the heights of these animals, which, it will be recalled, were not successive but partly contemporaneous, are as follows: Height of Lambdotherium and Eotitanops the estimated basal length of the skull; (2) cervicals with broad depressed centra, facets oblique (length Type Estimated height at shoulder .i2 = -2 J3 Smallest and most cursorial 350 450 660 800 14 Second smallest true titanothere known (Eotitanops gregoryi) Of intermediate size (E. princeps). Of largest size (E. major) 18 26 32 ABC D Figure 493. — Metatarsals of Eotitanops Median metatarsals of E. gregoryi (A), E. iorealis (B), E. princeps (C), E. major (D). Natural size. Eotitanops borealis and E. princeps The species and the mutations of E. horealis and E. princeps are so closely related that their skeletal of centra, C. 6, 25 mm.; C. 7, 26); (3) three scattered dorsals laterally compressed, centra elongate, meas- uring, ?D. 3, 26 millimeters; ?D. 4, 25; ?D. 10, 29. 594 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA nd lunar of type of Eotitanops borealis | Fore feet of Eotitanops princeps (type) The radius (Am. Mus. 4892) associated with the type upper molars is incomplete; it exhibits an asymmetrical proximal articular surface, and a prominent internal distal projection (styloid process) of the distal articular surface. This type (Am. Mus. 296), originally described as E. iorealis and now referred to the mutation E. princeps, is the only individual in which portions of the fore and hind limbs, vertebrae, and jaws are known, enabling us to establish the proportions. Figure 494. — Reconstructed skeleton and restoration of Eotitanops borealis Made by E. S. Christman under direction of W. K. Gregory. About one-tenth natural size. The provisionally recon- structed skeleton is based on the following American Museum specimens from the Wind River Basin, Wye: No. 14887 (E. borealis, Dry Muddy Creek 12 miles above mouth), skull, pelvis; No. 296 (.E. princeps),, lower jaw, humerus, manus, femur; No. 14888 (£. borealis, Alkali Greek, Davis ranch), part of scapula, pes. Remaining parts conjectural. The lunar is a very distinctive element in the lower Eocene titanotheres, especially in the fact that in the front view it rests mainly on the unciform and retains a narrow vertical facet anteriorly for the magnum, although in the back view it rests almost equally upon the magnum and the unciform. This is an adapta- tion to tridactylism or functional reduction of digits I and V. The manus is numerically tetradactyl, but a strik- ing feature is the slenderness of the fifth digit (Mtc V). The carpus is narrow and deep — transverse measure- ment 41 millimeters, vertical 34 (through cuneiform and unciform). The lunar presents anteriorly a superior transverse measurement of 17 millimeters as compared with a total vertical measurement of 22; it presents inferiorly a broad, oblique unciform facet EVOLUTION OF THE SKELETON OP EOCENE AND OLIGOCENE TITANOTHERES 595 and a narrow, nearly vertical magnum facet, similar to but relatively smaller than that seen in Mesatirhinus megarJiinus; posteriorly it rests half on the magnum, half on the unciform. As this'bone, therefore, practically distributes three-fourths of its front weight to the unciform, it exhibits an ex- treme displacement which appears to be a character common to the early Eocene Perissodactyla generally. Similarly the scaphoid is narrow (13 mm.) and deep anteroposteriorly (24 mm.) with a vertical measure- ment of 19 millimeters; it covers the entire superior face of the magnum. The magnum is a very distinc- tive bone as seen in front, being small, vertically ex- tended (12 mm.) and later- ally compressed (9 mm.). The imciform is a vertically deep element measuring 22 by 19 millimeters trans- versely. The metacarpal displacement is also extreme, Mtc II and Mtc III abut- ting widely against the magnum and unciform respectively. The metacar- pals are distinguished by the small size of Mtc V, Figure 495. — Atlas of Eotitanops borealis Am. Mus. 14887 (neotype atlas asse- dated with skull; cf. flg. 250). .^i. Anterior or condylar surface; A2, posterior or cervical surface; A3, dor ^sal; Ai, ventral; As, side view. OnC' third natural size. by 7 anteroposterior, being, therefore, relatively large- Mtc III is still larger; it measures 85 millimeters vertically and 13 transversely. Mtc IV measures 73 millimeters vertically and 11 transversely. Thus Figure 497. — Radius of Eotitanops borealis Am. Mus. 4892, fragments of the right radius associated with the type upper teeth. Ai, Distal part; A2, proximal part; A3, proximal view. One-third natural size. Figure 498. — Lunars of Eotitanops Left lunar of JE. borealis (.\m. Mus. 4892, type; Ai, front view, A2, top view) and E. princeps (Am. Mus. 296, type; Bi, front view, B2, top view). Figure 496. — Vertebrae of Eotitanops princeps Cervical (Ce, C7) and dorsal (D) vertebrae associated with the type lower jaw Mus. 296). One-half natural size. in which the shaft measures only 6 millimeters antero- posteriorly by 9 transversely. Mtc II has a length of 79 millimeters and a shaft diameter of 12 transverse Mtc IV is decidedly smaller than Mtc II. This lack of symmetry in the digits on either side of the third digit is characteristic of the early Eocene titanothere manus; it prepares the way for the secondary parax- onic condition in which Mtc II, III, IV, V become more nearly subequal. Humerus and femur The two limb bones known, the humerus and femur, are distinguished by the lateral compression of the shafts in harmony with the compression of the manus; they indicate that E. princeps as com- pared with the middle Eocene titanotheres was narrow chested and slender limbed, that its proportions most nearly resemble those of Mesa- tirhinus of the middle Eocene, and that the corresponding elements are much more primi- tive. The humerus (fig. 500) is relatively elongate, a km. primitive character; the great tuberosity is less developed than in more recent types. The shaft in the upper third measures 20 millimeters transversely, 35 anteroposteriorly, as Figure 499. — Manus of Eotitanops princeps m. Mus. 296; Wind Eiver Basin, Wyo.; Wind River formation, level B. Eight manus of type. One-half natural size. 596 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA compared with the total length, 205; the proximal measurement across the tuberosities is 49, while the measurement of the head to the front of the great tuberosity is 67; the moderately prominent deltoid crest extends 88 millimeters below the head, and the supinator ridge rises 60 millimeters above the interior border of the radial trochlea. The femur (fig. 500) has only partly assumed the distinctive titano there family characters; the shaft has not yet flattened; it more closely resembles that of below the trochanters, by the elevated patellar facet facing anteriorly and inferiorly. Pelvis of Eotitanops borealis (neotype) The proportions of the left innominate bone pre- served in the neotype of Eotitanops horealis (Am. Mus. 14887) indicate a subcursorial type of pelvis, slen- derly buUt, with the following characters: (1) Neck of ilium relatively narrow (tr. 29 mm.); (2) tuber- coxae and external border of ilium partly concave; (3) pubo-ischiadic fenestra elongate or vertically i ma/ Figure 500. — Humerus and femur Eotitanops princeps Am. Mus. 296, Wind River. Right humerus (Ai, A2) andtemur (B) of type. One-third natural size. other subcursorial Eocene perissodactyls, such as Hyrachyus, rather than the mediportal middle Eocene forms. Among the middle Eocene titanotheres its closest resemblances are to the femur of MesatirMnus. The head is lacking. From the great trochanter to the bottom of the internal condyle the shaft measures 250 millimeters. The bone is further distinguished from that of some of the more recent or middle Eocene titanotheres by the laterally compressed and deeply recurved great trochanter, by the very promi- nent second trochanter, by the lateral compression or convexity of the shaft anteriorly between the trochan- ters, a character which it shares with MesatirMnus, by the flattening of the shaft posteriorly between and Ai Figure 501. — Pelvis of Eotitanops borealis A, Am. Mus. 148S7' (neotype), incomplete pelvis associated with skull, ventral (Ai) and outer side (A2) views; B, Am. Mus. 14888, fragment of left ilium, indicating a somewhat wider ilium than that in the preceding specimen, asso- ciated with teeth and other parts, ventral view. One-third natural size. compressed; (4) plane of ilium at angle to that of pubis; (5) proportions of subcursorial rather than of mediportal type. Pes of Eotitanops The pes as compared with those of the other lower Eocene Perissodactyla (fig. 502) — namely, Eohippus (Equidae) and Heptodon (Lophiodontidae) — again exhibits Eotitanops as approaching the relatively large and slow-moving perissodactyls of the period, although possessing many characters in common with the con- temporary lighter-limbed forms. In the comparative outlines of Figure 502 it wUl be observed that Eotitanops horealis stands midway EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 597 between EoTiippus venticolus and Hyrachyus agrarius, as a transition between cursorial and mediportal adaptation. The changing proportions of the astrag- alus, calcaneum, and ectocuneiform are especially sig- nificant. Attention should be called to Eotitanops as isotridactyl — that is, the three metapodials are more uniform in size than those of either Eohippus or Hyrachyus. Peculiar features of the Eotitanops pes are (1) elongate neck of astragalus; (2) a vertically elongate the distal ends of the metapodials. These gradations correspond broadly with those we have observed in the dental series and with the successive increases in Eotitanops major Osborn Type.^Am. Mus. 14894, a left median metatarsal (fig. 493, D) ; also the distal end of the right tibia. Specific cTiaracters. — Of superior size; Mts III 104 millimeters longitudinal, 16 transverse, index 15. JSofiippus Jleptodo, Figure 502. — Left pes of cursorial and subcursorial Eocene Perissodactyla A, EoMppus venticolus, a primitive hippoid with narrow, slender foot and enlarged median metatarsal; B, Beptodon calciculus, a primi- tive lophiodont with side toes not reduced; C, Eotitanops borealis, a primitive titanothere with broader tarsals and metatarsals D, Hyrachyus agrarius, a primitive cursorial rhinoceros. One-half natural size. cuboid (inferential); (3) elevated ectocuneiform; (4) Mts III with cuboid facet narrow or wanting; (5) Mts IV with broad ectocuneiform facet; (6) meta- podials keeled posteriorly; (7) proximal median phalanges relatively short; (8) inferior astragalo- calcaneal facets usually small, entirely separate from sustentacular. The gradations in size of the as- sociated or referred foot bones of Eotitanops are well shown in the accompanying series (fig. 503) displaying This ill-defined species indicates the existence in Wind Eiver time of a relatively large, slender-footed titanothere, which is possibly ancestral to some of the short-footed middle Eocene types. A comparison of the measurements of E. major with those of E. horealis, from the Wind Eiver formation (Am. Mus. 14888), indicates an animal of much larger size but of the same foot proportions as shown by the index. There is a small cuboidal facet. 598 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements of Eoiitanops major and other species, in millimeters E. borealis E. major Mesati- rhinus peter- soni Tapiras tetrestris Median metatarsal. III, length Width of shaft _ 86 13 15 21 26 104 16 15 25 33 120 21 17 114 21 Index Tibio-astragalar facet, transverse- 18 bones of the carpus and tarsus, or in the shape of the small bones on the sides of the manus, such as the tra- pezium and pisiform, and of the pes, such as the entocuneiform. There are also certain proportions in the respective width or depth of the elements of the carpus and tarsus which remain highly distinctive. Every segment of each limb develops as a unit independent^ of every other segment. Allometric adaptation of each segment follows the general or uniform laws that are observed in other quadrupeds. These laws bring about the convergence or parallelism. Eokzppiis B^eptodon. -LamMd/}- N -EotiioTiops Figure 503. — Astragalus and calcaneum of cursorial and submediportal lower Eocene Perissodact^la Front and distal views. A, Eohippus sp., with narrow astragalus (in distal view the astragalus barely touches the cuboid); B, Heptodon calcicidus; C, Lambdotkeriiim popoagicum; D, Eotiianops gregoryi; E, Eoiitanops borealis; F, Eoiitanops sp. The cuboid facet (cb) on the astragalus is better developed in the later stages, though it is still much narrower than in middle Eocene titanotheres. Two-thirds natural size. SECTION 3. MIDDLE EOCENE GROUPS AND PHYLA The titanotheres of middle Eocene time may be grouped as follows: Palaeosyops (subgraviportal and brachj'podal) . Limnohyops (mediportal and mesatipodal) . Manteoceras and Dolichorhinus (subgraviportal and brachypodal) . Mesatirhinus (mediportal and mesatipodal) . DOUBLE PARALLELISM IN THE PALAEOSYOPINE AND MANTE0CERAS-D0LICH0RHINU3 GROUPS The double parallelism of graviportal and medi- portal proportions is so strong that it may again be said that truly ancestral (paleotelic) characters prove to be less conspicuous, obvious, or apparent than adaptive (cenotelic) characters. We must search for real subfamily relationships in inconspicuous parts of the limbs, as in the form of the facets between the Allometric change may progress entirely independ- ently of affiliation to remote ancestral stock or syngenesis; descendants of primitive mesatipodal forms may remain mesatipodal or may become either brachypodal or dolichopodal. Figure 504. — Astragalocalcaneal facets in lower Eocene Perissodactyla A, Eohippus sp.; B, Heptodon calciculus; C, Systemodon primac- vus; D, Lambdoiherium popoagicum. Two-thirds natural size. Like brachycephaly and dolichocephaly in the skull, an allometric tendency toward brachypody [or dolichopody generally proceeds to an extreme, '.but not invariably. EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 599 Figure 505. — Left astragalus and calcaneum of Eoiita- nops sp. FAMILY AND SUBFAMILY CHAEACTERS OF SKELETAL PAKTS IN MIDDLE EOCENE TITANOTHERES VERTEBRAE, ATLAS Palaeosyopinae . — In the atlas of LimnoJiyops, as in Palaeosyops, the spinal nerves (see fig. 508, A) issue nearer the median line than in Telmaiherium (fig. 508, B); the neural arch and the bridge over the vertebral artery are narrow. The axis is rather narrow, with a slender odontoid process. The postero-inferior tuber- osity is moderately devel- oped. In the atlas of Palaeo- syops (Am. Mus. 1580, fig. 508, A), as in LimnoJiyops, the spinal nerves issue nearer the median line than in Telmafherium. Unlike LimnoJiyops the neural arch is broad and powerful; the inferior vertebrarterial bridge is wide; the pleura- pophyses expand broadly Am. Mus. 14895, Wind River Basin, • u„i-u j;..„„t;„„„ T^kn Wyo.; Wind River formation, level '" both du-ections. The B. Facet for tibia (/ft). Two-tiiirds postcro-inferior tuberosity °^'""' '"■'■ is moderately strong. TelmatJieriinae. — In the atlas of Telmatlierium ulti- mum (Am. Mus. 2060; fig. 508, B) the spinal nerves are well separated, the pleurapophyses expand lat- erally but do not flare, the inferior vertebrarterial bridge is broad, but the canal itself is contracted, fore- shadowing the exclusion of the vertebral artery from the canal, which is a very distinctive character of some of the Oligocene titanotheres. The occipital cotylus is broad with massive borders; the odontoid facet is exceptionally broad and open (thus distin- guished from that of Manteoceras) . The postero- inferior tuberosity is moderately strong. Manteoceratinae. — The atlas of Manteoceras (Am. Mus. 12204, fig. 508, E) has the characters we should expect to find in a broad-skulled member of the Manteoceratinae: the vertebrarterial opening is very large posteriorly; the bridge is narrow (16 mm.) and rounded; the articular facets for the axis form a widely open angle; the postero-inferior tuberosity, which underlies the axis, is stout and prominent, as in the Oligocene titanotheres. Dolichorhininae. — In Mesatirhinus (Am. Mus. 1523. fig. 508, C) the pleurapophyseal wings are not known; the atlas presents an approximation to that of Doli- chorMnus: (1) the centrum is relatively elongate, (2) the spinous foramina are nearer together, (3) the ver- tebrarterial canal is covered by a bridge of medium length, (4) the facets for the axis are approximated and obliquely face each other, (5) the cotyli for the occip- ital condyles are relatively deep, (6) there is a reduced postero-inferior tuberosity. In DolicJiorhinus (fig. 508, D, Am. Mus. 1844, 13164 associated with skull), the body is relatively elongate, the occipital cotyli are very broad and deep, the spinous nerve notch is very deep and narrow, the pleurapophyses flare widely, the vertebraterial canal is more open than in Telmatlierium ultimum. As in Mesatirhinus prominent bony processes connect the transverse ligament above the odontoid process. Summary. — The atlas of Palaeosyops and Limno- Jiyops conforms to the brachycephahc types of skull, that of DolicJiorJiinus to the dolichocephalic type; those of Telmatlierium and Manteoceras are interme- diate between these extremes. From the limited materials in our possession {Palaeo- syops, DolichorJiinus, MetarMnus) we observe that the scapula of the Eocene titanotheres is subject to wide adaptive range from the more elongate mediportal type of the supposed MetarMnus to the broader sub- graviportal type of Palaeosyops. The scapula is, how- ever, quite distinctive in its family or syngenetic form as compared with that of other Perissodactyla, being characterized as follows: (1) a vertically elon- gated supraspinous fossa, which is equally broad above and below, giving a relatively straight anterior border; (2) a relatively large infraspinous fossa of subtriangu- lar form, in which the border rapidly contracts toward Figure 506. — Metatarsal and tibia of Eotitanops major Am. iVIus. 14894 (type), Wind River Basin, Wyo.; level B. Median metatarsal: Back (A>), front (A^), distal (A'), and proximal (A') views. Distal end of left tibia: Front (B') and distal (B-) views. One-iialf natural size. the neck — on this border is a distinct "angle" which becomes sharply marked in later types; (3) a broad, short neck, or collum scapulae. This is the graviportal prototype from which the scapula of the heavy Oligo- cene titanotheres is readily derived. On the whole, this middle Eocene scapula is analogous to that of the mediportal Tapirus, but it is distinctly tending toward and even modified in the graviportal direction. 600 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Only three scapulae are known, as follows : (1) Medi- portal, Dolichorhinus longiceps (figs. 509, 582); (2) subgraviportal, Palaeosyops (figs. 509, 545), with. In Mesatirhinus we should expect to find the scapula proportioned somewhat like that in Tapirus; unfor- tunately this scapula is unknown. i'lGUUE 5Ui. Restoration of Eotilanops borealis Modeled by E. S. Christman. About one-twelfth natural size height 345 millimeters, breadth 260; (3) supposed MetarUnus (figs. 509, 576, Am. Mus. 1873, Uinta B), elongate compared with that of Palaeosyops — namely, length 265 millimeters, breadth 165 (estimated). The supposed MetarJiinus may be taken as the mediportal type; it is less expanded toward the upper border. HUMERUS We hare observed (fig. 510) that the characters of the humerus of the titanothere family are rapidly intensified by adaptation. Each genus exhibits dis- tinctive ratios of length of the humerus and radius, as shown in the accompanying table. Measurements of fore limh of certain titanotJieres compared with tapirs, in millimeters Eadio- humeral ratio Metacarpo- humeral ratio Tapirus terrestris Tapirus indicus Eotitanops princeps (borealis), T^m. Mus. 296 Limnohyops? monoconus. Am. Mus. 11689 Palaeosyops leidyi, Am. Mus. 1544 Palaeosyops copei?. Am. Mus. 12205 Dolichorhinus h3'0gnathus, Am. Mus. 13164.. Menodus trigonoceras, Munich Mus Brontops robustus, Yale Mus. 12048 (type).. Brontops sp., Am. Mus. 518 205 250 203 293 325 340? ■315 620 608 528 177 228 228 235 237 '284 520 504 478 77 72 69? 81? 83 82 90 106 120 85 109 113? 106 50 48 41 37 34? 30 240 230 214 38 37 40 It wUl be observed that ia Palaeosyops and DolicJio- rliinus the humerus is much longer than the radius. These are subgraviportal types, but even in the medi- portal Limnohyops the humerus is somewhat longer than the radius. It is, however, a very marked distinction of the titanotheres, already pointed out, that as they become heavier the radius elongates more rapidly than the humerus. Thus the radiohumeral ratio rises from 72 in Palaeosyops leidyi to 90 in Brontotherium gigas. The rugose muscular attachments of the humerus rapidly assume progressive graviportal characters, which even in the lower Bridger enable us to distin- guish readily the titanothere humerus. These char- acters are strengthened in the Oligocene titanotheres and were undoubtedly correlated with certain dis- EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 601 tinctive motions of the fore limbs as follows: (a) The great tuberosity of the humerus expands into a broad, elevated plate; the deltoid crest descends into a dis- tinct tuberosity; the supinator crest secondarily increases in size and prominence. Of the above characters the platelike great tuberosity (fig. 510) is Primitively (Eotitanops iorealis) the deltoid tuber- osity of the humerus is near the upper part of the shaft; secondarily it is extended downward. Prim- itively the great tuberosity is a sessile prominence divided by a shallow median notch; secondarily it becomes very prominent and platelike and is divided fo.intei^v. E4 FiGUKE 508. — Atlas of Eocene titanotheres A, Palaeosyops rohustus, Am. Mus. 1580, upper Bridger; B, Telmatherium ullimum, Am. Mus. 2060 (type), Uinta C; C, Mesatirhinus megarhinus. Am. Mus. 1523, upper Bridger; D, DoUchorhinus sp., Am. Mus. IS44, Uinta C. A-D, Dorsal views. E, Manieoceras manteoceras, Am. Mus. 12204, upper Bridger: Ei, Dorsal view; Ej, ventral view; Ej, anterior view; Ei, posterior view. One-third natural size. The arrows indicate the course of the first spinal nerve and vertebral artery. the most distinctive, (b) Distally the radio-ulnar articulation (rotula and capitellum) is decidedly asymmetric; this asymmetry persists in the titano- theres; its significance is fully explained on page 602. (c) The ectocondylar or supinator crest is already quite prominent, (d) The entocondyle is less prominent. by a very deep median notch. Primitively the ecto- condylar crest, which is small, is distinctly defined, rising somewhat on the shaft (Eotitanops); second- arily it rises and widely expands. A distinctive fea- ture of the humerus as compared with that of the Hyracodontidae and Amynodontidae is the prominent 602 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA rugosity (fig. 500) on the inner side of the shaft for the tendon of the latissimus dorsi muscle. Figure 672 gives a comparison of the humeroradial articulations of Tapirus, Palaeosyops, and Rhinoceros. We observe that the asymmetry of the trochlea and capitellum persists in graviportal types, whereas the cursorial Equus acquires a more symmetrical form. Although this titanothere elbow joint has points of similarity with that of the tapirs, paleotheres, and other primitive perissodactyls, the form as a whole appears to be somewhat distinctive. Viewed from below, the trochlea is much broader than the capitel- lum; the trochlea is nearly plane, the capitellum sharply convex. In the Oligocene titanotheres (fig. 510) the capitellar area widens out so that there is less marked asymmetry with the trochlea. Correlated with this is the asymmetry of the upper end of the radius as seen from the front, the trochlear Figure 509 , Metark: -Types of scapula in middle Eocene titanotheres s, Am. Mus. 1873 (mediportal) ; B, DoUchorhinus sp., Am. Mus. 1833 (mediportal) C, Palaeosyops robusius, Am. Mus. 1680 (subgraviportal). portion being depressed, the capitellar portion ele- vated. A series of comparative views of the proximal end of the radius in several genera (fig. 511) brings this point out clearly. In the primitive ulna {Lambdotherium) the olecranon is decidedly erect and truncate at the top ; it becomes more depressed and pointed in the progressive weight- bearing forms. In Limnohyops and Mesatirhinus is seen an intermediate mediportal condition (fig. 511). In Mesatirhinus it is rounded and moderately rugose at the extremity; in Limnohyops more heavily rugose. In the graviportal forms {Palaeosyops, fig. 511) it be- comes pointed and heavily rugose at the extremity, prophetic of the condition in the heavy Oligocene titanotheres. In Dolichorhinus it is incurved, abbre- viated, and depressed. The shaft of the ulna is of the flattened, trihedral form. This olecranon process is cleft by a faint groove superiorly, which becomes very distinct in the Oligocene forms. Evolution of tJie manus. — The ancestral form of manus and pes is, as shown above, subcursorial. The principles of convergent mediportal and graviportal adaptation, observed in the arches aiid limbs, also dominate the foot structure; the divergence, in fact, is still more conspicuous. On the principles set forth above (pp. 583-584), vertical elongation of every element of the carpus and tarsus as a rule points to speed, while depression and transverse extension point to weight. The general trend of the evolution of the manus and pes in the titanotheres is from the relatively high and narrow subcursorial type {Eotitanops) through an intermediate or mediportal type {Mesatirhinus) into a relatively broad and low graviportal type {Palaeosyops). From the conspicuous adaptive divergences either in the mediportal or the graviportal direction, it is first necessary to select the relatively obscure family or universal characters of the manus and pes. The digital formula is that of all primitive perissodactyls. The manus has four digits (mesaxonic to subparaxonic); the pes has three digits (mesaxonic). One peculiar feature of the titanotheres is that (as in the amynodont rhinoceroses alone among all other perissodactyls) the primitive four digits persist in the manus. The pes in the titanotheres, as in all other perissodactyls, because it is connected with the chief loco- motor organ, the hind limb, is more reduced in size and more progressive in form than the manus. Primitive manus. — The manus of the ear- liest types known {Lambdotherium, Eotitan- ops) is mesaxonic — that is, the third, median digit (D. Ill) is distinctly the largest, as in Eohippus, Heptodon, and Lophiodon. The retention and expansion of the fifth digit (D. V) to share the weight appears to be partly secondary. Adaptive manus. — The titanothere family characters maybe summed up as follows: (1) Persistent tetra- dactylism, with considerable adaptive divergence among the genera in the relative length of the four digits, especially of D. V, which is relatively shorter in some forms {Palaeosyops, Limnohyops) and longer in others {Mesatirhinus, Manteoceras) ; (2) in the front view of the metacarpals, Mtc II, while supporting mainly the trapezoid, articulates broadly against the magnum; Mtc III, while mainly supporting the magnum, also articulates broadly against the unciform; (3) the trapezium is likewise present (in Eocene time) and articulates with the side of the trapezoid, also with Mtc III, and often (especiallj' in Manteoceratinae) with the scaphoid; (4) the proximal phalanges are of EVOLUTION' OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 603 Figure 510. — Types of fore limb in Eocene and Oligocene titanotheres Subcursorial: A, Lambdotherium; slightly less than one-third natural size. Mediportal: B1-B3, Limnohyops; Ei, £2, Mesathhrnu natural size. Subgraviportal: C1-C3, Palaeosyops; Di, X>2, Manteoceras; Fi-Fs, BoUchorhinus; slightly less than one-eigl G1-G5, Brontops; slightly less than one-twelfth natural size. slightly 1( h natural ss than one-eighth size. Graviportal: 604 TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA relatively broad type (mediportal and graviportal), never elongate (cursorial) as in the primitive horses; (5) the median phalanges are short; (6) the distal phalanges expand at the extremities and exhibit deep median clefts, indicating the attachment of broad horny hoofs like those of the rhinoceroses rather than narrow or appressed hoofs of the equine or hyracodont type. The phalanges differ considerably in length and in breadth in different genera. Manus oj the palaeosyopine group. — The palaeosyo- pine group includes three genera, TelmatJierium, Limnohyops, Palaeosyops; the manus of the last two named is known. Of these Limnohyops is more primitive and mediportal in its proportions and articu- lations and is thus analogous to the primitive and mediportal Mesatirhinus of the Dolichorhininae. -Characteristic details of radius and upper Eocene titanotheres ulna in middle and Lower row, proximal end of left ulna, outer view. Middle row, distal end of left radius, front view. Upper row, proximal end of left radius, front view. A, Limnohyops monoconus?; Limnohyops laticeps; C, Palaeosyops cf. tirhinus petersoni: G, Dolichorhinus hyognathus. leidyi; D, Palaeosyops Tobustus; One-sixth natural size. Figure 512 shows how similar these two forms are, yet a very close examination of the details of structure in the carpals and tarsals proves beyond question that they belong in separate subfamilies and diverged from each other at a remote period. The following dry descriptive details are of philo- sophic interest to the comparative anatomist because they demonstrate the assertion made above that we can discover a distinct syngenetic (common origin) character in each element of the carpus when closely examined and compared. They prove that the rela- tively light-limbed and more rapidly moving Limno- hyops retains more of the ancestral form and propor- tion (thus analogous to Mesatirhinus) while the short- footed Palaeosyops diverges most widely in the gravi- portal direction (thus analogous to Manteoceras) . Limnohyops characters. — Of mediportal proportions; mesatipodal; carpus more elevated and compressed than in the subgraviportal Palaeosyops; metacarpals and digits relatively longer than in Palaeosyops but relatively broader than in Mesatirhinus; digit V rather short; scaphoid and cuneiform relatively high; lunar high, resting mainly on unciform, obliquely on mag- num; magnum high, laterally compressed; second phalanges abbreviated; terminal phalanges cleft and spreading distally as in Mesatirhinus. There are two subtypes of Palaeosyops manus. (See figs. 537, 549.) Palaeosyops characters (Am. Mus. 12205). — Of graviportal proportions; brachypodal; carpus broad but less depressed than in Palaeosyops frohustus; digits more abbreviated and spreading than in Palaeo- syops leidyi; trapezium large, with no scaph- oid articulation apparent; magnum relatively high, rather broad, with but five distinct faceted angles; the lunar facet in front view is continuous with the unciform; terminal phalanges irregular, rounded distally. Manus oj the Manteoceras-Dolichorhinus group. — In this subfamily Mesatirhinus is the primitive and mediportal type analogous to Tapirus and Limnohyops, while Manteoceras is the modified subgraviportal type analogous to Palaeosyops, although less extreme. Close comparison of the carpals of Mesatirhinus and Dolichorhinus demonstrates the manteo- ceratine affinity and divergence from the palaeosyopine type. Mesatirhinus. — A mediportal carpus or relatively high, narrow fore foot, all the elements being vertically elongated but less so than in Tapirus terrestris. Mesatipodal; total breadth of carpus of a typical specimen {M. megarhinus) 65 millimeters, total depth 43 ; scaphoid less deep anteroposteriorly than in Palaeosyops, appearing relatively high and narrow; lunar very high, with oblique magnum facet, thus in front view bearing principally on unciform; trapezium narrow, with scaphoid, trapezoid, and Mtc II facets; trapezoid relatively large; magnum high, relatively narrow, and subquadrate, with high facets, hook of magnum more symmetrical, broadly spatulate, with deep median groove; unciform with its longest diameter oblique instead of horizontal, as in Palaeo- syops; Mtc V elongate, manus consequently more tetradactyl; metacarpals decidedly slender but less so than in Tapirus terrestris; distal phalanges elongate, cleft, and broadly expanded distally. Dolichorhinus. — The manus of Dolichorhinus repre- sents a sub-brachypodal specialization of the Mesati- rhinus type, paralleling that of Manteoceras. (See p. 606.) EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 605 Figure 512. — Manus of lower and middle Eocene titanotheres A, LamhdotheTium popoagicum; B, Eotitanops princeps; C, Limnohyops monoconus: D, Palaeosyops leidyi; E, Mante\ F, MesaiirMnus petersoni. One-tliird natural size. , ,, ^si_S^ \j:^ ABC Bi W^ Ci Dr Figure 513.— Comparison of the right scaphoid in middle Eocene Figure 514.— Terminal phalanges of the manus in middle Eocene titanotheres titanotheres and amynodonts Lower row, front view; upper row, top view. A, Palaeosyops leidyi: B, Limnohyops Generic differences in the terminal (ungual) phalanges of the third digit. A, monoconus; C, MesatirUnus petersoni: D, Manieoceras manteoceras. One-half nat- Mesatirhinus petersonit: B, Manteoceras manteoceras: C, Limnohyops monoconus?: jifal gijg_ D, Palaeosyops sp.; E, Amynodon sp. One-halt natural size. 606 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Manteoceras. — A graviportal, or short,- broad foot; digits and metacarpals, however, not spreading apart distally as in Palaeosyops. Brachypodal; de- tailed proportions and facets of carpals resembling a bi'oad, flat face, subquadrate as seen from in front, hook of magnum more symmetrical, broadly spatulate; second phalanges abbreviate; terminal phalanges dis- tinctly abbreviate, expanded distally, with a pro- D Figure 515. — Progressive graviportal adaptation in the pelvis of Eocene and Oligocene titanotheres A, Eotitanops borealis (subcursorial); B, Palaeosyops major (inediportal); C, Manteoceras sp. (subgraviportal); D, Brontotheriumlsp. (graviportal). One-eighth natural size. those of Mesatirhinus rather than those of the Palaeo- syopinae; lunar high, resting chiefly on unciform with a very oblique magnum facet; magnum relatively high and narrow, with five facets in front view, acuminate above, with lunar unciform facet continuous, forming nounced median groove. From the above description it appears that Manteoceras resembles Mesatirhinus in the proportion of the carpals. It differs widely in the abbreviation of the metacarpals and especially of the terminal phalanges. EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES Summary of contrasting cliaracters of the carpus and manus in middle Eocene titanotheres 607 Palaeosyopine group. Manteoceras-Dolichorhinus group Typical genera, Palaeosyops (subgraviportal), Limnohyops (mediportal) Typical genera, Mesatirhinus (mediportal), Manteoceras, Dolichorhinus (sub- graviportai) 1. Carpus broad, composed of relatively broad, low, and deep (anteroposteriorly) elements, correlated with broader spreading digits and spreading or rounded hoofs. 2. Scaphoid broad, deep anteroposteriorly, outer lateral face prolonged backward, radial facet rounded. 3. Cuneiform (Palaeosyops) broad, chief diameter horizontal. 4. Trapezium large, articulating with trapezoid, with Mtc II, but not generally with the scaphoid. 5. Trapezoid relatively large, subquadrate in form in front view. 6. Magnum large, with five to six facets, hook of magnum strongly asymmetrical, pointed. 7. Unciform more horizontal. 8. Metacarpals miore mesaxonic — that is, Mtc III relatively elongate; Mtc V abbreviate. 9. Metacarpals with deep proximal facets for the carpals at the extremities. 10. End phalanges rounded or spreading at the extremities. 1. Carpus composed of relatively -high elements, less deep anteroposteriorly, hoofs truncate and spreading dlstally. 2. Scaphoid relatively high, more shallow anteroposteriorly, radial facet flat. 2a. Lunar high and laterally compressed, resting chiefl.v on unciform. 3. Cuneiform relatively high and laterally compressed. 4. Trapezium large and deep vertically, articulating with trapezoid, with Mtc II, also with scaphoid (Mesatirhinus) . 5. Trapezoid relatively small. 6. Magnum high, relatively narrow, subquadrate, with five facets, hook of magnum more symmetrical. 7. Unciform more oblique. 8. Metacarpals more paraxonio — that is, Mtc V well devel- oped and podium more tetradactyl. 9. Metacarpals with proximal facets for carpals truncated rather than deep, with distal facets less globose. 10. End phalanges broadly expanded distally. The pelvis of the middle and later Eocene titano- theres has passed beyond the mediportal Tapirus stage and is in a stage between the subgraviportal and graviportal, conforming with the principles of the evolution of the pelvis set forth in Chapter IX (pp. 743-745). Although associated and complete pelves are rare, the material available throws a great deal of light on the characteristic form and develop- ment of this important organ. The chief materials and the geologic horizons at which they were collected are as follows: Eotitanops horealis, Am. Mus. 14887 (fig. 501): Wind River A or B. Limnohyops laticeps, Yale Mus. 11000 (fig. 532) : Bridger C or D. Limnohyops ? sp., Am. Mus. 2348 ' (fig. 538) : Washakie A. Palaeosyops frobustus, Princeton Mus. 10232 (fig. 539) : Bridger Cor D. Palaeosyops fmajor, Am. Mus. 13116 (fig. 533): Bridger B. f Manteoceras manteoceras, Am. Mus. 2358 (fig. 555) : Washakie B. Dolichorhinus hyognathus, Am. Mus. 1843 (figs. 579, 580): Uinta B. f Dolichorhinus ?hyognathus, Am. Mus. 1860: Uinta B. Of the above the pelvis of Eotitanops (fig. 501) shows a mingling of subcursorial and prophetic medi- portal characters. The complete associated pelvis of the type of Limnohyops laticeps in the Yale collection belongs to a juvenile individual, which may partly explain the fact that it is in a more primitive stage of development than any other middle Eocene titano- ' Specimen mounted with skeleton of P. leidyi in American Museum. 101959— 29— VOL 1 42 there pelvis known; the superior border of the iliac crest is partly thin and concave, representing the crista iliaca between the rugose borders of the tuber sacrale and tuber coxae. A similar pelvis in a somewhat more advanced stage (fig. 538) is that from Washakie A (Am. Mus. 2348), which has been mounted with the skeleton of Palaeosyops leidyi in the American Museum. This represents a slightly more advanced stage, which nevertheless retains the thin, slightly concave crista iliaca between the rugose areas of the tuber coxae and tuber sacrale. A pelvis certainly belonging to Palaeosyops, from Bridger B (Am. Mus. 13116, fig. 533), is distinguished by the elongate os innominatum and by the uniformly convex superior crest of the ilium. Of this type also is the pelvis (fig. 539) in the Prince- ton Museum (No. 10232), attributable to P. leidyi or P. rohustus, distinguished by a much broader ilium, with a uniformly convex superior border. Referable to Manteoceras is the finely preserved pelvis (Am. Mus. 2358) from Washakie A or B, characterized by very great iliac breadth (fig. 555) and the broad expansion of the supra-iliac border. This is an advanced subgraviportal type, prophetic of the form of the pelvis in the Oligocene titanotheres. The pelvis associated with Dolichor'hinus hyognathus from Uinta B (Am. Mus. 1843) is again elongate (fig. 580), with a uniformly convex superior border of the ilium. The comparative measurements of these pelves are shown in the accompanying table. 608 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Measurements oj pelves of Eocene titanoiheres compared, luitTi tapirs, in millimeters Tapirus indicus 7Palaeo- syops, Princeton Mus. 10232 Palaeo- syops major, Am. Mus. 13116 Limno- hyops laticeps, Yale Mus. 11000 (type) Limno- hyops, Am. Mus. 2348 ?Man- teoceras, Am. Mus. 23S8 Dolicho- rhinus, Am. Mus. 1843 Dolicho- rhinus, Am. Mus. 1860 TDiplaco- don elatus TProtitano- therium Total length of os innommatum.._ Total length of ilium 408 258 169 430 230 100 415 270 190 444 285 168 406 203 440 290 175 500 280 175 430 285 140 530 290 185 143 70 470 300 175 610 335 Total width of pelvis across ilia Total width of iUum (superior °665 265 235 253 340 Anteroposterior pubo-ischiadic Total breadth across ischiadic Breadth of peduncle of ilium 40 60 53 73 65 A comparison of these figures and measurements demonstrates that the main features of the progressive evolution of the titanothere pelvis in graviportal adaptation are the following: 1. Relative expansion of the iliac crest. 2. Reduction of the thin crista iliaca and expansion of the rugose superior borders of the ilium. 3. Relative abbreviation of the os innominatum. 4. Uniform rugose convexity of the superior crest of the ilium. The above are all progressive graviportal characters. It is demonstrated that even as far back as middle Eocene time the early weight-bearing or subgraviportal type of ilium was well established among the titano- theres, and that from the evidence afforded by the ilium alone these animals were heavier bodied and slower moving of limb than the modern tapirs. Our analysis (see below) of the graviportal adaptation in the ilium makes entirely clear the general functional or adaptive stages through which the pelvis of these Eocene titanotheres is passing. We observe that the relatively elongate ilium of Limnohyops is in the first stage, that Palaeosyops with its shorter ilium and heavier body has passed beyond this, and that Manteoceras possesses the fully developed graviportal type of ilium. The principal family characters of the pelvis appear to be as follows: (1) The metapophyses of the posterior lumbar vertebrae articulate with the anterior border of the ilium, as in Equus; (2) in LimnoTiyops the first and second and half of the third sacral vertebrae expand to articulate with the Uium, whereas in Tapirus only one and half of another unite with the ilium; similarly in Manteoceras three sacrals enter into the iliac union; (3) the ilium is much longer than the ischium; (4) the superior border of the ilium is slightly indented (Limnohyops) or uniformly convex {Palaeosyops, Manteoceras); (5) the peduncle of the ilium is broad and short, its dorsal border presenting a sharp ridge; (6) the ischia are not separated or cleft posteriorly as in the Amynodontidae, Rhinocerotidae, and other Perissodactyla; (7) the suprasacral area (tuber sacrale) progressively expands; (8) the rugose borders (tuber sacrale and tuber coxae) expand and unite to obliterate the thin intermediate crista iUaca. ILIUM The ilium of Eocene titanotheres exhibits a con- siderable range of generic characters, as follows: UIUM OF THE PAIAEOSYOPINE GROUP Limnohyops. — Ilia moderately expanded, with in- dented superior border. Palaeosyops. — Os innominatum elongate, iliac crest uniformly convex, expanding into a broad border in Bridger C or D stages. UIUM OF THE MANTEOCEEAS-DOLICHORHINUS GROUP Mesatirhinus. — The fragmentary specimen Am. Mus. 1571 exhibits a slender peduncle of the ischium. The ilium is unknown. Manteoceras. — The pelvis provisionally associated with this genus exhibits a very broad, uniformly convex superior border of the ilium; os innominatum relatively broadening and abbreviated. Dolichorhinus. — Superior borders of ilium mod- erately expanded or narrower than in the supposed Manteoceras; os innominatum moderately elongate. The detailed description of these various types of pelves is given in the succeeding section of this memoir. Materials. — A very large number of femora are preserved, many of which have the tibiae associated. On these it is possible to establish the femorotibial type of the titanotheres, which from the beginning to the end of their evolution is very distinctive. The femur of even the ancestral Eotitanops (fig. 500) displays some of the characteristic titanothere fea- tures, although it retains the flexed knee, distal EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 609 patella, and many of the proportions and characters of its primitive subcursorial ancestors. The femora of the middle Eocene palaeosyopines (Palaeosyops, Limnohyops) and manteoceratines (Mesatirhinus, Manteoceras) are seen from Figure 516 to display many special titanothere characters in common. The chief characters of the femur of the titanotheres are the following, the comparisons shown being made chiefly with the femur of the contemporary Amyno- dontidae, which are animals of the same size. posterior side is recurved, surrounding a deep, sharply characteristic pit. The second trochanter, t", is relatively less prominent than in primitive rhinoc- eroses; the third trochanter, t'", is placed high on the shaft, as in all the Equidae and in the primitive Rhinocerotidae (Hyrachyus) ; in Eocene titanotheres it never extends down to the middle of the shaft as in the progressive Rhinocerotidae; the third trochanter is less extensive than in Hyracodon or Amynodon. 4. The second and third trochanters are more nearly opposite each other, as in the primitive Equi- FiGURE 516. — Femora and tibiae of middle Eocene titanotheres C, Mesatirhinus petersoni; D and E, Manteoceras manteoceras; F, Telmatherium uliimum. One-sixth natural size. 1. The femur is longer than the tibia. This differ- ence is seen in the early mediportal types and increases with the graviportal adaptation. 2. The head of the femur is primitively more spherical but becomes progressively flatter or less spherical. 3. The shaft is rather straight and flat and has a slight forward curvature inferiorly. The great tro- chanter, t' , is not very high, being but slightly raised above the level of the head and relatively smaller and less prominent than in Amynodon. Its inner dae. This is a peculiar and rather characteristic feature. 5. The external patellar ridge is smaller and less produced superiorly; the distal condylar surfaces for the tibia are somewhat flatter; the internal one is larger. A most characteristic feature is the patellar trochlea. In the Palaeosyopinae the knee is straighter; in the Manteoceratinae it is more flexed. In the palaeosyo- pine subgraviportal types (Palaeosyops, Limnohyops) the patellar trochlea (fig. 518) is more vertical and 610 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA thus presents much more anteriorly than in the early rhinoceroses {Amynodon). This indicates that the femur was carried vertically at an early period. In the manteoceratines the patellar trochlea is more distal and oblique; thus in the more light-limbed Mesati- rhinus the patellar facet presents more obliquely downward and forward, and the femur was carried more obliquely, as in the rhinoceroses {Hyrachyus and Amynodon). Even in Manteoceras it was more downward than in Palaeosyops. Distally the internal and external tibial condyles are nearly subequal. In the amynodonts the external condyle is much more prominent, and the corresponding external tibial facet is produced backward. Femoi'otibial ratios. — In all the titanotheres the tibia is much shorter than the femur. This disparity is progressive (tibiofemoral ratio, 77 to 54) in the course of graviportal adaptation. Length of hones of Mnd limbs oj titanotheres and of tapir, in millimeters Tapirus indicus Eotitanops princeps, Am. Mus. 296 (t3'pe)__ Palaeosyops major, Am. Mus. 1316 Palaeosj'ops leidyi, Am. Mus. 1544 (type).. Limnohyops sp., Am. Mus. 11689 Manteoceras manteoceras, Am. Mus. 1587.- Mesatirhinus "petersoni," Am. Mus. 11659. Dolichorhinus hyognathus, Am. Mus. 13164 Menodus trigonoceras, Munich Mus Brontops robustus, Yale Mus. 12048 (type). Brontotherium gigas, Am. Mus. 519 320 250 433 370 355 390? 358 386 770 812 780 258 332 290 285 272 285 120 86? 137 110 111 37 34? 31 30 31 430 448 427 118 119 220 212 200 33 30 28 26 20 Contrasts with Amynodontidae. — The amynodonts are contemporary semiaquatic rhinoceroses. In Amyn- odon the great trochanter is more elevated ; the patellar facet is subhorizontal at the distal extremity of the shaft. TIBIA The tibia (fig. 516) is invariably shorter than the femur, the ratios in the various genera being as in the FiGTiRK 517. — Distal end of the femur in a middle Eocene titanothere and an upper Eocene amyno- dont A, Manteoceras manteoceras; B, Amynodon sp. One-third nat- ural size. above table. So far as the relative abbreviation of the tibia is indicative of speed and weight, Manteoceras is relatively the slowest and Mesatirhinus relatively the swiftest of the middle Eocene titanotheres. Among the special titanothere characters are the following: (1) External tuberosity of cnemial crest more prominent than internal tuberosity; (2) cnemial crest concave superiorly and deeply excavated on the external side; (3) shaft deeply trihedral in midsection and broadly flattened inferiorly. Characters 2 and 3 persist in the Oligocene titanotheres. As compared with the large contemporary Rhinoc- erotoidea (amynodonts and hyracodonts) of middle Eocene time, some of which attained the same size as titanotheres, we observe the following distinctions: (] ) In Palaeosyops the cnemial crest runs more obliquely inward, crossing the shaft; (2) the crest is broader at the summit and does not extend so far down the shaft as in Hyrachyus and Hyracodon; (3) the tibia of Palaeosyops and Manteoceras in proximal and distal views is shallow anteroposteriorly. As compared with the tibia of Amynodon, the titano- there tibia is more robust and lacks the posterior prolongation of the femoral condyle. The fibula was slender (but relatively less so than in Amynodon), subcylindrical, broadly expanded above for the articulation of the posterior inferior surfaces of the external head of the tibia, and closely appressed with the tibia below by a broad articulation, articu- lating broadly also with the astragalus and exhibiting a postero-inferior facet for the calcaneum during the extreme extension of the foot. In the pes, as in the manus, we observe certain syngenetic family characters which are peculiar to all titanotheres but which in various genera are more or less concealed by adaptations to speed or to weight. EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 611 This law of the dominance of teleogenetic (adaptive) over syngenetic (ancestral) character is in force in every single element of the pes as in the manus. For example, the articulations between the astra- galus and calcaneum exhibit both a mediportal type resembling that of tapirs, of light-limbed rhinoceroses, and of paleotheres and a graviportal type resembling that of other graviportal perissodactyls. We are therefore again compelled to examine minor and less cuboid, respectively. In one specimen of Palaeosyops Mts IV abuts against the ectocuneiform, but this is unusual. Similarly Mts III occasionally doe.-i not abut against the cuboid. In the two middle Eocene groups, namely, the Palaeosyopinae and the Manteoceras-Dolicliorhinus group, it is possible by very careful study to discover distinctive generic characters by which we may sepa- rate every bone of the manus and of the pes, although Figure 518.- Angulation of the knee joint: relation of patellar facet to long axis of femur Amynodon; Cae, A, C, D, and E, one-fourth natural size; B, one-half natural size. Lm, LimnoJiyops; Hy, Hyrachyus; Ms, MesatiThinus; A Caenopus. conspicuous characters in order to discover the real syngenetic family resemblance to be found in each element.^" Among family characters of the pes of the titano- theres are the following: 1. The small fibulocalcaneal facet, as in the Equidae, the fibula barely passing upon the calcaneum in the extreme extension of the foot. A median pit on the astragalus checks the flexion of the tibia by receiving its posterior process. 2. The entocuneiform is very large, articulating on the inner side of the mesocuneiform, of Mts II, and of the navicular, and freely projecting backward from the pes like a pisiform. 3. The mesocuneiform is invariably a very small bone as in primitive mammals. 4. The ectocuneiform is a large element. 5. Mts II and III typically abut on the outer proximal facets against the ectocuneiform and the '• The significance of the astragalocalcaneal facets as family characters in Perisso- dactyla was first pointed out by Osborn in the article "Evolution of the ungulate foot" (Scott and Osborn, 1890.1, pp. 531-569). through convergence and inheritance the bones are often brought to resemble each other closely. The h<^ Figure 519. — Inner side view of left fibula A, Palaeosyops sp.; B, Limnohyops monoconusf ; C, Brontotherium leidyl. One-sixth natural size. chief subfamily distinctions in typical genera are stated on the following page. 612 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA Contrast in features of tJie pes in the middle Eocene groups I. Palacosyopine group: Palaeosyops, Limnohyops 1. Tarsus broad, composed of anteroposteriorly deep elements, correlated with more or less spreading digits. 2. Calcaneum with somewhat rounded or oval shaft of tuber calcis moderately expanded at the summit. 3. Cuboid relatively broad. 4. Proximal facets on the metatarsals deeply extended antero- posteriorlv. II. Manteoceras-Dolichorhinus group: Manteoceras, Mesatirhinus, Dolichorhinus 1. Tarsus more shallow anteroposteriorly; more elevated in all its elements. Digits less spreading distally. 2. Calcaneum with laterally compressed and distally truncate tuber calcis. 3. Cuboid high, relatively narrow. 4. Metatarsals with proximal facets relatively shallow. Distal facets more flattened. PES OF THE PALAEOSYOPINE GEOTJP Limnohyops (figs. 520, 530). — The adaptive charac- ters of the pes of this animal, like those of the manus, are primitive, mediportal, and mesatipodal. Digits broader than in Mesatirhinus and stouter and heavier than in Tapirus — that is, of less swift type. Astraga- lus with elongate neck and vertically elongate sus- tentacular facet, unlike the rounded facet of Palaeo- syops. Calcaneum with tuber deep and expanding at summit. Cuboid deep (shallow in Palaeosyops). Entocuneiform very large. Mesocuneiform very small. Ectocuneiform very large, quadrilateral (not vertically extended) as in Mesatirhinus. Metatarsals of medium length; Mts III abutting against cuboid, Mts II abutting against ectocuneiform. Terminal phalanges cleft and expanding distally somewhat as in Mesati- rhinus but more robust. Palaeosyops. — The most robust and graviportal pes (figs. 520, 540) known among Eocene titanotheres. Digits stout and widely spreading; prominent muscu- lar rugosities indicating powerful flexor and extensor attachments; astragalus readily distinguished by its abbreviated neck, broad, shallow tibial trochlea, and especially by the large and oval sustentacular facet for the calcaneum, as well as by the broad cuboid facet; calcaneum with obliquely placed tuber calcis and ex- panded summit. Cuboid in two specimens very short, thus Mts IV articulating with ectocuneiform; the broad, abbreviated cuboid apparently a constant character. Other 'elements of tarsus relatively broad and low, deep anteroposteriorly; Metatarsals broad and stout, broadening at the lower extremities; ectocuneiform large and triangular; mesocuneiform very small, with rounded edges; entocuneiform large, of irregular shape. Terminal phalanges obtusely rounded, very distinctive. In the passage from Limnohyops to Palaeosyops we again note the transition from a rather heavy medi- portal type, of the proportions of the tapir, to a much heavier (subgraviportal) type with broad, spreading feet adapted to supporting the animal in swampy ground; the reduction of the terminal phalanges points to reduced hoofs, a possible amphibious adaptation. PES OF THE MANTEOCEEAS-DOIICHOKHINlrS GROUP As in the manus, we may distinguish the pes by the relatively high and narrow proportions of the podial elements. The mediportal Mesatirhinus presents a wide contrast with the subgraviportal Manteoceras. Mesatirhinus. — This mediportal pes (figs. 520, 569, 572) is readily distinguished by its relatively high and narrow proportions. The angles and facets are all sharply defined and sculptured. The bones of the second row of tarsals and proximal facets of meta- tarsals deep anteroposteriorly. Astragalus with elon- gate neck, narrow, vertically produced sustentacular facet for calcaneum; a narrow cuboidal facet. Cal- caneum very distinctive, with its deep, laterally compressed tuber and truncate distal extremity. Navicular relatively deep and narrow. Cuboid, also high, narrow and compressed in median line. Meso- cuneiform and ectocuneiform distinguished by their elevated and sharply quadrate form. Manteoceras (fig. 557). — Astragalus only known. Tarsus apparently of the same type as in Mesatirhinus but relatively broader, judging from the astragalus. Astragalus with a sharply defined tibial trochlea, subelongate neck, vertically elongate, straight-sided sustentacular facet for the calcaneum, of same type as in Mesatirhinus but relatively broader; a broad cuboidal facet (unlike Mesatirhinus). Dolichorhinus (PI. XXXI). — Mesatipodal — that is, metapodials relatively abbreviated, as in Manteoceras. A full description appears upon a subsequent page. Comparing these three forms with respect to the' elongation of the metapodials, Mesatirhinus is mesati- podal, Dolichorhinus is stiU mesatipodal but transi- tional, whde Manteoceras approaches the brachypodal condition. Another type of more elongated manteo- ceratine foot (Am. Mus. 2352) has been discovered and is described in full below. It is omitted here because its generic association is somewhat doubtful. SYSTEMATIC DESCRIPTIONS OF MIDDLE EOCENE TITANOTHERES SUBFAMILY PALAEOSYOPINAE Mediportal and progressively graviportal titano- theres of the lower and upper Bridger deposits and the lower deposits of the Washakie Basin. Feet mesa- tipodal to brachypodal. Ungual phalanges truncate to rounded. Astragalus progressively widening. Limnohyops The skeleton of Limnohyops, so far as laiown, is readily distinguished in all its parts from that of Palaeosyops by its mediportal type — that is, by its lighter construction. Yet the body was heavier and EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 613 A--V ! Figure 520. — Comparison of pes in four species of middle Eocene titanotheres A, Limnohyops monocorms; B, Palaeosyops leiiyi: C, Mesatirhinus petersoni; T>, Mesatirliinust (Seep. 643.) One-tbird natural size. The pes of TelmatJierium is unknown except in the upper Eocene T. ultimwrri' that of Manteoceras is linown only from the astragalus. 614 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA the motions were slower than in either the American or Asiatic species of Tapirus. Palaeosyops entered a graviportal line of evolution, but Limnohyops was more conservative. Its feet become mesatipodal Limnohyops? monoconus, Am. Mus. 11699 (manus, radius, and ulna, figs. 525 B, 527) ; Bridger B 2. Lhnnohyo-ps? monoconus, Am. Mus. 11690 (complete hind limb, fig. 529); Bridger B 1. Measurements of limb hones of Limnohyops laiiceps and L. monoconus, in millimeters L. lati- ceps, Yale Mus. 11000 (type) Humerus, length Radius, length I 230 Radius, breadth, I proximal I 53 Radius, breadth. distal "Ulna, length Carpus, width Mtc II, height Mtc III, height Mtc III, distal width (maximum) Mtc IV, height Mtc V, height Femur, length Tibia, length 60 307 L. ?monoconus Am. Mus. 11689, Bridger B2 295 230 58 55 312 "75 99 109 33 357 285 Am. Mus. 11690, Bridger Bl 55 308 103 34 97 79 387 297 Figure 521. — Astragali of Eocene titanotheres A, Lambdother'mm popoagkum; B, EotUanops borealis; C, Limnohyops monoconus: D, Palaeosyops robustiis; E, Manteoceras manteoceras; ¥ and G, Mesatirhinus petersoni; H, Metarhinus cf. M. earlei; I, DoUchorhinus hyognaikus. Ai, Bi, etc., front view; A2, B2, etc., back view. Astragalc- calcanealfacets: ectal (ed), sustentacuiar (stw), and inferior (in/); astragalon»vicalar facet (m); astragalocuboidal facet (c6). One-third natural size. rather than brachypodal, and the limbs throughout are narrower and more slender than those of Palaeo- syops. The hand and foot were relatively elongate — that is, they were appressed rather than spreading. The ungual phalanges are truncate, expanding dis- tally, rather than rounded and obtuse, as in Palaeo- syops. These animals present many adaptive resemblances to Mesatirhinus in the manteoceratine subfamily, yet so far as known the parts are somewhat heavier and more robust throughout. The materials of Limnohyops are as follows: Limnohyops laticeps, Yale Mus. 11000 (type skuU and parts of skeleton, PL LVII, figs. 261 B, 264, 531, 532); Bridger D (?). Limnohyops? monoconus, Am. Mus. 11689 (vertebrae, fore and hind limbs, figs. 525 A, 530) ; Bridger B 2. Two skulls of Limnohyops have been found in Bridger B, namely, L. prisons and L. monoconus, the latter an animal of consider- able size. Parts of three skeletons in the American Museum collection (Nos. 11689, 11699, 11690) belong, possibly, to L. mono- conus. The generic reference to Limnohyops is well assured; the specific reference is doubtful. The ground for the generic refer- ence of the skeletal specimens to Limnohyops is their agreement in many details with the correspond- ing bones of the type skeleton of L. laticeps, namely, (1) the distal end of Mtc III of the type agrees with Mtc III of Am. Mus. 11699; (2) the radius of Am. Mus. 11689 agrees in length and in details of character with the radius of the type; (3) the ulna of Am. Mus. 11689 agrees closely with that of the type of L. laticeps, especially in the rugose area of the olecranon. Limnohyops monoconus? Incomplete skeleton from Bridger B 2, Grizzly Buttes, Bridger Basin, Wye, Am. Mus. 11689 The material includes the atlas, a cervical, several anterior dorsal vertebrae, the humerus, radii, ulnae, incomplete manus and pes, femur, tibia (figs. 510,. EVOLUTION OF THE SKELETON OF EOCENE AND QLIGGCENE TITANOTHERES 615 Figure 522. — Calcanea of Eocene titanotheres A, Lambdotkerium popogagicum; B, Eotitanops borealis; C, Livinohyops monoco- nusf; D, Palaeosyops robustus; E, Mesatirhinus peter sonif; F, Metarhinus cf. M. earlei; G, f M esatirhinus sp. (Washakie). Astragalocalcaneal facets: ectal (ect), sustentacular (sus), and inferior (inf). One-third natural size. Figure 524. — Principal measurements of the carpus and tarsus A, Tarsus, transverse and vertical; astragalus, vertical (interior and median); calca- neum, vertical; metatarsal III, vertical and transverse, maximum v/idth near distal end. B, Carpus, transverse and vertical; lunar, transverse and vertical; metacarpal III, height and maximum width near distal end. C Figure 523. — Left ectocuneiform tarsi of lower and middle Eocene titanotheres A, Eotitanops borcaUs; B, Mesatirhinus petersoni; C, Limnohyops monoconus; D, Palaeosyops rohustus. Facets for second metatarsal (II), third metatarsal (III), navicular (nf), cuboid (c&).and mesocuneiform (cn^). One-half natmal size. Figure 525. — Humerus, radius, and ulna of Limnohyops monoconus? Am. Mus. 11689. Ai Left humerus, radius, and ulna, outer side view; A2, left radius, and As, left humerus, both front view; B, right humerus, radius, andulna, the last two partly restored from Am. Mus. )1699, front view. One-sixth natural size. (Compare fig. 527.) 616 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 511, 513, 514, 520-522, 685). Measurements of the limb bones are given above. Adaptive distinctions from Palaeosyops. — This mate- rial enables us to further distinguish LimnoTiyops from Palaeosyops. Manus and pes mesatipodal rather than brachypodal; all limb bones more slender; humerus relatively shorter; ulna more curved, with distinctive olecranon process; manus narrow; lunar Figure 526. — Left manus, radius, and ulna of Mesatirhinus pelenioni (doubtfully referred) Princeton Mus. 10013, upper Bridger, placed here for com- parison with the manus of Limnohyops (fig. 527). Aj, Front view of manus; As. top view of carpus; A3, inner side view of carpus; Bi, distal view of radius and ulna; Ba, proximal view of radius. One-third natural size. more acutely wedge-shaped distally. Magnum not so wide; metacarpals more slender, femur with more slender, curved shaft. Astragalus more slender, with narrow sustentacular and cuboidal facets; cal- caneum with narrow sustentaculum; metatarsals narrow, Mts V more curved, metatarsals not expand- ing distally; ungual phalanges truncate and square distally. Palaeosyopine syngenetic characters. — Among the more obscurely but syngenetically important char- acters tending to ally this animal to Palaeosyops are an astragalus having in common the following peculiarities: A pit for a ligament on the internal or tibial face just below the trochlear keel, a rather sharp extension of the superior edge of the navicular facet, a prominent protuberance near the distal end of the tibial face, an inward projection of a sinus or fossa tending to separate the sustentacular facet from the well-developed facet for the tibial sesamoid. Other resemblances with the Palaeosyopinae are seen in the subglobose shape of the distal facets of the metapodials, in the marked anteroposterior depth of the scaphoradial and other carpal facets, in the depth of the proximal metapodial facets, and in the femur with patellar facet facing anteriorly. Distinctions from the Manteoceras-Dolichorhinus group. — Limnohyops is separated from Mesatirhinus by the following characters: Limb bones of more graviportal type; humerus longer, radius stouter, more curved; ulna more curved and with larger olecranon; manus somewhat broader and shorter throughout; femur flatter, femur and tibia a little stouter but of about the same relative length; astra- galus broader, sustentacular facet farther in toward the middle of the bone; sustentacular facet broader, not so straight sided, forming a much more open angle with the navicular facet, inferior astragalocalcaneal facet smaller; a pit on the internal or tibial face of the astragalus just below the trochlear keel; metatarsals a little shorter and broader. Notwithstanding these differences there are many general adaptive resem- blances to Mesatirhinus, especially in the femur, tibia, and metatarsals. The differences, however, appear to indicate generic separation. Limnohyops is analogous to Manteoceras, especially in the general characters and proportions of the Figure 527. — Manus, radius, and ulna of Limnohyops mono- conus (doubtfully referred) Am. Mus. 11699, Bridger B 2. Ai, Right manus, front view; A2, right carpus, out- side view; As, phalanges of median digit; At, distal view of radius and ulna. One- third natural size. (Compare fig. 525.) humerus, tibia, astragalus, and pes — so much so, in fact, that it was formerly referred provisionally to that genus, from which, however, it is distinguished, so far as known, by the following characters: Scaph- oid deeper anteroposteriorly, trapezoid facet more EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 617 oblique, magnum facet narrower, lunar facet narrower; distal or phalangal facets of metacarpals and metatar- sals more globose, less flattened, proximal facets of same widely truncate posteriorly; tibia relatively longer with narrower proximal end; astragalus with narrower cuboid facet, navicular facet not so deep anteroposteriorly, inner keel of astragalotibial trochlea sharper, prominence on the distal part of the inner or tibial surface not ending in a budlike tuberosity, sustentacular facet less straight-sided. Forearm and manus from Bridger B 2, Grizzly Buttes west, Bridger Basin, Wye; Am. Mus. 11699. Limnohyops ref. .Figure 527. (For measurements see above) This well-preserved specimen resembles in general appearance the preceding one (Am. Mus. 11689) but differs in the following particulars: The radius is a little more slender; the olecranon of the ulna is deeper; the manus is smaller but agrees in so many close details with the other manus (Am. Mus. 11689) that there can scarcely be any doubt that the two belong to the same genus — namely, Limnohyops. The manus accordingly offers some further points of contrast with that of Manteoceras — namely, in the magnum the front face is not so sharply polyg- onal, the posterosuperior head or eminence is much narrower and extends obliquely backward and down- ward, whereas that of Manteoceras is subtruncate pos- teriorly and sharply ridged superiorly; the posterior hook of the magnum is much more slender and ends postero-inferiorly in a bluntly oval pitted surface; the facet for the third digit, Mtc III, is deeper antero- posteriorly and narrower posteriorly, that of Manteo- ceras being subrectangular in general outline; the unciform is narrower transversely and deeper verti- cally, its supero-external or cuneiform facet is less extensive transversely and narrower externally, it is less concave in front view, its supero-internal or lunar facet is less wide, especially posteriorly, all the ridges between adjacent facets are less angulate; the postero- external protuberance of the unciform is much more pointed posteriorly; the cuneiform carpi are much narrower transversely than in Manteoceras and reveal many striking detailed differences. Some, or even most, of the foregoing characters of the manus in question may be primitive characters which have been lost in Manteoceras in adaptation to the progres- sive broadening of the manus; but if the manus in question belonged to a direct ancestor of Manteo- ceras manteoceras of the succeeding horizon we should expect it to foreshadow that form a little more defi- nitely, and the very well-marked differences indicate again that it belongs to some other genus. Analogy to Mesatirhinus. — As compared with Mesa- iirJiinus (fig. 526) this manus exhibits a rather strik- ing general resemblance; but the carpals are broader, the scaphomagnum articulation is more oblique, the metacarpals are broader, and the ungual phalanges are larger and not so sharply flaring and truncate distally. Conclusions. — This manus, although it is more slender than that of Palaeosyops, shares with it certain palaeosyopine characters in which it contrasts with Mesatirhinus and Manteoceras, such as the greater anteroposterior depth of the scaphoid, the angular antero-internal border of the cuneiform, the pointed rather than spatulate hook of the magnum, the broader carpals, metacarpals, and phalanges. Hind limb from Bridger B 1, lower Cottonwood Creek, Bridger Basin, Wyo.; Am. Mus. 11690. Limnoiiyops ref . Figures 516, 518, 519, 523, 686. (For measurements see above) The material includes only the rather well pre- served left hind limb, namely, the femur, patella, tibia, fibula, and pes. As compared with the hind limb of Am. Mus. 11689, described above, the present specimen differs in its larger size and somewhat stouter proportions, especially of the femur; the metatarsals are noticeably longer and have straighter sides; in the astragalus the sustentacular facet ex- tends a little further dor- sad, the ectal facet is less deeply concave, the distal calcaneal facet is larger, and corresponding differ- ences occur in the calcane- um. These differences are accompanied by so many detailed resemblances (in contrast with other genera) that it appears probable that the hind limb in question belongs in the same genus (Limnohy- ops) with Am. Mus. 1 1689, 11699, above described, though possibly to a differ- ent species of that genus. As compared with other Palaeosyopinae the present specimen offers many resemblances and a few rather decisive differences: the femur, though smaller than that of P. major, does not differ greatly in its propor- tions and offers no clear-cut distinctions; the pes is noticeably higher and more slender; the astragalus is narrower with a narrower neck, more straight- sided sustentacular facet, narrower cuboid facet and sharper internal keel on the trochlea. Distinctions from Manteoceras-Dol ichorhinus group. — From Mesatirhinus the specimen differs in its larger size, more robust femur, tibia, and pes, distally diverging vertical sides of Mts III. The astragalus is wider, its ectal facet shallower and not produced downward at the lower internal corner, the susten- tacular facet was probably broader, not so straight- FiGURE 528. — Right scaphoid of Palaeosyops sp. (A) and Limnohyops monoconus (B) Front (Ai, Bi) and upper (As, B2) sur faces. One-half natural size. 618 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA sided, and it formed a more open angle with the cuboid facet; the calcanea show corresponding differ- ences. But these differences are accompanied by many general resemblances that indicate the annec- tent or primitive character of the pes in question. Comparison with Manteoceras is made difficult by the scantiness of the material referable to that genus. The femur is of nearly the same length as in the young male Manteoceras (Am. Mus. 1587), but the third trochanter Figure 529. — Left hind limb of Limnohyops monoconus (doubt- fully referred) Am. Mus. 11690; Bridger B 1. A', Front view; A^, outer side view. One-sixth natural size. seems smaller; the tibia is absolutely and relatively longer, its proximal end is narrower, its distal end is broader; the patellar facet is decidedly longer and more anterior than in Manteoceras. The astragalus, as compared with that of the young Manteoceras (Am. Mus. 1587), offers the following differences: The inter- nal trochlear ridge is angulate rather than broadly con- vex; the ectal facet is shallower, less produced on its lower internal border; the sustentacular facet is much longer: the cuboid facet is decidedly smaller; the navic- ular facet not so deep anteroposteriorly. As compared with the astragalus of the old female Manteoceras? (Am. Mus. 12204) most of the above-described differ- ences also hold good, save that the siz(3 in Am. Mus. 11690 is considerably greater and the navicular facet seems proportion- ately deeper rather than shallower. Conclusion. — In brief, the hind limb under consideration appears to belong neither with Palaeosyops, Mesatirhi- nus, nor Manteoceras. On the other hand, it appears not to differ gen- erically from Am. Mus. 11689, described above, and together with that specimen it may repre- sent a Bridger B mem- ber of the Limnoliyops phylum. Limnohyops laticeps Many parts of the skeleton of i. laticeps are fortunately preserved with the type skull in the Yale Museum (No. 11000). The geologic level is probably Bridger C or D, at Henrys Fork Hill, Bridger Basin, Wyo . Cervicals. — The axis and four other cervicals (C. 3, C. 5, C. 6, C. 7) are preserved; the fourth cervical is missing. The epiphyses are mostly detached, indicat- ing a juvenile condition. The axis (C. 2) and third vertebra (C. 3) especially have large inferior keels or hypapophyses. The third to sixth cervicals exhibit broadly depressed inferior lamellae. The seventh (C. 7) is irnper forate exhibiting narrow, rodlike transverse processes with- out inferior lamellae. Dorsals. — The median dorsals exhibit centra angulate but not keeled inferiorly. In one of the posterior dorsals the cen- trum is slightly keeled and the spine is elevated (112 mm.) above the base of the centrum. Lumhars. — The lumbars preserved are apparently the second and the fourth (L. 2, L. 4); the first, third Figure 530. — Right pes of lAm- nohyops monoconus? .\m. Mus. 11689; Bridger B 2. Ai, Front view; A2, outer side view of tarsus; A3 phalanges of median digit. One-third natural size. FiGuiiE 531. — Ventral surface of sacrum of Limnohyops laticeps Yale Mus. 11000 (type); Bridger C or D. One-third natural size. EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 619 and fifth lumbars are missing. They exhibit increas- ingly broad centra and moderately broad transverse processes. The zygapophyses are vertically placed, and unlike those of Palaeosyops are slightly if at all revolute. Sacrals. — The most distinctive character of the sacrals is that the sacrum includes five vertebrae, the fifth (S. 1) being due to the coalescence of an anterior caudal. They measure 171 millimeters anteropos- teriorly. The first and second sacrals and the anterior portion of a third sacral enter into union with the ilium. Pelvis. — The innominate bones are preserved almost entire (fig. 532). The left os innominatum measures 406 millimeters anteroposteriorly. The crest of the ilium measures 203 millimeters transversely; the superior border is thin and indented in the median portion or crista iliaca. The rugose tuber sacrale is narrower (80 mm.) than the rugose tuber coxae (140 mm.). The elongate proportions of the innomi- nate bones are partly due to the juvenile nature of this individual. Fore limb oj type. — The proximal and distal portions of the right humerus are preserved. The head and Figure 532. — Right os innominatum of Limno- hyops laticeps Yale Mus. 11000 (type); Bridger C or D. One-.sixth natural size. greater tuberosity measure 122 milHmeters (antero- posterior); the head measures 65 (transverse); there is a wide and deep bicipital groove ; the proportions are somewhat altered by crushing; the distal end of the humerus measures 68 (transverse) ; the total width of the distal articular surface is 55. The radius and ulna are complete. The radius is distinguished posteriorly by a characteristic median groove; it is 230 millimeters long; its humeral facets measure 53 millimeters transversely and 27 antero- posteriorly. The ulna is 307 millimeters in length; its distinctive character (see fig. 511) is the downward extension on the outer side of the olecranon of the rugose crest for the triceps muscle. Palaeosyops The Eocene titanothere skeleton is best known in members of the genus Palaeosyops, which was the largest and the most heavily built titanothere of that time, adapted to slow locomotion on soft ground along water borders and in marshes and swamps. We have the nearly complete skeleton of P. leidyi from the upper Bridger and parts of the more progressive P. rohustus and of the ancestral P. major. A titanothere family likeness is seen throughout the axial and appendicular parts, but generic and subfamily differ- ences, closely shared with Limnohyops, are apparent throughout. Up to the end of the lower Bridger deposition we may safely select the largest and most massive titano- FiGUKE 533. — Pelvis of Palaeosyops major Am. Mus. 13116; Bridger B 3. A, Ventral aspect: B, dorso-superior aspect. One-sixth natural size. there bones as belonging to Palaeosyops. The gravi- portal tendency is especially manifest in the hind limb, with its elongate femur and abbreviate tibia and slight angulation at the knee joint. The broad and spreading foot bones of both manus and pes are especially distinctive. The manus is functionally tridactyl rather than tetradactyl, owing to the reduced condition of the fifth digit, which is set apart from the other digits in a manner peculiar to this genus. The 620 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA /! terminal phalanges are obtusely rounded and not deeply cleft distally, indicating the presence of imperfect hoofs. Additional evi- dence of water-loving or semiaquatic habits is found in the weak spines of the dorsal and lumbar vertebrae, which are analogous to those in Coryphodon and Metamynodon. The dentition points to habits of feeding on the succulent plants which are characteristic of the borders of streams. The skeleton as a whole shows far more aquatic adaptations than that of the tapirs. Palaeosyops major Palaeosyops major is represented by some well-preserved limb bones and a pelvis associated with the large skull (Am. Mus. 13116) from Bridger B 3. The large size of these bones, coming as they do from so low a level, is very noticeable. They even exceed in total length those of P. leidyi (Am. Mus. 1544), but the long bones are more slender. The most characteristic feature of the pelvis (fig. 533) is the narrowness and ap- parently uniform convexity of the superior or anterior border of the ilium as compared with its expansion in subsequent stages of the develop- ment of the titanotheres. This nar- rowness is a primitive character, cor- responding with the low geologic level (Bridger B 3) at which this specimen was found. Other chief generic characters exhibited in this skeleton are the following: Radius (fig. 510) strongly arched forward and having a deep groove for the ex- tensor carpi radialis muscle; ulna (fig. 511) without the incurved olecranon of MesatirJii- nus; femur with a straight, flattened shaft, and a pa- tellar groove pre- senting anteriorly ; tibia with an out- curved cnemial crest. The short- necked astragalus and the stout cal- caneum are like- wise of Palaeo- syops type. The detailed measurements of this important skeleton as compared with the com posite P. leidyi (fig. 536) are as follows: