EX LIBRIS 
 
 NATHANIEL SOUTHGATE SHALER 
 
 LIBRARY 
 
 THE UNIVERSITY 
 
 OF CALIFORNIA 
 
 SANTA BARBARA 
 
 PRESENTED BY 
 
 PRESTON CLOUD
 
 Percy E. Raymond.
 
 ELEMENTARY 
 
 PALEONTOLOGY
 
 tLonHon : C. J. CLAY AND SONS, 
 
 CAMBRIDGE UNIVERSITY PRESS WAREHOUSE, 
 
 AVE MARIA LANE. 
 
 rics': DEIGHTON, BELL AND CO. 
 leipjig: F. A. BROCKHAUS. 
 fo $ork: MACMILLAN AND CO.
 
 ELEMENTARY 
 
 PALAEONTOLOGY 
 
 FOB GEOLOGICAL STUDENTS 
 
 BY 
 
 HENRY WOODS, B.A., F.G.S. 
 
 CAMBRIDGE: 
 
 AT THE UNIVERSITY PRESS. 
 1893 
 
 [All Rights reserved.]
 
 ambrtoge: 
 
 PRINTED BY C. J. CLAY, M.A. AND SONS, 
 AT THE UNIVERSITY PRESS.
 
 PREFACE. 
 
 TN the following pages I have endeavoured to give an 
 A elementary account of Invertebrate Palaeontology, 
 adapted to the requirements of geological students. 
 With this object in view, I have devoted the greater part 
 of the space at my disposal to a consideration of those 
 groups of fossil animals which are most useful to the 
 stratigraphist, and have treated very briefly, those which 
 are of interest mainly to the zoologist. My plan has been 
 to give, in each group, first an account of its general 
 zoological features with a full description of the hard 
 parts, secondly the classification and characters of those 
 genera which are important geologically, and thirdly a 
 sketch of the present and past distribution of the group. 
 
 Since Palaeontology can be learnt only by students 
 who have access to a collection of fossils, I have purposely 
 refrained from using figures to illustrate genera, and have 
 confined myself to those required to explain structure and 
 terminology.
 
 vi PREFACE. 
 
 For the benefit of those who may wish to obtain a 
 more detailed knowledge of the subject, or of any special 
 part of it, I have given at the end of the volume a list of 
 some of the more important and easily accessible palaeon- 
 tological works. 
 
 I gratefully acknowledge my indebtedness to Professor 
 McKenny Hughes and Dr G. J. Hinde for advice and 
 help. 
 
 H. WOODS. 
 
 ST JOHN'S COLLEGE, CAMBRIDGE, 
 August, 1893.
 
 CONTENTS. 
 
 GE 
 
 INTRODUCTION ...... j 
 
 PROTOZOA ..... 14 
 
 PORIPERA . 
 
 zb 
 
 COSLENTERATA . QQ 
 
 Jo 
 
 ECHINODERMATA 
 
 bo 
 
 VERMES . 
 
 yo 
 
 MOLLUSCOIDEA .....' JQJ 
 
 MOLLUSCA. . I( ,E 
 
 . . 1^0 
 
 ARTHROPODA , CQ 
 
 lOt> 
 
 LIST OF PAL^ONTOLOGICAL WORKS. . 211 
 
 INDEX .217
 
 
 INTRODUCTION. 
 
 Conditions necessary for the preservation of animals and plants as 
 fossils. Structure and composition of the hard parts of animals 
 and plants. Modes of fossilisation. Uses of fossils in geology. 
 Imperfection of the palseontological record. Distribution of fossils. 
 Classification. 
 
 THE fact that bodies resembling marine animals occur 
 embedded in the rocks has been known from the earliest 
 times. But for several centuries there were two views 
 held respecting their nature. By some persons they 
 were thought to have once formed part of living 
 animals and consequently to indicate that the spot 
 where they now occur was in past ages covered by 
 the sea. Others, feeling it difficult to account for so 
 much geographical change, considered that they were 
 not of organic origin at all, but had been formed by 
 some 'plastic force' within the earth, that they were in 
 fact ' Sports of Nature.' But since these bodies resemble 
 in every essential respect those now living, and since we 
 can conceive of no other means by which they could have 
 been formed except by animals, we may at once reject 
 this hypothesis. 
 
 The remains of animals and plants thus preserved in 
 the rocks are known as fossils, and their study forms the 
 subject of Palaeontology. The majority of fossils belong 
 w. p. 1
 
 2 CONDITIONS FOR FOSSILISATION. 
 
 to species which are not found living at the present day, 
 but at the same time a fossil is not necessarily now an 
 extinct form, thus in some of the later Cainozoic formations 
 as many as 90 per cent, of the species of mollusks are still 
 living. On the other hand an extinct animal or plant is 
 not always a fossil, as for instance in the case of Steller's 
 Sea-cow, Rhytina gigas, which formerly lived in the seas 
 off Kamtschatka, Alaska, and the Kurile Islands, but of 
 which no remains are found in the rocks. 
 
 In order that an animal or plant may be preserved as 
 a fossil, two conditions are generally necessary: First, it 
 must possess a skeleton of some kind or other, since the 
 soft parts rapidly decompose, consequently such animals 
 as jelly-fishes can leave no trace of their existence, unless 
 it be the imprint of their exterior. Second, it must be 
 covered up by a deposit, otherwise when exposed to the 
 air it will soon crumble to dust. Now, since there are 
 comparatively few places on land where material is being 
 deposited to any extent, it follows that terrestrial animals 
 will stand but little chance of being preserved, the greater 
 number after death will remain on the surface, and, if not 
 devoured by other animals, will in a short time entirely 
 decompose. A few, however, may become entombed in 
 peat-bogs, in the dust and ashes of volcanoes, in the sand 
 of sand-dunes, or by the falling of a landslip ; some will 
 be sealed up in deposits of carbonate of lime, such as the 
 travertine thrown down by calcareous springs, or the sta- 
 lagmite formed on the floor of caves ; and lastly, others 
 may be transported by running water and ultimately 
 buried in the bed of a river, of a lake, or of the sea. But 
 all these are of comparatively rare occurrence. With 
 aquatic animals the conditions are much more favourable, 
 since deposition is more universal in water than on laud.
 
 COMPOSITION OF THE SKELETON. 3 
 
 But of the aqueous deposits, those laid down in the sea 
 will enclose by far the larger number of animals on ac- 
 count of the greater area which they cover. 
 
 The structure and composition of the hard parts vary 
 considerably in different groups of animals and plants, 
 and as a result some are much more readily preserved as 
 fossils than others. Thus in some, like Argonauta, the 
 skeleton consists of a thin shell which is easily broken up; 
 then again in some sponges it is formed of needles of 
 silica which are held together by the soft parts only, and 
 consequently become easily scattered after the death of 
 the animal. But in other cases, as in most of the mollusks 
 and corals, the skeleton is very strong and not easily 
 broken up, and consequently these are abundant as fossils. 
 Perhaps even more important than the structure, is the 
 composition of the hard parts. These, in the case of 
 insects and some hydrozoans consist of a horny substance 
 known as chitin; in diatoms, in most radiolarians, and in 
 many sponges, of silica ; in the bones of vertebrates chiefly 
 of phosphate of lime ; in corals, echinoderms, mollusks and 
 many other animals -and some plants, of carbonate of lime ; 
 in most plants of woody or corky tissue. A larger or 
 smaller amount of organic matter is always combined with 
 the mineral. Of these substances, chitin is with difficulty 
 dissolved. Silica in its ordinary crystalline condition is 
 one of the most stable minerals, but when formed or- 
 ganically it is glassy and isotropic, and is dissolved with 
 comparative ease ; so that such skeletons may disappear 
 entirely, or another mineral may be deposited in the place 
 of the silica, or the isotropic silica may become crystalline. 
 In organisms with calcareous skeletons the carbonate of 
 lime is easily dissolved by water containing carbonic acid, 
 but the degree of solubility varies, according to the con- 
 
 12
 
 4 CALCAREOUS SKELETONS. 
 
 dition in which the carbonate of lime exists. In some 
 animals it occurs as aragonite, in others as calcite. Of 
 these two minerals, aragonite is the harder and heavier,, 
 its specific gravity being 2'93, whilst that of calcite is. 
 only 277. Aragonite is rhombic and optically biaxial, 
 calcite is hexagonal and uniaxial. Fossil calcite shells- 
 (e.g. Pecten opercularis) are translucent, their surface is. 
 compact, but their interior porous ; on the other hand the 
 aragonite shells (e.g. Pectunculus glycimeris) are opaque, 
 and have a chalky appearance and a compact structure 
 throughout. When a shell of each kind, having the same 
 size and weight, is suspended in water containing carbonic 
 acid, it is found that the one composed of aragonite loses 
 in the same time a much greater amount of matter than 
 the one of calcite. And further, the calcite shell remains 
 firm much longer than the aragonite, the latter soon 
 coming to have the consistency of kaolin, and is then 
 easily broken up and carried away by a stream of water. 
 This difference, however, does not appear to be due directly 
 to mineral composition, for Messrs Cornish and Kendall 
 found that when ordinary crystalline calcite and aragonite 
 were powdered and placed in carbonic acid solutions of 
 the same strength, the aragonite was not acted on more 
 rapidly than the calcite, and the same result was obtained 
 with powdered fossil shells. From all these considerations, 
 it is not surprising to find that in some strata the 
 aragonite skeletons have entirely disappeared, whereas 
 those formed of calcite remain. This will obviously be 
 most likely to occur where the bed is pervious and where 
 there is a flow of carbonated water. In the case of the 
 mussel, Mytilus edulis, in which the inner layer of the 
 shell is formed of aragonite and the outer of calcite, 
 Dr Sorby found that specimens from the raised beach at
 
 CALCAREOUS SKELETONS. 5 
 
 Hope's Nose, Torquay, had lost the inner layer but not 
 the outer. In some cases aragonite is replaced by calcite, 
 but then the organic structure is entirely destroyed, and 
 we get merely a mass of calcite crystals. But calcite is 
 never replaced by aragonite. The mineral character of the 
 skeleton of the chief calcareous organisms is as follows : 
 
 Foraminifera. The vitreous forms consist of calcite, 
 the porcellanous probably of aragonite. 
 
 Actinozoa. The Alcyonarians mainly of calcite, but 
 most of the Madreporaria of aragonite. 
 
 Echinodermata. All of calcite. 
 
 Polyzoa. Calcite and aragonite in layers, or sometimes 
 perhaps mixed. 
 
 Brachiopoda. All of calcite. 
 
 Lamellibranchiata. Many consist entirely of aragonite, 
 but Ostrea and Pecten of calcite. In Pinna, Mytilus and 
 Spondylus, the inner layer is of aragonite, the outer of 
 calcite. 
 
 Gasteropoda. The majority are of aragonite, but Sca- 
 laria and some species of Fusus are of calcite. In some 
 {e.g. Patella, Littorina, Fusus antiquus) the outer layer is 
 of calcite. 
 
 Cephalopoda. Nautilus and Sepia are mainly aragon- 
 ite, as also were probably the Ammonites. The guard of 
 Belemnites is calcite. 
 
 Crustacea. The shell consists of calcite, with some 
 phosphate of lime. In some cases there is a large amount 
 of organic matter. 
 
 The conditions in which fossils occur depend, as we 
 have seen, on their original composition and on the ma- 
 terial in which they are embedded. The chief types are 
 the following :
 
 6 MODES OF FOSSILISATION. 
 
 1. The entire organism preserved. Occasionally the 
 soft parts of the organism are preserved as well as the 
 skeleton, the whole having suffered very little change. 
 Instances of this are, (a) the woolly rhinoceros and mam- 
 moth frozen into the mud in Northern Siberia, and 
 (6) insects and plants encased in fossil resin known as 
 amber, found in the Oligocene beds on the Baltic shores of 
 Prussia and in the Pliocene beds of Cromer. 
 
 2. The skeleton preserved almost unchanged. Some- 
 times when the skeleton alone is preserved, it remains 
 almost in its original condition, except that it has lost its 
 organic matter. Thus the shells in the Pliocene beds of 
 England differ from living ones only in being lighter, 
 more porous, and generally colourless. In some instances 
 a certain amount of mineral matter, such as carbonate of 
 lime, is added to the skeleton, making it heavier and more 
 compact. 
 
 3. Carbonisation. In some plants, and in animals 
 with chitinous skeletons, such as graptolites, the original 
 material passes into a carbonaceous substance. The 
 organism undergoes decomposition where there is but 
 little access of air, it loses oxygen, and consequently the 
 percentage of carbon increases. 
 
 4. Incrustation. The organism becomes coated over 
 with a layer of carbonate of lime deposited from a cal- 
 careous spring, as in the case of those of Matlock : or of 
 silica, as in the thermal springs formerly existing in New 
 Zealand. 
 
 5. A mould of the skeleton. Sometimes the skeleton 
 disappears entirely, a mould only remaining. This is 
 especially the case with aragonite organisms embedded in 
 porous strata. When the shell of a moll ask becomes 
 covered up with sediment, after the soft parts have
 
 MODES OF FOSSILISATION. 7 
 
 decomposed, the interior also sometimes becomes filled. 
 Water containing carbonic acid subsequently percolates 
 through the rock and carries away the shell as bicarbonate 
 of lime, so that there is left only a mould of the interior 
 and of the exterior, the space between the two being that 
 which was originally occupied by the shell and which if 
 filled with wax will give an exact model of it. Excellent 
 examples of this mode of fossilisation are seen in the 
 mollusks Cerithium portlandicum and Trigonia gibbosa 
 from the Portland Oolite. Sometimes after the shell has 
 been removed, the space left becomes filled up with a 
 secondary deposit of mineral matter, but this although 
 having the form of the original skeleton will obviously 
 not have its internal structure. 
 
 6. Petrifaction. In some deposits the fossils show 
 the minute structure as well as the form of the organism, 
 but the original material of the skeleton has been replaced 
 by another mineral, so that it is truly ' petrified.' Thus 
 we find fossil wood which in thin sections shows the cells 
 and vessels just as in living trees, but with the walls 
 formed of silica instead of cellulose. The change has 
 gone on in such a manner that as each particle disappeared 
 its place was taken by a particle of silica. The chief 
 minerals which replace the original substance of organisms 
 in this manner are : 
 
 (i) Carbonate of lime ; calcite sometimes replaces the 
 
 silica of sponges, and the aragonite of mollusks. 
 (ii) Silica ; as in the fossils from the Blackdown Green- 
 sand, from the Thanet Sands near Faversham; 
 and the wood in the Purbeck dirt-bed in the Isle 
 of Portland. 
 
 (iii) Iron pyrites ; e.g. ammonites from the Oxford Clay, 
 Lias etc., some trilobites and graptolites.
 
 8 MODES OF FOSSILISATION. 
 
 (iv) Oxide of iron ; in the form of limonite in some 
 fossils from the Dogger (Inferior Oolite) of York- 
 shire and the Lower Greensand of Pottou etc., 
 and as haematite in fossils from the Carboniferous 
 Limestone of Cumberland. 
 
 (v) In rare cases there are other replacing minerals, 
 such as sulphate of lime, barytes, blende, galena, 
 malachite, vivianite, and spathic iron. 
 
 7. Infilling. The interior of foraminifera and some 
 other organisms soon after the death of the animal may 
 become filled with mineral matter, such as glauconite (sili- 
 cate of iron and alumina), calcite, or silica, subsequently 
 the shell often disappears leaving only the internal mineral 
 cast. Glauconite occurs in this way in the various green- 
 sand strata, and also in some of the deep-sea deposits at 
 the present day. In the Cambridge Greensand the inte- 
 rior of the organism was filled with phosphate of lime. 
 
 8. Imprints. Some animals leave only their foot- 
 prints, such as labyrinthodonts and reptiles from the 
 Trias. In the Solenhofen Slates*, jelly-fishes have left the 
 imprint of their exterior. 
 
 In Stratigraphical Geology fossils are of immense 
 importance. By their aid alone, any formation may be 
 identified in distant localities and under different litho- 
 logical conditions. This depends on the fact that the 
 genera and species of animals and plants are distributed 
 through the rocks in a definite and regular order. In the 
 geological record each great division (termed a System) 
 is characterised by a particular assemblage of genera and 
 species, some of which pass from one system to another, 
 but a large number are confined to each one, and these 
 * These are really shaly limestones.
 
 USES OF FOSSILS IN GEOLOGY. 9 
 
 enable us to identify the division. In a similar manner, 
 the smaller divisions of the system, the series and stages 
 (or beds), are each characterised by possessing certain 
 fossils which do not occur above or below. Further, it is 
 found that the fauna of the smallest division (stage or 
 beds) is not of uniform character throughout its thickness; 
 although there may be no change in its lithological nature, 
 some of the species which are abundant at one level will 
 become rare or altogether extinct in passing to higher or 
 lower horizons. Consequently, a set of beds may be 
 divided into belts or zones, the general aspect of the 
 fauna of each zone being somewhat different from that of 
 the others, but between these divisions there will be no 
 break either physical or palseontological. If then we 
 have determined the order of succession of the formations 
 in any one area by means of their relative positions, the 
 newer resting on the older, it is easy in any other district, 
 merely by examining the fossils, to refer any set of beds 
 to their proper position in the geological record. But 
 valuable as this law of the identification of strata by their 
 organic remains is, it must not be used without some 
 caution. For although two formations may have been 
 deposited at exactly the same time, it does not necessarily 
 follow that all the genera and species occurring in the 
 two will be identical. Thus for instance in the seas at 
 the present day the same forms of life do not occur in all 
 parts. There is a distribution in provinces, depending 
 largely on climatic conditions, each province possessing 
 some forms peculiar to itself. So that the organisms 
 now being entombed in deposits formed, say, off the 
 British coasts, will as a whole be different from those off 
 the Australian coasts, but still some of the species and 
 many of the genera will be common to both areas, and
 
 10 USES OF FOSSILS IN GEOLOGY. 
 
 sufficient to identify the two deposits as having been formed 
 within the same general period, although not sufficient to 
 say that they are absolutely synchronous. Then again 
 there is a distribution of organisms according to the depth 
 of the sea, and the nature of the sea-bottom, so that the 
 fauna of a deep-water formation will necessarily be dif- 
 ferent from that of a shallow-water one, and that of a 
 sandstone different from that of a clay. 
 
 In addition to their chronological value, fossils are 
 also important in indicating the conditions under which 
 the formations were deposited. In the case of the later 
 beds, where most of the genera are still existing, it is easy 
 to distinguish a marine deposit from a freshwater or 
 terrestrial one. And even in the earlier, where most of 
 the genera are extinct, it is not very difficult, since many 
 of the groups of animals still have representatives at the 
 present day, and these in some cases are entirely marine, 
 as for instance the corals, echinoderms, and cephalopods. 
 Then again none of the freshwater lamellibranchs possess 
 one adductor muscle only; and very few of the land or 
 freshwater gasteropods are siphonostomatous. 
 
 Fossils, especially plants, also furnish some evidence of 
 the climate of past times. But the evidence afforded by 
 marine animals is of comparatively little value, except 
 when we are dealing with existing species. This is owing 
 to the fact that at the present day the individual species 
 of the same genus have often a very different range in 
 space, some being tropical forms, others polar. But even 
 in the case of extinct species, the assemblage of the 
 genera is sometimes such as characterises some region at 
 the present day, as for instance in the tropical mollusks 
 of the London Clay. 
 
 The depth of the sea in which a formation was deposited
 
 IMPERFECTION OF THE RECORD. 11 
 
 can also be told with any degree of certainty only in the 
 case of species still living, or in the case of assemblages 
 of genera which are now found at some particular depth. 
 But in basing conclusions as to depth on fossils, it must 
 be remembered that the fauna of any locality may 
 include forms which have lived at higher or lower levels 
 and have been transported from their original home 
 after death. Perhaps the safest indication of shallow- 
 water and the proximity of land is given by the occur- 
 rence of land animals and plants in a marine formation. 
 
 If all the plants and animals which have lived in past 
 times had been preserved as fossils, we should have a 
 complete record of the succession of life on the earth, and 
 all questions of phylogeny could be easily answered. But 
 we have already seen several reasons why this record 
 the palffiontological record must be imperfect. Thus, 
 some animals have no skeleton. Others, particularly land 
 animals, do not become entombed. And some rocks which 
 originally contained fossils have had them dissolved by 
 the percolation of water containing carbonic acid. In 
 addition to this, the fossils in some of the older rocks 
 have been obliterated by metarnorphism. The palaeonto- 
 logical record is also incomplete because of the imperfection 
 of the geological record. Between some formations enor- 
 mous masses of strata are missing, having been removed 
 by denudation. And although these gaps may be 
 filled up elsewhere, it does not render the record of life 
 perfect, since in going from one district to another, we 
 almost certainly pass into a different marine province, or 
 into an area where the conditions of temperature, depth, 
 or the sea-bottom have changed, and where as a result we 
 shall meet with another assemblage of organisms.
 
 12 DISTRIBUTION OF FOSSILS. 
 
 The geological distribution of animals and plants 
 varies considerably, some have a long range, others a short 
 one. Thus a few species extend through several formations, 
 as for instance, Alrypa reticularis from the Llandovery 
 Beds to the Devonian, and Orthis calligrarnma from the 
 Arenig to the Wenlock Limestone. Others have a short 
 range, as in the case of many graptolites and ammonites, 
 which are often confined to a single band of rock. Many 
 instances of genera with long and with short ranges will be 
 seen in the following pages ; of the former we may mention 
 Lingula extending from the Cambrian, and Nautilus from 
 the Ordovician, to the present day. Of the latter Belern- 
 nitella found only in the Upper Chalk, Coenograptus in 
 the Upper Llandeilo, and Tetragraptus in the Lower 
 Arenig. Often the genera which have a long range in 
 time have also a wide distribution in existing seas, as in 
 the case of Lingula, which lives off the coasts of Japan, 
 China, Formosa, the Korean Archipelago, the Philippine 
 Islands, the Sandwich Islands, Australia, and in the Indian 
 Ocean. 
 
 With regard to the general succession of organisms, 
 we find, as might be expected, that those in the earlier 
 formations differ more from living animals and plants, 
 than do those in the later. Thus, in the Paleozoic, some 
 of the groups, many of the genera, and practically all the 
 species are extinct. But as we pass to more recent times 
 the proportion of extinct forms decreases, until in the 
 newer Cainozoic rocks almost all the genera and very 
 many of the species are still existing. The different 
 groups of organisms also appear in the geological series 
 in the order of their complexity of organisation, the 
 lower forms preceding the higher and more specialized. 
 Thus, taking the Vertebrata only, the Fishes first occur
 
 CLASSIFICATION. 13 
 
 in the Silurian, the Amphibians in the Carboniferous, 
 the Reptiles in the Permian, the Mammals in the Rhsetic. 
 The Birds are first found in the Solenhofen Slate (Upper 
 Jurassic), but they must be left out of account, since 
 owing to their mode of existence they rarely become 
 preserved. And similarly in each of these smaller groups, 
 the Fishes for instance, the genera of the earlier formations 
 are simpler and more generalized than those in the later. 
 
 The animal kingdom may be divided into groups, the 
 primary groups are termed sub-kingdoms, the members of 
 each of these agreeing in the general arrangement of their 
 parts. The sub-kingdoms are divided into classes, the 
 classes into orders, and the orders into families. The 
 animals included in each possess certain characters in 
 common, which are not found in the other groups. The 
 families are composed of genera, and the genera of 
 species. The sub-kingdoms are: 1. Protozoa. 2. Porifera. 
 3. Ccelenterata. 4. Echinodermata. 5. Vermes. 6. Mol- 
 luscoidea. 7. Mollusca. 8. Arthropoda. 9. Vertebrata. 
 But the student must be warned against considering 
 that this order of succession represents an increasing per- 
 fection of organisation. Some of the members of one sub- 
 kingdom may be more highly developed than the lower 
 members of another which as a whole occupies a higher 
 position in the zoological scale. In other cases, owing to 
 the difference in the general arrangement of the parts, it 
 is difficult to determine which of two divisions is the 
 higher.
 
 SUB-KINGDOM I. PROTOZOA. 
 
 Classes. Orders. Sub-Orders. 
 
 11. Imperforata. 
 II. Foramimfera | 2 Perforata. 
 
 1. Bhizopoda j 2. Radiolaria. 
 
 (3. Others not found fossil. 
 
 2. Infusoria. 
 
 3. Sporozoa or 
 
 Gregarinida. 
 
 THE Protozoa includes the lowest forms of animals, 
 such as Amceba, Vorticella and Globigerina. The body 
 consists in many cases of one cell only, in others of more 
 than one, but the cells never become differentiated to 
 form tissues as they do in all other animals, which are 
 for this reason grouped together as the Metazoa. 
 
 In some protozoans the protoplasm is, with the ex- 
 ception of the nucleus, of uniform character throughout ; 
 these are characterised by having no definite external 
 form, by being able to take in food at any part of their 
 body, and by possessing the power of throwing out lobular 
 or filamentous processes of protoplasm known as pseudo- 
 podia. In other forms the central uniform mass is sur- 
 rounded by a denser cortical layer, which gives the animal 
 a definite shape. In these the food is generally taken in 
 at one permanent aperture which functions as a mouth. 
 They never give out pseudopodia, but are in many cases 
 provided with cilia or flagella, which are threads of proto- 
 plasm having a definite form and a rhythmic movement. 
 
 Reproduction in the Protozoa takes place asexually by
 
 PROTOZOA. FORAMINIFERA. 15 
 
 means of fission and by the formation of spores. But in 
 some cases conjugation occurs, representing to some extent 
 the sexual method of the higher animals. In some of the 
 Protozoa there is no skeleton, but in others a shell, which 
 may be calcareous, siliceous, or chitinous, is secreted. 
 
 The Protozoa can be divided into three classes, (1) the 
 Rhizopoda, (2) the Infusoria, (3) the Sporozoa or Grega- 
 rinida. The Rliizopods are the only forms which have 
 been definitely recognised in the fossil state. 
 
 CLASS. RHIZOPODA. 
 
 Protozoa which possess no cortical layer and which are 
 able to throw out pseudopodia. 
 
 The Rhizopoda are divided into five or more orders, 
 but only two of these have been definitely found fossil, 
 namely, the Foraminifera (or Reticularia) and the Radio- 
 laria. 
 
 ORDER. FORAMINIFERA. 
 
 The Foraminifera are characterized by giving out 
 filamentous pseudopodia, which frequently branch and 
 anastomose, and by- possessing in most cases a shell or 
 test, which may be calcareous, arenaceous, siliceous, or 
 chitinous. 
 
 The calcareous forms are by far the commonest, and 
 in these two kinds of test may be distinguished, namely, 
 the vitreous and the porcellanous. In the vitreous, the 
 test is glassy and transparent, and is perforated by in- 
 numerable tubes for the passage of the pseudopodia; in 
 some forms {e.g. Rotaliti) these tubes are as much as 
 gifaf of an inch in diameter, but in others (e.g. Operculina) 
 they are only 16 ^ 00 of an inch. In the porcellanous forms, 
 the test, when viewed by reflected light, is opaque and 
 white, having the appearance of porcelain; it is perfectly
 
 16 PEOTOZOA. FORAMINIFERA. 
 
 homogeneous, and is not perforated by tubes, the whole 
 of the pseudopodia passing out by one or two large 
 apertures. Sometimes, however, the surface of the shell 
 is pitted, producing at first sight the appearance of per- 
 foration. 
 
 In the arenaceous forms the shell consists of grains of 
 sand united together by a cement, the appearance of 
 the shell varying according to the material of which the 
 grains are composed. The cement may be formed of 
 chitinous, or of calcareous and ferruginous material. Usu- 
 ally the test is imperforate. 
 
 The chitinous forms do not occur as fossils. In one 
 genus only is the test siliceous. 
 
 With regard to the form of the shell, in some genera 
 it consists of a single chamber, when it is said to be 
 unilocular, as in Orbulina, in which it is spherical, and in 
 Lagena (fig. 2 F), in which it is generally flask-shaped. 
 In other cases it consists of several chambers, which are 
 usually placed in communication with one another by 
 means of perforations in the walls (septa) between them*. 
 The arrangement of the chambers in these multilocular 
 forms is very varied, they may occur in a straight line as 
 in Nodosaria (fig. 2 H), in a curved line as in Dentalina, 
 in a plane spiral as in Cristellaria (fig. 2 G), or in a heli- 
 coid spiral as in Rotalia (fig. 2 L, M}. In some spiral 
 forms the earlier whorls are embraced partly or entirely 
 by the later ones, so that sometimes the last whorl only is 
 visible (e.g. Cristellaria); but when the later chambers 
 are merely attached to the extremities of the earlier ones, 
 all the whorls are visible (e.g. Rotalia). 
 
 * The communication in Nodosaria and its allies is through the 
 mouth-aperture of the chambers which open into one another, and only 
 to a small extent through perforations in the walls.
 
 PROTOZOA. FORAMINIFERA. 17 
 
 In the porcellanous and the lower vitreous forms 
 the septum between the chambers (fig. 1 A, b) is formed 
 entirely by the wall of the older chamber, but in the 
 higher vitreous forms each chamber possesses a wall of 
 its own, so that the septum (fig. 1 B, 6) is formed of two 
 lamellae. Also in the higher vitreous forms the walls of 
 the chambers are frequently strengthened by a deposit on 
 
 A /? 
 
 FIG. 1. A, section of a foraminifer in which each septum is formed of a 
 single lamella. B, in which the septum is formed of two lamellae, 
 a, passages between the chambers ; b, septum ; c, anterior wall of 
 last chamber; d, supplemental skeleton. (After Carpenter.) 
 
 their outer surfaces, filling up the outer hollows between 
 the chambers ; this deposit is often traversed by a system of 
 canals, and is known as the supplemental skeleton (fig. 1 B, d). 
 The Foraminifera are divided into two sub-orders, 
 (1) the Imperforata, (2) the Perforata. 
 
 SUB-ORDER 1. IMPERFORATA. 
 
 The test is not perforated by canals, but is provided 
 with one or two large apertures, through which the whole 
 of the pseudopodia pass out. The test is membranous, 
 porcellanous, or arenaceous. 
 
 w. p. 2
 
 FIG. 2. Foraminifera (recent). A, B, Hiliola (Biloculina) depressa. B, 
 section. C, Miliola (Miliolina) seminulum. D, Miliola (Spiro- 
 loculina) limbata. E, Textularia barretti. F, Lagena sulcata. G, 
 Cristellaria rotulata. H, Nodosaria radicula. I, K, Globigerina 
 bulloides. L, M, Eotalia beccari. (After Brady, " Challenger " Eeport.)
 
 PROTOZOA. FORAMINIFERA. 19 
 
 Miliola. (fig. 2, A D.) Test calcareous and por- 
 cellanous, multilocular, the chambers being coiled on an 
 elongated axis, each chamber forming half of a convolu- 
 tion. Sometimes all the chambers are visible externally 
 on both sides of the shell, at other times, owing to the 
 lateral prolongations of the chambers, only the last one or 
 two are seen, or it may be that more chambers are shown 
 on one side than on the other. The external features of 
 the shell consequently vary considerably, and on this 
 account the genus has been divided into a number of 
 subgenera. Trias to present day. 
 
 Saccamina. Test arenaceous, compact, thick, formed 
 of a single spherical, pyriform, or fusiform chamber, or of 
 a number of chambers united end to end. Surface smooth 
 or nearly so. Carboniferous and living. 
 
 SUB-ORDER 2. PERFORATA. 
 
 The test is perforated by numerous minute canals for 
 the passage of pseudopodia ; it is usually calcareous and 
 vitreous, but sometimes arenaceous. 
 
 Lagena. (fig. 2, F.) Test unilocular, calcareous, 
 vitreous, very finely perforated. Form spherical, ovate 
 or flask-shaped. A single terminal aperture, sometimes 
 at the end of a long neck ; rarely two apertures. Sur- 
 face smooth, ribbed, or spinous. Wenlock Beds to present 
 day. 
 
 Nodosaria. (fig. 12, H.) Test vitreous, composed 
 of a number of chambers arranged in a straight line, and 
 separated by constrictions. Surface smooth or ornamented 
 with granules, spines, or ribs. Permian to present day. 
 
 Textularia. (fig. 2, E.) Test vitreous, sometimes 
 arenaceous ; form variable, conical, pyriform, or cuneiform ; 
 
 22
 
 20 PROTOZOA. FORAMINIFERA. 
 
 composed of numerous chambers in two alternating 
 parallel series. Aperture slit-like on the inner edge 
 of the last chamber. Carboniferous to present day. 
 
 Globigerina. (fig. 2, /, K.) Test calcareous, vitreous, 
 perforated by large canals. Multilocular, chambers glo- 
 bular, few, arranged in a plane or helicoid spiral, each 
 chamber opens by a large aperture into a large central 
 cavity. No supplemental skeleton. Trias to present day. 
 
 Rotalia. (fig. 2, L, M.} Test calcareous, very finely 
 perforated, multilocular. The chambers arranged in a 
 helicoid spiral, so that on the upper surface all the whorls 
 are seen, on the lower, only the last one. The aperture is 
 in the form of a curved fissure on the lower surface of the 
 last segment. The septa are generally double. A supple- 
 mental skeleton is sometimes present. Upper Jurassic 
 to present day. 
 
 Nummulites. (fig. 3.) Test calcareous, lenticular 
 in form, and composed of a large number of whorls coiled 
 
 cb d i 
 
 FIG. 3. Nummulites, showing vertical and horizontal sections, a, mar- 
 ginal cord, with system of canals; b, septum, with canals; c, cham- 
 bers; d, test; e, pillars coming from the supplemental skeleton. 
 (After Zittel.)
 
 PROTOZOA. FORAMINIFERA. 21 
 
 in a plane spiral. Each whorl completely encloses the 
 preceding one, by means of the lateral prolongations of 
 the chambers, so that externally only the last whorl of 
 the shell is visible. The whorls are divided into chambers 
 by septa which are slightly curved backwards ; each 
 chamber communicates with the neighbouring one by 
 means of a median fissure placed next to the margin of 
 the previous whorl. Each septum is formed by two 
 lamellae. A supplemental skeleton is present, the larger 
 part of it forming what has been termed the ' marginal 
 cord.' The general shell-substance is minutely perforated, 
 and a system of canals also traverses the septa and supple- 
 mental skeleton. Carboniferous Limestone to present 
 day. It attains its maximum in the Eocene, only one 
 or two rather rare forms are living, one of which (N. cum- 
 mingi) is found in shallow water in tropical and sub- 
 tropical regions. 
 
 EOZOON. In the year 1864 the name Eozoon cana- 
 dense was given by Dawson to a peculiar body found in 
 the Laurentian rocks of Canada, and referred by Dr Car- 
 penter and himself to the Foraminifera. Other 'species' 
 were subsequently found in Bavaria, Bohemia, Silesia and 
 elsewhere. 
 
 The Laurentian rocks of Canada consist of two divi- 
 sions, an upper and a lower. The thickness of the lower 
 is estimated at 20,000 feet; it is composed of gneisses 
 and schists, with intercalated beds of crystalline limestone. 
 In the latter, Eozoon occurs, frequently having the form 
 of extensive sheets or masses ; but according to Dawson, 
 recently collected examples " have established the fact 
 that the normal shape of young and isolated specimens of 
 Eozoon canadense is a broadly-turbinate, funnel-shaped or
 
 22 
 
 EOZOON. 
 
 top-shaped form, sometimes with a depression on the 
 upper surface, giving it the appearance of the ordinary 
 cup-shaped Mediterranean sponges." 
 
 A hand specimen of Eozoon shows it to consist of 
 alternating layers of a green mineral, serpentine, and a 
 white mineral, calcite. In thin sections viewed under the 
 microscope, the serpentine is seen to have at times a 
 fairly regular and more or less beaded form. This layer 
 is considered by Dawson and Carpenter to fill the space 
 which was originally occupied by the soft parts of the 
 animal (fig. 4, b), the serpentine having been introduced 
 subsequently by infiltration, and thus forming a cast of 
 the body chambers. On each side of the serpentine and 
 between it and the layer of calcite, there may be seen in 
 good specimens a thin lamina (a), said to be calcareous 
 and perforated by numerous tubules ; this lamina is com- 
 
 FIG. 4. Diagram to explain the organic theory of Eozoon. a, body- wall ; 
 b, body chamber ; c, supplemental skeleton ; d, canals in the sup- 
 plemental skeleton ; e, stolon-passages. 
 
 pared with the body-wall of the Foraminifera such as 
 GloUgerina, the tubules being the perforations for the 
 passage of the pseudopodia. External to the body-wall 
 is the thick layer of calcite (c), having the characteristic 
 cleavage of that mineral; this is traversed by numerous
 
 EOZOON. 23 
 
 branching canals (d), and by others which pass directly 
 from one chamber to another placing them in communica- 
 tion. These canals are filled with serpentine or occasion- 
 ally calcite. Since this layer comes between the body- walls, 
 it will of course correspond to the supplemental skeleton, 
 such as that which occurs in Nummulina ; the branching 
 canals will represent the complicated canal-system, and 
 the canals passing between the chambers the stolon- 
 passages (e). 
 
 If Eozoon be organic, it must certainly be placed 
 among the Foraminifera, of which it will be a gigantic 
 example. Several authors, however, especially King and 
 Rowney in this country, and Mb'bius in Germany, have 
 brought forward weighty objections to this view. They 
 state that the chamber casts of serpentine are so extremely 
 irregular in form that they cannot be compared with those 
 of the Foraminifera, and they consider them to be merely 
 granules of serpentine in a matrix of calcite. The body- 
 wall is shown to be not calcareous but to consist of 
 chrysolite, a fibrous variety of serpentine, derived from the 
 alteration of the latter, and in some cases a gradual 
 passage can be traced from the unaltered serpentine to 
 the fibrous chrysolite. A layer of the same mineral is 
 frequently found surrounding nodules of serpentine in 
 ophites. Then again the fibres are often in actual contact, 
 but if they represented the casts of the pseudopodial 
 foramina, they ought to be isolated. With regard to the 
 supplemental skeleton, this is found not to be of uniform 
 structure throughout, but to contain masses of pyrites and 
 other minerals, and in fact to correspond exactly to the 
 matrix in which pargasite and coccolite occur. The 
 branching canals are much too irregular both in form and 
 distribution to be at all comparable to the canal system
 
 24 PROTOZOA. FORAMINIFERA. 
 
 of a supplemental skeleton, and they are regarded merely 
 as crystals of metaxite. Similarly the stolon passages are 
 found to be simply crystals of pyrosclerite. The mode of 
 occurrence of Eozoon also furnishes corroborative evidence. 
 It has never been found in any unaltered rock, and no other 
 fossils have been discovered in association with it. In the 
 case of the specimens found at Chelmsford and Bolton, 
 U.S.A., it has been shown that the limestone in which the 
 Eozoon occurs was not formed at the same time as the 
 gneiss with which it is associated, but that it has been 
 introduced subsequently in the manner of a vein, so that 
 here at any rate Eozoon cannot possibly be organic. 
 
 Distribution of the Foraminifera. 
 
 The majority of the Foraminifera are marine, most of 
 them living on the sea-bottom. A few however, as for 
 instance Globigerina, exist at or near the surface in the 
 open ocean, and these few forms are very important on 
 account of their great abundance. The earliest examples 
 of the Foraminifera are found in the Ordovician rocks, 
 but they are rare until we reach the Carboniferous, some 
 of the limestones of which are formed largely of them, as 
 for instance, the $accamma-limestone of the North of 
 England and Scotland, and the Fusulina-Umestoue of 
 Russia, America etc. The order continues to be well 
 represented throughout the Mesozoic formations, particu- 
 larly in the Chalk, and it attains its greatest development 
 in Tertiary and recent times. In the Eocene the genus 
 Nummulites is extremely abundant, forming the massive 
 Nummulitic Limestone of Southern Europe, Egypt, Asia 
 Minor, and the Himalayas.
 
 PROTOZOA. RADIOLARIA. 25 
 
 The genera and species of the Foraminifera have as a 
 general rule a long range in time, as might be expected 
 from their low organization ; some of the species which 
 occur in the Palaeozoic are still living. 
 
 ORDER. RADIOLARIA. 
 
 In the Radiolaria the body consists of a central mass 
 of protoplasm enclosed in a definite membrane, known as 
 the central capsule. The intracapsular protoplasm contains 
 one or more nuclei, and is continuous, through pores in 
 the capsule, with a layer of protoplasm outside the 
 capsule ; this layer gives off filamentous pseudopodia, 
 which occasionally anastomose. A skeleton is generally 
 developed, which is composed either of silica, a silicate of 
 carbon, or a peculiar horny substance known as acanthin. 
 The skeleton may be entirely outside the central capsule or 
 partly within, and it consists either of isolated spicules, 
 or of a lattice-like structure frequently with projecting 
 spines. The earliest British examples occur in the 
 Ordovician rocks of Scotland, others have been found 
 in the Carboniferous, and they become abundant in 
 the Mesozoic and Cainozoic formations. Those forms 
 in which the skeleton is composed of acanthin do 
 not occur fossil.
 
 SUB-KINGDOM II. POBIFERA. 
 
 Orders. 
 
 1. Myxospongiffi. 6. Hexactinellidae. 
 
 2. Ceratospongiae. 7. Octactinellidse. 
 
 3. Monactinellidae. 8. Heteractinellidse. 
 
 4. Tetractinellidae. 9. Calcispongiae. 
 
 5. Lithistidae. 
 
 THE Porifera includes the Sponges. They are dis- 
 tinguished from the Protozoans by the body being always 
 multicellular, by the cells being arranged in definite 
 layers, and by the occurrence of sexual reproduction. 
 
 In a simple sponge the body has the form of a hollow 
 sac open at the upper end, and in nearly all cases attached 
 to the sea-bottom or to a foreign object, by the lower. 
 
 FIG. 5. Diagram of the Structure of a simple Sponge, one canal only 
 shown, os, osculum; p, pores; i, incurrent or inhalent canal; 
 c, globular chamber; e, excurrent or exhalent canal; o, ostium.
 
 PORIFERA. 27 
 
 In the living state a large number of small openings 
 termed pores (fig. 5, p) may be seen on the surface, these 
 in many forms are not permanent, disappearing and being 
 replaced by others, they are the openings of canals leading 
 to the interior, known as the incurrent canals (fig. 5, i). 
 These dilate into globular chambers (g), from which 
 canals pass to the central cavity of the sponge known as 
 the excurrent canals (e), their openings on the inner 
 surface being the ostia (o). The opening of the central 
 cavity to the exterior is termed the osculum (os\ in some 
 cases there is only one, which is then placed at the 
 summit of the sponge ; in others there are several. A 
 continual current of water flows through the canal-system, 
 passing in by the pores and out by the osculum. By 
 means of this the processes of respiration, nutrition and 
 excretion go on. 
 
 The wall of the sponge is formed of three layers, 
 the outer one is the ectoderm and consists of a single 
 layer of flattened cells. The inner is the endoderm, it 
 lines the globular chambers and the excurrent canals and 
 is similar to the ectoderm, except in the chambers, where 
 each cell is provided with a collar and a flagellum, the 
 movements of the latter producing the current of water. 
 Between the endoderm and the ectoderm comes the 
 mesoderm which is much thicker than the other layers 
 and forms the greater part of the sponge; in it the 
 skeleton is produced. 
 
 In nearly all sponges there is a skeleton, which serves 
 to support the soft parts and to prevent the canals from 
 closing. For systematic purposes its structure is found to 
 be of immense importance, more especially since the 
 external form of the sponge varies enormously even in the 
 same species. The skeleton may consist of fibres of a
 
 28 PORIFERA. 
 
 horny substance known as spongin, or of needles (termed 
 spicules} of various forms (fig. 6), composed of carbonate 
 of lime or of silica, or it may consist of both siliceous 
 spicules and spongin. It is only those forms which have 
 either a siliceous or calcareous skeleton that are definitely 
 known as fossils. In some groups the spicules are not 
 united, as for instance the Monactinellidse and Tetracti- 
 nellidse, but in others they are fused together or inter- 
 locked so as to form a complete scaffolding, and generally 
 it is in these only that the external form of the sponge 
 can be preserved in the fossil state. In most sponges, 
 two kinds of spicules may be distinguished, the skeletal- 
 spicules which go to build the main part of the skeleton, 
 and the flesh-spicules which are smaller and isolated and 
 are seldom preserved fossil. In the centre of each spicule, 
 there is a tube known as the axial canal (fig. 6c), which 
 in the living animal is occupied by a thread of organic 
 matter. The spicules of recent siliceous sponges are 
 characterised by the glassy appearance of their surface, 
 and by the silica being colloidal, isotropic, and soluble in 
 heated caustic potash. But in the fossil state the spicules 
 have generally undergone considerable change, occasionally 
 their silica is still colloidal but the surface has no longer 
 the glassy appearance, and the axial canal is frequently 
 filled with silica in a crystalline or crypto-crystalline 
 condition, and is consequently easily distinguished between 
 crossed Nicols when the spicule itself still remains colloidal 
 and consequently isotropic ; generally however the spicule 
 has become crystalline or crypto-crystalline, and when 
 this is the case the axial canals can rarely be detected 
 since they are filled with material in the same condition. 
 Sometimes however the silica of the spicules has been 
 entirely removed, in some cases a hollow cast only remain-
 
 PORIFERA. 29 
 
 ing ; in others its place is taken by another mineral, as for 
 instance by calcite in the sponges from the Lower Chalk 
 of Dover and Folkestone, by iron pyrites in Protospongia 
 from the Menevian Beds of St Davids, by iron peroxide in 
 the sponges of the Upper Chalk of the South of England, 
 by glauconite in those from the Upper Greensand. 
 Obviously then silica in the colloidal state in which it 
 occurs in recent sponge spicules is anything but a stable 
 substance, thus differing widely from its crystalline and 
 crypto-crystalline condition. 
 
 The spicules of the calcareous sponges are usually 
 smaller than those of the siliceous forms, and their material 
 is not in an isotropic state, each one being optically a crys- 
 tal of calcite but having the external form and internal 
 structure of a spicule, consequently in polarised light they 
 are readily distinguished from the siliceous forms. Then 
 again the fossil calcareous spicules have undergone much 
 less chemical change than the siliceous ones, generally 
 they are still composed of carbonate of lime, it is only 
 in rare cases that this is replaced by silica, but at the 
 same time the external form of the individual calcareous 
 spicules has very often disappeared. 
 
 The canal-system is indicated in the skeleton of both 
 recent and fossil forms, by spaces in the spicular frame- 
 work, but these spaces represent only the larger canals, 
 the smaller existing in the soft parts alone. 
 
 Reproduction in the sponges takes place by budding 
 or gemmation, and sexually. 
 
 The Porifera may be divided into the following nine 
 orders. 
 
 ORDER 1. MYXOSPONGLE. Sponges with no skeleton 
 or occasionally a few isolated siliceous spicules. Not 
 known in the fossil state.
 
 FIG. 6. Sponge spicules (skeletal), a, monactinellid, Halichondria 
 panicea, Kecent. b, tetractinellid, Pachastrella sp. Upper Green- 
 sand, c, tetractinellid, Geodites sp. Eocene, d, lithistid, Scytalia 
 radiciformis, Chalk, e, lithistid, Seliscothon mantelli. f, hexactinellid 
 Cceloptychium agaricoides, Chalk, g, octactinellid, Astrceospongia 
 sp. Silurian, h, heteractinellid, Asteractinella expansa, Carbo- 
 niferous. .;', calcisponge, Grantia compressa, Kecent. All magnified.
 
 PORIFERA. 31 
 
 ORDER 2. CERATOSPONGI^E. Sponges with a skeleton 
 composed of a fibrous network of spongin. This includes 
 the ordinary bath sponges. A few siliceous spicules are 
 occasionally present. Not definitely known in the fossil 
 state. 
 
 ORDER 3. MONACTINELLID.E. Sponges in which 
 the skeleton consists of spongin and siliceous spicules. 
 The spicules (fig. 6, a) are uniaxial, that is to say, they 
 consist of a single rod or axis ; they vary considerably in 
 shape, being straight or curved, and with sharp or blunt 
 ends. Flesh-spicules may also occur. Since in this order 
 the spicules are only united by decomposable material, 
 it is extremely rare to find the form of the sponge 
 preserved fossil, usually detached spicules only occur. 
 The earliest forms belonging to this order are found 
 in the Silurian, they become more abundant in the 
 Carboniferous where the genus Reniera occurs. The 
 freshwater form Spongilla is found in the Purbeck Beds 
 of the South of England. A large number of examples 
 are still living. 
 
 ORDER 4. TETRACTINELLID.E. The spicules (fig. 6, 
 6, c) are siliceous and are held together by spongin, they 
 consist of four rays given off from a common centre, the 
 angle between each of the rays being 120. The rays 
 may be equal or unequal in length, frequently one is very 
 much elongated, and sometimes their terminations are 
 bifurcated. Like the Monactinellids, the Tetractinellids 
 are seldom preserved in anything like a perfect condition 
 as fossils. The oldest forms occur in the Carboniferous 
 Limestone, where they are represented by the genus 
 Geodites. 
 
 ORDER 5. LrraiSTnxE. Sponges with thick stony 
 walls and very variable external form. The spicules
 
 32 
 
 PORIFERA. 
 
 (fig. 6, d, e) are siliceous and consist of four rays, or are 
 irregular in form, the extremities of the rays branch or 
 expand and firmly interlock with one another but do not 
 fuse together. In addition to the skeletal-spicules there 
 is generally a surface layer of trifid spicules or of discs. 
 Flesh-spicules are also present. Several different types 
 of canal-system occur. Owing to their solidity the 
 Lithistids are preserved abundantly as fossils. They are 
 rare in the Palaeozoic, the earliest form being Archceo- 
 scyphia found in the Upper Cambrian ; in the Ordovician 
 and Silurian we get Astylospongia. No forms belonging 
 to this order have been found in the Devonian, Permian, 
 or Trias, they are numerous in the Jurassic, attain their 
 maximum in the Cretaceous, and are scarce in the 
 Tertiary. 
 
 Siphonia. (fig. 7.) Pear-shaped, usually provided 
 with a stalk, which is given off from the broad end of 
 the body and terminates in rootlets. The incurrent 
 canals are small, slightly curved, and extend radially from 
 
 FIG. 7. Siphonia tulipa. Upper Greensand, Warminster. A, vertical 
 section. B, horizontal section, e, excurrent canals; f, incurrent 
 canals. Two-thirds natural size.
 
 PORIFERA. 33 
 
 the centre of the sponge to the surface. The excurrent 
 canals are larger, and are arranged parallel to the surface 
 of the sponge, extending from the base to the summit, 
 where they open into the large central cavity by means 
 of a series of parallel ostia. The skeletal-spicules possess 
 four rays with bifurcated and expanded extremities, by 
 means of which they interlock. Upper Greensand to 
 Upper Chalk. 
 
 ORDER 6. HEXACTINELLID.E. The spicules (fig. 6,/) 
 are siliceous and consist of six rays (three axes) at right 
 angles to one another. Each ray is traversed by an 
 axial canal, and these unite at the point of junction of the 
 six rays. In some cases one ray is longer or shorter than 
 the others, or almost absent. The spicules are united 
 so as to form a framework of skeleton cubes, either by 
 being fused, or simply held in position by the soft parts. 
 Flesh-spicules are abundant, but rare as fossils. The 
 canal-system is usually simple. The earliest form is 
 Protospongia from the Menevian Beds of St Davids. In 
 the Ordovician and Silurian we get the genera Dictyophyton 
 and Phormosella ; in the Carboniferous Hyalostelia. There 
 are none in the Permian and Trias, but they become 
 abundant in the Jurassic, especially in the upper part, 
 and also in the Cretaceous ; they are rare in the Tertiary. 
 The sponge -character of the genera Ischadites, Receptacu- 
 lites, and Sphcerospongia, from the Silurian and Devonian 
 is now disputed. 
 
 Protospongia. Form unknown but probably cup- 
 shaped. Spicules cruciform, owing to the reduction of 
 one axis, and arranged in a quadrate manner, the larger 
 ones forming a framework, in the squares of which are 
 smaller spicules of two or three sizes, arranged in the 
 same regular way, so that the larger squares enclose 
 w. P. 3
 
 34 PORIFERA. 
 
 four or five series of smaller ones. The spicules were 
 either free or probably partly fused together. Menevian 
 Beds and Lingula Flags. 
 
 Ventriculites. Simple, form variable, but usually 
 cup-shaped, funnel-shaped, or cylindrical. Central cavity 
 large and deep. Walls folded so as to form a series of 
 vertical grooves and ridges. Canal-system well developed, 
 the radial canals are large and start from the surface of 
 the central cavity, but end before reaching the outer 
 surface, others start from the outer surface and end before 
 reaching the central cavity. Spicules six-rayed and fused 
 with one another so as to form a mesh-work. The node 
 where the axes of the spicule cross is hollow, having the 
 form of a negative octahedron, the central part of each 
 face of which is absent; the axial canals cross in the 
 centre of the space. The sponge was provided with a 
 root consisting of siliceous fibres. Upper Chalk. 
 
 ORDER 7. OCTACTINELLID^. Spicules siliceous (fig. 
 6, g), consisting of eight rays, six of which are in one 
 plane diverging at equal angles, the other two are at right 
 angles to this plane, forming a vertical axis. Frequently, 
 however, the vertical axis is only slightly developed or 
 altogether absent. The spicules are not united. The 
 only genus is Astrceospongia found in the Silurian and 
 Devonian. 
 
 ORDER 8. HETERACTINELLHXE. Spicules siliceous 
 and unusually large (fig.. 6, h), the number of rays varying 
 from six to thirty. The body spicules are not fused, but 
 there is a dermal layer in which the spicules are inter- 
 woven and more or less fused. The only genera are 
 Tholiasterella and Asteractinella found in the Carbon- 
 iferous rocks of Ayrshire. 
 
 ORDER 9. CALCISPONGLE. Spicules composed of car-
 
 PORIFERA. 35 
 
 bonate of lime (fig. 6, j); usually much smaller and less 
 varied in form than those of the siliceous sponges ; there 
 are three kinds, the simple uniaxial, the three-rayed, and 
 the four-rayed ; they are never fused with one another, 
 and they are either arranged close together so as to form 
 fibres, or they are loosely distributed. The earliest forms in 
 Britain occur in the Carboniferous rocks of Fifeshire. 
 
 Peronella. Cylindrical, simple or branched ; central 
 cavity tubular and extending from the summit to the 
 base. Walls thick and with no definite canals, the current 
 of water having probably passed through the irregular 
 spaces between the spicular fibres. Spicules three or four- 
 rayed forming anastomosing fibres. Carboniferous (pos- 
 sibly also Devonian) to Cretaceous ; most abundant in the 
 Jurassic and Cretaceous. 
 
 Distribution of the Porifera. 
 
 The Sponges are all aquatic, and, with the exception of 
 the Monactinellid genus Spongilla, and its allies, all marine. 
 Fhey are found in the seas of all parts of the world, many 
 jf the genera and species having a very wide distribution. 
 AJ1 the orders except the OctactinellidaB and the Heterac- 
 iinellidse are still living. The Lithistids are found mainly 
 between 10 and 150 fathoms, but some species occur 
 it greater depths. The Hexactinellids are deeper water 
 forms than the Lithistids, being found down to a depth 
 )f 2900 fathoms, but they are most abundant between 
 100 and 200 fathoms. The CalcispongiaB are mainly 
 shallow water forms. 
 
 The fossil forms are comparatively rare in the Palaeozoic 
 ocks, until we reach the Carboniferous ; and throughout 
 
 32
 
 36 PORIFERA. 
 
 the geological formations they are much less abundant 
 in argillaceous than in calcareous and arenaceous rocks. 
 Sponges are first found in the Cambrian System, the 
 earliest form being Protospongia from the Menevian Beds 
 and Lingula Flags. In the Tremadoc the Hexactinellid 
 Hyalostelia occurs, ranging onwards as far as the 
 'Chalk. In the Ordovician we have in the Llandeilo 
 Beds the first appearance of Ischadites associated with 
 Hyalostelia. In the Bala Beds we meet with Astylospon- 
 gia. The most abundant Silurian form is Ischadites; 
 Astrceospongia and Hyalostelia also occur. Sponges 
 are rare in the Devonian, but Sphcerospongia and Re- 
 ceptaculites have been recorded. In the Carboniferous 
 rocks, sponges become much more common, the siliceous 
 spicules often forming thick beds of chert : the Mon- 
 actinellids are represented by Reniera, the Tetractinellids 
 by Oeodites, the Lithistids by Doryderma, the Hexacti- 
 nellids by Hyalostelia, and the Heteractinellids by the 
 two component genera. Sponges appear to be absent in 
 the Permian and they are rare in the Trias, except in the 
 St Cassian Beds of the Tyrol, where Calcispongise are 
 numerous. In the Jurassic sponges are extremely abun- 
 dant; the only Monactinellid is Spongilla from the Purbeck 
 Beds; Lithistids and Hexactinellids although abundant 
 in Germany and Switzerland are rare in England; the 
 Calcispongiae are fairly numerous in this country as well as 
 in France and Germany. But of all the systems, it is the 
 Cretaceous in which sponges are most abundant. They are 
 found chiefly at four horizons: (1) in the Lower Greensand 
 of Faringdon, Upware, Kent,, and Surrey ; (2) in the Upper 
 Greensand and Chloritic Marl of Warminster, Blackdown, 
 Haldon, and the Isle of Wight ; the Lithistids being here ' 
 very abundant, exceeding the Hexactinellids; the Calci- ;
 
 PORIFERA. 37 
 
 sponges are also common in some places ; (3) in the 
 Lower Chalk of the South of England, the Calcisponges 
 are however not represented here ; (4) in the Upper 
 Chalk, where the siliceous sponges are very common, the 
 Calcisponges rare. Lastly in the Tertiary formations few 
 sponges have been found.
 
 SUB-KINGDOM III. CCELENTERATA. 
 
 Classes. 
 
 1. Hydrozoa. 
 
 Sub-Classes. 
 
 1. Hydromedusa?. 
 
 2. Scyphomedusae. 
 
 3. Siphonophora. 
 
 4. Graptolitoidea. 
 
 5. Stromatoporoidea. 
 
 Orders. 
 
 Sub-Orders. 
 Actinaria. 
 Antipatharia. 
 
 3. Madreporaria. 
 
 1. Aporosa. 
 
 2. Rugosa. 
 
 3. Fungida. 
 
 4. Perforata. 
 
 (1. Zoantharia. 
 2. Alcyonaria. 
 
 THE Co3lenterates include the jelly-fishes, corals, and 
 allied forms. They differ from the sponges in the absence 
 of a canal-system, in the middle layer of the body wall 
 being very thin, and in the presence of thread-cells or 
 nematocysts. This sub-kingdom can be divided into two 
 classes, (1) the Hydrozoa, (2) the Actinozoa. 
 
 CLASS. HYDROZOA. 
 
 The Hydrozoans are characterised by not having the 
 digestive tract separate from the general cavity of the body 
 (ccelenteron), and by the latter not being divided into
 
 C(ELENTERATA. HYDROZOA. 39 
 
 compartments by partitions (mesenteries). The simplest 
 form of hydrozoan is the common freshwater Hydra. 
 In this the body consists of an elongated sac, at one end 
 of which there is an opening, serving as the mouth, the 
 animal being attached by the opposite end. The interior 
 of this sac is hollow and undivided, it is produced above 
 into a row of tentacles surrounding the mouth ; the whole 
 body is very contractile and constantly changing its shape. 
 Multiplication takes place (1) by the giving off of buds 
 which ultimately separate from the parent, (2) by fission, 
 and (3) sexually, by the production of ova and spermatozoa. 
 The sexual elements are developed in the ectoderm. In 
 many hydrozoans however, the sexual elements are pro- 
 duced in another individual (medusa), so that there is an 
 alternation of a sexual with an asexual generation. In a 
 large number of forms there are no hard parts what- 
 ever, others possess a skeleton which is produced by 
 the ectoderm, and may be chitinous as in Sertularia, or 
 calcareous as in Millepora. A simple hydrozoan, like 
 Hydra, is known as a polyp, but many forms are compound, 
 consisting of a number of polypites united together. All 
 the hydrozoans, except Hydra and Cordylophora are 
 marine. They may be divided into five sub-classes, 
 (1) HydromedusaB (or Hydroidea), (2) Scyphomedusse, 
 (3) Siphonophora, (4) Graptolitoidea, (5) Stromatoporoidea. 
 The Hydromedusae includes such modern genera as 
 Tubularia and Sertularia. Although there is frequently 
 a chitinous or calcareous skeleton, this sub-class is but 
 poorly represented in the fossil state. The living form 
 Hydractinia is found in the Cretaceous and later forma- 
 tions. Probably the genera Dendrograptus found in the 
 Cambrian and Ordovician, and Dictyonema which ranges 
 from the Cambrian to the Devonian, also belong to this
 
 40 CCELENTERATA. HYDROZOA. 
 
 group. The Hydrocorallinse in which the skeleton is 
 calcareous, are also placed here, Millepora being the best 
 known form, it ranges from the Eocene to the present day. 
 
 The Scyphomedusse includes such jelly-fishes as 
 Aurelia, Rhizostoma, and Pelagia, but as they possess no 
 hard parts it is not to be expected that they should be 
 found fossil ; nevertheless the impressions of some forms 
 (e.g. Rhizostomites) have been found in the Lithographic 
 Slate of Solenhofen. 
 
 The Siphonophora includes Physalia ('Portuguese 
 Man-of-War'), Physophora, and other forms, and is 
 unknown in the fossil state. 
 
 SUB-CLASS. GRAPTOLITOEDEA. 
 
 The Graptolitoidea includes the graptolites ; these are 
 altogether extinct, being found only in the Lower Palaeozoic 
 rocks, where owing to their abundance and to the limited 
 range in time of both genera and species, they are of 
 immense importance to the stratigraphical geologist. 
 They occur most commonly in argillaceous rocks, especially 
 in black shales, whilst they are rare in sandstones and 
 limestones. The graptolites were, like the modern 
 Sertularia, compound animals, consisting of a number of 
 polypites united' by a common ccenosarc, the whole being 
 protected by a skeleton of chitin. But the original 
 material of the skeleton is never preserved in the fossil 
 state, its place being taken by carbon or iron pyrites. 
 The entire skeleton is termed the polypary; this in a 
 simple form like Monograptus, consists of the following 
 parts. A more or less cylindrical tube known as the 
 common canal (fig. 8 b, c), extends the whole length of 
 the animal, the wall being termed the periderm. From
 
 HYDROZOA. GRAPTOLITOIDEA. 41 
 
 one side of the common canal a row of small cups or 
 tooth-like projections is given off, these are the hydrothecce 
 (fig. 8 b, A), and each one opens on the one hand into the 
 common canal and on the other to the exterior ; the latter 
 aperture, known as the mouth (m) of the hydrotheca, is 
 
 a b c 
 
 FIQ. 8. Morphology of Graptolites. a, Portion of Monograptus perso- 
 natus ; b, vertical section of the same ; c, Monograptus showing the 
 dorsal groove in which the sicula was placed ; d, transverse section 
 of Monograptus with dorsal groove and virgula ; e, Monograptus 
 colonus, Coniston Grits, with sicula; f, Didymograptus murchisoni, 
 Llandeilo Beds ; g, Diplograptus foliaceus, Llandeilo Beds, with the 
 position of the embedded sicula indicated ; h, sicula of Diplograptus, 
 the earliest stage ; i, the same with virgula and two buds developed. 
 (All enlarged except f.) 
 
 a, axis or virgula ; c, common canal ; h, hydrotheca ; m, mouth of 
 hydrotheca ; s, sicula.
 
 42 HYDROZOA. GRAPTOLITOIDEA. 
 
 frequently circular, but sometimes quadrangular or slit- 
 like. On the dorsal side of the polypary, that is, opposite 
 the hydrothecae, there is on the outer surface of the 
 periderm a longitudinal groove (fig. 8 c), in which is 
 placed a chitinous thread or rod, termed the axis or 
 virgula (fig. 8 d, a) ; in some cases this is solid, in others it 
 appears to be hollow. In some species of Monograptus 
 the virgula projects beyond the distal end of the common 
 canal. At the proximal end of the polypary there is a 
 small elongated cone-like body, of an embryonic nature, 
 termed the sicula (fig. 8 e, s). 
 
 With regard to the soft parts of the animal, speaking 
 from analogy with recent allied forms, we may suppose, 
 that each hydrotheca lodged an individual polypite, and 
 these were connected by means of the ccenosarc which 
 occupied the common canal. 
 
 In the form just described (Monograptus) the polypary 
 is always simple, but in many genera it is compound, 
 consisting of two or more branches. When the branches 
 are given off from a common centre, their proximal 
 parts are sometimes enclosed in a horny sheath termed 
 the central disc 1 , as in some species of Tetragraptus. 
 In some cases (e.g. Monograptus, fig. 8 e) the polypary 
 possesses only a single row of hydrothecae, such forms are 
 said to be monoprionidian ; others however (e.g. Diplo- 
 graptus, fig. 8 g) possess two rows on opposite sides of 
 the polypary, these are the diprionidian forms, and they 
 may have a single common canal (e.g. Retiolites), but 
 more usually there are two (e.g. Diplograptus), and when 
 this is the case, the polypary appears to consist of two 
 monoprionidian forms united by their dorsal surfaces. In 
 
 1 In some simple forms (e.g. Climacograptus bicornis) there is fre- 
 quently a similar disc at the proximal end.
 
 HYDROZOA. GRAPTOLITOIDEA. 43 
 
 the genus Dicranograptus, however, there is a transition 
 between these two types, its proximal end being diprioni- 
 dian, whilst its distal end is monoprionidian. The hydro- 
 thecse vary considerably in different genera, and even in 
 different species of the same genus; but in the same 
 species they are similar, except that they diminish in size 
 towards the proximal end of the polypary ; they may be 
 rectangular (e.g. Phyllograptus), tubular (e.g. Rastrites), 
 conical (e.g. Monograptus spiralis), or recurved (e.g. Mono- 
 graptus lobiferus). They may be in contact throughout 
 their entire length (e.g. Phyllograptus), at their bases only 
 (e.g. Ccenograptus), or entirely separate (e.g. Rastrites). 
 Frequently they are provided with one or more spines 
 near the mouth. By most authors the hydrothecse are 
 said to communicate freely with the common canal, in 
 this respect differing from the allied living forms, in which 
 there is a constriction, or a diaphragm, at the base of each 
 hydrotheca separating it from the common canal. But 
 McCoy and Hopkinson state that in some specimens, 
 as for instance Didymograptus extensus, Didymograptus 
 patulus, and Tetragraptus serra, from the Skiddaw Slates, 
 there is a distinct septum between the hydrothecse and 
 the common canal, and moreover in some cases the com- 
 mon canal itself is divided up by transverse septa. 
 
 The periderm of the graptolite is said by some authors 
 to be formed of three layers. In the genus Retiolites the 
 two inner layers are modified so as to form a fibrous net- 
 work, upon which the outer layer is stretched as a kind of 
 epiderm. 
 
 We saw that in Monograptus, the virgula sometimes 
 extends beyond the distal end of the polypary ; in some 
 genera (e.g. Diplograptus) it sometimes also extends beyond 
 the proximal extremity. In the majority of cases the vir-
 
 44 HYDROZOA. GRAPTOLITOIDEA. 
 
 gula is single, and is placed dorsally in the monoprionidian 
 genera, but in the diprionidian it is central, being situated 
 in the middle of the wall separating the two common 
 canals. Occasionally, however, it is double, as in Retio- 
 lites, in which the axes are separated and placed in the 
 wall on opposite sides of the common canal. The position 
 of the sicula also varies. In Monograptus it is united 
 to the dorsal surface of the polypary, the pointed end 
 being directed distally (fig. 8 e, s). In Didymograptus its 
 broad end only is united to the polypary, the pointed 
 end being directed downwards (fig. 8 f, s). In Dicello- 
 graptus, in which there are two branches diverging at 
 a wide angle, it forms a kind of axial spine. In Diplo- 
 graptus it is enclosed by the hydrothecas (fig. 8 g, s). 
 Lastly, in some forms it does not persist in the adult 
 state. In those genera which have two branches, the 
 term angle of divergence is applied to the angle between 
 the hydrothecal margins. 
 
 The appearance of even the same species of graptolite 
 varies considerably according to its mode of preservation. 
 Frequently it is flattened to a film, and when this is the 
 case we may get a side view, a front view showing the 
 mouths of the hydrothecse, or a back view ; in the two 
 latter cases the margins will be parallel. But when the 
 original material has been replaced by iron pyrites, the 
 natural form of the polypary is preserved. 
 
 The reproduction of the graptolites appears to have 
 been similar to that which takes place in some modern 
 hydrozoans. In a few diprionidian forms (fig. 9) sac-like 
 bodies have been found attached to the sides of the 
 polypary, these appear when perfect to be pear-shaped, 
 and in some cases at any rate, they are connected with 
 the virgula by means of fibres. These bodies resemble
 
 HYDROZOA. GRAPTOLITOIDEA. 45 
 
 in many respects the gonangia of living Sertularians, 
 and very probably represent them. In the latter the 
 
 FIG. 9. Diplograptus whitfieldi, with reproductive sacs. (After Hall.) 
 
 gonangia are chitinous sacs, within which in the calypto- 
 blastic hydroids 1 , the gonophores are developed, and these 
 give rise directly to the generative elements. The earliest 
 condition of the graptolite at present known, is the sicula 
 (fig. 8 h), this probably arises indirectly from the sac-like 
 bodies which have been supposed to be gonangia. The 
 first change which takes place in the sicula, is the forma- 
 tion of the virgula in the wall of one side ; this frequently 
 projects beyond one or both ends (fig. 8 i). Next there 
 appears a bud on one face of the sicula, frequently near 
 its broader extremity but sometimes near the point ; 
 this developes into the first hydrotheca, and is soon 
 
 1 These include such genera as Plumularia, Sertularia and Campanu- 
 laria.
 
 46 HYDROZOA. GRAPTOLITOIDEA. 
 
 followed by other buds, which in like manner pass into 
 hydrothecse. In the diprionidian forms the buds are 
 produced on two sides of the sicula ; they are sometimes 
 directed towards the point of the sicula, at other times 
 away from it. When two series are formed they may 
 either remain free or be united by their dorsal surfaces. 
 Owing to the fact that the soft parts of the graptolites 
 are entirely unknown, it is difficult to speak of their 
 affinities with any degree of certainty. There can be no 
 doubt, however, that they belong to the Hydrozoa ; and 
 Allman and others have referred them doubtfully to the 
 Hydromedusa3, being nearest to such forms as Sertularia 
 and Plumularia, with which they agree in the general 
 arrangement of the hydrothecae, and in having gonangia. 
 But they differ from them in possessing a virgula, in 
 being free forms, in the diminution in size of the hydro- 
 thecse towards the proximal end of the polypary, and in 
 the free communication which exists in many, if not all, 
 cases between the hydrothecae and the common canal. 
 The graptolites also resemble slightly some forms of 
 Polyzoa, as for instance Rhabdopleura, which possesses a 
 chitinous axis. 
 
 Monograptus. (fig. 8 e.) Polypary simple, unilateral, 
 monoprionidian, straight, curved or spiral. Hydrothecse 
 vary in form in different species. The pointed end of 
 the sicula is thrown back upon and attached to the dorsal 
 side of the proximal end of the polypary. Lower Llan- 
 dovery to Lower Ludlow. 
 
 Rastrites. Polypary similar to Monograptus, but 
 the common canal is very narrow, and the hydrothecffi are 
 long and widely separated. Lower Llandovery to Tarannon. 
 
 Cyrtograptus. Polypary similar to Monograptus, but
 
 HYDROZOA. GRAPTOLIT01DEA. 47 
 
 always curved and with branches given off from the 
 hydrothecal margin. Upper Tarannon to Lower Ludlow. 
 
 Ccenograptus. Polypary bilateral, monoprionidian. 
 The central non-celluliferous portion bent into an S-shape, 
 giving off branches from the convex portions. Hydrothecae 
 in contact only, not overlapping. Sicula median, tri- 
 angular. Upper Llandeilo. 
 
 Didymograptus. (fig. 8 f.) Polypary bilateral, mo- 
 noprionidian, consisting of two simple branches diverging 
 at an angle which varies from to 180 in different 
 species. HydrothecaB rectangular, in contact. The pointed 
 end of the sicula directed downwards. Lower Arenig to 
 Upper Llandeilo. 
 
 Tetragraptus. Polypary bilateral, monoprionidian, 
 consisting of four simple radiating branches. Hydrothecse 
 rectangular, in contact. A central disc may or may not 
 be present. Lower Arenig. 
 
 Dicranograptus. Proximal portion of the polypary 
 diprionidian, but dividing distally into two monoprionidian 
 branches. Terminations of the hydrothecse isolated and 
 incurved. Upper Llandeilo to Lower Bala. 
 
 Dicellograptus. Like Dicranograptus, but mono- 
 prionidian throughout, the two branches united by the 
 sicula only. Angle of divergence greater than 180. 
 Upper Arenig to Upper Bala. 
 
 Diplograptus. (fig. 8 g.) Polypary diprionidian. 
 Hydrothecse overlapping, and placed obliquely. Virgula 
 prolonged beyond the distal and sometimes also the 
 proximal extremity of the polypary. Sicula imbedded. 
 Lower Arenig to Tarannon.
 
 48 HYDROZOA. GRAPTOLITOIDEA. 
 
 Climacograptus. Polypary diprionidian. Hydro- 
 thecse placed vertically and separated by deep depressions. 
 Virgula prolonged beyond both extremities. Lower 
 Arenig to Tarannon. 
 
 Phyllograptus. Polypary leaf-like, formed of four 
 monoprionidian branches united along the whole of their 
 dorsal surfaces. Hydrothecae rectangular, in contact 
 throughout their entire length, and furnished with two 
 strong spines. Arenig. 
 
 Retiolites. Polypary simple, diprionidian, narrower 
 at the ends. There are two virgulas, which are feebly 
 developed, one being placed in each of the lateral faces of 
 the polypary. Hydrothecse rectangular, alternating, given 
 off from a single common canal. Inner layer of the 
 periderm is in the form of a chitinous network. Lower 
 Bala to Wenlock. 
 
 Distribution of the Graptolitoidea. 
 
 The oldest graptolites in Britain occur in the Tremadoc 
 Beds, where we get the genus Bryograptus ; in the 
 OZewws-shales of Gothland, Dichograptus and Clonograptus 
 also occur. These forms are also found in the lower part 
 of the Arenig Beds, but other genera are associated with 
 them and soon become abundant, such as Tetragraptus 
 and Didymograptus ; Phyllograptus also occurs here. 
 The Llandeilo graptolites are transitional between the 
 Arenig and Bala forms, Phyllograptus has died out, 
 Didymograptus is fairly common in the lower part, and 
 the genera Dicellograptus, Dicranograptus, Diplograptus 
 and Climacograptus now appear for the first time. In 
 the Bala Beds, these four last-mentioned genera become
 
 HYDROZOA. STROM ATOPOROIDEA. 49 
 
 much more abundant, and with them occur Leptograptus 
 and Pleurograptus. The only genera which pass up from 
 the Bala to the Silurian are Climacograptus, Diplograptus, 
 and Retiolites ; not a single Bala species (except perhaps 
 a variety of Climacograptus scalaris) is found in the 
 Llandovery Beds, so that between the Ordovician and 
 Silurian there is an enormous break in the graptolitic 
 succession. As a whole the Silurian formations are 
 characterised by the presence of the family Monograp- 
 tidce 1 , which appears first at the base of the Llandovery 
 Beds. In the lower part of the Llandovery the genera 
 Diplograptus and Climacograptus are fairly abundant, but 
 they become extinct in the Tarannon, and in the Wenlock 
 and Ludlow Beds the only forms are Monograptus, Cyrto- 
 graptus, and Retiolites. The last traces of graptolites 
 occur in the Downtonian Beds, but they are too imperfect 
 for determination. 
 
 SUB-CLASS. STROMATOPOROIDEA. 
 
 In the Stromatoporoids the skeleton is calcareous, and 
 very variable in form ; it may be hemispherical, spheroidal, 
 dendroid, encrusting, or altogether irregular, frequently 
 forming large masses. It consists of a series of concentric 
 laminae separated by interlaminar spaces ; both are crossed 
 at right angles by a series of rods or pillars, which give off 
 horizontal processes at definite intervals, and these joining 
 together really form the porous lamina. The stromatopo- 
 roids were compound animals, the polypites in some cases 
 simply occupying pores in the external lamina, in others 
 there were definite tubes. The Stromatoporoids are 
 most closely related to such living forms as Hydractinia 
 
 1 This includes the genera, Monograptus, Rastrites and Cyrtograptus. 
 
 w. p. 4
 
 50 
 
 CCELENTERATA. ACTINOZOA. 
 
 and Millepora. They are found only in the Ordovician, 
 Silurian, and Devonian Systems, being most abundant in 
 the last ; frequently they are of considerable importance 
 as rock-builders. Some of the best known genera are 
 Labechia, Stromatopora, Actinostroma, and Clathrodictyon. 
 
 CLASS. ACTINOZOA. 
 
 This includes the corals and sea-anemones. They 
 differ from the Hydrozoa (1) in possessing an alimentary 
 tube, the oesophagus, which is distinct from the coelenteron 
 but at the same time communicates freely with it ; (2) in 
 the ccelenteron being divided up into chambers by vertical 
 radiating partitions known as mesenteries; (3) in the 
 reproductive elements being developed within the polyp 
 and never on a medusa. 
 
 In a typical actinozoan, such as the common sea- 
 anemone Actinaria, the body has a more or less cylindrical 
 
 VLV- 
 
 FIG. 10. Diagram-sections of a sea-anemone. A, vertical. B, horizon- 
 tal, through the upper part of the polyp. C, through the lower 
 part, a, tentacles ; 6, mouth ; c, cesophageal tube ; d, coelenteron ; 
 
 form, and is attached by one end, the other having an 
 opening, the mouth (fig. 10 6), surrounded by tentacles (a). 
 The mouth leads into the oesophageal tube (c), which opens
 
 CCELENTERATA. ACTINOZOA. 51 
 
 at its lower end into the body-cavity (d). The latter is 
 divided into chambers by folds of the endoderm, the mesen- 
 teries (e) ; in the upper part of the polyp, their inner 
 edges join the oesophageal tube, in the lower part they 
 remain free, so that a section in the former region (fig. 
 10 B} will show the body wall and also the oesophageal 
 wall, but in the latter (G) the body wall only. The tenta- 
 cles are placed immediately above the intermesenteric 
 chambers, and the space in each tentacle is continuous 
 with that of the chamber below. In Actinaria as in the 
 other sea-anemones there is no skeleton, but the majority 
 of actinozoans possess one, which in many cases is external 
 to the body, being formed of carbonate of lime, secreted 
 by the ectoderm ; this is said to be sclerodermic : in others 
 it is formed internally and consists of calcareous spicules, 
 or there is a coenosarcal axial rod of horny or calcareous 
 material, this last being a sclerobasic skeleton. The Acti- 
 nozoa can be divided into three orders, (1) Zoantharia, 
 (2) Alcyonaria, (3) Ctenophora. The Monticuliporoidea 
 may also perhaps belong to this class, but they are con- 
 sidered by several authors to be Polyzoa. 
 
 OBDEK. ZOANTHAKIA. 
 
 In the Zoantharia, the tentacles and mesenteries are 
 generally numerous and are never eight in number as is 
 the case in the Alcyonaria, but frequently a multiple of 
 six or sometimes four. The tentacles are always simple. 
 A skeleton is usually present and it may be calcareous 
 or horny. 
 
 There are three sub-orders (1) the Actinaria, including 
 the sea-anemones, which possess no skeleton and are 
 consequently unknown in the fossil state ; (2) the Anti- 
 
 42
 
 52 ACTINOZOA. MADREPORARIA. 
 
 patharia, in which the skeleton consists of an internal 
 horny rod ; these are also not found fossil ; (3) the 
 Madreporaria, including the well-known stony corals, in 
 which the skeleton is calcareous, these are very abundant 
 as fossils. 
 
 SUB-ORDER. MADREPORARIA. 
 
 There is a sclerodermic skeleton, consisting of carbo- 
 nate of lime, secreted by the ectoderm of the lower part 
 of the polyp. The entire skeleton is spoken of as the 
 corallum] it may be simple or compound, in the latter 
 case each individual is termed a corallite. 
 
 In a typical simple coral, the skeleton has a more or 
 less conical form, the base of the cone is depressed and 
 termed the calyx, in it the polyp was placed. The wall 
 bounding the corallum is known as the theca (fig. 11 A, d) 
 sometimes there is outside this, another calcareous layer, 
 
 FIG. 11. A. Diagram-section (horizontal) of a simple coral, a, colum- 
 ella; 6, primary septa; c, pali ; d, theca; e, dissepiments. 
 
 B. Portion of a horizontal section of Cyathophyllum murchisoni, 
 Carboniferous Limestone, showing the septa composed of a thin 
 middle layer with a thicker layer on each side (magnified). 
 
 the epitheca. The whole space enclosed by the theca is 
 termed the visceral chamber; this is divided up by
 
 ACTINOZOA. MADREPORARIA. 53 
 
 numerous calcareous partitions, the most important of 
 which are the septa (fig. 11 A, 6). These are vertical 
 plates running from the theca towards the centre, and 
 alternating with the mesenteries. They are of different 
 sizes, some reaching the centre, others going only part of 
 the distance ; they frequently occur in cycles, of which 
 three or more may often be distinguished, the largest 
 being the primary (6), the others the secondary, tertiary 
 and so on. In some forms one or more of the primary 
 septa are absent, and we have what are known as septal 
 fossulce. When the septa project upwards above the edge 
 of the theca, they are said to be exsert. Sometimes they are 
 only feebly developed, being represented by rows of spines. 
 When seen in section the septa are frequently found 
 to consist of three layers (fig. 11 B), the middle being 
 darker and thinner than the outer, and apparently repre- 
 senting the original septum, the outer layers having been 
 formed subsequently. In the centre of the visceral 
 chamber, where the larger septa come together, there is 
 often a vertical rod, which extends from the base of the 
 chamber to the bottom of the calyx, this is the columella 
 (figs. 11 A, a; 13 c). Its structure varies considerably; 
 when it is solid and ends in a knob or point in the calyx, 
 it is termed styliform ; if the top is porous, it is said to 
 be spongy ; when it consists of twisted laminae it is termed 
 trabeculate ; if formed by the twisting together of processes 
 given off from the inner edges of the septa, it is false ; 
 and lastly in some forms it is altogether absent. Another 
 set of vertical partitions somewhat similar to the septa are 
 the pali (fig. 11 A, c), these are radiating plates attached 
 to the columella and placed opposite the inner edges of 
 some of the shorter septa, but not joining them. Passing 
 from one septum to another, there are often rods known as
 
 54 
 
 ACTINOZOA. MADREPORARIA. 
 
 synapticulce. Similarly uniting two septa, there are plates, 
 which may be horizontal or oblique, straight or curved, 
 these are the dissepiments (figs. 1 1 A, e ; 13d). Lastly we 
 have the tabulce (fig. 13 B, t), which instead of merely 
 passing across the space between two septa, cut across 
 several, in a typical case going right across the visceral 
 chamber horizontally; but frequently they extend only 
 partly across. The tabulae are arranged one above 
 the other, so that the visceral chamber is divided into 
 horizontal compartments. On the outside of the wall of 
 the coral there are in some forms vertical ridges, which 
 may be smooth or spiny, these are known as the costce, and 
 they usually correspond with the septa. 
 
 A simple coral like that just described may become 
 compound either by budding or by fission. Three kinds 
 of budding may be distinguished, (1) the lateral (fig. 12 A), 
 in which the buds are given off from the sides of the 
 
 FIG. 12. A. Dendrophyllia nigrescens, showing lateral budding, Recent, 
 (reduced one-half). B. Cyathophyllum truncatum, showing calicular 
 budding, Wenlock Limestone, (natural size). C. Syringopora, 
 showing basal budding, Carboniferous Limestone, (natural size). 
 
 polyp, (2) the calicular (fig. 12 B), in which they arise 
 from the calyx, (3) the basal (fig. 12 C), in which the
 
 ACTINOZOA. MADREPORARIA. 55 
 
 buds spring from branches or stolons, which are given off 
 from the base of the corallite. Fission commences by 
 the mouth becoming slightly constricted, this goes on 
 until two mouths and two polyps are formed ; and this 
 division is of course repeated in the skeleton. When in 
 a compound form, the individual corallites are free from 
 one another, the corallum is termed dendroid (fig. 12 A] ; 
 when they are in contact, it is massive. Sometimes the 
 spaces between the individual corallites are filled up with 
 calcareous material formed by the coenosarc, and known as 
 the casnenchyma. 
 
 The Madreporaria 1 are divided into four sections (1) 
 Aporosa, (2) Rugosa, (3) Fungida, (4) Perforata. 
 
 Section /. Aporosa. 
 
 The theca and septa are not perforated but solid. 
 Dissepiments, and sometimes also tabulae and synapticulse, 
 are present. The septa are generally in multiples of six. 
 
 Montlivaltia. Corallum simple, fixed or free; tur- 
 binate, cylindrical, or discoidal. Epitheca well developed, 
 theca feeble. Columella absent. Septa numerous, strong 
 and dentate, often exsert. Dissepiments abundant. In 
 England, Lias to Corallian; foreign, Trias to Recent. 
 
 Isastraea. Compound, massive ; calyces polygonal. 
 Walls of the corallites fused along their entire length. 
 Columella rudimentary or absent. Septa thin and close 
 
 1 Until recently one of the divisions of the Madreporaria was the 
 Tabulata. This was characterised by the well-developed tabulae, and the 
 rudimentary septa ; but it is now considered to be an artificial group and 
 its genera are placed in the Aporosa, Perforata, and Alcyonaria. It 
 included amongst others the following, Heliolites, Favosites, Syringopora, 
 and Halysites.
 
 56 ACTINOZOA. MADREPORARIA. 
 
 together. Dissepiments fairly abundant. Lias to Upper 
 Greensand. 
 
 Holocystis. Compound, massive, convex ; calyces 
 polygonal. Columella small, styliform. Corallites united 
 by their walls or by costae. The four primary septa are 
 much better developed than the others. Tabula3 present. 
 Lower Greensand. 
 
 Thecosmilia. Compound, branching or rarely almost 
 massive. Multiplication by fission. Margin of calyces 
 irregular. Columella rudimentary or absent. Septa, 
 strong, dentate, and more or less exsert. Dissepiments 
 abundant. Epitheca thick and folded, but often not 
 preserved. Lias to Corallian. 
 
 Section II. Rugosa. 
 
 The corallum is compact and imperforate, and gene- 
 rally bilaterally symmetrical. The septa are usually in 
 multiples of four, there being four primary septa. In 
 most cases the septa are of two sizes, the longer alternating 
 with the shorter. Dissepiments are abundant and tabulae 
 are often present. One or more septal fossulae usually 
 occur. There is no coenenchyma. 
 
 Cyathophyllum. Simple or compound ; branching 
 or massive. Septa numerous, of two sizes, alternating, 
 the longer giving rise to a false columella. Tabulae small, 
 occupying the central part only of the visceral chamber. 
 Dissepiments form an extensive peripheral zone of vesi- 
 cular tissue. Bala to Carboniferous Limestone. 
 
 Acervularia. Compound, massive ; corallites with 
 two walls, the outer polygonal, frequently hexagonal, the 
 inner circular. Septa well developed, reaching the centre.
 
 ACTINOZOA. MADREPORARIA. 
 
 57 
 
 Columella absent. Tabulae extending across the central 
 part of the visceral chamber. Dissepiments form a peri- 
 pheral vesicular zone. Budding calicular. Silurian to 
 Devonian. 
 
 Lithostrotion (fig. 13). Compound, massive or den- 
 droid. When massive the corallites are prismatic, when 
 dendroid they are cylindrical. Septa well developed, 
 alternately long and short. Columella styliform, laterally 
 compressed. Peripheral zone of dissepiments narrow. 
 Tabulae wide, occupying the centre of the visceral 
 
 d w c t d 
 
 FIG. 13. Lithostrotion basaltiforme, Carboniferous Limestone. A. Hori- 
 zontal section of a- single corallite, x 2. B. Vertical section, x 5; 
 c, columella ; t, tabulae ; d, dissepiments ; w. theca. 
 
 chamber. Septal fossulae well marked. Carboniferous 
 Limestone. 
 
 Omphyma. Simple, turbinate. Septa numerous, 
 alternately long and short, but extending only a short 
 distance into the visceral chamber, the central part being 
 occupied by the tabulae. The four primary septa are 
 placed in shallow fossulse. Peripheral zone of dissepiments 
 narrow. The theca gives off root-like processes. Bala to 
 Lower Ludlow. 
 
 Zaphrentis. Simple, free, bilateral ; turbinate, coni- 
 cal or cylindrical, often curved; calyx deep. A well
 
 58 ACTINOZOA. MADREPORARIA. 
 
 marked septal fossula, containing a septum which is 
 generally small. Septa numerous. Tabulae well developed, 
 extending quite across the visceral chamber. Dissepiments 
 few, never forming a peripheral vesicular zone. Columella 
 absent. Theca thick. Wenlock Limestone to Carboni- 
 ferous Limestone. 
 
 Calceola. Simple, conical or slipper-shaped, one side 
 is flat, the opposite convex ; calyx very deep and closed 
 by a semi-lunar operculum, which has on its inner surface 
 a strongly marked median ridge and several less prominent 
 lateral ridges. Septa rudimentary, being represented by 
 striae. An epitheca is present. Middle Devonian. 
 
 Section III. Fungida. 
 
 The corallum is simple or compound. The calyces are 
 united by their costa3. Synapticulae are present, and 
 sometimes also a columella and dissepiments. Septa 
 generally solid. 
 
 Thamnastraea. Compound, massive ; convex or 
 laminar. Walls of the corallites indistinct. Calyces 
 shallow. Septa of adjoining corallites confluent. Colu- 
 mella variable. Dissepiments and synapticulae present. 
 In England, Lias to Upper Greensand ; foreign, Trias to 
 Miocene. 
 
 Section IV. Perforata. 
 
 The corallum is porous or reticulate. The visceral 
 cavities of adjoining corallites are placed in communication 
 with one another by means of perforations in their walls. 
 The septa sometimes consist of trabeculae, or are repre- 
 sented by spines only. Dissepiments and tabulae may or 
 may not be present.
 
 ACTINOZOA. MADREPORARIA. 59 
 
 Litharaea. Compound, massive. Calyces more or 
 less polygonal. Septa well developed, the faces spiny, the 
 upper edges dentate. Walls of the corallites reticulate. 
 Columella trabecular. In England, Bracklesham Beds ; 
 foreign, Eocene to Miocene. 
 
 Favosites. Compound, massive, sometimes branched. 
 Corallites long and polygonal, the walls are in contact but 
 not fused, and they are perforated by pores arranged in 
 a row along each face, known as ' mural pores.' Tabulae 
 numerous, regular, generally extending quite across the 
 visceral chamber. Septa represented by rows of spines. 
 Bala to Carboniferous Limestone. 
 
 Syringopora. Compound, fasciculate; corallites tu- 
 bular, for the most part not in contact. The visceral 
 chambers of the different corallites are placed in communi- 
 cation by means of horizontal connecting tubes. Tabula3 
 numerous, funnel-shaped. Septa feebly developed, gene- 
 rally represented by rows of spines. Budding basal. 
 Llandovery to Carboniferous Limestone. 
 
 OBDEK. ALCYONAEIA. 
 
 There are eight mesenteries and eight tentacles, the 
 latter being pinnate. The skeleton varies considerably, it 
 may consist of isolated calcareous spicules in the soft parts 
 (e.g. Alcyoniuni), of an internal axial rod formed of horny 
 material (e.g. Gorgonia) or of carbonate of lime (e.g. 
 Pennatula), or of both (e.g. Isis), or it may be external in 
 the form of calcareous tubes (e.g. Tubipora). 
 
 Heliolites. Corallum compound, massive or branching, 
 not spicular, formed of tubes of two sizes, the larger 
 circular ones are known as autopores, between these come 
 the smaller polygonal siphonopores. Tabula? occur in both, 

 
 60 ACTINOZOA. 
 
 and in the autopores there are septa also, which may be 
 lamellar, spiny, or rudimentary, and are usually twelve in 
 number. Bala to Devonian. 
 
 Halysites. Compound, corallites long and tubular, 
 arranged in a single row and united at their sides so as to 
 form laminae, which intersect ; in some species the coral- 
 lites are of two sizes. Epitheca thick. Septa absent or 
 represented by spines. Tabulae well developed, horizontal 
 or concave. Llandeilo to Wenlock. 
 
 Distribution of the Actinozoa. 
 
 From the point of view of their distribution at the 
 present day, the Madreporaria may be divided into two 
 groups, the solitary and the reef-building. Each of these 
 includes representatives of the three sections, Aporosa, 
 Fungida, and Perforata. The solitary corals are almost 
 entirely confined to deep water, occurring down to at 
 least 2,900 fathoms, and are found in almost all latitudes. 
 The species likewise have a wide distribution, apparently 
 not being affected by conditions of temperature and depth. 
 It might therefore be expected that they would also have 
 a long range in time ; this however is not the case, for 
 existing forms extend but a short way back into the 
 geological record, thus not a single living species is found 
 fossil in the English Cainozoic formations. The distribu- 
 tion of the reef-building corals unlike that of the solitary 
 forms is limited by both depth and temperature. Thus 
 they are found only in shallow water, not usually extending 
 lower than depths of 20 or 30 fathoms, and only where the 
 temperature of the ocean is not less than 68 F., flourishing 
 best in water even warmer than this. Like the solitary 
 corals, the reef-building genera of the present day have
 
 ACTINOZOA. 61 
 
 but a very limited geological range. As a general rule, 
 corals can only exist in salt water, there are however one 
 or two exceptions, thus Cylicia rubeola flourishes in the 
 river Thames, New Zealand. Clear water is likewise 
 generally necessary, but one species, Porites limosa, 
 thrives in muddy situations. In geological times the 
 reef-building corals had a much wider geographical range 
 than they have at the present day. Thus they occur 
 abundantly in various formations in temperate and also in 
 polar regions. But it does not necessarily follow from 
 this, that these forms required the same temperature as 
 their living representatives. 
 
 The only extinct section of the Madreporaria is the 
 Rugosa ; this is confined to the Palaeozoic period, ranging 
 from the Ordovician to the Carboniferous. The Aporose 
 corals also appear first in the Ordovician system, the 
 earliest genus being Columnaria ; but they are very much 
 more abundant in the Mesozoic and Cainozoic periods 
 than in the Palaeozoic. The Perforata commence in the 
 Ordovician and range on to the present day, but the 
 forms occurring in' the Palaeozoic are almost entirely 
 confined to that period. The Palaeozoic families of the 
 Alcyonaria are also almost entirely unrepresented in 
 later formations, and very few of the modern forms occur 
 fossil, but Corallium is found in the Jurassic, and forms 
 allied to Pennatula and Gorgonia occur in the Cretaceous. 
 The Ctenophora do not occur fossil. The Monticuliporoids 
 (regarded by some as Polyzoa) range from the Ordovician 
 to the Permian, and they include Monticulipora, Fistulipora 
 and other forms. From all these considerations it follows 
 that there must be a marked contrast between the coral 
 faunas of the Palaeozoic and the Neozoic periods. 
 
 Fossil corals are most abundant in calcareous rocks
 
 62 ACTINOZOA. 
 
 and comparatively rare in argillaceous and arenaceous 
 beds ; this is indeed what might be expected, since 
 existing forms can, as a general rule, live only in clear 
 water ; consequently very many limestone rocks are formed 
 almost entirely of coral remains. In North America corals 
 are abundant in the Ordovician, but in England only a 
 few forms have been found; they are much more abundant 
 in the Silurian, especially in the Wenlock Limestone, the 
 chief genera being Haly sites, Omphyma, Acervularia, Fa- 
 vosites, Heliolites, and Cyathophyllum. In the Devonian, 
 the last four are still well represented ; Zaphrentis and 
 Calceola also occur. The genera Lithostrotion, Amplexus, 
 and Michelinia are confined to the Carboniferous System 
 in England, and the following do not extend beyond 
 it, Zaphrentis, Syringopora, Favosites, and Cyathophyl- 
 lum. In the Permian and Trias corals are rare. In the 
 Lias a few forms have been found, such as Montlivaltia, 
 Thamnastrcea, Thecosmilia, and Isastrcea. In the Oolites, 
 these four genera become extremely abundant in England, 
 and many others also occur. In the Cretaceous formations, 
 corals become much rarer, the chief being Holocystis, Micra- 
 bacia, and Parasmilia. Corals are also rare in the English 
 Cainozoic ; Litharcea, Dendrophyllia, and Oculina occurring 
 in the Eocene, Madrepora in the Oligocene, and Flabellum 
 in the Pliocene.
 
 SUB-KINGDOM IV. ECHINODERMATA. 
 
 Classes. Orders. 
 
 1. Asteroidea 
 
 J 1. Encrinasteriffi. 
 {2. Eusasteroidea. 
 
 2. Ophiuroidea. 
 
 
 3. Echinoidea 
 
 J 1. Palffiechinoidea. 
 1 2. Euechinoidea. 
 
 4. Holothuroidea. 
 
 
 5. Crinoidea , 
 
 ( 1. Palaeocrinoidea. 
 " 1 2. Neocrinoidea. 
 
 6. Cystidea. 
 
 
 7. Blastoidea. 
 
 
 THE animals included in this sub-kingdom are all 
 marine; they comprise the starfishes, brittle-stars, sea- 
 urchins, sea-lilies, sea-cucumbers, and the extinct blas- 
 toids and eystideans. The body is very often radially 
 symmetrical, the symmetry being generally pentamerous. 
 But in many cases there is a more or less well-marked 
 bilateral arrangement of parts. Unlike so many of the 
 coelenterates the echinoderms are never colonial forms. 
 In the majority of cases the alimentary cavity terminates 
 in an anus. A distinct vascular system is present, and 
 also a set of vessels containing a watery fluid, known as 
 the ivater-vascular system. This consists of a ring round 
 the ossophagus from which five radiating trunks are given 
 off, and it functions generally in locomotion, and probably
 
 64 ECHINODERMATA. 
 
 also in respiration. A nervous system is also found 
 having a distribution similar to that of the water-vascular 
 system. 
 
 In all the echinoderms there is a dermal skeleton. 
 This is calcareous and consists of isolated pieces, or more 
 usually of rods or plates, often forming a complete exo- 
 skeleton, which may be either flexible or rigid; spines 
 
 FIG. 14. A. Portion of transverse section of a spine of a sea-urchin 
 Echinometra, Eecent (magnified). B. Section of interambulacral 
 plate of recent Cidaris parallel to the surface (magnified). 
 
 and other processes are also often present, attached to 
 the plates. When examined microscopically each part 
 of the skeleton is found to be formed of a network of 
 calcareous rods (fig. 14). The details of the structure 
 vary in different forms, depending on the size and shape 
 of the spaces between the rods. In the spines of the sea- 
 urchins the network of rods is frequently arranged so as 
 to form radiating perforated plates (fig. 14 A), which are 
 united by rods placed at right angles to them. The 
 axial portion of the spine often has a coarser structure 
 than the other parts. Another characteristic feature of 
 the skeleton is that each component part shows a uniform 
 optic orientation, being in fact a crystal of calcite, differing 
 only from an ordinary crystal in not having crystal
 
 ECHINODERMATA. 65 
 
 contours and in possessing the netted structure. In a 
 plate the principal crystallographic axis is at right angles 
 to the surface, in a spine it is parallel to the length. In 
 fossil forms the spaces in the network of rods generally 
 become filled with calcite, this being deposited in crystalline 
 continuity with that forming the plate or spine. In such 
 cases the characteristic cleavage of calcite becomes well 
 marked, so that when the plate or spine is broken, the 
 fracture passes along the cleavage planes, instead of being 
 irregular as in the recent forms. By the infiltration of 
 calcite and the development of cleavage, the organic 
 structure in the fossils is sometimes more or less com- 
 pletely destroyed. 
 
 Reproduction in the echinoderms is mainly sexual, 
 and as a rule the sexes are separate, but they do not 
 differ externally. The Echinodermata are divided into the 
 following seven classes: (1) Asteroidea; (2) Ophiuroidea ; 
 (3) Echinoidea; (4) Holothuroidea ; (5) Crinoidea; (6) 
 Cystidea; (7) Blastoidea. 
 
 CLASS. ASTEROIDEA. 
 
 The star-fishes constitute the class Asteroidea. In 
 these the body is sometimes pentagonal in outline as in 
 Goniaster, but in the majority of cases it is more or less 
 star-shaped, consisting of a central part, the disc, which is 
 produced into five arms or rays. In a few forms there are 
 more than five arms, as in Solaster papposus which has 
 thirteen. 
 
 On the under surface of the animal are placed the 
 
 mouth and tube-feet. The mouth is in the centre of the 
 
 disc, and along the arms are rows of tube-feet, these are 
 
 tubular processes connected with the radial water-vascular 
 
 w. P. 5
 
 66 ECHINODERMATA. ASTEROIDEA. 
 
 vessel. This surface then is the oral, ambulacral, or 
 ventral. The upper surface is aboral, anti- ambulacral, or 
 dorsal ; on it occurs the anus, a small opening placed near 
 the centre of the disc. In some forms the anus is absent. 
 Also on the dorsal surface between two of the rays there 
 is a porous plate, the madreporic tubercle; sometimes more 
 than one is present. 
 
 On the ventral surface of each arm, there is a deep 
 longitudinal groove, the ambulacral groove ; this is formed 
 by two rows of plates known as the ambulacral ossicles, 
 (fig. 15 a) which are inclined to each other so as to form 
 
 b e e 
 
 FIG. 15. Section of the arm of a star-fish (Astropecten). a, ambulacral 
 ossicles; b, adambulacral plates; c, inferior marginal plates with 
 spines ; d, superior marginals ; e, radial water vessel : /, ampulla ; 
 <7, tube-feet. 
 
 an obtuse angle ; the ossicles on opposite sides of the groove 
 are movably articulated at their dorsal surfaces. Along 
 each row of ambulacral ossicles, there is a row of pores, 
 each pore coming between two plates and being formed by 
 the apposition of two notches on the oral and distal faces 
 of adjoining plates ; generally the pores are arranged in a 
 straight line, but in some forms as in the common star- 
 fish, Asterias rubens, they are zigzag, being alternately 
 near to, and distant from, the middle of the ambulacral 
 groove. External to the rows of ambulacral ossicles there 
 is on each side of the groove, another row of plates, the 
 adambulacral plates (b). In some forms beyond this row,
 
 ECHINODERMATA. ASTEROIDEA. 67 
 
 and coming between the dorsal and ventral surfaces of the 
 arms there are one or two rows known as the marginal 
 plates (c, d}. The dorsal surface of the skeleton is formed 
 of a meshwork of calcareous rods with membranous inter- 
 spaces. Short spines are often abundant on the plates. 
 Pedicellariae also occur, especially on the dorsal surface ;' 
 they are spines having a pincer-like termination, and 
 probably serve to remove excrement from the surface of 
 the body. Projecting into the mouth at the angles formed 
 by the ambulacral grooves are five pairs of plates, which 
 give the mouth a star-shaped form. The mouth leads 
 into the globular stomach, from which a small projection 
 passes a short way into each arm, the stomach narrows 
 and passes into the pentagonal pyloric sac, from the 
 angles of which five processes are given off, and pass into 
 the arms near the aboral surface and soon divide. From 
 the pyloric sac a short narrow intestine leads to the anus. 
 The water-vascular system consists of a vessel forming a 
 ring round the mouth (fig. 16 a), from which five branches 
 (6) radiate off, one down each arm, placed in the ambula- 
 cral groove and consequently outside the skeleton. Each 
 of these radial vessels gives off two rows of processes on 
 opposite sides, which pass on the one hand through the 
 pores between the ambulacral ossicles into vesicles situ- 
 ated dorsally to the ossicles and known as the ampullce (g} } 
 and on the other hand into the tubular processes known 
 as the tube-feet (f), which project along the ventral 
 surface of the arm and are provided at their extremities 
 with sucking discs. It is by means of these tube-feet 
 that the animal moves. The ampullae contract, forcing 
 water into the tube-feet, which then expand and become 
 attached to some foreign object, afterwards the ampullae 
 become distended causing the tube-feet to contract. The 
 
 52
 
 68 
 
 ECHINODERMATA. ASTEROIDEA. 
 
 water-vascular system is placed in communication with 
 the sea- water by means of a canal (fig. 16 d) passing from 
 the circular vessel to the dorsal surface of the disc and 
 
 FIG. 16. Diagram of the water- vascular system of a starfish, a, circular 
 vessel round the mouth ; b, radial vessels ; c, Polian vesicles ; 
 d, stone-canal; e, madreporic plate; /, tube-feet (only a few shown); 
 <7, ampulla. 
 
 ending in the madreporic tubercle. This is known as the 
 stone-canal on account of the deposit of carbonate of lime 
 in its walls. 
 
 The distribution of the nervous system is similar to 
 that of the water-vascular system. It consists of a ring 
 round the mouth from which a branch goes down the 
 ambulacral groove of each arm, being situated immediately 
 under the epidermis, and external to the water-vascular 
 vessel ; each radial nerve terminates at the end of the ray 
 in an eye. The blood vascular system consists of a dorsal 
 and a ventral ring, one round the mouth and the other 
 round the anus, from each of which radial branches are
 
 ECHINODERMATA. ASTEROIDEA. 69 
 
 given off; these two circular vessels are connected by 
 means of a tube passing along the stone-canal, and some- 
 times regarded as a heart*. The genital glands occur in 
 pairs at the base of each arm and they open to the 
 exterior between the rays on the dorsal surface. 
 
 ORDER. ENCRINASTERI^. 
 
 The ambulacral ossicles alternate on either side of the 
 ambulacral groove. 
 
 Palaeaster. Arms five, thick, convex, short or of 
 moderate length. The dorsal surface formed of rows of 
 small ossicles provided with spines. There is a row of 
 adambulacral plates and also a row of large marginal 
 plates. Madreporic tubercle small. Bala Beds to Car- 
 boniferous. 
 
 ORDER. EUASTEROIDEA. 
 
 The ambulacral plates are placed opposite each other. 
 
 Astropecten. Body flattened. Arms five, long. 
 Two rows of marginal plates, the ventral row carries 
 spine-like scales, which increase in size outwards and 
 terminate in long spines ; the dorsal row granulated and 
 often spinous. Dorsal surface of the disc and rays fur- 
 nished with small appendages which are covered with 
 bristly prolongations. Anus absent. Lias to present day. 
 
 Distribution of the Asteroidea. 
 
 The Asteroidea have a wide distribution in the ocean 
 at the present day ; they are most abundant at moderate 
 depths, but also occur in abyssal regions. 
 
 The fossil forms do not differ greatly from the modern 
 
 * A different interpretation of the vascular system has been given 
 by CuSnot.
 
 70 ECHINODERMATA. ASTEROIDEA. 
 
 ones. The Encrinasteroidea are confined to the Palaeozoic. 
 A few Euasteroids occur in the Devonian and Carbon- 
 iferous, but they are mainly Neozoic forms. The star- fishes 
 occur first in the Bala Beds, where we get the genus 
 Palceaster. They are more abundant in the Silurian, 
 where Palceaster, Palceasterina, and Palceocoma occur. 
 Palceaster and other forms are found in the Devonian ; in 
 the Jurassic, Uraster, Solaster, Astropecten, Plumaster, 
 Oreaster and Goniaster. The two last also occur in the 
 Cretaceous. In the Cainozoic rocks star-fishes are rare. 
 
 CLASS. OPHIUROIDEA. 
 
 In the Ophiuroids or brittle-stars, the body consists 
 of a disc and arms. The arms are generally five in 
 number, usually simple, but in some cases branched ; they 
 are much smaller than in the star-fishes and are merely 
 appendages of the disc and not prolongations. Usually 
 they are long, cylindrical and very flexible, serving for 
 locomotion by means of movements which take place 
 chiefly in a horizontal direction. Unlike the arms of the 
 star-fish they do not contain any prolongations of the 
 alimentary canal, and the ambulacral groove is not open 
 to the exterior. The arrangement of the nervous, the 
 water-vascular, and the vascular systems is similar to that 
 found in the Asteroidea; but the tube-feet are not pro- 
 vided with sucking-discs and there are no ampullae. There 
 is no anus and pedicellariae are absent. 
 
 Generally there is a well-developed skeleton, but in 
 some cases granules only are found. In a typical case the 
 following parts are found. Encasing the arms, there are 
 four rows of plates (fig. 17), one dorsal, one ventral and 
 two lateral. The lateral are provided with rows of spines.
 
 ECHINODERMATA. OPHIUROIDEA. 71 
 
 Between the lateral plates and the ventral there are 
 apertures for the passage of the tube-feet. The greater 
 part of the space enclosed by the four rows of plates is 
 filled up by a longitudinal row of ossicles (d) ; each ossicle 
 consists of two halves fused together along the median 
 vertical line and in this part is immovable. But each 
 ossicle in the row articulates with the preceding and 
 succeeding one, so that the arm is flexible. On the 
 
 FIG. 17. Section of the arm of an Ophiuroid (Ophioglypha). a, dorsal 
 plate; b, lateral plate; c, ventral plate; d, ambulacral ossicles fused 
 along the median vertical line ; e, ambulacral groove. 
 
 ventral surface of the row there is a groove (e) in which 
 the radial vascular and water-vascular vessels, and the 
 nerve cord are placed ; this corresponds to the ambulacral 
 groove of the star-fish and the plates above to the ambula- 
 cral ossicles. 
 
 Coming now to the disc, this is usually round or 
 pentagonal. On its ventral surface (fig. 18 A) the arms 
 are prolonged so as to reach the mouth. The last pair of 
 ambulacral ossicles, those next the mouth, instead of 
 being fused like the others, become free, and each one 
 unites with that of the adjoining arm. The parts of the 
 disc that come between the arms are known as the inter- 
 brachial areas ; these are covered mainly with scales and 
 granules, but in each of the areas there is a large plate, 
 the buccal plate (b), and other smaller plates, projecting
 
 72 ECHINODERMATA. OPHIUROIDEA. 
 
 into the mouth, giving it a stellate form. One of the 
 buccal plates serves as the madreporic plate. The genital 
 openings (g) also occur on the ventral surface of the disc, 
 they are in the form of slits in the interbrachial areas, 
 usually two in each, situated near the bases of the arms ; 
 occasionally there are four. The dorsal surface of the 
 
 V 
 
 FIG. 18. A, Ophioderma longicauda, Eecent. Ventral surface of disc 
 and part of the arms, b, buccal plates ; g, genital slits ; v, ventral 
 plates of arms. B, Ophioglypha, Eecent. Dorsal surface, r, radial 
 plates ; I, lateral plates of arms ; d, dorsal plates of arms. 
 
 disc (fig. 17 B) is covered with scales or granules; and 
 usually there is at the bases of the arms on each side, a 
 large plate, the radial plate (r); the arms end abruptly 
 at the margin of the disc. 
 
 Ophioderma (fig. 18 A). Arms long, round, smooth ; 
 the lateral plates provided with short spines or papillae. 
 Four genital slits in each area, two near the border of the 
 disc, two near the mouth arranged radially. Buccal plates 
 undivided. Dorsal surface of the disc finely granulated. 
 Lias to present day.
 
 ECHINODERMATA. OPHIUROIDEA. 73 
 
 Distribution of the Ophiuroidea. 
 
 The majority of the Ophiuroids at the present day 
 occur in shallow water, more than half of the known 
 species being found above a depth of 30 fathoms, and 
 most of these not extending lower. Other forms occur 
 at greater depths, as many as 69 species being found 
 below 1000 fathoms. Ophiuroids are rare as fossils, the 
 earliest form occurs in the Bala Beds, and belongs to the 
 genus Protaster, this ranges on to the Silurian, where 
 Eudadia and other forms appear. In the Mesozoic we 
 get Ophioderma, Ophiolepis, and Geocoma. All the Eocene 
 genera are still living. 
 
 CLASS. ECHINOIDEA. 
 
 The Echinoids or sea-urchins have usually a globular, 
 heart-shaped, or discoidal body. The soft parts are en- 
 closed in a shell or test, which is formed in the mesoderm 
 and is covered by a thin ectoderm. This test consists of 
 numerous calcareous plates, which in the majority of cases, 
 are immoveably united. There is nothing corresponding 
 to the ambulacral groove of the star-fish, the water- 
 vascular system being internal, as a result the tube-feet in 
 order to reach the exterior, must pierce the plates of the 
 test. The mouth is always on the inferior surface and 
 often central, but when excentral it is placed anteriorly. 
 The anus is either at the summit of the test or posterior 
 to it, somewhere along a line drawn from the summit to 
 the mouth. In the test we may distinguish two parts, a 
 small patch of plates placed at the summit, known as the 
 apical disc, and the remainder of the test termed the 
 corona. The apical disc represents the whole of the
 
 74 ECHINODERMATA. ECHINOIDEA. 
 
 dorsal surface of the starfish, and the corona the ambula- 
 cral surface. 
 
 b 
 
 FIG. 19. A. Diagram of the upper surface of a regular echinoid, with 
 the tubercles and spines omitted, a, ambulacral areas; b, inter- 
 ambulacral areas; p, pores in the ambulacral plates. B. Apical 
 disc of Echinus esculentus, Recent, a, anus ; p, periproct; g, genital 
 plates, each with a pore; m, madreporic plate; o, ocular plates, 
 each with a pore. 
 
 In a typical regular echinoid (e.g. Echinus) the anus is 
 placed within the apical disc (fig. 19), which then consists 
 of the following parts. At or near the centre is the 
 anus (a), this is surrounded by a membrane covered with 
 scaly plates and known as the periproct {p}. The peri- 
 proct is encircled by a ring formed of ten plates, five 
 called genital (g} and five ocular (o). The genital plates 
 form the inner part of the ring, they are often more or 
 less hexagonal in outline, and are provided with a pore 
 which serves as the opening for the genital ducts. In 
 some cases one or more of the plates is imperforate, and
 
 ECHINODERMATA. ECHINOIDEA. 75 
 
 one has a porous structure serving as the madreporic 
 plate (ra). Outside the genital plates and alternating 
 with them come the ocular plates, these are smaller than 
 the genital and usually triangular or pentagonal and they 
 each have a perforation for the eye- spot. In the irregular 
 echinoids, the anus is not placed within the apical disc, 
 which then becomes more compact. In some forms it is 
 considerably elongated, as in Collyrites, in which two of 
 the ocular plates are placed some distance posterior to the 
 other portion of the disc. 
 
 The corona consists of twenty rows of plates, each row 
 extending from the apical disc to the mouth. The plates 
 are of two kinds, ambulacral (fig. 19 A, a) and interam- 
 bulacral (b) and they are arranged in pairs, there being 
 five double rows of ambulacrals separated by five double 
 rows of interambulacrals, each double row being termed 
 an area. The former end against the ocular plates, the 
 latter against the genital, and in each case fresh plates 
 are developed next the apical disc. In each area the 
 plates alternate on either side, and since their inner ends 
 are triangular, the line between the two rows is zig-zag. 
 The ambulacral plates are smaller and more numerous 
 than the interambulacral, and they are perforated by 
 pores (p) for the passage of the tube feet to the exterior, 
 a radial water-vessel being placed under each ambulacral 
 area. The pores may be round or elongated and they are 
 situated in the outer portion of the plates and are almost 
 always in pairs, sometimes there is one pair on each plate, 
 sometimes more. When there is one pair, the pores are 
 said to be unigeminal, two pairs bigeminal, three pairs 
 trigeminal. In the two latter cases the pairs are arranged 
 obliquely. Sometimes the pores in each pair are united 
 by a groove, they are then termed conjugate. In some
 
 76 ECHINODERMATA. ECHINOIDEA. 
 
 sea-urchins the rows of pores extend from the apical 
 disc to the mouth without any important alteration, the 
 ambulacral areas are then said to be simple (e.g. Cidaris}. 
 In others the rows of pores diverge soon after leaving 
 the apical disc, and then come together again, so as to 
 form a rosette on the upper surface of the corona, the 
 pores only being irregularly developed on the lower, the 
 ambulacral areas are then termed petaloid (e.g. Scutella), 
 and when the rows of pores diverge but do not come in 
 contact at their lower ends they are sub-petaloid (e.g. 
 Echinobrissus). The plates of both the ambulacral and 
 interambulacral areas are often provided with rounded 
 elevations known as tubercles and granules. The tubercles 
 are of various sizes, the largest being the primary. In 
 
 FIG. 20. A. Spine of Cidaris florigemma, from the Corallian Rocks, 
 a, acetabulum ; h, head ; c, collar ; 6, body or stem. B. Ambu- 
 lacral plate of Cidaris (recent) with a large primary tubercle and 
 secondary tubercles. In the primary tubercle, m, mamelon ; b, boss ; 
 , areola. 
 
 these the following parts may be distinguished ; at the 
 summit there is a spheroidal piece, which is sometimes 
 perforated, this is the mamelon (fig. 20 B, m ). The 
 mamelon rests on a short pillar termed the boss (b), the
 
 ECHINODERMATA. ECHINOIDEA. 77 
 
 upper margin of which is sometimes smooth, sometimes 
 crenulated. The base of the boss is frequently surrounded 
 by a smooth excavated space known as the areola (a). 
 The granules are smaller than the tubercles. 
 
 Attached to the tubercles are the spines, these are of 
 different sizes and shapes in different genera and species 
 and even in the same individual, being needle-like, rod- 
 like, or flask- shaped. At the end of the spine where it 
 articulates with the mamelon there is a rounded cavity, 
 the acetabulum (fig. 20 A, a), above this comes the head (h), 
 this is limited above by a ring or collar (c) which may be 
 smooth or crenulate and which serves for the attachment 
 of the muscles that move the spine. Beyond the collar 
 and forming the greater part of the spine is the stem or 
 body (6), which may be smooth, or ornamented with ridges 
 or rows of spiny processes. Pedicellarise also occur. 
 On the surface of some sea-urchins, there are nearly 
 smooth bands covered with very minute tubercles, bearing 
 slender modified spines, these bands are termed fascioles, 
 sometimes they form a ring beneath the anus {e.g. 
 Micraster) when -they are said to be sub-anal, at other 
 times they encircle the rosette formed by the petaloid 
 ambulacra (e.g. Hemiaster) and are said to be peri- 
 petalous. 
 
 The mouth is placed in the centre of a membrane, 
 which is covered with plates or granules, and is known as 
 the peristome. Consequently the aperture in the corona 
 of fossil specimens does not represent the mouth but the 
 peristome. The shape of the peristome varies in different 
 forms, it may be circular, oval, pentagonal, or decagonal. 
 In some genera there is at the commencement of the 
 alimentary canal, a complicated calcareous apparatus, 
 which functions in mastication ; this is connected with
 
 78 ECHINODERMATA. ECHINOIDEA. 
 
 arches, known as auricles, placed over the ambulacra just 
 within the peristome. 
 
 In the irregular echinoids there is a well-marked 
 bilateral symmetry; a plane which passes through the anus 
 (which is posterior), the apical disc, and the mouth, will 
 divide the body into two similar parts. The ambulacra are 
 often markedly dissimilar, the anterior one differing from 
 the others which are paired. The bilaterality is quite 
 inconspicuous in the regular sea-urchins, but the plane 
 of symmetry may be found by means of the madreporic 
 plate, which is always placed at the upper end of the 
 right anterior interambulacral area. 
 
 OEDER. PAL2EECHINOLDEA. 
 
 The number of rows of plates forming the test varies. 
 In each interambulacral area there is one, or more than 
 two ; in each ambulacral two or many. The plates 
 frequently overlap, giving the test a certain amount of 
 flexibility. All the genera, except Echinocystites, are 
 regular. In the genital plates there are from three to 
 five pores, in the oculars two or none. Jaws are present. 
 
 Palaeechinus. Test spheroidal, rigid. Apical disc 
 central, enclosing the anus. Five large genital plates, 
 each with three perforations, one may have one only. 
 Ocular plates, five, small, each with two perforations. 
 Ambulacral areas narrow, with two rows of plates, pores 
 unigeminal. Interambulacral areas wide, with four to 
 eight rows of plates at the equator, fewer towards the 
 poles, plates hexagonal, except those next the ambulacral 
 area, which are pentagonal ; surface of plates covered with 
 granules. Spines small. Upper Silurian to Carboniferous 
 Limestone.
 
 ECHINODERMATA. ECHINOIDEA. 79 
 
 ORDER. EUECHINOIDEA. 
 
 The test is always formed of twenty rows of plates, 
 two in each area. The plates very rarely overlap. 
 Regular and irregular forms occur. There is usually 
 one pore only in each genital arid ocular plate. Jaws are 
 present in some forms, absent in others. 
 
 (a) Regular Euechinoids. 
 
 The test is radially symmetrical and generally sphe- 
 roidal. The apical disc encloses the anus. The mouth 
 is central. The ambulacral areas are simple. Jaws 
 present. 
 
 Cidaris. Test spheroidal, the summit and base 
 equally flattened. Apical disc very large, rarely preserved 
 fossil, ocular plates large. Ambulacral areas narrow, 
 undulating, pores unigeminal, between the rows of 
 pores are vertical rows of small tubercles and granules. 
 Interambulacral areas wide, plates few, each with a 
 primary tubercle which is generally perforated, and may 
 be crenulated or smooth ; areola large, surrounded by 
 secondary tubercles, beyond which are granules. Peri- 
 stome rounded. Spines large, of various forms, generally 
 ornamented with rows of granules. Lias to present day. 
 
 Peltastes. Test circular in outline, somewhat de- 
 pressed. Apical disc very large, prominent, containing 
 one extra plate, the sur-anal, placed in front of the 
 periproct ; the madreporic plate has an oblique fissure. 
 Ambulacral areas narrow, straight or slightly flexuous, 
 with small tubercles; pores unigeminal except near the 
 mouth. Interambulacral areas wide, with large primary
 
 80 ECHINODERMATA. ECHINOIDEA. 
 
 tubercles, which are imperforate, and may be crenulate. 
 Peristome slightly notched. Lower Greensand to Gault. 
 
 Hemicidaris. Test spheroidal, inferior surface flat- 
 tened. Apical disc small. Ambulacral areas narrow, 
 slightly flexuous, with two rows of tubercles on the under 
 surface, which pass into granules on the upper. Pores 
 unigeminal, but bigeminal or trigeminal near the mouth. 
 Interambulacral areas broad ; plates large and few, each 
 with a large perforated and crenulated tubercle, and also 
 smaller tubercles and granules. Spines cylindrical, long. 
 Peristome large, decagonal. Inferior Oolite to Purbeck 
 Beds. 
 
 (6) Irregular Euechinoids. 
 
 The test is bilaterally symmetrical. The anus is 
 placed outside the apical disc. The mouth is central or 
 excentral. Ambulacra simple or petaloid. Jaws may be 
 present or absent. 
 
 Echinoconus (= Galerites). Test conical, or almost 
 hemispherical, inferior surface flat, outline pentagonal or 
 oval. Apical disc small, with four genital plates. Ambu- 
 lacral areas, narrow, straight, simple ; pores bigeminal, 
 trigeminal near the mouth. Interambulacral areas with 
 broad plates, tubercles very small, perforated and crenu- 
 lated. Peristome small, central, decagonal. Jaws absent. 
 Periproct marginal or submarginal. Upper Greensand to 
 Upper Chalk. 
 
 Holectypus. Test hemispherical, depressed, base 
 excavated. Apical disc small. Ambulacral areas narrow, 
 straight, simple ; pores unigeminal, tubercles small. In- 
 terambulacral areas with rather large plates, tubercles 
 small. Peristome central, decagonal. Jaws present.
 
 ECHINODERMATA. ECHINOIDEA. 81 
 
 Periproct large, placed between the peristome and the 
 posterior margin of the test. Inferior Oolite to Corallian. 
 Pygaster. Test large, depressed, outline pentagonal 
 or circular, base concave. Apical disc small, madre- 
 poric plate large. Ambulacral areas straight, simple; 
 pores unigeminal ; tubercles in vertical rows. Interam- 
 bulacral areas wide, tubercles perforate, crenulate or not. 
 Peristome central, large, decagonal. Jaws present. Peri- 
 proct very large, placed behind the apical disc. Inferior 
 Oolite to Cretaceous. 
 
 Echinobrissus (= Nucleolites). Test depressed ; out- 
 line oval or quadrilateral, rounded anteriorly, truncated 
 and broadest posteriorly ; inferior surface concave. Apical 
 disc compact, four perforate genital plates, and one 
 imperforate. Ambulacral areas subpetaloid, pores uni- 
 geminal, the outer pore elongated. Interambulacral 
 plates wide, tubercles small. Peristome oval or pentagonal, 
 excentric, a little anterior. Jaws absent. Periproct 
 placed in a sulcus on the dorsal surface. Inferior Oolite 
 to Recent. 
 
 Echinocorys (= Ananchytes). Test very convex 
 above, inferior surface flattened, outline oval. Apical 
 disc a little elongated ; the four genital plates perforated, 
 the two anterior separated from the two posterior by two 
 ocular plates. Ambulacral areas simple, pores unigeminal. 
 Interambulacral plates large, tubercles small. Peristome 
 bilabiate, placed anteriorly. Jaws absent. Periproct 
 oval, inframarginal. Upper Chalk. 
 
 Holaster. Test cordiform, inferior surface more or 
 
 less flattened, superior surface with a broad shallow 
 
 groove anteriorly. Apical disc elongate, the two pairs of 
 
 genital plates separated by the two oculars. Ambulacral 
 
 w. p. 6
 
 82 ECHINODERMATA. ECHINOIDEA. 
 
 areas large, simple ; pores unigeminal, round or elongate, 
 the anterior ambulacrum in the groove. Interambulacral 
 areas with small tubercles and granules. Peristome 
 excentric and elliptical. Jaws absent. Periproct supra- 
 marginal. Upper Greensand and Lower Chalk ; also Ter- 
 tiary in Australia. 
 
 Micraster. Test cordiform or oval. Apical disc 
 small, excentric. Ambulacral areas subpetaloid, placed in 
 sunken areas, the subpetaloid part of the two anterior- 
 lateral longer than the two posterior-lateral ; pores 
 unigeminal. The anterior unpaired ambulacrum in a 
 deep groove, pores round. Interambulacral areas with 
 large plates ; tubercles small, perforate and crenulate. 
 Subanal fasciole. Peristome excentral, near the anterior 
 border, with a projecting lip. Jaws present. Periproct 
 supramarginal. Middle and Upper Chalk ; also Tertiary 
 in Australia. 
 
 Distribution of the Echinoidea. 
 
 Some Echinoids live at great depths in the ocean, one 
 species being found at 2900 fathoms, but by far the larger- 
 number occur near the coasts in shallow water. They 
 are most abundant where the sea-bottom is rocky, sandy, 
 or calcareous, and less common where it is muddy ; 
 consequently fossil forms are rare in clayey strata. Those 
 found in deep water have a much wider range in space 
 than those found in shallow water, and many of the 
 genera, especially those with a considerable range in 
 depth, have also a long range in time, some extending 
 back to the Cretaceous period and even further. 
 
 There is a marked contrast in the distribution of the 
 two orders of the Echinoidea. The Palseechinoids are
 
 ECHINODERMATA. ECHINOIDEA. 83 
 
 found almost exclusively in the Palaeozoic, only one genus 
 occurring in the Neozoic, viz. Anaulocidaris from the 
 Trias. The Euechinoids are nearly confined to the 
 Neozoic ; Eocidaris from the Permian, being the only 
 genus found in the Palaeozoic. 
 
 The earliest echinoid is the genus Bothriocidaris from 
 the Ordovician rocks of Esthonia in Russia. In the 
 Silurian we get Palceechinus and Echinocystites ; in the 
 Devonian, Lepidocentrus; in the Carboniferous, Palceechinus, 
 Melonites, and Archceocidaris. In the Jurassic the 
 echinoids are much more numerous, relatively to the 
 other groups of animals, than in the earlier formations, 
 they are rare in the Lias and the other clayey divisions, 
 but very abundant in the calcareous beds, especially in 
 the Inferior Oolite and Coral Rag. The genera which are 
 best represented in the Jurassic are, of the regular group, 
 Gidaris, Hemicidaris, Acrosalenia, Pseudodiadema, and 
 Stomechinus ; of the irregular group, Collyrites, Echino- 
 brissus, Holectypus, and Pygaster. In the Cretaceous the 
 echinoids are even more abundant than in the Jurassic ; 
 many of the genera found in the Lower Cretaceous occur 
 also in the Upper Jurassic. The most important genera 
 are, (1) regular, Cidaris, Pseudodiadema, Cyphosoma, 
 Peltastes, Salenia, (2) irregular, Discoidea, Echinoconus 
 (= Galerites), Hemiaster, Micraster, Cardiaster, Holaster, 
 Echinocorys (= Ananchytes). Between the Cretaceous 
 and the Eocene there is an enormous break in the 
 succession of the echinoids, not a single species being 
 common to the two systems, and the genera also are 
 mostly different. This change is due in part to the great 
 difference in the conditions under which the deposits were 
 formed, the Chalk being a comparatively deep water 
 formation, and the Eocene beds, shallow water; but the 
 
 62
 
 84 ECHINODERMATA. ECHINOIDEA. 
 
 Eocene forms differ much more from those of the Upper 
 Chalk than they do from those of the Chalk Marl, the 
 latter being a shallower water deposit. Throughout the 
 English Tertiaries the echinoids are much rarer than in 
 the Cretaceous ; thus in the Cretaceous there are thirty 
 genera, in the Eocene seven, in the Pliocene eleven. In 
 the Eocene this can be accounted for largely by the fact 
 that the sea-bottom was for the most part muddy ; in the 
 Pliocene by the lower temperature of the ocean. The 
 London Clay echinoids belong to tropical or sub-tropical 
 genera. The commonest Eocene forms are Hemiaster and 
 Schizaster. The Pliocene forms present a considerable 
 affinity to those now living in the West Indian seas, 
 indicating the existence of shallow water between Europe 
 and America in which migration could take place. The 
 most important genera in the Pliocene are Echinus, 
 Echinocyamus, Spatangus, and Temnechinus. 
 
 CLASS. HOLOTHUROIDEA. 
 
 This includes the sea-cucumbers. They possess an 
 elongated cylindrical body provided with a circle of 
 tentacles around the mouth connected with the water- 
 vascular ring ; from the radial vessels tube-feet are given 
 off, except in the genus Synapta. The integument is 
 leathery, and the calcareous skeleton is very poorly develop- 
 ed, consisting of minute isolated pieces of various shapes, 
 such as spicules, anchors, and wheels. At the present day 
 the Holothurians are widely distributed, but owing to the 
 nature of their hard parts, they are rarely found fossil. 
 The earliest forms occur in the Carboniferous rocks of 
 Scotland, and a few have also been recorded from the 
 Jurassic and later formations.
 
 ECHINODERMATA. CRINOIDEA. 85 
 
 CLASS. CRINOIDEA. 
 
 The Crinoidea includes the sea-lilies or feather-stars. 
 Unlike the star-fishes and sea-urchins, these have the oral 
 surface directed upwards and the dorsal surface down- 
 wards. In very many forms the animal is fixed by means 
 of a stem given off from the dorsal surface, but in others 
 this is only found in the young state, the adult form 
 being free. The skeleton consists of a stem, and of a cup 
 or calyx formed of calcareous plates closely united and 
 arranged in a regular manner; from the margin of this, 
 branches or arms are given off. 
 
 The soft parts consist of a central mass known as the 
 disc, placed in the calyx, and of arms which come off from 
 the disc ; these may be five in number or there may be as 
 many as two hundred produced by repeated branching. 
 The whole of the alimentary canal is situated within the 
 disc, the mouth and anus being on the upper surface. On 
 each side of the arms there is a row of small appendages, 
 which alternate ; these are the pinnules. On the upper 
 surface of each pinnule and of each arm there is a groove, 
 which is continued on to the disc, where they unite, 
 ultimately forming five main channels which meet in the 
 mouth, this being usually central. The grooves are lined 
 with cilia, by the movements of which food particles are 
 conveyed to the mouth. The anus is placed in one of the 
 spaces between the main channels on the disc, usually at 
 the end of a tubular process. In some forms the surface of 
 the disc is covered with small calcareous plates, in others 
 there are only isolated granules. Immediately under the 
 groove of each arm there is a radial nerve cord, these like 
 the grooves unite to form larger trunks at the margin of 
 the disc and ultimately join as a ring round the mouth.
 
 86 ECHINODERMATA. CRINOIDEA. 
 
 The blood -vascular system has a similar distribution, the 
 radial vessels being immediately under the nerve cord ; 
 but there is in addition to this a bundle of blood vessels 
 situated in the axis of the disc and known as the central 
 plexus, this gives off branches to the oral ring and 
 elsewhere ; dorsally it is connected with what is known as 
 the chambered organ, which is placed in the centro-dorsal 
 plate, or sometimes between the basal plates of the calyx. 
 In addition to the nervous system already mentioned, 
 there is another placed dorsally, consisting of a fibrous 
 sheath surrounding the chambered organ, and of cords 
 given oft' from it, which pass through canals in the plates 
 of the calyx to the arms and pinnules. The fibrous 
 sheath is also prolonged into the stem when present and 
 contains in its centre a vessel from the chambered organ. 
 The water- vascular system consists of radial vessels placed 
 dorsally to the blood vessels and of a ring round the 
 mouth ; from the latter, tubes hang down and open into 
 the body-cavity, which communicates with the water of 
 the exterior by means of pores. In connection with the 
 radial vessels are tubular processes, the tentacles, which 
 form a row on each side of the food-grooves, and correspond 
 with the tube feet of the star-fish, but do not function in 
 locomotion. 
 
 We come now to the structure of the skeleton. In 
 the majority of crinoids a stem is present, this is more or 
 less flexible and is sometimes several feet in length. It is 
 formed of a number of circular or pentagonal (occasionally 
 elliptical) plates, articulated by their broad surfaces. Each 
 plate is pierced in the centre by a canal which is circular 
 or pentagonal and contains a prolongation of the cham- 
 bered organ and its fibrous sheath. The articular faces of 
 the plates are often provided with radiating striae or with
 
 ECHINODERMATA. CRINOIDEA. 
 
 87 
 
 ridges in the form of a rosette. From the stem small 
 branches known as cirri are sometimes given off, these 
 have a structure similar to that of the stem and are also 
 pierced by a central canal. The inferior part of the stem 
 often expands and branches in a root-like manner, and 
 serves to fix the animal. Sometimes the plates near the 
 top of the stem become very much larger than the others. 
 
 FIG. 21. Diagram of the plates of Actinocrinus triacontadactylus, Carboni- 
 ferous Limestone. 6, basal plates ; rl, first cycle of radials; r2, r3, 
 second and third cycles of radials; br, brachial plates; ir, inter- 
 radials ; a, anal inter- radial. 
 
 Resting on the topmost joint of the stem is the calyx 
 (figs. 21, 22), this consists at its base of a cycle of five 
 (sometimes fewer) plates, known as the basals (6), occasion- 
 ally these are not visible, being covered by the radials. In 
 some forms there is below the basals and alternating with 
 them another cycle of plates, termed the underbasals, and 
 the base is then said to be dicyclic ; when basals only are 
 present, it is monocydic. Above the basals and alternat- 
 ing with them, comes a cycle of five radial plates (ri); 
 often there are two or three cycles of radials (r2, ra), in
 
 88 
 
 ECHINODERMATA. CRINOIDEA. 
 
 which case the plates are placed vertically above one 
 another. In some genera there are between the rows of 
 radials, other smaller plates, the inter-radials (fig. 21, ir) ; 
 and in the inter-radius which leads up to the anus there 
 is an extra series of plates, the anal inter-radials (fig. 21, a). 
 From the highest row of radials the arms arise ; these are 
 generally movable, they have a groove on the upper 
 surface, and are formed either of a single row of plates, 
 
 FIG. 22. Apiocrinus parkinsoni, from the Bradford Clay, s, topmost 
 plate of the stem ; b, basal plates ; r 1, r 2, r 3, first, second and 
 third cycles of radial plates; br, brachial plates. (Three-quarters 
 natural size.) 
 
 the brachials (br), or of a double row, in which case the 
 plates alternate. The pinnules also consist of calcareous 
 plates, with an arrangement similar to those of the arms. 
 It has already been stated that the upper surface of 
 the disc is sometimes furnished with small plates, but 
 these are seldom preserved fossil ; they do not how- 
 ever cover over the food grooves or mouth, which are 
 still visible externally. But in most of the Palaeozoic 
 crinoids there is also a more or less complete roof of
 
 ECHINODERMATA. CRINOIDEA. 89 
 
 plates over the upper surface of the calyx, hiding the 
 grooves and the mouth. This roof is in some genera 
 provided with a conical process, the anal proboscis, at the 
 summit of which is the anus. In these forms, as well as 
 in a few others, the grooves in the arms are also roofed 
 over with plates. 
 
 There are two orders, (1) Palseocrinoidea, (2) Neocri- 
 noidea. 
 
 ORDER. PAL^OCRINOIDEA. 
 
 The calyx is large and the arms small, relatively. 
 Inter-radials are generally present. On the ventral surface 
 there is a roof of plates hiding the food grooves and the 
 mouth. 
 
 Actinocrinus. Calyx variable in form, pear-shaped, 
 ovoid, or more or less spherical ; monocyclic. Basals three, 
 forming a hexagon. Three cycles of radials firmly united, 
 the first high, the third bearing the double series of 
 brachials. Inter-radials present and also the anal inter- 
 radial, the first plate of the latter resting on the basals. 
 The ventral roof formed of thick, tubercled, hexagonal 
 plates ; anal proboscis sometimes present. Arms ten to 
 thirty, with two rows of plates. Stem circular, canal 
 pentagonal. Silurian to Carboniferous Limestone. 
 
 Crotalocrinus. Dicyclic. Underbasals five, small, 
 pentagonal. Basals very large. One cycle of radials, very 
 large. Inter-radials absent, except a small anal inter-radial. 
 Arms five, with a single row of plates ; dichotomous, the 
 branches uniting so as to form lamellar expansions or 
 networks, which are enrolled. Ventral roof formed of 
 small plates ; in the centre five large oral plates. Anal 
 proboscis marginal. Stem thick, circular; canal penta- 
 gonal ; roots thick. Wenlock Limestone.
 
 90 ECHINODERMATA. CRINOIDEA. 
 
 OBDEB. NEOCBINOIDEA. 
 
 The calyx is small and the arms large, relatively. 
 Usually monocyclic, basals five. Inter-radials are seldom 
 present, there is no anal inter- radial. The radials of the 
 higher cycles are generally movable. There is no ventral 
 roof of plates. 
 
 Apiocrinus. Calyx pear-shaped. Basals five, thick. 
 Three cycles of low radials, excavated on their upper 
 surfaces, all the plates firmly united. Arms ten, bifurcat- 
 ing once or twice, formed of a single row of plates. Stem 
 long, cylindrical, base expanded ; the articular surfaces of 
 the plates radiately striated. The plates of the upper 
 part of the stem are in contact at the periphery only, 
 leaving a space between each plate. The upper part of 
 the stem expands and passes gradually into the calyx, the 
 upper surface of the last plate is provided with five 
 radiating ridges between which the basals lie. Lias to 
 Lower Cretaceous. 
 
 Marsupites. Calyx large, globular, plates large and 
 thin. Dicyclic. Stem absent. Base formed of a large 
 central pentagonal plate, the centro-dorsal plate. Under- 
 basals five, pentagonal. Basals five, hexagonal. Radials 
 five, pentagonal. Arms bifurcating, formed of a single 
 row of plates. Ventral surface of calyx covered with 
 small plates. Anus sub-central. Upper Chalk. 
 
 Distribution of the Crinoidea. 
 
 In comparison with its abundance in former times the 
 class Crinoidea is but poorly represented at the present 
 day. The most important family is the Comatulidse (e.g.
 
 ECHINODERMATA. CRINOIDEA. 91 
 
 Antedon, Actinometra), this has a wide range in space and 
 depth, the genus Antedon being found from the shore 
 down to 2900 fathoms. The stalked crinoids are much 
 less abundant than the Comatulidae, and are found mainly 
 at great depths. In most cases the species of crinoids 
 have only a limited distribution in space. 
 
 The Palaeocrinoids are found only in the Palaeozoic 
 formations, the Neocrinoids only in the Neozoic and at 
 the present day. In the Palaeozoic the crinoids are much 
 more numerous than the other Echinoderms, their remains 
 in some cases forming the main part of some limestone 
 beds (crinoidal limestone), as for instance in the Carboni- 
 ferous. None of the other classes are ever sufficiently 
 abundant to be of importance as rock-builders. 
 
 The crinoids occur first in the Tremadoc Beds ; in the 
 Upper Ordovician Glyptocrinus and a few others have 
 been found. In the Silurian they become very much 
 more abundant, attaining their maximum development, 
 the most important genera being Gyathocrinus, Ichthyo- 
 crinus, and Crotalocrinus. In the Devonian, Cyathocrinus, 
 Actinocrinus and others are common ; in the Carboniferous, 
 Actinocrinus, Poteriocrinus, and Platycrinus. Crinoids 
 are rare in the Permian. In the Trias we meet with a 
 new phase in their development, the Palaeocrinoids being 
 replaced by the Neocrinoids ; the characteristic form is 
 Encrinus. In the Jurassic we get Antedon, Pentacrinus, 
 Extracrinus, Apiocrinus, and Millericrinus, the first two 
 living on to the present day. In the Cretaceous the chief 
 forms are Marsupites and Bourgueticrinus. In the Caino- 
 zoic, crinoids are very rare.
 
 ECHINODERMATA. CYSTIDEA. 
 
 CLASS. CYSTIDEA. 
 
 The Cystideans in general appearance resemble the 
 crinoids. They consist of a calyx and generally a stem, 
 the latter as a rule being very sho i ' The calyx is in 
 most cases more or less spherical or ovate, with a depres- 
 sion at its base for the articulation of the stem. It is 
 formed of plates which vary in number from thirteen to a 
 hundred or more, these are not arranged in a regular 
 manner as they are in crinoids, and at best the calyx 
 shows but little radial symmetry. Arms are present, and 
 vary from three to five in number, but they are very 
 feebly developed or sometimes quite rudimentary; in 
 some forms they are free, but in other cases fused to the 
 calyx. Or they may be represented merely by scattered 
 filaments, articulated to the calyx by sockets on some of 
 the plates. At the summit of the calyx (fig. 23 A) either 
 at or near the centre, there is an aperture (a) from which 
 
 B 
 
 FIG. 23. A, Glyptosphcerites letichtenbergi, from the Ordovician of Eussia. 
 a, mouth covered by oral plates; b, ambulacral grooves; c, socket for 
 the armlet; d, anus; e, ovarian aperture, (after Volborth). B, a 
 few plates of the same enlarged, showing double pores. C, Plates of 
 Echinosphcerites, with pore-rhombs.
 
 ECHINODERMATA. CYSTIDEA. 93 
 
 radiate food-grooves (6), this may therefore be regarded as 
 the mouth. On the upper surface also there is another 
 opening regarded as the anus (d), it is covered with plates 
 which form a cone. In some forms, near the mouth there 
 is a smaller aperture, which may be a genital pore (e). 
 A characteristic feature of the plates of the calyx is the 
 presence of pores which pass right through the plates. 
 In a few forms they are absent. They may either be 
 arranged in pairs (fig. 23 B) placed in oval depressions or 
 on elevations ; in other cases they form what are known 
 as pore-rhombs (fig. 23 (7), these consist of rhombic figures, 
 the outline of which is formed by the pores, half the 
 rhomb being on one plate, and half on the adjoining, and 
 the pores of the two plates are united by grooves. The 
 number of these pore-rhombs varies considerably in 
 different genera. 
 
 The cystideans then resemble the crinoids in the 
 general form of the calyx and stem, in the position of the 
 mouth and anus, and in the possession of food-grooves 
 and arms. But they differ in having the arms feebly 
 developed, in the irregular arrangement of the plates and 
 in the presence of pores. 
 
 Lepadocrinus (= Pseudocrinites). Calyx ovate, rhom- 
 bic in outline, with rounded edges ; at the base is a cycle 
 of four plates followed by three cycles of five plates each. 
 Mouth central. Two to four straight food-grooves, ex- 
 tending to about the middle of the calyx, and with 
 projecting edges. Anal aperture near the summit, covered 
 with six triangular plates. Three pore-rhombs, one near 
 the base, two on the upper surface of the calyx. Stem 
 thick, circular, rather long, becoming thinner towards the 
 lower extremity. Silurian.
 
 94 
 
 ECHINODERMATA. CYSTIDEA. 
 
 Distribution of the 
 The Cystideans are comparatively rare fossils, they 
 range from the Menevian Beds to the Carboniferous 
 Limestone, and attain their maximum development in the 
 Upper Ordovician. In the Menevian we get Protocystites ; 
 this also occurs in the Tremadoc, and with it Macrocystella. 
 In the Upper Ordovician we get amongst others, Echino- 
 sphcerites, Ateleocystites, and Pleurocystites ; in the Silu- 
 rian Lepadocrinus and Ateleocystites. In the Devonian 
 there are fewer, and in the Carboniferous only one or two 
 genera are known. 
 
 CLASS. BLASTOIDEA. 
 
 In the Blastoids, as in the cystideans and many of 
 the crinoids, the body consists of a stem and calyx, but 
 the former is rarely found attached. The calyx may be 
 spherical, oval, pear-shaped, or bud-like ; it is formed 
 chiefly of thirteen plates, arranged in a regular manner. 
 True arms are not present. 
 
 In a typical form (Pentremites) the calyx (fig. 24) has 
 
 BC 
 
 b b an 
 
 FIG. 24. Pentremites godoni, Carboniferous. A, lateral; B, upper sur- 
 face; <7, under surface, a, ambulacral areas; b, basal plates; r, 
 radials ; d, deltoids ; s, spiracles around the mouth ; an, anus. 
 (Twice natural size.)
 
 ECHINODERMATA. BLASTOIDEA. 
 
 95 
 
 the following structure. The dorsal part is formed of a 
 cycle of three plates the basals (6), two of which are 
 alike, and the third smaller. Above the basals, comes a 
 cycle of five radial plates (r), these are larger than the 
 basals and form the main part of the calyx. At the upper 
 end of each there is a deep incision, which serves for the 
 reception of the ambulacral area (a). Above the radials 
 and alternating with them comes a cycle of smaller plates 
 the deltoids (d). The mouth is placed at the summit 
 of the calyx, in the centre, and around it are five other 
 apertures termed spiracles (s), one of which is larger than 
 the others and includes the anus (an). From the summit 
 the five ambulacral areas (a) radiate towards the dorsal 
 surface, they are bordered partly by the deltoids but 
 mainly by the radials. Each ambulacral area (fig. 25) 
 
 FIG. 25. Ambulacral area of Pentremites godoni, Carboniferous. I, lancet 
 plate ; s, side plate ; p, pore ; a, ambulacral groove ; sp, spiracle. 
 (Three times natural size.) 
 
 consists of the following plates. Occupying the middle is 
 a long pointed plate (I), the lancet plate, which is traversed 
 by a longitudinal canal, in which the radial vessel of the
 
 96 ECHINODERMATA. BLASTOIDEA. 
 
 water-vascular system was placed. On each side of the 
 lancet plate is a row of small plates, the side plates (s). 
 Extending down the middle of each ambulacral area is a 
 groove, the ambulacral groove (a) ; this in some specimens 
 is covered over by a roof of small calcareous plates. At 
 right angles to the ambulacral groove, on each side of it, 
 are numerous transverse grooves. Along the outer margin 
 of the side plates there is a row of pores, the marginal 
 pores (p), formed by spaces between adjoining plates. When 
 
 f 
 
 FIG. 26. Pentremites sulcatus, Carboniferous. Horizontal section of the 
 calyx. I, lancet plate ; p, side plates ; r, radial plates ; hy, hydrospires. 
 (After Zittel.) 
 
 the lancet plate and the side plates are removed from an 
 ambulacral area, we find underneath, in the middle line, 
 an elongated trough-like plate, known as the under lancet 
 plate. At the sides of this may be seen the edges of the 
 hydrospire. The latter consists (fig. 26 A) of a flattened and 
 folded calcareous tube, communicating with the exterior 
 by means of the marginal pores, and also ventrally by the 
 spiracles. A current of water probably passed in through 
 the former openings and out by the latter. In well 
 preserved specimens, the mouth, as in many crinoids, is
 
 ECHINODERMATA. BLASTOIDEA. 97 
 
 not visible externally, but is covered over by a roof of 
 small plates. Arms are represented by pinnules only, 
 which are given off from the margins of the ambulacral 
 areas. 
 
 The other blastoids differ from the one described 
 mainly in the relative sizes of the plates, in the distance 
 which the ambulacra go towards the base, and in the 
 amount of folding of the hydrospires. 
 
 Granatocrinus. Calyx elliptical, oval, or more or 
 less spherical, in section pentagonal or round. Basals 
 small, not seen in a side view. Radials of variable size. 
 Deltoids generally rhombic, large in some species, small 
 in others. Ambulacra narrow, straight. Lancet-plate 
 narrow. Hydrospires simple, often not folded, dilated at 
 the end. Spiracles round or oval, piercing the apices of 
 the deltoids, the posterior one including the anus. Car- 
 boniferous Limestone. 
 
 Distribution of the Blastoidea. 
 
 In England the Blastoids range from the Devonian to 
 the Carboniferous, being most abundant in the latter. 
 But in North America they are also found in the Silurian. 
 The English Devonian forms are rare and but little 
 known. In the Carboniferous Limestone the blastoids 
 attain their maximum, ten genera are represented, the 
 most important being Granatocrinus and Codaster. Pen- 
 tremites does not occur in Britain. 
 
 w. p.
 
 SUB-KINGDOM V. VERMES. 
 
 Classes. Orders. 
 
 1. Platyhelminthes. 
 
 2. Nemathelminthes. 
 
 3. Rotifera. 
 
 (1. Chaetopoda.... \ 
 I 2. Oligochseta. 
 2. Gephyrea. 
 3. Hirudinea. 
 
 THE worms include a large number of animals which 
 differ considerably in their plan of organisation and thus 
 form a very artificial group. The body is bilaterally 
 symmetrical and has generally an elongated form. In some 
 it is unsegmented, in others it is divided into somites. 
 Lateral appendages are often present, but they differ from 
 those of the Arthropods in being unsegmented. A nervous 
 system and generally a blood vascular system and ali- 
 mentary canal are present. A large number possess no 
 skeleton and consequently the palseontological history of 
 the sub-kingdom is extremely imperfect. 
 
 The worms may be divided into four classes, (1) Platy- 
 helminthes, (2) Nemathelminthes, (3) Rotifera, (4) An- 
 nelida. The Platyhelminthes and Rotifera are unknown 
 in the fossil state, the former includes the liver-fluke, 
 tape- worms etc. (e.g. Distomum, Tcenia, Planaria), the
 
 Private Library of 
 Percy E, Raymond, VERMES. 99 
 
 latter the wheel-animalcules. The Nemathelminthes 
 includes the thread-worms (e.g. Ascaris), only two or three 
 forms are found fossil. 
 
 CLASS. t ANNELIDA. 
 
 The body is segmented and generally the segments 
 are similar. There is a ventral nerve cord and a ring 
 round the oesophagus. A vascular system is generally 
 present. 
 
 There are three orders (1) Chaetopoda, (2) Gephyrea, 
 (3) Hirudinea. 
 
 The Gephyrea includes Sipunculus, Echiurus, etc.; the 
 Hirudinea includes the leeches. Both these orders are 
 represented in the fossil condition by doubtful examples 
 only. 
 
 ORDER. CH^TOPODA. 
 
 The Chsetopoda possess bristle-like appendages termed 
 setae. There are two sub-orders (1) Oligochseta, (2) Poly- 
 chaeta. The Oligochseta is unknown in the fossil state, 
 it includes the common earth-worm Lumbricus, 
 
 SUB-ORDER. POLYCILETA. 
 
 The members of this sub-order are nearly all marine, 
 they are characterised by the possession of numerous setae 
 which are placed on special processes termed parapodia. 
 Tentacles are present on the head. Many forms live in 
 tubes, which may consist of carbonate of lime, of chitinous 
 material, or of grains of sand cemented together by a 
 secretion. On account of the possession of this tube the 
 polychaetous annelids are often found fossil. Other forms, 
 
 72
 
 100 VERMES. 
 
 which do not live in tubes, are provided with minute 
 chitinous jaws, and in some formations, especially the 
 Ordovician and Silurian, these are abundantly preserved. 
 
 Serpula. Tube calcareous, long, round, angular or 
 flattened; straight, curved irregularly, or sometimes spiral, 
 closed at one end; generally 'attached to some foreign 
 object by a portion of its surface. Carboniferous to present 
 day. 
 
 Spirorbis. Tube calcareous, small, spiral, attached 
 by one side. The spiral either left-handed or right- 
 handed, the last whorl often produced into a free tube. 
 Ordovician to the present day. 
 
 Distribution of the Vermes. 
 
 Practically all the worms found fossil belong to the 
 Polychseta, the earliest examples occurring in the Cam- 
 brian beds. In addition to the worm-tubes and jaws, we 
 find in the rocks numerous trails and burrows which are 
 regarded as having been formed by worms, but it is 
 impossible to speak definitely of the affinities of these 
 forms. And in many cases it is quite probable that the 
 tracks may have been made by other animals such as 
 crustaceans and gasteropods.
 
 SUB-KINGDOM VI. MOLLUSCOIDEA. 
 
 Classes. Sub-Classes. Orders. Sub-Orders. 
 
 1. Phylactolaema. 
 1. EctoproctaJ /I. Cyclostomata. 
 
 / 
 
 1. Polyzoa ....... J ^ 2 - G y mnolaem a- J 2. CheUostomata. 
 
 2. Entoprocta. ( 3. Ctenostomata. 
 
 V 3. Aspidophora. 
 
 Orders. 
 
 _ , . , II. Inarticulata. 
 
 2. Brachiopoda. ^ _ 
 
 (2. Articulata. 
 
 WHETHER the two classes Polyzoa and Brachiopoda are 
 very closely related to one another is rather doubtful. 
 They have affinities on the one hand with the worms and 
 on the other with the mollusks. For the present they 
 may be retained as a separate sub-kingdom. 
 
 CLASS. POLYZOA. 
 
 With the exception of the genus Loxosoma, all the 
 Polyzoa are colonial animals, numerous individuals living 
 in association. The colony may be arborescent, laminar, 
 almost massive, or encrusting shells and stones. A dermal 
 skeleton, consisting of calcareous or chitinous material, is 
 usually present. The entire colony is known as the
 
 102 
 
 POLYZOA. 
 
 zoarium, and each individual of the colony as a polypide. 
 The entire skeleton is termed the ccenceciitm, and the 
 skeleton of each polypide a zoosdum. 
 
 The polypide (fig. 27 A) has a sac-like form ; at the 
 upper end there is a platform or disc, the lophophore, on 
 which tentacles (t) are placed, arranged either in a circle 
 
 \ 
 
 FIG. 27. A, Diagram of the structure of a single polypide of a 
 Polyzoan. s, body-wall; t, tentacles; o, mouth; CBS, oasophagus ; 
 st, stomach ; int, intestine ; a, anus ; g, ganglion ; /, funiculus ; 
 ov, ovary ; sp, testis. B, Avicularium of Bugula, enlarged, b, beak ; 
 md, mandible ; C, chamber ; p, peduncle ; om, occlusor muscles ; 
 dm, divaricator muscles. (After Hincks, Brit. Mar. Polyz.) 
 
 or in the form of a horse- shoe. The tentacles are not con- 
 tractile, but they are provided with cilia, which produce 
 a current of water that conveys food to the central 
 mouth (o). The anal aperture (a) is near the mouth, 
 generally below the lophophore, but in some forms within
 
 POLYZOA. 
 
 103 
 
 the circle of tentacles. On account of this approximation 
 of the mouth and anus the alimentary canal is bent into a 
 U-shape, and in it may be distinguished oesophagus (ces), 
 stomach (st) and intestine (int). Between the alimentary 
 canal and the body wall there is a considerable space, 
 filled with fluid, this is the body-cavity or coelom. The 
 nervous system consists of a single ganglion (g) placed 
 on the side of the oesophagus facing the intestine. The 
 polyzoan multiplies sexually and by budding; generally 
 the polypide is hermaphrodite, the ovary and testie occur- 
 ring in the same individual. Blood vessels and excretory 
 organs are absent in almost all forms. 
 
 The outer layer of the body wall in the lower part of 
 the polypide generally becomes hardened by calcareous or 
 chitinous matter, and after the death of the animal, this 
 alone remains, constituting the zocecium. The zocecium 
 therefore is not distinct from the body wall, but is merely 
 its outer hardened layer. The anterior part of the poly- 
 pide can be withdrawn by means of longitudinal muscles 
 into the zocecium, just as the finger of a glove can be 
 pulled into the hand. In some Polyzoa (the Cyclostomata 
 
 FIG. 28. A, Portion of Si Mia landslorovi, a Cheilostomatous Polyzoan, 
 Recent, o, ocecium m, aperture of the cell; o, avicularium. 
 B, Portion of Tubulipora fimbria, a Cyclostomatous Polyzoan, 
 Recent. Enlarged.
 
 104 POLYZOA. 
 
 fig. 28 J5) the zocecium is tube-like, the aperture is at the 
 end and is of the same diameter, or nearly so, as the rest 
 of the tube. In others (the Cheilostomata, fig. 28 A) 
 the zocecium is more or less oval, the aperture (m) is con- 
 tracted and is not terminal, but is situated in front near 
 the anterior end, and is provided with a movable lid or 
 operculum. In many of the Cheilostomata, there is at 
 the anterior end of the zocecium, above the aperture, a 
 projecting chamber (o), into which the ova pass, termed 
 the ooecium. Many of the Cheilostomata also possess 
 appendages to the zotecia, termed avicularia (fig. 27 B) 
 and vibracula, but these are rarely preserved fossil. The 
 avicularium may be sessile or placed on a peduncle (p), 
 and in the more specialized forms, has somewhat the 
 appearance of a bird's head, consisting of a chamber 
 (C) produced into a beak and provided with a mandible 
 (md) which is kept constantly snapping by means of 
 muscles in the chamber. The vibraculum consists of 
 a long seta kept in motion by means of muscles at its 
 base. The individual polypides of a colony may commu- 
 nicate with one another, either directly, or by means of 
 communication-plates ; these are portions of the zooecium 
 which are thinner and perforated. The surface of the 
 zocecium may be smooth or punctate, or ornamented with 
 spines, granules, or ribs. 
 
 The Polyzoa are divided into three sub-classes, (1) 
 Ectoprocta, (2) Entoprocta, (3) Aspidophora. The Ecto- 
 procta are the only forms found fossil. 
 
 SUB-CLASS. ECTOPKOCTA. 
 
 The anal aperture is not placed within the area of the 
 lophophore. There are two orders, (1) Phylactoloema, 
 (2) Gymnolsema.
 
 POLYZOA. 105 
 
 OEDEE. PHYLACTOL2EMA. 
 
 The lophophore is horse-shoe shaped. There is a 
 tongue-shaped lobe in front of the mouth, known as the 
 epistome. The forms included in this order are found 
 only in fresh-water and do not occur fossil. 
 
 OEDEE. GYMNOL^MA. 
 
 The lophophore is circular. An epistome is present. 
 There are three sub-orders, (1) Cyclostomata, (2) Cheilos- 
 tomata, (3) Ctenostomata. The last is not known in the 
 fossil state. 
 
 SUB-OEDEE. CYCLOSTOMATA. 
 
 The zocecia are tubular, their apertures are round and 
 terminal, not constricted and not provided with opercula. 
 Vibracula are absent, and generally also avicularia and 
 ooecia. 
 
 Fenestella. Zoarium funnel-shaped or fan-shaped. 
 Branches straight, united by cross-bars, so as to form a 
 network. The cross-bars do not bear zocecia. On each 
 branch there is a median ridge or carina, on the sides of 
 which the zocecia occur. Openings of zocecia round. 
 Silurian to Permian. 
 
 Fascicularia. Zoarium large, generally massive and 
 globose. Zocecia in the form of long tubes, with horizontal 
 tabulae, in contact laterally, and forming bundles which are 
 either distinct and radiate from the base to the periphery, 
 or fuse into laminae which intersect. Coralline Crag. 
 
 SUB-OEDEE. CHEILOSTOMATA. 
 
 The zocecia are more or less oval, never tubular, their 
 apertures are contracted and provided with a movable oper- 
 culum. Avicularia, vibracula, and ooecia are often present.
 
 106 POLYZOA. 
 
 Membranipora. Zoarium encrusting, generally irre- 
 gular in form. Zocecia quincuncial, or arranged irregularly, 
 their margins raised, the anterior part depressed and more 
 or less membranous. Chalk to present day. 
 
 Distribution of the Polyzoa. 
 
 By far the larger number of the Polyzoa are marine, 
 they occur both in shallow and in deep water. The deep- 
 water forms belong mainly to the Cheilostomata, the 
 Ctenostomata being but poorly represented. The Cyclos- 
 tomata are comparatively rare at the present day, except 
 in the Northern seas. 
 
 The earliest Polyzoa occur in the Ordovician rocks. 
 Practically all the Palseozoic genera are extinct, and 
 they belong almost entirely to the Cyclostomata. The 
 Cheilostomata do not become abundant until we reach 
 the Cretaceous, and in the Tertiary they are more im- 
 portant than the Cyclostomata. Very many of the Pliocene 
 forms belong to species still living. 
 
 The chief genera found in the Palaeozoic are Fenestella, 
 Ptilodictya, Glauconome ; in the Jurassic, Stomatopora, 
 Spiropora, Berenicea, Idmonea ; in the Cretaceous, Cerio- 
 pora, Heteropora, Stomatopora, Membranipora, Eschara ; 
 in the Eocene, Hornera ; in the Pliocene, Membranipora, 
 Eschara, Hornera, Fascicularia. 
 
 CLASS. BRACHIOPODA. 
 
 In the Brachiopods the soft parts of the animal are 
 enclosed in a shell which is formed of two parts termed 
 valves, one is placed on the dorsal surface, the other on 
 the ventral. The interior of the shell is lined by a
 
 BRACHIOPODA. 107 
 
 membrane, the mantle, which is divided into two lobes, 
 one occurring in each valve, and the space between the 
 two is known as the pallial chamber. The margin of the 
 mantle is thickened and carries numerous chitinous setae 
 The alimentary canal is placed in a portion of the body- 
 cavity (coelom) in the posterior part of the shell; the 
 mouth (fig. 29, v) opens into the pallial chamber, and 
 leads into an oesophagus which is followed by a stomach 
 
 FIG. 29. Waldheimiajlavescem, Recent. Longitudinal section, d, arms; 
 h, cirri; a, adductor muscle; c, c', divaricator muscles; ss, septum; 
 r, mouth ; z, terminal part of alimentary canal. (After Davidson.) 
 
 surrounded by liver-lobes, and an intestine ; in the articu- 
 late brachiopods the latter is short and ends blindly, in 
 the inarticulate it is long and ends in an anus which opens 
 into the pallial chamber. The nervous system consists of 
 a ring round the oesophagus, with ganglionic enlargements 
 from which nerves are given off to the arms, mantle, etc. 
 The part of the body-cavity which surrounds the alimentary 
 canal communicates with the pallial chamber by means 
 of two or four funnel-shaped canals. The body-cavity
 
 108 BRACH1OPODA. 
 
 extends into the mantle as a series of spaces or sinuses ; 
 these produce slight depressions on the interior of the 
 valves, and can often be traced in casts of the interior 
 of fossil specimens as ridges (fig. 40). The body-cavity is 
 filled with a fluid which is kept in motion by means of 
 cilia. A heart appears to be absent, but a vesicle on the 
 dorsal surface of the stomach has been described as 
 such. 
 
 The brachiopods are never colonial animals. Repro- 
 duction takes place sexually, never by budding or fission. 
 The genital organs are placed in the body-cavity, in 
 the sinuses of the mantle, or in both. Some forms are 
 hermaphrodite, others diceceous. 
 
 The greater part of the pallial chamber is occupied by 
 two long processes, the arms (fig. 29, d), given off from 
 the sides of the mouth. These are covered with cirri (h), 
 and they, with the mantle serve as organs of respiration. 
 The cirri also produce a current of water, which carries 
 food to the mouth. 
 
 Of the two valves of the brachiopod, the ventral is 
 always larger than the dorsal, and it is produced into a 
 beak or umbone, the termination of which is generally 
 perforated by a foramen (fig. 32 A, /). Each valve is 
 equilateral, that is to say, a line drawn from the umbone 
 to the opposite margin of the shell divides the valves into 
 two equal and similar parts. This character, combined 
 with the inequality in the size of the valves, renders it 
 easy to distinguish the shell of a brachiopod from that of 
 a lamellibranch. In many forms the two valves are joined 
 together by means of a hinge, these constitute the group 
 Articulata; in others they are held together by muscles 
 only, these form the Inarticulata. The hinge consists of 
 two short curved processes or teeth given off from the
 
 BRACHIOPODA. 109 
 
 ventral valve near the umbone, which fit into correspond- 
 ing sockets in the dorsal valve. The line near the teeth 
 on which the two valves move in the opening and closing 
 of the shell, is termed the hinge-line (fig. 30, g A). In 
 some genera (Terebratula) this is short and curved, in 
 others (Spin/era) it is long and straight. The posterior 
 part of the shell is that near the hinge (fig. 30, a), the 
 
 FIG. 30. Terebratula obesa, Upper Chalk. A, dorsal view. B, lateral 
 view, a, posterior; b, anterior; a b, length; cd, breadth; e f, 
 thickness ; g h, hinge-line. Two-thirds natural size. 
 
 anterior is the opposite margin (6). The length of the 
 shell is measured from the posterior to the anterior 
 border (a 6). The breadth is at right angles to this, from 
 one side of the shell to the other (c d). The thickness is 
 measured from one valve to the other perpendicular to the 
 length and breadth e -f. In some genera (e.g. Terebratula) 
 the length is greater than the breadth, in others (e.g. 
 Strophomena) the bread this greater. Between the hinge- 
 line and the umbone, there is in some brachiopods (e.g. 
 Cyrtia, fig. 31) a flat or slightly concave portion of the 
 shell, usually triangular, on which the ornamentation of 
 the rest of the shell is absent, the surface being either 
 smooth or striated ; this is known as the area. It is
 
 110 BRACHIOPODA. 
 
 generally found on the ventral valve, but may also occur 
 on the dorsal. The margin of the foramen is sometimes 
 
 
 FIG. 31. Cyrtia exporrecta, Wenlock Limestone, ale, area; b c, 
 hinge-line. Natural size. 
 
 formed partly or entirely by the deltidium (fig. 32 A, d). 
 This consists usually of two small plates ; in Rhynchwiella 
 the two plates form a triangle, in the centre of which is 
 the foramen ; in Waldheimia the two plates form the 
 lower margin only of the foramen ; in Terebratella the 
 two plates are completely separated by the foramen. 
 
 Almost all living brachiopods are fixed to a foreign 
 object, but some of the fossil forms were free. The 
 attachment takes place very often by means of the ped- 
 uncle ; this is a cylindrical process, sometimes long, 
 sometimes short, connected with the mantle, and passing 
 either through the foramen, or between the apices of the 
 valves. The peduncle consists of horny material, with at 
 times some muscular layers also. Some forms (e.g. 
 Strophalosia) are fixed by spines given off from the surface 
 of the shell ; others (e.g. Crania) by means of one valve 
 which is soldered to the rock or other object. 
 
 The two valves of the brachiopod can be opened and 
 closed by means of muscles (fig. 29), those which open them 
 are called the divaricators (cc'), those which close them, 
 the adductors (a). When the soft parts of the animal have 
 been removed the places where the muscles were attached 
 to the interior of the shell are indicated by a difference in
 
 BRACHIOPODA. 
 
 Ill 
 
 the surface such as striation, or by slight depressions or 
 elevations, these markings are termed the muscular im- 
 pressions. In the articulate brachiopods there are 
 generally five or six pairs of muscles. In the genus Wald- 
 heimia there are two pairs of divaricators (fig. 29 A, c, c') 
 and one of adductors (a). Both pairs of the former are 
 attached to a process (the cardinal process, fig. 32 B, c, c) 
 
 FIG. 32. Waldheimia flavescem, Becent. A, Interior of ventral valve. 
 /, foramen; d, deltidium; t, teeth; a, impressions of adductor 
 muscles ; c, c', impressions of divaricator muscles ; b, b", muscles 
 of the peduncle. B, Interior of dorsal valve, c, c', cardinal process ; 
 b", hinge-plate ; s, dental sockets ; I, loop ; a, a', adductor 
 impressions; c, point of attachment of the smaller divaricator. 
 (After Davidson.) 
 
 on the dorsal valve between the teeth sockets, and one 
 pair of these joins the ventral valve near its centre 
 (fig. 32 A, c), and the other, which is smaller, is attached 
 nearer the posterior border (c'). Hence these two pairs 
 of muscles form with the shell a lever of the first order. 
 The adductor muscles are united to the ventral valve near 
 the centre (fig. 32 A, a) and form a single impression
 
 112 BRACHIOPODA. 
 
 divided by a median line ; these muscles bifurcate before 
 reaching the dorsal valve and there form four impressions 
 (fig. 32 B, a, a'). In the Inarticulata the muscles are 
 much more complicated. 
 
 The arms, already mentioned as occupying the main 
 part of the pallial chamber, are generally coiled up. In 
 some forms they can be protruded a greater or shorter 
 distance. Sometimes they are supported on a calcareous 
 framework the brachial skeleton which is attached to 
 the posterior part of the dorsal valve. In Rhynchonella it 
 consists of two short curved processes. In Terebratula 
 (fig. 42) the processes are longer, ribbon-like, and united 
 so as to form a short loop. In String ocephalus (fig. 43) 
 the loop is more extensive and forms a band parallel to 
 and near the margin of the valves. In Waldheimia (fig. 
 32 B) the loop extends nearly to the anterior margin of 
 the shell and is then bent back upon itself. In other 
 genera, the brachial skeleton is in the form of two spiral 
 ribbons; in Spirifera (fig. 38 A) the apices of the spirals 
 are directed towards the lateral margins of the shell, in 
 Glassia they point inwards, in Atrypa (fig. 39 A) upwards 
 to the centre of the dorsal surface. 
 
 The largest brachiopod is Productus giganteus from 
 the Carboniferous Limestone, which has a breadth of 
 twelve inches ; the size varies from this down to about a 
 quarter of an inch. Generally the shell is very thin, but 
 in some forms (e.g. Productus llangollensis) it is thick 
 and massive, leaving but little space for the soft parts of 
 the animal. The external form varies immensely, it may 
 be globular, ovoid, hemispherical, quadrilateral, or tri- 
 angular. Usually both valves are convex, but in some, 
 one is plane the other convex, or one may be concave and 
 the other convex, and in the last case the space in the
 
 BRACHIOPODA. 113 
 
 interior is small. Sometimes there is a depression or 
 sulcus on one valve (generally the ventral) and a corre- 
 sponding ridge on the other valve. The surface of the 
 shell is sometimes quite smooth, or it may be punctate, 
 or ornamented with striae or ribs, which generally radiate 
 from the umbones, but are occasionally concentric. A 
 few forms possess spines. 
 
 In the Articulata the shell is mainly calcareous. In 
 the genus Waldheimia it is formed of three layers (fig. 33), 
 the inner (a), next the mantle, is the thickest and most 
 
 FIG. 33. Vertical section of shell of Waldheimia, flavescens, Eecent. 
 a, prismatic layer ; b, epidermal layer ; c, outer calcareous layer ; e, d, 
 canals traversing the calcareous layers. Magnified. (After King.) 
 
 important, it consists of flattened calcareous prisms, 
 arranged obliquely to the surface of the shell, each prism 
 being encased in a chitinous membrane, which of course 
 has disappeared in the fossil examples. The middle layer 
 (c) is lamellated and also calcareous. The outer (6) is the 
 epidermal layer and consists of chitinous material. The 
 
 FIG. 34. Horizontal section through the prismatic layer of Terebratula 
 maxillata, from the Great Oolite, showing prisms and canals. 
 Magnified. 
 W. P. 8
 
 114 BRACHIOPODA. 
 
 inner and middle layers are traversed by canals (fig. 33, 
 e, d, 34) running at right angles to the surface of the 
 shell, and contain prolongations of the mantle. The shell 
 is secreted by the mantle, its outermost border producing 
 the epidermal layer, a zone just within this producing the 
 lamellated layer, and the remainder producing the prismatic 
 layer which gradually encroaches on the preceding. So 
 that the last layer is the only one which can subsequently 
 increase in thickness. In many forms the lamellated 
 layer is absent; and some (e.g. Rhynchonella) possess no 
 canals traversing the calcareous layers. 
 
 The shell of the Inarticulata has a different structure. 
 In Lingula it consists of alternating calcareous and chiti- 
 nous layers, the calcareous material being mainly phos- 
 phate of lime. The canals which traverse these layers 
 are more numerous and more minute than those found in 
 the Articulate forms. The Brachiopods are divided into 
 two orders, (1) Inarticulata, and (2) Articulata. 
 
 ORDER INARTICULATA. 
 
 The valves are not provided with teeth but are held 
 together by muscles only. The intestine is long and ends 
 in an anus. There is no brachial skeleton. 
 
 Lingula. Shell thin, nearly equivalve, compressed, 
 elongate-ovate or quadrilateral, tapering towards the um- 
 bones, slightly gaping at the extremities. Dorsal valve 
 a little shorter than the ventral. Hinge-line slightly 
 thickened. Twelve muscular impressions in each valve, 
 but only feebly marked. Surface of shell smooth or 
 concentrically or radiately striated. Fixation by means of 
 a long peduncle passing between the umbones. Shell
 
 BRACHIOPODA. 
 
 115 
 
 composed mainly of alternating layers of phosphate of 
 lime and chitinous material. Cambrian to present day. 
 
 Lingulella. Separated from Lingula by the presence 
 of a distinct slit in the ventral valve, and of a hinge-area. 
 Cambrian to Ordovician. 
 
 Discina. Shell sub-orbicular, surface smooth or 
 covered with striae of growth. Valves conical, almost 
 equal, the dorsal valve deeper than the ventral, the 
 summits of both sub-central. Animal attached by a 
 peduncle passing through a foramen just behind the 
 summit of the ventral valve. Four adductor impressions, 
 the anterior prominent, the posterior small and widely 
 separated. Cambrian to present day. 
 
 Crania (fig. 35). Shell calcareous, quadrangular or 
 subcircular, smooth or furnished with radiating ribs, fixed 
 
 A B 
 
 FIG. 35. Crania anomala, Recent. A, interior of ventral valve; B, dorsal 
 valve, a, anterior adductors ; a', posterior adductors ; c, posterior 
 adjusters ; c', cardinal muscle ; r, o, central and external adjusters. 
 (From Woodward.) 
 
 by the ventral valve. Ventral valve depressed-conical; 
 dorsal conical with a sub-central apex. Internal border 
 of both valves covered with coarse granulations. Two 
 pairs of well-marked adductor impressions in each valve 
 
 82
 
 116 
 
 BRACHIOPODA. 
 
 (ad) ; the posterior pair near the margin, the anterior 
 near the centre of the valves and close together, especially 
 so in the ventral valve. Also other smaller muscular 
 impressions. Ordovician to present day. 
 
 ORDER. ARTICULATA. 
 
 The valves articulate by means of two teeth on the 
 ventral valve which fit into sockets on the dorsal. The 
 intestine is short and ends blindly. A brachial skeleton 
 may or may not be present. 
 
 Productus (fig. 36). Shell free, generally transverse 
 (i.e. broader than long) sometimes elongated, often pro- 
 duced into ears in the umbonal region. Dorsal valve 
 
 FIG. 36. Productus giganteus, Carboniferous Limestone. A, interior of 
 dorsal valve ; B, interior of ventral valve ; C, ideal section of both 
 valves ; D, dorsal hinge-line, j, cardinal process ; a, adductor ; 
 r, divaricator ; h, ventral area ; b, brachial prominence (?) ; s, hollows 
 occupied by the spiral arms ; v, reniform impressions. (From 
 Woodward.)
 
 BRACHIOPODA. 117 
 
 concave. Ventral valve very convex, often geniculate, 
 sometimes with a median sinus ; umbone large, recurved, 
 imperforate. Surface ornamented with radiating ribs, 
 crossed by concentric folds, especially in the umbonal 
 region. Hinge-line straight, teeth rudimentary. Area 
 linear or absent. Tubular spines, especially in the region 
 of the umbone and ears. Muscular impressions strongly 
 marked. In the interior of the dorsal valve a median 
 process. No brachial skeleton. Devonian to Permian. 
 
 Chonetes. Shell transverse, semicircular, concavo- 
 convex, or sometimes plano-convex ; area double. Hinge- 
 line straight forming the greatest width of the shell. 
 Teeth strong. Upper margin of area of ventral valve 
 furnished with a row of hollow spines, which increase in 
 length towards the terminations of the hinge-line. Surface 
 ornamented with radial striae. Silurian to Carboniferous. 
 
 Strophomena. Shell concavo-convex, semicircular 
 or nearly quadrangular, ornamented with small radiating 
 ribs, sometimes also concentric folds, rarely smooth. Space 
 between the two valves very small. Hinge-line straight 
 forming the greatest width of the shell. Area double, 
 that of the dorsal valve linear, that of the ventral valve 
 with a median notch covered by a pseudo-deltidium. 
 Umbone of ventral valve perforated by a small circular 
 foramen except in old age. Two strong diverging teeth 
 in the ventral valve. Four muscular impressions. Ordo- 
 vician to Carboniferous. 
 
 Leptaena. Shell concavo-convex, semicircular, orna- 
 mented with radiating ribs. Hinge-line straight forming 
 the greatest width. Area double, the dorsal linear. 
 Ventral valve very convex, with two diverging teeth. 
 Dorsal valve with four strongly marked muscular impres-
 
 118 
 
 BRACHIOPODA. 
 
 sions, which are elongated, extending nearly two-thirds of 
 the length of the shell. Muscular impressions in the 
 ventral valve very small. Radiating vascular impressions. 
 Ordovician to Carboniferous ; perhaps also Lias. 
 
 Orthis (fig. 37). Outline circular or quadrate. Both 
 valves convex, the dorsal sometimes flattened. Surface 
 radiately ribbed or striated. Hinge-line straight, generally 
 
 A B 
 
 FIG. 37. Orthis striatula, Devonian. A, interior of dorsal valve; 
 B, ventral valve, c, curved brachial processes (cruras) ; v, genital 
 impressions; h, area; t, teeth; a, adductors; d, divaricators. 
 (From Woodward.) 
 
 shorter than the width of the shell. Area double, each 
 divided by a median triangular fissure. In the ventral 
 valve, two large teeth, supported by dental plates. Four 
 muscular impressions in the dorsal valve ; two long 
 narrow ones in the ventral. Cambrian to Carboniferous. 
 
 Spirifer (fig. 38). Shell transverse, more or less 
 triangular, biconvex, smooth or ornamented with ribs or 
 striae. Often with a sinus on the ventral valve and a 
 ridge on the dorsal. Hinge-line long and straight. Area 
 double, that of the ventral valve triangular, often trans- 
 versely striated, divided by a median triangular slit. 
 Dorsal area small, also with a median slit. Teeth sup-
 
 BRACHIOPODA. 
 
 119 
 
 ported by dental plates in the ventral valve. Brachial 
 skeleton often filling a great part of the interior of the 
 
 FIG. 38. Spirifer striatus, Carboniferous. A, interior of dorsal valve ; 
 showing brachial skeleton. B, interior of ventral valve. (From 
 Woodward.) 
 
 shell, in the form of two spiral ribbons, with their apices 
 directed laterally. Silurian to Permian. The sub-genus 
 Spiriferina occurs in the Lias. 
 
 Uncites. Shell elongate-oval, biconvex, striated. 
 Hinge-line curved, no area. Umbone of the ventral valve 
 prominent and incurved, foramen closed in the adult. 
 Apex of dorsal valve recurved and partly hidden in the 
 ventral valve. Brachial skeleton spiral, the apices of the 
 spires directed laterally. Devonian. 
 
 Atrypa (fig. 39). Shell sub-circular, biconvex, orna- 
 
 A B 
 
 FIG. 39. Atrypa reticularis, Wenlock Limestone. A, dorsal valve show- 
 ing brachial skeleton. B, interior of ventral valve, a, impressions 
 of adductor muscles ; c, divaricator muscles ; p, muscles of peduncle ; 
 o, ovarian impression ; d, deltidium. (From Woodward.)
 
 120 BRACHIOPODA. 
 
 mented with radiating ribs arid often with lines of growth. 
 Ventral valve depressed in front. Dorsal valve often 
 much inflated. Hinge-line curved, no area. A small 
 circular foramen, sometimes with a small deltidium often 
 concealed. Ventral valve with two strong crenulate teeth, 
 muscular impressions grouped in the centre of the valve. 
 Brachial skeleton in the form of two spirals with their 
 apices directed toward the centre of the dorsal valve. 
 Ordovician to Trias. 
 
 Rhynchonella (fig. 40). Shell triangular, elongated 
 or rounded, very convex, ornamented with radiating 
 ribs. A sinus on the ventral valve and a corresponding 
 ridge on the dorsal. Umbone small, acute, recurved; 
 
 A B 
 
 FIG. 40. Rhynchonella acuminata, Carboniferous Limestone. Internal 
 casts. V, 'vascular' impressions; 0, ovarian impressions; A, adduc- 
 tors ; B, divaricators ; P, muscles of the peduncle. (From Wood- 
 ward.) 
 
 foramen enclosed by deltidium. Ventral valve with two 
 strong teeth, muscular impression in middle of valve. 
 Brachial skeleton reduced to two short, free, curved, 
 lamellae. Ordovician to present day.
 
 BRACHIOPODA. 
 
 121 
 
 Pentamerus (fig. 41). Shell oval, ornamented with 
 ribs, rarely smooth. Ventral valve the most convex. 
 Dorsal valve usually with a fold in front and on the 
 ventral valve a corresponding depression. Umbone of 
 
 A B 
 
 FIG. 41. Pentamerus knighti, Aymestry Limestone. A, longitudinal 
 section ; B, transverse section, ss, dorsal septa ; , ventral septum ; 
 d, dental plates. (From Woodward.) 
 
 ventral valve sharp, strongly incurved, usually touching 
 the dorsal valve and concealing the triangular aperture. 
 No area, nor deltidium. Dental-plates (d) trough-like, 
 converging in the ventral valve to form a median septum 
 (s), in the dorsal valve two contiguous septa (ss). Silurian 
 to Devonian. 
 
 Terebratula (fig. 42). Shell biconvex ; oval, elongate, 
 or rounded ; surface smooth. Often with two folds on 
 
 FIG. 42. Terebratula vitrea, Kecent. Interior of dorsal valve, showing 
 the brachial skeleton. (From Woodward.)
 
 122 
 
 BRACHIOPODA. 
 
 the dorsal valve and two corresponding sinuses on the 
 ventral. Hinge-line curved. Umbone of ventral valve 
 truncated by a circular foramen, having a deltidium 
 at its base. Brachial skeleton in the form of a short loop 
 extending to only about a third of the length of the shell. 
 Devonian to present day. 
 
 Waldheimia (fig. 32). Distinguished from Terebra- 
 tula by the longer brachial loop, extending to at least 
 half the length of the shell, and by a median septum in the 
 dorsal valve, which is however sometimes rudimentary. 
 Lias to present day. 
 
 Stringocephalus (fig. 43). Shell nearly circular in 
 outline ; smooth. Ventral valve with a sharp prominent 
 umbone, area present. Foramen large and triangular in 
 the young individuals, smaller and oval in the adult on 
 account of the development of the deltidium. Teeth 
 large. Ventral valve with a median septum (vs), which 
 
 A B 
 
 FIG. 43. Stringocephalus burtini, Devonian. A, Dorsal valve. B, Profile. 
 a, adductor; c, crura; I, loop; j, cardinal process; p, hinge-plate; 
 s, dorsal septum ; vs, ventral septum ; t, dental sockets. (From 
 Woodward.)
 
 BRACHIOPODA. 123 
 
 extends from the umbone almost to the front of the valve, 
 and increases in height towards the latter. Dorsal valve 
 less convex, with a small septum (), and a long slightly 
 curved cardinal process (f), divided at its extremity to 
 embrace the ventral septum. The brachial skeleton 
 consists of two branches arising from the hinge-plate (p), 
 which pass to the middle of the shell (c) and are then 
 sharply bent back and form a ribbon (I) parallel to the 
 margin of the valve. Silurian and Devonian. 
 
 Distribution of the Brachiopoda. 
 
 The Brachiopods are all marine and are found in all 
 parts of the world ; at the present time they are much 
 less numerous than in former periods of the earth's history. 
 About half of the existing species are found at depths 
 of less than 100 fathoms, several of these do not extend 
 beyond this limit. Below 150 fathoms they soon become 
 comparatively rare, but a few species occur down to 2900 
 fathoms. Some genera, as for instance Lingula, do not 
 occur below a depth of a few fathoms. A few species, as 
 Terebratulina caput-serpentis, have a very wide geograph- 
 ical distribution, extending from polar to tropical regions, 
 and these have also a great range in depth, the form 
 mentioned being found from the shore down to 1180 
 fathoms. 
 
 Generally the species confined to shallow water have 
 a much more limited geographical range than those found 
 in deeper water ; and the polar or boreal species have a 
 wider range than the tropical, since they can find the 
 same temperature in lower latitudes at great depths. 
 
 The Brachiopods are very abundant as fossils, the 
 earliest forms occur in the Lower Cambrian beds. In the
 
 124 BRACHIOPODA. 
 
 Ordovician the Brachiopods are much more numerous 
 than in the Cambrian, and they attain their maximum in 
 the Silurian, their decline begins with the Devonian and 
 becomes especially marked in the Mesozoic. The most 
 important genera in the Cambrian are Lingula, Obolella, 
 Discina, and Orthis (the last being the only representative 
 of the Articulata) ; in the Ordovician, Lingidella, Obolella, 
 Orthis, Strophomena, Leptcena ; in the Silurian, Lingula, 
 Atrypa, Orthis, Meristella, Rhynchonella, Pentamerus, 
 Strophomena, Chonetes ; in the Devonian, Uncites, Stringo- 
 cephalus, Atrypa, Orthis, Spirifer ; in the Carboniferous, 
 Chonetes, Athyris, Spirifer, Productus, Terebratula, Rhyn- 
 chonella; in the Permian, Productus, Spirifer, Rhynchonella. 
 The majority of the Palaeozoic genera die out before the 
 commencement of the Mesozoic, the most important 
 genera now being Terebratula, Terebratulina, Waldheimia, 
 Rhynchonella, and Crania. In the Tertiary the Brachio- 
 pods are rare, the chief forms being Terebratula, Wald- 
 heimia, and Rhynchonella.
 
 SUB-KINGDOM VII. MOLLUSCA. 
 
 Classes. 
 
 1. Lamellibranchiata. 
 
 2. Gasteropoda 
 
 3. Scaphopoda. 
 
 4. Cephalopoda 
 
 Orders. 
 
 1. Asiphonida ............. 
 
 2. Siphonida ................ 
 
 1. Polyplacophora. 
 
 2. Prosobranchiata. 
 
 3. Opisthobranchiata. 
 
 4. Heteropoda. 
 
 5. Pulmonata. 
 
 6. Pteropoda. 
 
 :1. Tetrabranchiata 
 2. Dibranchiata. . . . 
 
 Sections. 
 (\. Monomyaria. 
 2. Heteromyaria. 
 \B. Homomyaria. 
 j 1 - Integripalliata. 
 1 2. Sinupalliata. 
 
 Sub- Orders. 
 
 fl. Nautiloidea. 
 (2. Ammonoidea. 
 fl. Decapoda. 
 (2. Octopoda. 
 
 THE majority of the mollusks (Oysters, Whelks, Cuttle- 
 fishes) are marine, but some live on land, others in 
 fresh-water. Unlike the worms and arthropods, they are 
 unsegmented animals and bear no appendages. Typically 
 the body is bilaterally symmetrical and there is con- 
 sequently a repetition of the same organs on each side. 
 But in many gasteropods this symmetry is more or less 
 obliterated. From the dorsal surface there is a fold of the
 
 126 MOLLUSCA. 
 
 skin forming what is known as the mantle ; this generally 
 secretes a calcareous shell, consisting of one or two pieces. 
 There is a ventral protrusion of the body forming the foot, 
 a muscular organ used generally in locomotion. Res- 
 piration takes place in most cases by means of gills ; in 
 some forms there is instead a pulmonary sac. A heart is 
 present and is placed on the dorsal surface, and it is 
 systemic (i.e. receives arterial blood). The mouth is 
 situated anteriorly, and typically the anus is posterior, the 
 former may or may not be provided with organs of 
 mastication. Renal organs (nephridia) are present and 
 communicate with the body-cavity. The nervous system 
 consists of three pairs of ganglia connected by cords. 
 Sexual reproduction only occurs; some forms being uni- 
 sexual, others hermaphrodite. 
 
 The Mollusca are divided into four classes, (1) Lamel- 
 libranchiata, (2) Gasteropoda, (3) Scaphopoda, (4) Cepha- 
 lopoda. 
 
 CLASS. LAMELLIBRANCHIATA. 
 
 In the lamellibranch as in the brachiopod the shell is 
 generally calcareous and consists of two valves, but these 
 instead of being dorsal and ventral as in the latter, are 
 placed one on the right, the other on the left side of the 
 animal, and the two are joined together by means of a 
 hinge and ligament at the dorsal margin. The interior of 
 the shell is lined by a fold of the skin, the mantle (fig. 44 
 m), which is divided into two lobes, one being placed in 
 each valve. In the middle of the space enclosed by the 
 mantle (the mantle-cavity) is the foot (6). This is a 
 laterally flattened muscular organ, frequently hatchet 1 or 
 1 Hence the name Pelecypoda used by some authors for this class.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 127 
 
 plough-share shaped, and is used for crawling, or for 
 burrowing in sand or mud. Sometimes as in the cases of 
 Trigonia and Cardium, by means of a rapid movement, it 
 
 FIG. 44. Mya arenaria. The left valve and mantle, and also half the 
 siphons, have been removed, a, anterior adductor muscle; a', 
 posterior adductor; b, visceral mass; c, cloacal chamber into which 
 the anus opens ; /, foot ; g, branchiae ; h, heart ; m, cut edge of the 
 mantle ; o, mouth ; s, branchial siphon ; s', anal siphon ; t, labial 
 palps ; v, anus. (From Woodward.)
 
 128 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 enables the animal to jump to a considerable distance. 
 In the genus Mytilus the foot is very much reduced ; in 
 others (e.g. Ostrea) it is absent altogether, but this occurs 
 only in those forms which have lost the power of loco- 
 motion. In some genera (e.g. Mytilus) there is on the 
 posterior part of the foot a gland which secretes a bundle 
 of horny fibres, known as the byssus, by means of which 
 the animal moors itself to foreign objects. On each side 
 of the foot, between it and the mantle, and attached to 
 the body dorsally, are the gills or branchiae (fig. 44 g). 
 These are usually leaf-like, hence the name Lamelli- 
 branchiata; there are two pairs, one on each side of the 
 body, and they are covered with cilia. 
 
 In some forms, like Anodon, the margins of the two 
 mantle-lobes although in contact are not united, and 
 when this is the case, there are at the posterior margin, 
 two adjoining openings leading from the exterior to the 
 mantle- cavity ; these are produced by the apposition of 
 contiguous excavations in the mantle-lobes. A current 
 of water caused by the cilia on the gills and mantle flows 
 in through the ventral opening, and provides the animal 
 with food and oxygen ; another current flows out through 
 the dorsal opening, carrying with it faecal and indigestible 
 matters. In many cases, however, the two lobes of the 
 mantle are fused to a greater or less extent at their 
 margins. This union occurs at two places, one between 
 the exhalent and inhalent openings, and the other ventral 
 to the latter opening. In this way the mantle comes to 
 form a kind of bag, having three openings, a ventral for 
 the protrusion of the foot, and two posterior for the in- 
 halent and exhalent currents of water. Frequently the 
 mantle is greatly produced at the posterior openings, so 
 as to form two complete tubes, known as siphons (fig. 44
 
 MOLLUSCA. LAMELLIBRANCHIATA. 129 
 
 s, s') ; these are sometimes free from one another, some- 
 times united, and they may be as much as four times the 
 length of the shell. The ventral is generally the longer, 
 it is furnished with tactile papillae, and is known as the 
 branchial siphon (s), the dorsal being the anal siphon (s'). 
 In many forms the siphons can be withdrawn into the 
 shell by means of muscles. Occasionally, as in Teredo, 
 the siphons are surrounded by a calcareous tube. 
 
 The two valves of the shell can be closed by means of 
 the adductor muscles (a, a'), which pass from the interior 
 of one valve to the other. In many genera there are two 
 adductors, and these forms are frequently spoken of as 
 Dimyaria ; others, known as the Monomyaria, possess one 
 only, and when this is the case it is the posterior which is 
 present, the anterior having atrophied ; this occurs in the 
 oyster, but in this form the anterior muscle is found to be 
 present in the young state. 
 
 The mouth (o) is placed in the middle line of 
 the body, ventral to the anterior adductor muscle, 
 and it is not provided with organs of mastication or 
 prehension. At its sides, are two leaf-like processes, 
 the labial palps (). The mouth leads into a short 
 oesophagus, which passes into a globular stomach sur- 
 rounded by the liver ; then comes the intestine, which 
 after undergoing many convolutions, reaches the dorsal 
 surface of the body, where it passes through the peri- 
 cardium and is surrounded by the ventricle of the heart, 
 and afterwards ends in an anus (v) situated dorsally to the 
 posterior adductor muscle. In the lamellibranchs there 
 is no head, hence the class is sometimes spoken of, as the 
 Acephala. The nervous system consists of three pairs of 
 ganglia. One pair is placed at the sides of the mouth and 
 is connected by cords with a pair in the foot, and with a 
 w. p. 9
 
 130 MOLLUSC A. LAMELLIBRANCHIATA. 
 
 third pair placed beneath the posterior adductor muscle. 
 From these ganglia, nerves are given off to the muscles, 
 gills, etc. Tactile organs are present on the margin of 
 the mantle and especially on the ventral siphon. In some 
 forms, eyes occur at the ventral margin of the mantle- 
 lobes, they are especially well-developed in the genus 
 Pecten. The heart (h) is placed dorsally, just below the 
 hinge ; it is surrounded by a large pericardial cavity, 
 and consists of two auricles, and a ventricle, which as 
 already mentioned, extends round the intestine. From 
 the ventricle, an anterior and a posterior aorta are given 
 off. In nearly all cases the blood is colourless. The renal 
 organs consist of a pair of glandular bodies underneath 
 the ventricle. In almost all forms the sexes are separate, 
 but a few are hermaphrodite. 
 
 As already mentioned the two valves of the shell are 
 placed on the sides of the animal and not dorsally and 
 ventrally as in the brachiopods. The margin near the 
 hinge (fig. 45 d) is dorsal, the opposite (v), where the 
 valves open, is ventral, that near the mouth is anterior (a), 
 that near the anus and siphons posterior (p). In the 
 majority of cases the two valves are equal or practically 
 equal, and each valve is generally inequilateral. But 
 in some (e.g. Pectunculus) the shell is equilateral, in 
 others (e.g. Ostrea) it is inequivalve. Each valve may be 
 regarded as a greatly depressed hollow cone, the apices of 
 which form the umbones (u) ; these are sometimes straight 
 (e.g. Pecten), but generally curved towards the anterior 
 margin. In a few genera (e.g. Nucula, Trigonia) they are 
 directed posteriorly. In Diceras the umbones are spiral. 
 There is sometimes, placed in front of the umbone and 
 bounded by a groove, an oval depressed area (lu), half 
 occurring on each valve, this is termed the lunule, and it 

 
 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 131 
 
 is important to remember that it is always anterior to the 
 umbones. Behind the umbones there is sometimes a 
 somewhat similar, but larger area, bounded by a ridge, 
 and known as the escutcheon. 
 
 In the interior of the valves various markings will be 
 noticed (fig. 45 B), these are produced by the union of the 
 muscles with the shell. The adductors form prominent 
 
 FIG. 45. Cytherea chione. Eecent. A, dorsal view of the two valves. 
 B, interior of left valve. 
 
 a, anterior border ; p, posterior ; d, dorsal ; v, ventral ; lu, 
 lunule ; u, umbone ; I, ligament ; aa, anterior adductor impression ; 
 pa, posterior adductor ; pi, pallial line ; , pallial sinus ; w, x, y, 
 cardinal teeth ; z, anterior-lateral tooth. 
 
 oval or round, sometimes elongated, depressions (the 
 adductor impressions, aa, pa,), there are two in each valve 
 in the Dimyaria, one being anterior, the other posterior ; 
 sometimes they are of the same size, in others the anterior 
 is smaller. In the Monomyaria there is one impression 
 only which is then placed posteriorly or sometimes almost 
 central. Less important than the adductor impressions 
 are those produced by the muscles for the retraction of 
 the foot, of which there are two in each valve ; of these 
 
 92
 
 132 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 one occurs close to the anterior adductor, the other just 
 above the posterior adductor. Passing from one adductor 
 to the other in each valve, is a linear depression, the 
 pallial line (pi) ; this is caused by the attachment of the 
 muscles of the mantle to the shell. In some forms this 
 line runs evenly between the two adductor impressions 
 and parallel to the margin of the valve, it is then said to 
 be simple or entire. But in those genera which possess 
 retractile siphons, there is a bending in of the pallial line 
 just before reaching the posterior adductor, this inden- 
 tation is known as the pallial sinus (s), and is caused by 
 the muscles of the siphons. 
 
 The hinge is formed by means of projections known as 
 teeth, which alternate in the two valves, the teeth of one 
 valve fitting into depressions between those of the other. 
 The teeth may be distinguished as cardinal and lateral. 
 The cardinal are those which occur under the umbones 
 (fig. 45 w, x, y) ; the lateral at the sides, those in front 
 being the anterior-lateral (z\ those behind, the posterior- 
 lateral. In some genera cardinal teeth only are found, in 
 others lateral or some of the lateral only. In a few 
 forms the two kinds of teeth cannot be separated, thus 
 Area possesses a row of numerous small teeth, all of 
 which are alike. Lastly some forms (e.g. Anodon) are 
 without teeth. The margin on which the teeth occur is 
 known as the hinge-line; generally it is curved, but in 
 some genera like Area it is straight. Sometimes (e.g. 
 Area) there is on each valve a flattened triangular space 
 between the hinge-line and the umbone, known as the 
 area ; when this is present the umbones of the two valves 
 are widely separated. 
 
 In the brachiopods the valves are opened by the 
 divaricator muscles, but in the lamellibranchs the work of
 
 MOLLUSCA. LAMELLIBRANCHIATA. 133 
 
 these muscles is performed by the ligament. This 
 consists of two parts, the external (fig. 45 I), and the 
 internal (sometimes erroneously termed the cartilage.) 
 One or other may be absent. The external ligament is 
 composed of horny material ; it is placed outside the 
 shell and is either under the umbones or posterior to 
 them, it passes from one valve to the other and is 
 frequently attached to more or less prominent ridges. 
 When the valves of the shell are closed the external 
 ligament is stretched, and consequently in order to open 
 the shell, the animal has merely to relax its adductor 
 muscles. The internal ligament consists of parallel elastic 
 fibres, and is placed in grooves or sockets along the hinge- 
 line, so that when the valves are closed it is compressed, 
 and being elastic tends to force the valves apart. Its 
 action is similar to that of a piece of india-rubber placed 
 in the hinge-line of a door. Occasionally the ligament is 
 preserved in fossil specimens. The length of a lamelli- 
 branch shell is measured from the anterior to the 
 posterior margin (fig. 45 B, a p), the breadth or height 
 from the umbone to the ventral margin (d v), the thick- 
 ness from one valve to the other at right angles to the 
 lines of length and breadth. 
 
 The shell is secreted by the mantle and consists of 
 two calcareous layers, the inner is the pearly or nacreous 
 layer, and consists of numerous lamellae ; the outer is the 
 prismatic layer (fig. 46), and consists of prisms placed at 
 right angles to the surface of the shell. The external 
 surface is covered by a thin green or brownish epidermis. 
 The prismatic layer is formed by the margin of the 
 mantle only, the pearly layer by the whole of the rest of 
 the mantle, and this layer gradually encroaches on the 
 former, which consequently cannot afterwards increase in
 
 134 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 thickness, whereas the pearly layer may do so throughout 
 the life of the animal. In some genera both layers 
 consist of calcite, in others of aragonite, or one may be of 
 calcite and the other of aragonite. The surface of the 
 
 Fro. 46. Section of prismatic layer of recent Pinna, parallel to the 
 surface of the shell and at right angles to the prisms. Magnified. 
 
 shell may be smooth, or may be ornamented with radi- 
 ating or concentric ribs, with stria? or spines. Often the 
 exterior shows concentric lamellae which are considered to 
 represent periods of growth. The part of the shell at the 
 umbone is that which was first formed and it sometimes 
 shows different characters to the other parts. The 
 margins of the valves may be smooth or crenulated. In 
 many genera the two valves can be completely closed, in 
 others they are always open at some part, and are then 
 said to be gaping; this gape occurs most frequently at 
 the posterior part but sometimes also anteriorly. 
 
 It is of importance to be able to distinguish the right 
 and left valves. In order to do this we must determine 
 first the anterior and posterior margins. When the soft 
 parts of the animal are present this is easily done. But 
 when the shell only is before the observer the points to 
 be noticed are these :
 
 MOLLUSC A. LAMELLIBRANCHIATA. 135 
 
 (1) The umbones are generally directed anteriorly. 
 
 (2) The lunule is always anterior to the umbones. 
 
 (3) The external ligament is never anterior. 
 
 (4) The pallial sinus is posterior. 
 
 (5) When one adductor impression only is present, it 
 
 is the posterior. 
 
 Having found the anterior and posterior margins, the 
 shell should be placed with the dorsal margin uppermost 
 and the anterior margin pointing away from the observer, 
 then the right and left valves will be on his right and left 
 hand sides respectively. 
 
 Most of the lamellibranchs are free, but a few forms 
 like the oyster are permanently attached by one valve. In 
 some cases it is the right valve which is fixed, in others 
 the left. In the free forms movement takes place usually 
 by means of the foot, but some genera (Pecten, Lima) 
 move by the alternate rapid opening and shutting of the 
 valves. A few are capable of making borings into various 
 substances ; thus Teredo, the ship-worm, bores into 
 wood, Lithodomus'inio limestone, and Pholas into various 
 materials such as sandstone, limestone, gneiss, peat, and 
 amber. Wood perforated by Teredo has been found fossil 
 in various formations of Eocene and Oligocene age. 
 
 The features which more especially characterise the 
 lamellibranchs as a class, are, the bilateral symmetry, the 
 mantle being divided into two lobes, the bivalve shell, the 
 lamellar gills, and the absence of a head and of organs of 
 mastication. Although at first sight the shell appears to 
 resemble closely that of the brachiopod, it differs in 
 several important respects, namely, (1) in the valves being 
 right and left, instead of dorsal and ventral, (2) in being 
 generally inequilateral and equivalve, (3) in the presence
 
 136 MOLLUSCA. LAMELLIBEANCHIATA. 
 
 of teeth on both valves, (4) in the presence of a ligament, 
 (5) in the umbones never being perforated for a peduncle. 
 The classification followed here is based on the presence 
 or absence of siphons, and on the adductor muscles ; this 
 is not altogether a natural one, but it is convenient for 
 palseontological purposes. There are two orders, (1) 
 Asiphonida, (2) Siphonida. 
 
 OEDEE. ASIPHONIDA. 
 
 Animal without siphons, mantle lobes free or united 
 at one point only, pallial line simple. 
 
 Section 1. Monomyaria. 
 One adductor (the posterior) present only. 
 
 Ostrea*. Shell irregular, lamellar, inequivalve, slightly 
 inequilateral, fixed by the left valve. Left valve convex, 
 with radiating ribs or striae, umbone prominent ; right 
 valve flat or concave, often smooth and without stria3 or 
 ribs. Ligamental cavity triangular or elongated. Hinge 
 without teeth. Adductor impression sub-central ; pallial 
 line indistinct. Trias (? also Carboniferous) to present 
 day. 
 
 Gryphaea. Shell inequivalve, free in the adult. Left 
 valve large, with a prominent incurved umbone. Right 
 valve flattened or concave. Lias to present day. 
 
 Exogyra. Shell fixed by the left (larger) valve. 
 Right valve flat, operculiform. Umbones more or less 
 spiral, directed posteriorly. Upper Jurassic to Chalk. 
 
 * All the genera of Mollusca described are marine unless otherwise stated.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 137 
 
 Plicatula. Shell irregular, inequivalve, depressed ; 
 smooth, folded or scaly, without ears, fixed by the umbone 
 of the right valve. Hinge-area obscure. Hinge with 
 strong teeth in each valve ; ligament internal, between 
 the teeth. Adductor impression simple, excentral, near 
 the pallial line. Trias to present day. 
 
 Lima. Shell obliquely oval, equivalve, compressed, 
 with radiating strise or ribs, the ribs scaly. Valves gaping 
 anteriorly and sometimes posteriorly. Umbones distant, 
 sharp, prominent, auriculate, the ears unequal. Hinge 
 straight without teeth. A triangular hinge-area, furnished 
 with a central ligamental cavity. Adductor impression 
 large, divided, excentral. Two small pedal impressions. 
 Carboniferous to present day. 
 
 Pecten. Shell suborbicular or oblong, free, closed, 
 equilateral, inequivalve, umbones approximated. Surface 
 usually with radiating ribs, auriculate, the anterior ears 
 larger than the posterior, the right one with a byssal 
 sinus. Right valve more convex than the left. Hinge-line 
 straight. Ligamental cavity central, narrow, triangular, 
 internal. Adductor impression large, sub-central. Lias 
 to present day. (Possibly also Carboniferous.) 
 
 Aviculopecten. Shell a little inequilateral ; right 
 valve less convex than the left. Anterior ear smaller than 
 the posterior ; a byssal sinus beneath the anterior ear in 
 the right valve. Ligament in a narrow groove parallel to 
 the hinge-line. Adductor impression large, sub-central. 
 Surface generally radiately ribbed. Devonian to Per-
 
 138 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 Section 2. Heteromyaria. 
 The anterior muscle small, the posterior large. 
 
 Avicula. Shell oblique, inequilateral, inequivalve, left 
 valve more convex. Interior nacreous. Hinge straight, 
 with one or two small cardinal teeth. Auriculate, the 
 posterior ear, wing-like and longer than the anterior. A 
 byssal sinus under the right anterior ear. Hinge-area 
 small, striate. Ligament partly internal, partly external, 
 the internal in a deep groove. Posterior adductor im- 
 pression large, sub-central. Ordovician to present day. 
 
 Pterinea. Shell inequilateral, oblique, equivalve, 
 auriculate, the anterior ear shorter than the posterior ; a 
 byssal sinus under the right anterior ear. Hinge-line 
 long, straight ; with anterior small teeth, and posterior 
 elongated, laminar, teeth. Ligamental area large, longi- 
 tudinally striated. Posterior muscular impression large, 
 shallow; anterior impression small, deep, below the an- 
 terior ear. Ordovician to Carboniferous. 
 
 Gervillia. Shell obliquely elongated, slightly inequi- 
 valve, inequilateral, the left valve a little more convex. 
 Umbones almost terminal. Hinge-line straight, broad, 
 with numerous perpendicular widely separated ligamental 
 pits ; its posterior edge with two or more obscure teeth 
 diverging behind. Ears not very distinct from the rest of 
 the shell, anterior ear short, posterior long. Adductor 
 impressions as in Avicula. Trias to Eocene. 
 
 Inoceramus. Shell variable in form, circular, oval, 
 or oblong, inequilateral, inequivalve, ventricose or com- 
 pressed, not auriculate. Umbones prominent. No teeth.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 139 
 
 Surface with concentric (or rarely radiating) grooves. 
 Hinge-line straight, or slightly curved, elongated. Nume- 
 rous parallel close-set cartilage pits. Adductor impression 
 rarely visible. Inner layer of shell thin and nacreous, 
 outer layer thick, fibrous and prismatic. Trias to Chalk. 
 
 Perna. Shell nearly equivalve, inequilateral, com- 
 pressed, subquadrate or sub-circular, auriculate. Hinge- 
 line straight, no teeth. Hinge-area large. Cartilage pits 
 numerous, parallel, close together and elongated. Right 
 valve with a byssal sinus. Adductor impression large, 
 sub-central, double. Posterior ear often large, anterior 
 rudimentary. Trias to present day. 
 
 Mytilus. Shell equivalve, inequilateral, sub-triangular, 
 posterior border rounded. Umbones sharp, terminal, 
 anterior. Teeth small or obsolete. Ligament linear, 
 marginal, sub-internal. Anterior adductor impression 
 small, posterior large. Surface of shell with an epidermis. 
 Trias to present day. 
 
 Modiola. Shell equivalve, inequilateral, oblong, in- 
 flated in front. Umbones obtuse, anterior, but not terminal. 
 No teeth. Ligament linear, marginal. Adductor im- 
 pressions like Mytilus. Devonian to present day. 
 
 Lithodomus. Shell sub-cylindrical, very inequi- 
 lateral, rounded in front, wedge-like behind. Umbones 
 anterior, not prominent. Ligament internal. No teeth. 
 A thin nacreous layer. Carboniferous to present day. 
 
 Pinna. Shell equivalve, inequilateral, wedge-shaped, 
 generally thin, not auriculate. Umbones sharp, anterior, 
 terminal. Valves gaping posteriorly. Hinge-line straight, 
 long. No teeth. Ligament linear, almost entirely in-
 
 140 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 ternal, lodged in a groove. Posterior adductor large, 
 sub-central ; anterior apical. Devonian to present day. 
 
 Section 3. Homomyaria. 
 
 The two muscles of equal size. Mantle-lobes free or 
 sometimes united posteriorly at one point. 
 
 Area. Shell thick, generally equivalve, sub-quad- 
 rangular, ventricose, costulate or cancellate. Margins 
 smooth or dentate ; closed or open ventrally. Hinge-line 
 straight. Teeth numerous, equal, transverse. Umbones 
 prominent, remote, separated by a lozenge-shaped area, 
 which has numerous ligamental grooves converging from 
 the hinge-margin to the umbones. Adductor impressions 
 subequal, the anterior rounded, the posterior divided. 
 Pallial line simple. Ordovician to present day. 
 
 Cucullaea. Shell rhombic or trapezoidal, ventricose, 
 nearly or quite equivalve. Valves closed, ornamented 
 with radiating striae. Umbones incurved. Hinge-area 
 large, well defined, with diverging ligamental grooves ; 
 hinge-line straight, with two short central transverse teeth, 
 and two to five lateral teeth nearly parallel to the hinge- 
 margin. Ligament external. Posterior adductor impres- 
 sion on a thin raised plate. Pallial line simple. Carbon- 
 iferous to present day. 
 
 Cardiola. Shell thin, convex, oval, generally inequi- 
 lateral ; umbones prominent, incurved. Surface with 
 radiating and concentric grooves. Hinge-line straight ; 
 ligamental area large, horizontally grooved. Muscular 
 impressions unknown. Silurian and Devonian.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 141 
 
 Pectunculus. Shell thick, solid, sub-orbicular, equi- 
 valve, almost equilateral. Surface smooth or radiately 
 striated. Ligament external. Umbones central, slightly 
 curved towards each other, separated by a triangular 
 ligamental area provided with diverging striae. Hinge 
 with a semi-circular row of numerous, small, strong, 
 transverse teeth, obliterated at the centre in the older 
 forms by the growth of the ligamental area. Margins 
 crenulate inside, adductor impressions sub-equal. Pallial 
 line simple. Lower Cretaceous to present day. 
 
 Nucula. Shell equivalve, trigonal, oval, closed, pos- 
 terior side very short ; umbones directed posteriorly. 
 Surface smooth or ornamented. Interior nacreous. Margin 
 of valves smooth or crenulated. Hinge-line angulated, 
 with a median internal triangular ligamental pit, on each 
 side of which is a row of numerous sharp teeth. Adductor 
 impressions sub-equal. Pallial line simple. Silurian to 
 present day. 
 
 Leda Shell oblong, similar to Nucula, produced and 
 pointed posteriorly. Surface smooth or striated. Teeth 
 numerous. Ligamental pit internal. Pallial line slightly 
 sinuated. Margins smooth. Lunule lanceolate. Silurian 
 to present day. 
 
 Trigonia. Shell thick, ornamented with tubercles or 
 with radiating or concentric ribs ; trigonal, very inequi- 
 lateral, anterior margin rounded, posterior produced and 
 angular. Generally with a ridge extending from the 
 umbones to the posterior border, cutting off an area which 
 has a different ornamentation. Umbones anterior, directed 
 posteriorly. Cardinal teeth diverging, striated, two in the 
 right valve, three in the left, the central tooth in the 
 latter being bifid. Ligament marginal, thick. Adductor
 
 142 MOLLUSC A. LAMELLIBRANCHIATA. 
 
 impression deep, the anterior much smaller than the 
 posterior, and placed near the umbones. Pedal impres- 
 sions in front of the posterior adductor and one in the 
 umbone of the left valve. Pallial line simple. Interior 
 nacreous. Lias to present day. 
 
 Unio. Shell oval or elongated, with a thin epidermis. 
 Surface smooth, striated or folded. Umbones more or 
 less anterior, often corroded. Ligament external, elon- 
 gated. In the right valve there are two anterior lateral 
 teeth and a long lamellar posterior-lateral tooth ; in the 
 left valve one anterior-lateral, one cardinal, and two long 
 lamellar posterior lateral teeth. Adductor impressions 
 very deep, especially the anterior. Pallial line simple. 
 Purbeck Beds to present day. Lives in freshwater. 
 
 Anthracosia. Shell elongate, oval, inequilateral. 
 Umbones anterior, not prominent. A strong cardinal 
 tooth in each valve. Ligament external in a linear groove. 
 Muscular impressions deep. Pedal impression above the 
 anterior adductor. Carboniferous and Permian. 
 
 Cardinia. Shell trigonal, oval or oblong, very inequi- 
 lateral, compressed, thick, marked by striae of growth. 
 Umbones small, close together. Ligament external. 
 Cardinal teeth small or obsolete ; in the right valve one 
 anterior- lateral tooth, in the left, one posterior-lateral. 
 Impression of anterior adductor very deep. Pallial line 
 simple. Trias to Lower Oolites. 
 
 OBDEE. SIPHONIDA. 
 
 Animal with siphons. Mantle-lobes more or less 
 united at the margins. Both muscles well developed.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 143 
 
 Section 1. Integripalliata. 
 
 Siphons short, not retractile. Pallial line without 
 sinus. 
 
 Cardita. Shell oblong, elongated, very inequilateral, 
 ornamented with prominent radiating ribs. Often a little 
 gaping and sinuous at its ventral margin. Lunule present. 
 Ligament external. In the right valve two cardinal teeth 
 and a small anterior-lateral tooth. In the left valve one 
 cardinal and a posterior elongated lateral tooth. Adductor 
 impressions very large. Pallial line simple. Trias to 
 present day. 
 
 Astarte. Shell solid, inequilateral, compressed, closed, 
 more or less trigonal or sub-orbicular. Surface smooth or 
 with concentric furrows or striae. A thick epidermis. 
 Umbones prominent. Lunule distinct. Escutcheon elon- 
 gated. Ligament external. Two cardinal teeth in each 
 valve, lateral teeth rudimentary. Adductor impressions 
 strongly marked, above the anterior one is a pedal im- 
 pression. Pallial- line simple. Silurian to present day. 
 (Those in the Palaeozoic doubtful.) 
 
 Opis. Shell trigonal, cordiform, convex, with an 
 oblique keel extending from the umbone to the posterior 
 border. Umbones prominent, incurved, or sub-spiral. 
 Lunule large. Surface generally with concentric furrows. 
 A cardinal tooth in each valve. Ligament external. 
 Adductor impressions very deep. Pallial line simple. 
 Trias to Chalk. 
 
 Crassatella. Shell solid, oblong or sub-trigonal, 
 attenuated behind. Surface smooth or concentrically 
 furrowed. Margins of valves smooth or crenulated. Um-
 
 144 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 bones small, close together. Lunule distinct. Ligament 
 internal. Hinge with two cardinal teeth, and one posterior- 
 lateral and sometimes one anterior-lateral. Adductor 
 impressions deep; pedal impressions small, distinct. Pallial 
 line simple. Lower Cretaceous to present day. 
 
 Chama. Shell irregular, thick, inequivalve, fixed by 
 the umbone of the larger valve (generally the left, some- 
 times the right). Umbones spiral or subspiral, that of 
 the fixed valve longer than the other. Surface with 
 concentric lamella or spines. The fixed valve larger and 
 deeper than the other. In each valve a strong cardinal 
 tooth, and sometimes in the inferior valve a narrow 
 curved posterior tooth also. Ligament external, in a deep 
 groove, prolonged towards the umbones. Adductor im- 
 pressions large, often rough. Pallial line simple. Cre- 
 taceous to present day. 
 
 Hippurites. Shell large, massive, conical or sub- 
 cylindrical, very inequivalve, fixed by the apex of the 
 larger valve. The larger (right) valve, conical, striated or 
 smooth, and with three parallel furrows on the cardinal 
 side, and three corresponding folds in the interior. Smaller 
 (left) valve flattened or slightly convex, operculiform, 
 porous, the pores leading into canals; with a central 
 umbone, two large cardinal teeth ; two large adductor 
 impressions ; pallial line simple. The smaller valve is 
 formed of two layers, the outer is thin and prismatic, the 
 inner is porcellanous and traversed by numerous canals. 
 The larger valve is also composed of two layers, the outer 
 is formed of small prisms arranged in parallel layers 
 obliquely to the surface of the shell, the inner is porcel- 
 lanous, and formed of thin leaflets. Middle and Upper 
 Cretaceous.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 145 
 
 Lucina. Shell orbicular or oval, slightly inequilateral, 
 more or less compressed. Lunule distinct. Umbonal 
 area with an oblique posterior furrow. Hinge with 
 usually two cardinal teeth and one or two lateral in each 
 valve, the lateral and sometimes the cardinal are absent. 
 Ligament external, sometimes partly internal. Adductor 
 impressions well-marked, rugose, the anterior elongated 
 within the pallial line, the posterior oblong. Margins of 
 valves smooth or finely crenulated. Silurian to present 
 day. 
 
 Cardium. Shell convex, nearly equilateral, cordate 
 or oval, generally closed. Umbones prominent, incurved, 
 nearly central. Surface with radiating ribs, which are 
 often spiny. Margins of valves crenulated. Two cardinal 
 teeth, and one anterior-lateral and one posterior-lateral in 
 each valve. Ligament external. Adductor impressions 
 shallow. Pallial line more or less sinuated. Trias to 
 present day. (Perhaps also Palaeozoic.) 
 
 Cyrena. Shell cordifonn, oval or trigonal ; umbones 
 often eroded. Hinge with three cardinal teeth and one 
 anterior- and one posterior-lateral in each valve. Liga- 
 ment prominent, external. Pallial line usually entire. 
 Lias to present day. Lives in fresh and brackish water. 
 
 Cyprina. Shell orbicular or oval, convex, with 
 concentric stria?; a thick epidermis. Umbones prominent, 
 incurved. Ligament external, prominent. No lunule. 
 In each valve three cardinal teeth and one posterior- 
 lateral. Adductor impressions oval. Pallial line entire. 
 Margins of valves smooth. Lias to present day. 
 
 w. P. 10
 
 146 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 Section 2. Sinupalliata. 
 
 Siphons long, partly or completely retractile. Pallial 
 line with a sinus. 
 
 Venus. Shell thick, oval, inflated; surface smooth or 
 with ribs or concentric lamellae ; lunule distinct. Margin 
 of valves finely crenulate. Hinge-plate wide with three 
 cardinal teeth, often bifid. Ligament external, prominent. 
 Pallial sinus small. Middle Jurassic to present day. 
 
 Cytherea (=Meretrix). Shell thick, ovate, sub- 
 trigonal, smooth or with concentric furrows. Margin of 
 valves not crenulate. Lunule well-marked. Ligament 
 external. Hinge thick, with three cardinal teeth in each 
 valve, two anterior-lateral in the right, and one in the left 
 valve. Pallial sinus angular. Jurassic to present day. 
 
 Artemis (= Dosinia). Shell orbicular, compressed, 
 with concentric striae. Lunule depressed. Escutcheon 
 narrow. Ligament sunk. Three cardinal teeth in each 
 valve, one anterior-lateral in the left valve, and two 
 (rudimentary) in the right. Margins smooth. Pallial 
 sinus very deep. Cretaceous to present day. 
 
 Tellina. Shell transverse, oval, elongate, or sub- 
 orbicular, slightly inequivalved, compressed, rounded in 
 front, attenuated behind, and furnished with an oblique 
 fold from the umbone to the posterior border. Margin of 
 valves smooth. Two cardinal teeth in each valve, and 
 one anterior- and one posterior-lateral which are often 
 indistinct in the left valve. Ligament external, promi- 
 nent. Pallial sinus very deep. Lower Cretaceous to 
 present day.
 
 MOLLUSCA. LAMELLIBRANCHIATA. 147 
 
 Solen. Shell very long, sub-cylindrical, straight, 
 smooth or finely striated, the dorsal and ventral margins 
 parallel. Gaping at the anterior and posterior extremities. 
 Margin of valves smooth. Umbones anterior, terminal. 
 Hinge terminal, with one cardinal tooth in each valve. 
 Ligament long, external. Anterior adductor impression 
 elongated. Pallial sinus short, square. Trias ( ? also 
 Devonian and Carboniferous) to present day. 
 
 Glycimeris (=Panopea). Shell equivalve, inequi- 
 lateral, oblong, thick, concentrically striated, gaping at 
 each end. Ligament external on a prominent ridge. 
 One strong cardinal tooth in each valve. Pallial sinus 
 very deep. Cretaceous to present day. 
 
 Pholadomya. Shell thin, translucent, oblong or 
 oval, ventricose, equivalve, gaping posteriorly and some- 
 times anteriorly. Anterior side short and rounded. 
 Surface with radiating ribs crossed by concentric striae. 
 Umbones prominent. Ligament external. Hinge with- 
 out teeth or with a small transverse dentary tubercle in 
 each valve. Adductor impressions very faint. Pallial 
 sinus very deep. Lias to present day. 
 
 Goniomya. Shell ovate-elongate, nearly equilateral, 
 gaping ; surface with fine granulations and with V-shaped 
 ribs pointing ventrally. Hinge without teeth. Ligament 
 external. Adductor impressions near the dorsal margin. 
 Lias to Cretaceous. 
 
 Myacites (= Arcomya). Shell thin, oblong, inequi- 
 lateral, ventricose, gaping at the ends; surface granulated 
 and often with concentric furrows. Umbones small, 
 anterior. Ligament external. Hinge without teeth. 
 Adductor impressions faint. Pallial line sinuous. Trias 
 to Chalk. 
 
 102
 
 148 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 Thracia. Shell oblong, slightly compressed, atten- 
 uated and gaping posteriorly ; surface smooth, concen- 
 trically striated or somewhat granular. Right valve 
 larger than the left. Hinge with a cartilage-process in 
 each valve and a crescentic ossicle. External ligament 
 short and prominent. Adductor impressions small. Pal- 
 lial sinus deep. Trias to present day. 
 
 Mactra. Shell oval, trigonal, nearly equilateral, 
 slightly gaping behind, smooth or with concentric striae. 
 Internal ligament in a large triangular pit. External 
 ligament in a groove. In front of the internal ligament 
 pit is a bifid cardinal tooth. Anterior- and posterior- 
 lateral teeth well-marked, compressed. Adductor im- 
 pressions semicircular. Pallial sinus round or angular. 
 Corallian to present day. 
 
 Corbula. Shell oval, very inequivalve, closed, round- 
 ed in front, truncated behind. Surface generally with 
 concentric grooves. Umbones prominent. Right valve 
 larger and more convex than the left, and with a strong 
 cardinal tooth in front of the cartilage pit, and a posterior 
 cardinal tooth. Left valve with a projecting cartilage - 
 process. Adductor impressions well-marked. Pallial 
 sinus very small. Trias to present day. 
 
 Pholas. Shell elongate, cylindrical, gaping at both 
 ends. Surface with spiny ridges, best marked in front. 
 Dorsal region with accessory valves. No teeth ; no liga- 
 ment. In the interior under the umbones is a process for 
 the insertion of the muscle of the foot. Pallial sinus very 
 deep. Lias to present day. 
 
 Teredo. Shell more or less globular, gaping at the 
 ends, valves tri-lobed, with concentric striae. In the
 
 MOLLUSCA. LAMELLIBRANCHIATA. 149 
 
 interior under the umbones is a long narrow plate for the 
 insertion of the pedal muscle. Tube long, calcareous, 
 sub-cylindrical, straight or curved, often with partitions. 
 Jurassic (? Carboniferous) to present day. 
 
 Distribution of the Lamellibranchiata. 
 
 All the Lamellibranchs are aquatic animals, and by far 
 the larger number are marine. The marine forms range 
 from the shore down to a depth of 2900 fathoms ; they 
 are most abundant in shallow water, at depths greater 
 than 500 fathoms they are scarce. 
 
 The lamellibranchs appear first in the Tremadoc Beds, 
 and they gradually increase in importance, attaining their 
 maximum at the present day. Many of the genera have 
 a rather extended range in time. In the Palaeozoic 
 formations the Heteromyaria and the Homomyaria are 
 predominant, the former attaining thus early its greatest 
 development. In the Mesozoic, the Integripalliata reach 
 their maximum, the Monomyaria are also abundant, 
 whereas the Sinupalliata are represented by a few forms 
 only. In the Tertiary, the Monomyaria and Heteromyaria 
 are much less abundant, the prevailing forms belong- 
 ing to the Sinupalliata. 
 
 The most important genera in the different systems 
 are as follows : 
 
 Cambrian. Modiolopsis, Glyptarca, Ctenodonta. 
 
 Ordovician. Ambonychia, Modiolopsis, Palaarca, Ctenodonta. 
 
 Silurian. Avicula, Pterinea, Modiolopsis, Cardiola, Ctenodonta, 
 Grammy sia. 
 
 Devonian. Pterinea, Cucullaa, Cardiola, Ctenodonta, Megalodon. 
 
 Carboniferous. Aviculopecten, Posidonomya, Anthracosia, Cono- 
 cardium, Edmondia, Sanguinolites. 
 
 Permian. Monotis, Bakevellia, Axinus.
 
 150 MOLLUSCA. LAMELLIBRANCHIATA. 
 
 Trias. Ostrea, Lima, Pecten, Gervillia, Area, Nucula, Myoplioria, 
 Cardita, Cardium. 
 
 Jurassic. Ostrea, Grypheea, Lima, Pecten, Avicula, Gervillia, Tri- 
 gonia, Modiola, Hippopodium, Pinna, Area, Nucula, Cardinia, Myacites, 
 Astarte, Cardium, Pholadomya, Thracia. 
 
 Cretaceous. Ostrea, Exogyra, Spondylus, Lima, Pecten, Gervillia, 
 Inoceramus, Perna, Tngonia, Hippurites, Radiolites, Cardium. 
 
 Eocene. Pecten, Pectunculus, Astarte, Cardita, Crassatella, Cliama, 
 Lucina, Cardium, Cyprina, Cytherea, Panopcea, Pholadomya. 
 
 Oligocene. Ostrea, Mytilus, Cyrena, Cytherea, Psammobia, Corbula. 
 
 Pliocene. Pecten, Mytilus, Pectunculus, Nucula, Astarte, Lucina, 
 Cardium, Cyprina, Venus, Artemis, Tellina, Hya. 
 
 CLASS. GASTEROPODA. 
 
 The Gasteropoda contains even a larger number of 
 animals than the preceding class ; well-known examples 
 are the snail, the whelk and the cowry. The bilateral 
 symmetry, so characteristic of the lamellibranchs, is 
 generally obliterated, owing to the twisting of the 
 visceral-sac and the atrophy of some of the organs on one 
 side of the body. There is a distinct head, which bears 
 tentacles, and usually eyes. On the ventral surface of the 
 body is a large fleshy foot ; this is usually sole-like and 
 used for crawling, but in the Heteropods it is in the form 
 of a flattened fin, in the Pteropods it is wing-like. The 
 mantle is never divided into two lobes. Respiration 
 takes place by means of a pulmonary chamber, or by gills; 
 the latter are placed in a sac formed by the mantle ; 
 sometimes they are present on both sides of the body, but 
 very often those on the left have disappeared. In some 
 forms the mantle at the opening of the gill-sac is 
 produced into a tube, known as the siphon, by means of 
 which water passes to the gills. The mouth is at the 
 anterior end of the body; the anus is occasionally
 
 MOLLUSCA. GASTEROPODA. 151 
 
 posterior, but as a rule it is placed near the opening of 
 the gill chamber. Placed just within the mouth on the 
 floor of the cavity is a dental apparatus, known as the 
 odontophore ; this consists of a chitinous ribbon, on which 
 numerous teeth are placed in rows, it is moved by means 
 of muscles and serves as a rasping organ. The arrange- 
 ment of the teeth varies in different genera and is of 
 considerable importance in the classification of recent 
 gasteropods, but since the odontophore has never been 
 definitely recognised in the fossil forms, it is of no service 
 to the palaeontologist. The nervous system consists of 
 three pairs of ganglia united by cords. Some of the 
 gasteropods are unisexual, others hermaphrodite. 
 
 In the majority of the gasteropods a shell is present, 
 secreted by the mantle ; in a few forms, as for instance 
 the slugs, it is internal, but usually it is external. The 
 shell, except in Chiton, and its allies, consists of a single 
 piece, and is hence said to be univalve. In the limpet 
 (Patella) it has the form of a hollow cone. But in most 
 cases it consists of a long tube, open at one end, and 
 tapering to a point at the other. This tube is generally 
 coiled into a spiral, each coil being termed a whorl ; in a 
 few genera (e.g. Vermetus, Siliquaria) the whorls are 
 separated, but as a rule they are in contact (fig. 47), the 
 line between two contiguous whorls being known as the 
 suture (su). All the whorls except the last one form 
 together the spire (S) of the shell, the point of which is 
 termed the apex (a). The last whorl is nearly always 
 larger than the preceding, frequently very much so, and 
 the part of it farthest from the apex forms the base of the 
 shell. The form of the spire varies in different genera 
 and species, sometimes it is composed of a large number 
 of whorls, sometimes of few, and it may be elongated,
 
 152 
 
 MOLLUSCA. GASTEROPODA. 
 
 short, or depressed. The angle of the spire (spiral angle) 
 will consequently also vary; this is measured by lines 
 drawn from the apex to the base of the shell on opposite 
 sides of the exterior of the whorls. The coiling of the 
 shell is usually dextral ; that is when the apex of the 
 shell is pointed away from the observer (as in fig. 47), the 
 aperture will be on the right hand-side ; in a few cases it 
 is sinistral, when the aperture will be on the left. 
 
 FIG. 47. Longitudinal section of Triton corrugatus. The upper part of 
 the spire has been partitioned off many times successively, a, apex; 
 su, suture ; S, spire ; L, outer lip of the aperture ; ac, anterior 
 canal; pc, posterior canal. (From Woodward.) 
 
 Frequently the inner parts of the whorls coalesce, and 
 form an axial pillar extending from the apex to the base 
 of the shell (fig. 47), and known as the columella. In 
 other cases the inner parts do not fuse and in the place of
 
 MOLLUSCA. GASTEROPODA. 153 
 
 the columella there is left a space, extending from the 
 base of the shell a greater or less distance towards the 
 apex ; the opening of this space at the base of the shell is 
 called the umbilicus. When there is a columella the 
 shell is said to be imperforate, when instead there is a 
 space it is perforate; sometimes in the latter case the 
 space becomes filled up with a shelly growth, known as 
 callus. 
 
 Usually the cavity of the gasteropod shell is con- 
 tinuous from the apex to the aperture, but in a few cases 
 partitions are thrown across the earlier parts of the shell 
 (fig. 47), forming chambers which afterwards remain 
 empty. The form of the aperture varies considerably in 
 different genera and is of great importance in classifi- 
 cation ; in shape, it may be circular, oval, elongate, 
 oblong, etc. Its margin is termed the peristome; the 
 outer part of which forms the outer lip (L), the inner 
 (that next the columella) the inner lip. As the 
 gasteropod crawls along, the shell is carried on the dorsal 
 surface of its body with the apex directed backwards; 
 consequently the- part of the aperture farthest from the 
 apex is anterior, the opposite (nearest the apex) is pos- 
 terior. Sometimes, as in Natica, there is no break in the 
 aperture, it is then said to be entire or ho los tomato us ; 
 in others the anterior border is notched or produced into 
 a tube (ac) in which the siphon is placed, these forms 
 are said to be siphonostomatous ; sometimes also there is 
 at the posterior border another canal (pc), in which the 
 excurrent or anal siphon is placed. The lip may be thin 
 and sharp, or thickened. Sometimes it is curved out- 
 wards, and is then said to be reflected ; or it is curved in- 
 wards and is inflected. It may be even, or crenulated, 
 or produced into processes.
 
 154 MOLLUSCA. GASTEROPODA. 
 
 In many genera there is attached to the posterior part 
 of the foot a calcareous or horny plate, the operculum ; 
 this is so arranged that when the animal withdraws into 
 its shell, the operculum closes more or less completely the 
 aperture. It has been considered by some as a second 
 valve, but it probably represents the byssus of the 
 lamellibranch, rather than this. The operculum is seldom 
 preserved fossil ; its form varies considerably in different 
 genera, in some (Turbo) it is of very large size ; it 
 may be spiral, and is then sometimes formed of a large 
 number of whorls (multispiral) as in Trochus, or of a few 
 whorls (paucispiral) as in Littorina. When not spiral it 
 may be concentric when growth is equal all round and it 
 is marked with concentric lines, the nucleus being nearly 
 central, as in Paludina ; or it may be unguiculate or claw- 
 shaped when the nucleus is at the apex as in Fusus. 
 
 The form of the shell in the spiral gasteropods varies 
 considerably, depending on whether the whorls are in one 
 plane or in a helicoid spiral, on the spiral angle, on the 
 number and form of the whorls, on the size of the last 
 whorl and whether it embraces the earlier whorls or not ; 
 the chief types are the following : 
 
 1. Discoidal ; the whorls all in one plane, as in 
 Planorbis. 
 
 2. Conical or trochiform ; conical with a flat base, as 
 in Trochus. 
 
 3. Turbinate ; conical with a convex base, as in Turbo. 
 
 4. Turreted or elongated ; as in Turritella. 
 
 5. Fusiform ; tapering to each end, as in Fusus. 
 
 6. Cylindrical ; as in Pupa. 
 
 7. Globular ; as in Natica.
 
 MOLLUSC A. GASTEROPODA. 155 
 
 8. Convolute; when the last whorl covers all the 
 others and the aperture is consequently as long as the 
 shell, as in Cyprcea, 
 
 9. Auriform; aperture very large and spire very 
 short, as in Haliotis. 
 
 The surface of the shell is frequently ornamented with 
 spines, or with ribs and striae, these are said to be 
 longitudinal when they run parallel to the sutures from 
 the aperture to the apex ; they are transverse when they 
 run across the whorls from suture to suture. When rows 
 of spines or processes extend across all the whorls from 
 the apex to the base of the shell, as in Murex, we have 
 what are termed varices. The colours with which the 
 shells of recent gasteropods are provided are only pre- 
 served fossil in rare cases. The shell consists of an outer 
 epidermal layer, and of a thick porcellanous layer; in 
 some cases there is also an inner nacreous or pearly layer. 
 
 The Gasteropoda are divided into the following six 
 orders, (1) Polyplacophora, (2) Prosobranchiata, (3) Opis- 
 thobranchiata, (4) Heteropoda, (5) Pulmonata, (6) Ptero- 
 poda. 
 
 OEDEE. POLYPLACOPHOEA. 
 
 The body is bilaterally symmetrical, and the shell is 
 formed of eight calcareous plates. This includes the 
 genus Chiton and allied forms. The order is only poorly 
 represented in the fossil state ; the earliest representatives 
 occurring in the Ordovician rocks. 
 
 OEDEE. PEOSOBEANCHIATA. 
 
 The gills and auricle are in front of the ventricle of 
 the heart ; there is always a shell.
 
 156 MOLLUSCA. GASTEROPODA. 
 
 Patella. Shell conical, oval or sub-circular; apex 
 sub-central or excentral, recurved anteriorly ; surface with 
 radiating ribs or striae, rarely smooth. Margin simple or 
 spinose. Muscular impression horse-shoe shaped, open in 
 front. Jurassic to present day. 
 
 Emarginula. Shell conical, surface cancellated, 
 apex recurved posteriorly. Anterior border with a well- 
 marked slit. Muscular impression horse-shoe shaped. 
 Carboniferous Limestone to present day. 
 
 Pleurotomaria. Shell trochiform, conical, turbinate, 
 or discoidal ; interior nacreous. Umbilicus present or 
 absent. Aperture subquadrate or oval, outer lip sharp, 
 with a slit, which as the shell grows becomes filled up, 
 leaving a band on the whorls, toward which the lines of 
 growth converge, being directed obliquely backwards. 
 Operculum horny. Upper Cambrian to present day. 
 
 Murchisonia. Shell turreted, with many whorls, 
 provided with a band as in Pleurotomaria, Aperture 
 oval, with a slit, and a short anterior canal. Ordovician 
 to Permian. 
 
 Bellerophon. Shell globular or discoidal; whorls 
 few, symmetrically coiled in one plane. Aperture sub- 
 circular or oval, with a deep median sinus, which is 
 replaced by a band or keel dividing the shell into two 
 similar parts; columellar edge often with callus. Upper 
 Cambrian to Permian. 
 
 Fhasianella. Shell elongated, oval or oblong, 
 smooth, polished, imperforate, interior not nacreous. 
 Aperture oval, entire, rounded anteriorly, angular pos- 
 teriorly ; outer lip thin, simple, sharp. Columella
 
 MOLLUSCA. GASTEROPODA. 157 
 
 flattened. Operculum calcareous, convex externally, 
 concave internally. Devonian to present day. 
 
 Turbo. Shell solid, turbinate or conical, whorls 
 convex, interior nacreous. Aperture large, circular, entire, 
 slightly produced anteriorly; outer lip sharp. Imper- 
 tbrate or with a small umbilicus. Operculum thick, 
 calcareous, exterior convex, interior flat and spiral, 
 nucleus central or sub-central. Silurian to present day. 
 
 Trochus. Shell conical, whorls numerous and flat or 
 slightly convex, the last whorl angular; base flat or nearly 
 so. Aperture oblique, rhomboidal, outer lip sharp. 
 Columella twisted, with a prominent anterior protuber- 
 ance. Operculum horny, multispiral. Silurian to present 
 day. 
 
 Nerita. Shell thick, solid, ovoid or semiglobose, 
 imperforate. Spire short. Surface smooth or with spiral 
 ribs. Aperture semicircular; outer lip thick, denticulate; 
 inner lip callous, flattened with a straight denticulate 
 border. Operculum calcareous. Cretaceous to present 
 day. 
 
 Solarium. Shell conical, depressed, angular at the 
 periphery. Aperture entire, subquadrate ; lip sharp. 
 Umbilicus wide and deep, limited by a crenulated margin. 
 Operculum horny, spiral. Cretaceous to present day. 
 
 Euomphalus. Shell depressed, discoidal, with a 
 wide and large umbilicus ; whorls convex with a ridge on 
 the upper surface. Aperture polygonal ; outer lip with a 
 sinus on its upper surface. Silurian to Carboniferous. 
 
 Scalaria. Shell more or less umbilicate, turreted, 
 spire elongate ; whorls numerous, convex, sometimes
 
 158 MOLLUSCA. GASTEROPODA. 
 
 separated, ornamented with transverse ribs or sometimes 
 with longitudinal lamellae. Aperture circular, entire, 
 margin thickened. Operculum horny, paucispiral. Trias 
 to present day. 
 
 Turritella. Shell imperforate, turreted, many- 
 whorled, whorls flat or slightly convex, ornamented with 
 spiral ribs and with striae of growth; spire very long. 
 Aperture oval or subquadrate, entire, outer lip thin. 
 Operculum horny. Trias to present day. 
 
 Calyptraea. Shell conical, limpet-shaped, spiral, apex 
 central ; interior with a half-cup shaped process under the 
 apex and attached posteriorly. Cretaceous to present 
 day. 
 
 Natica. Shell oval, globular, generally smooth, spire 
 short, last whorl very large. Aperture semilunar or oval, 
 entire ; outer lip sharp, inner lip callous, not crenulate. 
 Frequently an umbilicus, often filled with callus. Oper- 
 culum of same size as aperture, horny or calcareous, 
 paucispiral, nucleus excentric. Carboniferous (perhaps 
 Silurian) to present day. 
 
 Paludina. Shell thin, turbinate; whorls convex, 
 smooth or with faint ribs. Aperture entire, oval, slightly 
 angulated behind. Operculum horny with concentric 
 striae. Lower Cretaceous to present day. Lives in fresh- 
 water. 
 
 Littorina. Shell thick, not nacreous internally, tur- 
 binate, few-whorled, imperforate. Aperture rounded, an- 
 gular behind, outer lip sharp. Columella flattened. 
 Operculum horny, paucispiral. Lias to present day. 
 
 Pseudomelania. Shell many-whorled, elongate, tur- 
 riculate, imperforate, spire long. Aperture oval, entire, a
 
 MOLLUSCA. GASTEROPODA. 159 
 
 little inflated in front, narrowed behind ; outer lip sharp. 
 Columella smooth. Lias to Tertiary. 
 
 Melania. Shell elongate, turreted, many-whorled, 
 imperforate, apex sharp but usually corroded. Surface 
 smooth, ribbed or spiny. Aperture entire, narrow behind, 
 rounded in front; outer lip sharp, slightly sinuous. 
 Operculum horny, subspiral. Wealden to present day. 
 Lives in freshwater. 
 
 Nerinea. Shell elongate, perforate or not, whorls 
 numerous. Aperture sub-quadrangular, oval or elongate, 
 with a short anterior canal and a posterior slit near the 
 suture, which forms as it becomes filled up a continuous 
 band. Columella and also the interior of the whorls 
 furnished with folds which are continuous to the apex. 
 Inferior Oolite to Upper Cretaceous. 
 
 Cerithium. Shell imperforate, turreted, without epi- 
 dermis. Whorls numerous, the last whorl always shorter 
 than the spire. Aperture oblong or semi-oval, with a 
 short posterior canal and a well-marked recurved anterior 
 canal; outer lip expanded. Operculum horny, oval, 
 paucispiral. Trias to present day. 
 
 Strombus. Shell ovoid, ventricose, tuberculate or 
 spiny, imperforate. Spire with numerous whorls, the last 
 very large. Aperture long, narrow, with a short anterior 
 canal ; often canaliculate posteriorly ; outer lip expanded, 
 thick, often lobed, with a deep notch near the anterior 
 canal. Operculum small, horny, claw-shaped, edge serrated. 
 Lower Cretaceous to present day. 
 
 Rostellaria. Shell fusiform, spire elongated, com- 
 posed of many whorls, which are smooth or faintly ribbed. 
 Aperture oblong with a long straight or slightly curved
 
 160 MOLLUSCA. GASTEROPODA. 
 
 anterior canal and a posterior canal applied to the spire ; 
 outer lip expanded, thick or crenulate, with a notch in 
 front. Operculum small, oval or claw-shaped, edge not 
 serrated. Eocene to present day. 
 
 Cypraea. Shell ovoid or elongate, convex, convolute, 
 surface covered with shining enamel. Spire concealed. 
 Aperture long and narrow, with a short canal at each end ; 
 outer lip, inflected and crenulated ; inner lip crenulated. 
 In the young forms the outer lip is thin and the spire 
 prominent. Cretaceous to present day. 
 
 Buccinum. Shell oval or elongate, imperforate. 
 Spire long. Whorls ventricose, smooth or with longi- 
 tudinal folds. Aperture oval, large ; outer lip simple ; 
 anterior canal short, truncated, a little reflected. Oper- 
 culum small. Cretaceous to present day. 
 
 Nassa. Shell ovate, ventricose, generally ornamented. 
 Aperture oval, with a short reflected anterior canal. Inner 
 lip callous, forming a tooth- like projection near the anterior 
 canal ; outer lip crenulate internally. Columella with an 
 oblique fold at its base. Cretaceous to present day. 
 
 Purpura. Shell tuberculate, striated or lamellar, not 
 varicose. Spire short, last whorl large. Aperture oval, 
 large, with an oblique groove in front. Columella flattened, 
 callous. Operculum lamellar, nucleus external. Pliocene 
 to present day. 
 
 Fusus. Shell imperforate, fusiform, elongate, spire 
 many-whorled. Aperture oval ; outer lip simple. A long 
 straight anterior canal, not closed. Columella smooth, 
 without folds. Operculum oval. Cretaceous to present 
 day.
 
 MOLLUSC A. GASTEROPODA. 161 
 
 Murex. Shell oval or elongate, solid, spire prominent, 
 whorls convex, carrying three or more continuous longi- 
 tudinal varices, which may be spiny, foliaceous, or tuber- 
 cular. Aperture rounded, anterior canal more or less long, 
 narrow and tubular, often partly closed; no posterior 
 canal ; outer lip thick. Operculum oval, nucleus sub- 
 apical. Upper Cretaceous to present day. 
 
 Mitra. Shell fusiform, thick. Spire elevated, summit 
 acute. Aperture narrow, elongate, notched in front. 
 Columella with oblique folds, of which the posterior are 
 the stronger ; outer lip not reflected, thickened internally. 
 No operculum. Eocene to present day. 
 
 Voluta. Shell ovate or fusiform, solid, thick, summit 
 of spire mammilated. Last whorl very large, aperture 
 elongate, oval ; canal very short, recurved ; outer lip 
 simple, often thick, rarely reflected. Columella with ob- 
 lique folds, of which the anterior are the stronger. No 
 operculum. Upper Cretaceous to present day. 
 
 Pleurotoma. - Shell turreted, fusiform, spire long. 
 Aperture oval; outer lip curved, provided with a deep 
 slit near the suture ; inner lip smooth. Canal long, 
 straight, narrow. Operculum horny, ovate, acute, nucleus 
 apical. Cretaceous to present day. 
 
 Conus. Shell inversely conical, generally smooth. 
 Spire short, many-whorled. Aperture narrow, straight, 
 with parallel or sub-parallel borders; outer lip thin, 
 simple, no folds or teeth, notched at the suture. Columella 
 straight, smooth. Operculum horny, much smaller than 
 the aperture. Cretaceous to present day. 
 
 
 w. p. 11
 
 162 MOLLUSCA. GASTEROPODA. 
 
 ORDER. OPISTHOBRANCHIATA. 
 
 The gills and auricle are placed behind the ventricle. 
 A shell is absent in some forms. All are hermaphrodite 
 and all marine. 
 
 Bulla. Shell solid, smooth, sub-globular or ovoid, 
 convolute. Spire concave. Aperture as long as the last 
 whorl, rounded at the ends, widest in front ; outer lip 
 sharp. Cretaceous to present day. 
 
 ORDER. HETEROPODA. 
 
 The forms included in this order live in the open ocean 
 at the surface. The foot is modified so as to form a 
 vertical fin ; a shell is absent in some and when present is 
 always thin. Only a very few forms occur fossil. 
 
 ORDER. PULMONATA. 
 
 Respiration takes place by means of a pulmonary 
 chamber. A shell may be present or absent, and is 
 holostomatous. All the genera are hermaphrodite, and 
 nearly all live on land or in fresh water. 
 
 Limnaea. Shell spiral, thin, horny; last whorl very 
 large, spire sharp. Aperture large, oval, rounded in front. 
 Columella more or less twisted. Peristome sharp, entire. 
 Purbeck Beds to present day. Lives in fresh water. 
 
 Flanorbis. Shell discoidal, horny, whorls numerous. 
 Aperture oblique ; peristome simple, sharp. Jurassic to 
 present day. Freshwater. 
 
 Helix. Shell variable, conical, discoidal, or globular; 
 perforate or not ; aperture oblique. Peristome simple or 
 reflected. Eocene to present day. Lives on land.
 
 MOLLUSC A. GASTEROPODA. 163 
 
 ORDER. PTEROPODA. 
 
 The Pteropods are all pelagic animals. The head is 
 not well marked, and the foot is represented by two 
 lateral wing-like fins. A shell may or may not be present. 
 
 Hyolithes (= Theca). Shell straight, rarely curved, 
 pyramidal, triangular in section, smooth or striated ; 
 posterior part sometimes crossed by walls. Aperture 
 sometimes oblique ; provided with an operculum having 
 the form of a half cone. Upper Cambrian to Permian. 
 
 Conularia. Shell generally straight, pyramidal, sec- 
 tion quadrangular, each lateral face with a median longi- 
 tudinal groove ; posterior part of shell sometimes provided 
 with concave partitions. Surface ornamented with nume- 
 rous transverse, parallel, angulated ridges. Aperture 
 partly closed by triangular or lobed processes. Ordovician 
 to Trias. 
 
 Tentaculites. Shell thick, solid, in the form of a 
 greatly elongated cone, straight or slightly curved, section 
 circular, posterior extremity sharp or with a vesicular 
 enlargement. Surface provided with prominent, trans- 
 verse, parallel rings, and with transverse and longitudinal 
 striae. Ordovician to Devonian. 
 
 Distribution of the Gasteropoda. 
 
 Some of the Gasteropoda live on land, others in fresh 
 water, but the majority are marine ; they are found in the 
 seas of all parts of the world, but are especially abundant 
 in warm regions and in comparatively shallow water. A 
 few forms can exist both on land and in water, e.g. 
 Ampullaria, which commonly lives in lakes and rivers, is 
 
 112
 
 164 MOLLUSCA. GASTEROPODA. 
 
 also found on land. Some marine genera, such as Littorina, 
 Cerithium, and Purpura, are able to live in fresh as well 
 as in salt water ; on the other hand some freshwater forms 
 are at times found living in the sea, e.g. Limncea, Neritina, 
 Bithynia, and Planorbis; this is especially the case in 
 places where the water is less salt than the main mass of 
 the ocean, as for instance in the Baltic, where we find the 
 genera just mentioned living side by side with Littorina 
 and with the marine lamellibranchs Cardium, Tellina, and 
 Mya. Of the six orders of the Gasteropoda, four are 
 entirely marine, viz. the Polyplacophora, Heteropoda, 
 Opisthobranchiata, and Pteropoda; arid the Prosobranchiata 
 are mainly marine. The Pulmonata are found only on 
 land and in fresh water. The Heteropoda and Pteropoda 
 live in mid-ocean at or near the surface, whereas the other 
 marine forms occur mainly at no very great distance from 
 the coasts and in comparatively shallow water. 
 
 In connection with the Gasteropoda a few words may 
 be said with regard to the distribution of the marine 
 Mollusca generally. Just as we find on land, that the 
 surface of the earth can be divided into regions, each 
 characterised by the presence of certain animals riot found 
 elsewhere, so also in the oceans we have similar life- 
 provinces, caused mainly by conditions of climate and 
 food supply. These are marked by the abundance of 
 certain genera and the presence of some species which do 
 not extend into other provinces ; but they are less distinct 
 than the terrestrial regions, since very few of the genera 
 are confined to one province. In the European region the 
 chief provinces are : the Arctic, which includes the polar 
 seas and extends as far south as the North of Iceland and 
 the North Cape ; the Boreal, extending from the last 
 down to near the southern end of Norway and including
 
 MOLLUSC A. GASTEROPODA. 165 
 
 the Shetland Islands ; the Celtic, including the coasts of 
 Southern Sweden, the Baltic, Denmark, Northern France 
 and the British Isles; and the Lusitanian, the coasts of 
 the Bay of Biscay, Portugal, the Mediterranean, and North- 
 west Africa. In each province the character of the 
 molluscan fauna also varies with the nature of the sea- 
 bottom, some genera (e.g. Mya, Lutraria, Scrobicularia) 
 being found especially on muddy bottoms, others (e.g. 
 Natica, Turritella, Cyprcea, Cardium) on sandy, and others 
 (e.g. Buccinum, Littorina, Patella, Area) on rocky. There 
 is also a distribution according to depth, caused by 
 temperature (which as a whole decreases with the depth), 
 and by the absence of light; in this respect five zones 
 have been traced : 
 
 (1) Littoral Zone, extending between high and low 
 water marks ; in Europe this is especially characterised 
 by the abundance of the genera Littorina, Trochus, 
 Patella, Hydrobia, Haliotis, Fissurella, Solen, Mya, and 
 Cardium. 
 
 (2) Laminarian Zone, ranging from low water down 
 to about 14 fathoms; in it algae (Laminaria, etc.) are 
 particularly luxuriant, and afford food for numerous phyto- 
 phagous mollusks. Some of the commonest genera met 
 with are Trochus, Nassa, Rissoa, and Ostrea. Nudibranchs 
 are also very numerous. 
 
 (3) Zone of Nullipores or Corallines, from 14 to about 
 35 fathoms, where the calcareous algae (Nullipores) are 
 very abundant. It is characterised by the abundance of 
 Pleurotoma, Fusus, Buccinum, Natica, Eulima, Venus, 
 Dosinia, Astarte, Nucula, Area, Lima, and Pecten. 
 
 (4) Zone of Brachiopods and deep-sea Corals, from 
 35 to about 230 fathoms; off Europe Oculina is the 
 common coral; brachiopods and Polyzoa are also abundant.
 
 166 MOLLUSCA. GASTEROPODA. 
 
 Some of the chief mollusks are Turritella, Odostomia, 
 Dentalium, Tellina, Necera, and Yoldia. 
 
 (5) Abysmal Zone, extending from the last down to 
 the greatest depths at which life has been found. In this 
 zone the shells are mostly thin, colourless, transparent and 
 of small size ; it is especially characterised by numerous 
 Scaphopods ; other common forms are Pleurotoma, Fusus, 
 Actceon, Scaphander, Philine, Area, Nucula, Limopsis, 
 Leda, Lima, and Pecten. The remains of Pteropods which 
 have fallen from the surface of the ocean after death are 
 also numerous. 
 
 In the fossil state gasteropods are less numerous than 
 lamellibranchs, although they exceed them at the present 
 day. The earliest forms occur in the Upper Cambrian, 
 they belong to the holostomatous Prosobranchiata, and 
 throughout the Palaeozoic this is the predominant group, 
 no genera belonging to the siphonostomatous section are 
 known with certainty until the Trias is reached ; the latter 
 become more abundant in the Oolites, still more so in the 
 Cretaceous, and in the Tertiary they are the predominant 
 forms. The other orders are not nearly so well represented 
 as the Prosobranchiata. The Polyplacophora range from 
 the Ordovician to the present day, but they are more 
 abundant in the Palaeozoic than in later formations. The 
 Heteropoda are represented by a few forms only, the first 
 occurring in the Miocene. The Opisthobranchiata range 
 from the Carboniferous to the present day, they are 
 moderately well represented in the Jurassic and Cretaceous 
 and become more abundant in the Tertiary. The Pul- 
 monata are first found in the Carboniferous, but they are 
 quite rare until the Tertiary is reached. The Pteropoda 
 occur first in the Lower Cambrian, but the nature of some 
 of the earlier forms (Hyolithes, Conularia, Tentaculites) is
 
 MOLLUSCA. GASTEROPODA. 167 
 
 somewhat doubtful; it is possible that they may not be 
 Pteropods; but representatives of the order certainly occur 
 in the Silurian and Devonian. 
 
 The most important genera found in the different 
 systems are : 
 
 Cambrian. Bellerophon, Hyolithes, Conularia. 
 
 Ordovician. Bellerophon, Holopcea, Cyclonema, Mac- 
 lurea, Conularia, Tentaculites. 
 
 Silurian. Pleurotomaria, Murchisonia, Bellerophon, 
 Euomphalus, Holopella, Acroculia. 
 
 Devonian. Pleurotomaria, Murchisonia, Bellerophon, 
 Loxonema, Euomphalus, Macrocheilus, Tentaculites. 
 
 Carboniferous. Pleurotomaria, Murchisonia, Bellero- 
 phon, Loxonema, Euomphalus, Naticopsis, Macrocheilus, 
 Conularia. 
 
 Permian. Pleurotomaria, Murchisonia, Turbo, Loxo- 
 nema, Macrocheilus. 
 
 Trias. Pleurotomaria, Turbo, Loxonema, Natica, Turri- 
 tella. 
 
 Jurassic. Pleurotomaria, Amberleya, Cirrus, Phasia- 
 nella, Trochus, Natica, Pseudomelania, Nerinea, Cerithium, 
 Alaria, Purpuroidea. 
 
 Cretaceous. Pleurotomaria, Solarium, Turritella, Na- 
 tica, Paludina, Cerithium, Aporrhais, Cinulia. 
 
 Eocene. Phorus, Calyptrcea, Natica, Melania, Ceri- 
 thium, Rostellaria, Cyprcea, Cassidaria, Triton, Fusus, 
 Pyrula, Murex, Typhis, Voluta, Oliva, Pleurotoma, 
 Conus. 
 
 Oligocene. Neritina, Natica, Paludina, Melania, Ceri- 
 thium, Potamides, Murex, Voluta, Ancillaria, Pleurotoma, 
 Limncea, Planorbis, Rissoa, Helix, Bulimus.
 
 168 MOLLUSCA. GASTEROPODA. 
 
 Pliocene. Trochus, Scalaria, Turritella, Emarginula, 
 Natica, Littorina, Cerithium, Buccinum, Nassa, Purpura, 
 Fusus, Valuta. 
 
 CLASS. SOAPHOPODA. 
 
 The Scaphopoda include only a few genera, which in 
 some respects resemble the lamellibranchs. The body is 
 bilaterally symmetrical ; the mantle is cylindrical and 
 open at both ends, and it secretes a straight or slightly 
 curved tubular shell which is also open at both ends. 
 The foot is greatly elongated and can be protruded through 
 the larger aperture of the mantle and shell. The animal 
 is attached to the posterior part of the shell by means of 
 a muscle ; an odontophore is present, but the head is not 
 well marked, and eyes and heart are absent. The sexes 
 are separate. All the Scaphopods are marine, and they 
 often live buried in sand or mud. The earliest forms 
 occur in the Ordoviciau rocks. 
 
 Dentalium. Shell conical or sub-cylindrical, at- 
 tenuated posteriorly, slightly curved. Anterior aperture 
 simple, not constricted ; posterior aperture smaller. Sur- 
 face smooth, annulated or ornamented with longitudinal 
 striae or ribs. Devonian to present day. 
 
 CLASS. CEPHALOPODA. 
 
 The Cephalopods are more highly organised than any 
 of the other mollusks ; well-known forms are the cuttle- 
 fishes, belemnites, ammonites, and the nautilus. They 
 are always bilaterally symmetrical. The head is well 
 marked, it is separated from the body by a constriction, 
 and is especially characterised by the fact that the fore-
 
 MOLLUSCA. CEPHALOPODA. 169 
 
 foot extends round the mouth, and its margin is produced 
 into eight or ten (many in Nautilus) processes or arms 
 (fig. 48 e,f), which are provided either with sucking discs 
 or tentacles, and are used in prehension and locomotion. 
 The mid-foot is in the form of a tube termed the funnel 
 (d) which opens in front to the exterior, and behind into 
 
 FIG. 48. Diagram of a vertical median antero-posterior section of Sepia 
 qfficinalis. a, shell; b, mouth of mantle-cavity; c, mantle-cavity; 
 d, funnel ; e, lobes (arms) of the fore-foot ; /, long arm of the fore- 
 foot ; g, the upper beak or jaw ; h, the lower beak or jaw ; i, odonto- 
 phore; k, the viscero-pericardial sac; I, the nerve-collar; m, the 
 crop ; n, the gizzard ; o, the anus ; p, left gill ; q, ventricle of the 
 heart; r, renal glandular mass; s, left nephridial aperture; t, viscero- 
 pericardial aperture ; u, branchial heart ; w, ink-sac. (After Lan- 
 kester, Zool. Art. Encyc. Britt.) 
 
 the mantle-cavity (c). The funnel may be either a perfect 
 tube or may be formed by the apposition of two troughs. 
 The hind- foot is absent. On the upper surface of the 
 head there are two large eyes, which, except in Nautilus, 
 are almost as highly developed as in vertebrate animals. 
 The mantle is formed by a single fold of the integument, 
 which passes quite round the body of the animal; but 
 dorsally the fold is very shallow so that the mantle- cavity 
 exists mainly on the ventral surface. The gills (p) are placed
 
 170 MOLLUSCA. CEPHALOPODA. 
 
 in the mantle-cavity, in some forms there is one pair, in 
 others two. A current of water flows in at the sides of 
 the mantle-cavity, and passes out through the funnel, by 
 means of the contraction of the walls of the mantle-cavity. 
 In the forms belonging to the order Dibranchiata, a gland 
 (known as the ink-sac, w) which secretes a black fluid (sepia) 
 is present ; the duct from this opens together with the 
 anus (o) into the mantle-cavity ; the ink is ejected at times 
 and passes out through the funnel, rendering the water 
 cloudy, and by this means facilitating the escape of the 
 animal from its enemies. Just within the mouth there 
 are two jaws (g, h) which have the form of a parrot's beak, 
 and are either horny or calcareous. An odontophore (i) is 
 also present, but the arrangement of the teeth is less 
 variable than in the gasteropods, and is of little value for 
 systematic purposes. The heart consists of a median 
 ventricle (q), and of lateral auricles, which are either two 
 or four in number, according as there are two or four gills ; 
 it gives off in front an artery to the head, and one behind 
 to the viscera. The nervous system is remarkable in that 
 the three pairs of ganglia are placed close together, 
 forming a central mass (I) ; one part is placed above the 
 oesophagus, and is connected by cords with the other part 
 beneath it. This central nervous system is enclosed in a 
 cartilaginous box and gives off nerves to the arms, viscera, 
 etc. The sexes are always separate, and the genital ducts 
 open into the mantle-cavity. The shell may be external 
 or internal (a), in the latter case it is usually placed in a 
 sac in the mantle on the dorsal side ; in some forms it is 
 absent.
 
 MOLLUSCA. CEPHALOPODA. 171 
 
 ORDER. TETRABRANCHIATA. 
 
 The only genus of the Tetrabranchiata existing at the 
 present day is Nautilus, but in past geological times the 
 order was represented by very numerous and varied forms, 
 such as Orthoceras, Nautilus, and Ammonites. The Tetra- 
 branchs are characterised by possessing two pairs of gills, 
 two pairs of auricles, and two pairs of kidneys. An ink- 
 sac is absent ; the funnel is not a complete tube, but is 
 formed by the apposition of two troughs. The arms are 
 numerous and do not bear sucking-discs. 
 
 A shell is present in all forms and is always external. 
 It consists of a tube, which tapers to a point at one end, 
 and may be straight, arched, or spiral ; in the spiral forms 
 the whorls may be separate, or partly free, or in contact 
 throughout ; in some cases they are placed all in one plane, 
 in others arranged obliquely. The cavity of the shell, 
 unlike that in the gasteropods, is not continuous through- 
 out from the aperture to the apex, but is divided up into 
 
 FIG. 49. Section of shell of Nautilus, a, body-chamber ; b, septum ; 
 c, septal neck ; d, siphuncle.
 
 172 MOLLUSC A. CEPHALOPODA. 
 
 a number of chambers by means of partitions termed 
 septa (fig. 49 6); the size of the chambers increases 
 towards the aperture of the shell. The body of the animal 
 was placed in the last chamber (a), all the earlier ones 
 being filled with air only. Originally the earlier chambers 
 were occupied by the animal, but as growth went on, it 
 successively withdrew to the larger parts of the shell, each 
 time depositing behind it a septum and thus forming a 
 new chamber. All the chambers are traversed by a tube 
 known as the siphunde (d), which is given off from the 
 posterior part of the animal. The position of the siphuncle 
 varies in different genera ; in some (Nautilus) it pierces 
 the septa at or near their centre, in others (Ammonites) 
 at the external margin, in others (Clymenia) at the internal 
 margin. In the modern Nautilus the siphuncle is mem- 
 branous, except that it is coated with calcareous granules ; 
 but in some fossil forms it was completely invested by a 
 calcareous tube. In addition to this the septa are fre- 
 quently produced in the form of funnels around the 
 siphuncle, so as to more or less completely eusheathe it. 
 These funnels are termed septal necks (c) ; in some genera 
 (Nautilus) they are directed backwards, in others (Ammo- 
 nites) forwards. 
 
 In the genus Nautilus, the outer convex portion of the 
 shell is the ventral side, and the inner the dorsal ; but in 
 some fossil forms the opposite was the case, the outer 
 being dorsal, and the inner ventral. It will therefore be 
 more convenient to speak of the external and internal 
 margins, instead of the dorsal and ventral. The margin 
 of the aperture is in some cases even all round ; in others 
 a process is given off from the external margin, or the 
 lateral margins are produced into ' ears,' or the external 
 margin may possess a sinus. In a few forms the aperture
 
 MOLLUSCA. CEPHALOPODA. 
 
 173 
 
 is constricted. The line where the edges of the septa 
 unite with the shell is known as the suture (fig. 50), 
 obviously this will only be seen when the shell is removed ; 
 but the fossil forms frequently occur as casts and in these 
 the sutures are admirably shown. The form of the sutures 
 varies considerably in different genera and is of great 
 
 FIG. 50. A, suture of an Ammonite (Parkinsonia parkinsoni) from the 
 Inferior Oolite. B, Suture of Ceratites nodosus, from the Muschel- 
 kalk. I, external lobe ; II, 2 1, superior and inferior lateral lobes; al, 
 auxiliary lobes ; , external saddle ; 1 s, 2s, superior and inferior lateral 
 saddles ; as, auxiliary saddles. In each case the straight line on the 
 left represents the position of the siphuncle, and the curved one on 
 the right the line of contact with the next whorl. 
 
 importance for systematic purposes; in some cases they 
 are straight or only slightly curved, in others they are 
 folded many times, the portions which are convex towards 
 the mouth of the shell being termed the saddles (fig. 50 s), 
 whilst the intervening concave portions are known as the 
 lobes (I). In many forms the lobes and saddles exhibit 
 secondary foldings, these may be slight, producing merely 
 a denticulate suture, or they may be deep and provided 
 with other foldings, giving a foliaceous appearance to the
 
 174 MOLLUSCA. CEPHALOPODA. 
 
 suture. The lobes and saddles are similar on opposite 
 sides of the shell ; commonly there is first the external lobe 
 (fig. 50 I) on the external margin, then the superior and 
 inferior lateral lobes (1 1, 2 1), and other lobes known as 
 auxiliary lobes (al). The saddles are arranged in a similar 
 manner, we have the external saddle (s), the lateral saddles 
 (1 s, 2 s), and auxiliary saddles (as). 
 
 The surface of the shell may be smooth or variously 
 ornamented with striae, ribs, tubercles or spines. 
 
 There are two sub-orders (1) Nautiloidea, (2) Ammon- 
 oidea. 
 
 SUB-OBDEE. NAUTILOIDEA. 
 
 The form of the shell is variable ; it may be straight, 
 arched, or spiral. The sutures are usually simple, but 
 occasionally undulate or denticulate. The septal necks 
 are (except in the genera Bathmoceras and Nothoceras) 
 directed backwards. The position of the siphuncle is 
 variable ; its interior is frequently contracted by calcare- 
 ous deposits. 
 
 Orthoceras. Shell straight, occasionally slightly 
 curved, conical, transverse section usually circular. Septa 
 simple, concave; the last chamber large; aperture not 
 contracted. Siphuncle usually slender, cylindrical, and 
 central, or sub-central, sometimes ex-central. Ornamen- 
 tation variable. Tremadoc Beds to Trias. 
 
 Actinoceras. Similar to the preceding, but the 
 siphuncle is large and is inflated between the septa, and 
 contains in the interior a large amount of calcareous 
 deposit. Ordovician to Carboniferous Limestone. 
 
 Gomphoceras (= Phragmoceras). Shell fusiform, 
 straight or slightly curved ; section circular, sometimes 
 oval; last chamber very long, aperture contracted, T-shaped.
 
 MOLLUSC A. CEPHALOPODA. 175 
 
 Siphuncle sub-central, placed towards the ventral (usually) 
 convex side. Surface smooth or with transverse ribs or 
 striae. Silurian (perhaps also Ordovician, Devonian and 
 Carboniferous). 
 
 Cyrtoceras. Like Orthoceras but always curved, and 
 with the siph uncle usually sub-marginal. Cambrian to 
 Carboniferous. 
 
 Nautilus. Shell spiral, coiled in one plane, whorls 
 few and more or less embracing. Last chamber large, 
 aperture simple, with an external sinus. Septa concave, 
 sutures simple or with slight lobes and saddles. Siphuncle 
 sub-central, septal necks short and directed backwards. 
 Surface of shell smooth or ornamented with striae or ribs. 
 Ordovician to present day. 
 
 Aturia. Shell discoidal, whorls completely embrac- 
 ing ; suture-line zig-zag, with a deep lateral lobe on each 
 side. Siphuncle on the internal margin; septal necks 
 elongated, completely covering the siphuncle. Eocene 
 and Miocene. 
 
 SUB-OBDER. AMMONOIDEA. 
 
 The shell is generally coiled into a plane spiral. The 
 sutures are undulated or bent into lobes and saddles. 
 The siphuncle is marginal and does not contain internal 
 deposits. The septal necks (except in Clymenia and 
 Goniatites) are directed forwards. 
 
 Clymenia. Shell discoidal; whorls numerous, all 
 visible, but each partly embraces the preceding one ; the 
 last chamber is long, generally occupying three-quarters 
 of the last whorl. Aperture with a ventral sinus. Sutures 
 simply lobed. Siphuncle on the internal margin ; septal
 
 176 MOLLUSCA. CEPHALOPODA. 
 
 necks directed backwards. Surface usually ornamented 
 with transverse strias. Upper Devonian. 
 
 Goniatites. Shell discoidal ; whorls embracing, um- 
 bilicus sometimes narrow, sometimes wide. ' Last chamber 
 usually large, aperture with a ventral sinus. Sutures 
 never foliaceous, sometimes angular, sometimes rounded; 
 siphuncle small, on the external margin ; septal necks 
 usually directed backwards. Upper Silurian to Carboni- 
 ferous. 
 
 Ceratites. Shell discoidal, ornamented with ribs or 
 tubercles ; umbilicus large ; last chamber short. Saddles 
 rounded, lobes denticulate. Trias, especially Muschelkalk. 
 
 Ammonites. Shell coiled in a plane spiral ; the 
 whorls sometimes embracing. Sutures lobed and folia- 
 ceous. Siphuncle on the external margin. Surface smooth, 
 or ornamented with striae, ribs, tubercles or spines. The 
 external margin of the shell may be rounded or provided 
 with a ridge or keel. Lias to Chalk. 
 
 In the last chamber of some Ammonites and a few 
 other genera, a pair of calcareous or horny plates, known 
 as the Aptychus, are occasionally found ; in shape they are 
 triangular or semicircular, and the edges where the two 
 plates are in contact are straight, the others curved. The 
 Aptychus is thought to have served as an operculum. 
 
 The species of Ammonites are very numerous and differ 
 so much from one another that they are now regarded as 
 representing many distinct genera ; these are based mainly 
 on the length of the last chamber, on the form of the 
 sutures, and the presence or absence of an aptychus, and 
 on the aperture. But in an elementary work like the 
 present it will be convenient to retain the old genus 
 Ammonites.
 
 MOLLUSCA. CEPHALOPODA. 177 
 
 Hamites. Shell bent upon itself three times, the 
 parts not in contact ; the last chamber long. Suture-line 
 with the saddles and lobes deeply incised, the superior- 
 lateral lobe and often the inferior- lateral lobe being 
 divided into two paired parts. Surface smooth or orna- 
 mented with ribs, tubercles, or spines. Lower Greensand 
 to Chalk. 
 
 Turrilites. Shell spiral, turreted, usually sinistral, all 
 the whorls in contact. Sutures with the lateral lobes 
 symmetrically divided. Surface ornamented with ribs or 
 tubercles. Gault to Chalk. 
 
 Baculites. Shell straight, elongate-conical ; last 
 chamber large, aperture produced ventrally. Sutures 
 with the lobes symmetrically divided. Lower Greensand 
 to Upper Chalk. 
 
 Scaphites. Shell coiled in a plane spiral ; the whorls 
 in contact and embracing, except the last, which is 
 detached from the spiral and then recurved. Last cham- 
 ber long. Surface ornamented with simple or nodular 
 ribs. Gault to Chalk. 
 
 Crioceras. Shell coiled in a plane spiral ; the whorls 
 not in contact. Surface ornamented with ribs and often 
 with tubercles and spines. Lower Greensand to Gault. 
 
 Ancyloceras. Like Crioceras, but the last whorl is 
 produced in a straight line and then bent back in the 
 form of a hook. Inferior Oolite to Lower Greensand. 
 
 ORDER. DLBRANCHIATA. 
 
 The Dibranchiates are represented at the present day 
 
 by the various forms of cuttle-fishes ; but they are of much 
 
 less importance geologically than the Tetrabranchs, the 
 
 only really common fossil genus being Belemnites. Some 
 
 w. P. 12
 
 178 MOLLUSC A. CEPHALOPODA. 
 
 of the modern cuttle-fishes attain an enormous size, for 
 instance Loligo sometimes has a length of forty feet or 
 more. 
 
 The Dibranchiata, as the name indicates, possess one 
 pair of gills only ; there is also one pair of auricles and 
 one pair of kidneys. The number of arms is limited to 
 eight or ten ; and on the inner side that facing the 
 mouth they are provided with rows of sucking discs, 
 which sometimes possess horny hooks. The jaws are not 
 calcified. An ink-sac is always present and is occasionally 
 found preserved fossil; and even in this condition it is 
 capable of being used for artistic purposes. The funnel is 
 in the form of a complete tube. A skeleton is absent in 
 some forms ; when present it is internal, with the single 
 exception of the genus Argonauta ; it may be either horny 
 or calcareous. In some (Sepia etc.) it has the form of a 
 flattened spoon-like body, known as the pen, which is 
 composed mainly of laminated calcareous material ; occa- 
 sionally at the posterior end there is a small chambered 
 portion. The pen is placed on the dorsal surface of the 
 animal in a sac formed by the mantle. In Spirula the 
 shell is situated in the posterior part of the body, and 
 consists of a tube coiled in a plane spiral and divided up 
 into chambers by means of septa, which are traversed by 
 a siphuncle placed near the inner margin. In Belemnites 
 it consists of a solid portion (the "guard") and of a 
 chambered part with a siphuncle. The shell in Argonauta 
 is of quite a different nature to that found in other forms ; 
 it is spiral but not chambered, and is secreted, not by the 
 mantle, but by the two dorsal arms, and is found only in 
 the female, serving for the reception of the eggs. 
 
 The Dibranchiata are divided into two sub-orders 
 (1) the Decapoda, (2) the Octopoda.
 
 MOLLUSC A. CEPHALOPODA. 
 
 179 
 
 SUB-OKDER. DECAPODA. 
 
 There are ten arms, eight of equal length and two 
 longer than the others : the suckers are stalked and are 
 provided with a horny ring. An 
 internal shell is always present. 
 
 The genera Spirula and Sepia 
 already mentioned belong to this 
 sub-order; the former does not 
 occur fossil and the latter only 
 rarely. Allied to Sepia is the genus 
 Belosepia found in the Eocene. The 
 most important fossil form is Belem- 
 nites. 
 
 Belemnites. The shell in this 
 genus consists of two parts the 
 guard (fig. 51 a) and the phragmo- 
 cone (6). The former is solid and 
 is much more commonly preserved 
 than the latter; it varies consider- 
 ably in shape and size, being cylin- 
 drical, fusiform, conical, etc. The 
 posterior end is always pointed; at 
 the anterior end there is a conical 
 cavity, termed the alveolus. The 
 length of the guard varies from 
 one to fifteen inches. 
 
 When sliced transversely or longi- FIG. 51. Diagram- 
 tudinally it is seen to be formed of a section of Belemnites. 
 
 . fr' . i i a, guard; b, phragmo- 
 
 number of layers arranged concen- cone . c< siphuncle tra . 
 trically around an axial line, which versing the chambers of 
 is not quite central but is placed the phragmocone; d, pro- 
 nearer the ventral surface; these ostracum - 
 
 122
 
 180 MOLLUSC A. CEPHALOPODA. 
 
 layers are not of uniform thickness along the whole length 
 of the guard, they become thicker towards the posterior 
 end and thinner towards the anterior. Each layer is 
 formed of minute prisms of calcite, which are placed 
 perpendicular to the axial line, thus producing a radiating 
 fibrous appearance. The surface of the guard is some- 
 times smooth, or it may be granular, or furnished with 
 ramifying vascular impressions ; in some species there is 
 a longitudinal groove on the ventral surface. 
 
 The phragmocone (6) is a conical tube which fits into 
 the alveolus at the anterior end of the guard, it is divided 
 up into chambers by septa which are concave in front ; a 
 siphuncle (c) traverses the chambers along the ventral 
 margin. The wall of the phragmocone (sometimes termed 
 the conotheca) is very thin, and in well preserved specimens 
 the dorsal surface (or sometimes the lateral) is found to 
 be produced in front into a large laminar expansion 
 (d)] this prolongation is known as the pro-ostracum, 
 and corresponds to the "pen" of the cuttle-fish. The 
 suckers on the tentacles were provided with horny hooks, 
 which are occasionally preserved fossil ; the ink-sac and 
 mandibles have also been found in a few specimens. Lias 
 to Upper Cretaceous. 
 
 Belemnitella. This differs from the preceding genus 
 in having a slit in the guard on the ventral side of the 
 alveolus. There are well-marked vascular impressions on 
 the ventral surface. Chalk. 
 
 SUB-ORDER. OCTOPODA. 
 
 There are eight arms only ; the suckers are sessile and 
 possess no horny ring. The shell is rudimentary or 
 absent. This sub-order is very poorly represented in the
 
 MOLLUSC A. CEPHALOPODA. 181 
 
 fossil state ; examples of it are Octopus and Argonauta, 
 the latter has been found in the Pliocene Beds. 
 
 Distribution of the Cephalopoda. 
 
 The Nautiloidea at the present day are represented by 
 four species of Nautilus only ; these occur in comparatively 
 shallow water in the Indian Ocean, the Persian Gulf, the 
 China Sea, and off New Guinea and Fiji. The group 
 appears much earlier in the geological series than either 
 the Ammonoidea or the Dibranchiata, and soon attained 
 its maximum development. Two genera, Orthoceras and 
 Cyrtoceras, are found in the Cambrian (Upper Tremadoc 
 Beds), and these, especially the first, continue to be im- 
 portant in the succeeding Paleozoic formations as far as 
 the Carboniferous. In the Ordovician the group is very 
 much better represented than in the Cambrian, the chief 
 genera being Orthoceras, Cyrtoceras, Endoceras, and Nau- 
 tilus ; but it is most abundant in the Silurian, where the 
 number of species is very great; afterwards in the De- 
 vonian and Carboniferous, although still very numerous, 
 there is a slight decline. In the Permian it is poorly 
 represented; and only two genera extend beyond the 
 Palaeozoic, of these, one (Orthoceras) becomes extinct in 
 the Trias, the other (Nautilus) lives on to the present day, 
 and becomes especially abundant in the Cretaceous, in 
 the Tertiary it is rare, and is associated with one other 
 form only, namely, Aturia. 
 
 The distribution of the Ammonoidea differs from that 
 of the Nautiloidea in that the former are mainly Mesozoic. 
 In the Palaeozoic the only genera are Goniatites, Clymenia, 
 and (in the Carboniferous of India) Ammonites. Through- 
 out the Mesozoic the Ammonites are extremely abundant ;
 
 182 MOLLUSCA. CEPHALOPODA. 
 
 in the Trias Ceratites is characteristic ; in the Cretaceous 
 we get a remarkable development of new genera, Turri- 
 lites, Hamites, Scaphites, Crioceras, and Baculites, all of 
 these except the first, being evolute forms. At the end of 
 the Cretaceous, the Ammonoids become extinct in a 
 remarkably sudden manner, except in California, where 
 some Ammonites have been found in the lower Tertiary 
 beds. 
 
 The Dibranchiata are more numerous and more varied 
 in existing seas, than they were at any former period. 
 They are most abundant in littoral regions and are distri- 
 buted in provinces similar to the other mollusks ; typical 
 littoral genera are Octopus, Sepia and Loligo. Some 
 forms are pelagic, others abyssal. The Dibranchiata are 
 unknown in the Palaeozoic, the earliest examples appearing 
 in the Trias. Throughout the Mesozoic, Bekmnites is the 
 chief form, and is very abundant especially in the clayey 
 beds. In the Jurassic, Geoteuthis and Belemnoteuthis also 
 occur; and in the Upper Cretaceous, Belemnitella. In 
 the Tertiary, Belosepia and Beloptera, are the principal 
 genera.
 
 SUB-KINGDOM VIII. ARTHROPODA. 
 
 Orders. 
 (I. Phyllopoda. 
 
 . Entomostraca. 2 " stracoda - 
 ] 3. Copepoda. 
 
 U- Cirripedia. 
 1. Leptostraca. 
 
 2. Malacostraca. . J 2. Thoracostraca. 
 
 Sub-Orders. 
 
 1. Cumacea. 
 
 2. Stomatopoda. 
 
 3. Schizopoda. 
 
 4. Decapoda. 
 
 1. Amphipoda. 
 
 2. Isopoda. 
 
 Classes. 
 
 1. Crustacea. .. 
 
 3. Arthrostraca. 
 
 2. Prototracheata. 
 
 3. Myriapoda. 
 
 4. Insecta. 
 
 5. Arachnida. 
 
 {1. Trilobita. 
 2. Xiphosura. 
 3. Eurypterida. 
 
 IN the Arthropods the body is bilaterally symmetrical 
 and segmented, and some or all of the segments bear a 
 pair of appendages each, which differ from those of the 
 worms in being jointed. Some of the appendages near 
 the mouth are modified so as to function in mastication 
 and prehension. A chitinous exoskeleton is always present 
 and is often strengthened by the deposition of carbonate
 
 184 ARTHROPODA. 
 
 or phosphate of lime. A heart is found in most forms ; it 
 is placed dorsally, and is provided with paired slits, termed 
 ostia. The body-cavity contains blood. In some forms 
 respiration takes place by means of the general surface of 
 the body ; others are provided with special organs gills 
 (or branchiae), tracheae, pulmonary sacs (or lung-books). 
 The gills are generally processes given off from some of 
 the appendages; the tracheae are branching tubes filled 
 with air and opening to the exterior ; the pulmonary sacs 
 are infoldings of the integument. The nervous system 
 consists of a supra-oasophageal ganglion, a ring round the 
 oesophagus, and a ventral cord, usually with ganglia, placed 
 beneath the intestine. Cilia are absent in all members of 
 the sub-kingdom, with the exception of Peripatus. The 
 sexes are separate in almost all forms. 
 
 The Arthropoda are divided into the following classes, 
 (1) Crustacea, (2) Prototracheata, (3) Myriapoda, (4) In- 
 secta, (5) Arachnida. The last four breathe by means of 
 tracheae or pulmonary sacs, and are therefore often grouped 
 together as the Tracheata, the Crustacea which breathe 
 by means of gills, forming the Branchiata. The Tracheate 
 forms are comparatively rare as fossils, and consequently 
 no description of them will be given here : the Myriapoda 
 include the millipedes and centipedes, the oldest form is 
 from the Old Red Sandstone; the Insects occur first in 
 the Upper Silurian, they become more abundant in the 
 Coal Measures, and are fairly common in some of the 
 Mesozoic and Cainozoic deposits, as for instance the Lias, 
 the Solenhofen Slates, the Purbeck, and the Bembridge 
 Beds ; the Arachnida include amongst others the scorpions 
 and spiders, the earliest scorpion is found in the Silurian, 
 the earliest spider in the Coal Measures.
 
 
 ARTHROPOD A. CRUSTACEA. 185 
 
 CLASS. CRUSTACEA. 
 
 The members of this class are mainly aquatic and are 
 very abundant as fossils ; they breathe generally by means 
 of gills, but in some cases by the general surface of the 
 body. The chitinous exoskeleton is frequently hardened 
 by a calcareous deposit, hence the name Crustacea. Seg- 
 mentation is usually well-marked, but is absent in the 
 Ostracoda. Three regions may be distinguished in the 
 body : the head, the thorax, and the abdomen. There are 
 five segments in the head, but these are indicated only by 
 the appendages ; the number in the thorax and abdomen 
 is variable in one of the two great divisions, namely the 
 Entomostraca, but is constant in the other, the Mala- 
 costraca. In most forms some or all of the segments of 
 the thorax (and sometimes also the anterior segments of 
 the abdomen) fuse with those of the head to form a 
 cephalothorax, this is covered dorsally by a large, unseg- 
 mented shield or carapace, termed the cephalothoracic 
 shield. The head usually bears fi ve pairs of appendages, viz. 
 two pairs of antennae, one of mandibles, and two of maxillae. 
 The thorax is also provided with appendages, and very often 
 the abdomen too. The mandibles and maxillae serve as 
 jaws, and often also some of the anterior thoracic appen- 
 dages. Eyes are generally present and are placed on the 
 head ; in some cases they are simple, in others compound, 
 consisting of a number of lenses. The sexes are separate 
 except in some cirripeds and isopods. In some cases 
 development is direct, that is to say, the young individual 
 has the same form as the adult, but generally this is not 
 the case, the young undergoing a metamorphosis before 
 reaching the adult stage. The two chief larval forms are 
 known as the Nauplius and the Zocea. In the Nauplius
 
 186 CRUSTACEA. PHYLLOPODA. 
 
 form the body is unsegmented and possesses three pairs 
 of appendages representing the two pairs of antennae and 
 the mandibles. In the Zocea stage some of the thoracic 
 appendages are also present, and the abdomen is seg- 
 mented but possesses no appendages. 
 
 The Crustacea are divided into two large groups, 
 (1) the Eritomostraca, (2) the Malacostraca. 
 
 ENTOMOSTRACA. 
 
 The forms included in this group are mostly of small 
 size and of comparatively simple organisation. The num- 
 ber of segments in the body is variable, and also the 
 number and form of the appendages. In the development 
 there is nearly always a Nauplius stage. There are four 
 orders, (1) Phyllopoda, (2) Ostracoda, (3) Copepoda, 
 (4) Cirripedia. The Copepods are not definitely known 
 as fossils. 
 
 ORDER. PHYLLOPODA. 
 
 The Phyllopoda includes the water-fleas (Daphnia) 
 and other forms. The body is segmented and the greater 
 part of it is covered by a shield-like carapace or by a 
 bivalved shell. The number of segments in the abdomen 
 and thorax varies very widely. On the head there are 
 generally two pairs of antennae, one of mandibles, and one 
 or two of maxillae. The thorax bears four or more pairs 
 of swimming-feet, which are leaf-like. The abdomen is 
 frequently without appendages ; it often ends in a caudal 
 fork. Two large eyes are present and often also a small 
 unpaired eye. The Phyllopods live mainly in fresh or 
 brackish water. Only a very few genera are found fossil ; 
 the earliest occur in the Devonian.
 
 CRUSTACEA. OSTRACODA. 187 
 
 Estheria. Valves thin, horny ; ovate, oblong or quad- 
 rilateral, united at the straight dorsal border, and with 
 the apices placed anteriorly or nearly central. Surface 
 generally covered with concentric stria?. Devonian to 
 present day. Living in fresh or brackish water. 
 
 OBDER. OSTRACODA. 
 
 The Ostracods are generally minute. The body is not 
 segmented and is usually compressed laterally and com- 
 pletely enclosed in a bivalved shell (or carapace), which 
 may be horny or calcareous ; one valve is placed on each 
 side of the animal. Dorsally the two valves are joined 
 together by means of an elastic ligament which serves to 
 open the shell ; an adductor muscle passes from the 
 interior of one valve to the other. There are seven pairs 
 of appendages which can be protruded when the shell is 
 opened. In the marine forms the shell is notched an- 
 teriorly so as to allow the antenna? to pass through when 
 the shell is closed. The head carries two pairs of antennae, 
 one of mandibles, two of maxilte. The thorax has two or 
 three pairs of appendages, which are not leaf-like. The 
 abdomen is rudimentary and is without appendages; it 
 terminates either in a fork or a spiny plate. Respiration 
 takes place by means of the general surface of the body. 
 The carapace is in almost all cases the only part which 
 occurs fossil ; its surface may be smooth or variously 
 ornamented. 
 
 Leperditia. Carapace smooth, convex, elongated, a 
 little wider posteriorly. The right valve larger than the 
 left. Hinge-line straight. There is a small tubercle 
 (eye-spot) placed anteriorly near the hinge ; and posterior 
 to it is a slightly elevated circular area. Ordovician to 
 Carboniferous.
 
 188 CRUSTACEA. OSTRACODA. 
 
 Primitia. Carapace equi valve, convex, oblong. Hinge - 
 line straight. Each valve has a dorsal groove which 
 starts from the hinge-line. Cambrian to Carboniferous. 
 
 Beyrichia. Carapace elongated, inflated; dorsal 
 border straight, ventral border semicircular. On the 
 surface there are two or three large furrows which pass 
 from the dorsal towards the ventral edge, the parts be- 
 tween being convex and often tuberculate. Cambrian to 
 Devonian. 
 
 Cypris. Carapace thin, smooth or punctate, kidney- 
 shaped or oval ; ventral edge often concave. Oligocene 
 to present day. Freshwater. 
 
 Cypridea. Differs from Cypris in having a beak-like 
 process at the anterior ventral angle. Purbeck and 
 Wealden. 
 
 Distribution of the Ostracoda. 
 
 The Ostracods have a very wide distribution at the 
 present day ; they are mainly marine, and often occur in 
 shoals. They are most abundant in shallow water, only 
 fifty-two species being found below the 500 fathom line. 
 The fossil forms are very numerous, the earliest occurring 
 in the Cambrian. 
 
 ORDER. CIRRIPEDIA. 
 
 The Cirripeds include the barnacles, acorn-shells, etc. ; 
 forms which differ considerably in appearance from the 
 other crustaceans, and which were for a long time regarded 
 as mollusks. The body is completely enclosed in a fold of 
 the integument, which commonly forms a calcareous shell. 
 The animal, in the adult state, is fixed to a foreign object 
 by the anterior end of the head, either directly or by
 
 CRUSTACEA. CIRRIPEDIA. 
 
 189 
 
 b 
 
 means of a stalk or peduncle. The head is not well- 
 marked off from the thorax ; it bears two pairs of antennae, 
 one pair of mandibles, and two pairs of maxillae. The 
 thorax has usually six pairs of bira- 
 mous feathery limbs. The abdomen 
 is rudimentary. A heart and vas- 
 cular system are absent ; nearly all 
 forms are hermaphrodite. The shell 
 consists of several pieces ; in Lepas 
 (which possesses a stalk) there are 
 five, two are placed on each side 
 of the body, those near the stalk 
 being termed the scuta (fig. 5 2 a), 
 those at the upper end the terga 
 (b), and there is also one unpaired 
 part placed dorsally, the carina (c). 
 Balanus has no stalk ; in this the 
 shell consists of a tube formed of six 
 pieces, in the upper part of which 
 the scuta and terga are placed. 
 
 FIG. 52. Lepas austra- 
 lis, Recent, a, scutum ; 
 b, tergum; c, carina; d, 
 peduncle. Natural size. 
 (After Darwin.) 
 
 Distribution of the Cirripedia. 
 
 The Cirripeds are all marine, and the greater number 
 are found in shallow water. Balanus is especially charac- 
 teristic of littoral regions. At a depth of greater than 
 1000 fathoms, only two genera, Scalpellum and Verrucosa 
 have been found, but these are not confined to deep water. 
 The earliest genus is Turrilepas, which ranges from the 
 Ordovician to the Devonian. Pollicipes commences in 
 the Rhaetic, Scalpellum in the Cretaceous, and Balanus in 
 the Eocene ; all three are still living.
 
 190 CRUSTACEA. PHYLLOCARIDA. 
 
 MALACOSTRACA. 
 
 The Malacostraca are of large or moderate size and 
 more highly organised than the Entomostraca. The num- 
 ber of segments is constant throughout the group, there 
 being five in the head, eight in the thorax, and six in the 
 abdomen ; each segment bears one pair of appendages. 
 In some cases the development is direct, in a few the 
 Nauplius stage occurs, but most commonly the Zoaea larva 
 is found. 
 
 There are three Orders, (1) Leptostraca, (2) Thoracos- 
 traca, (3) Arthrostraca. 
 
 ORDER. LEPTOSTRACA (PHYLLOCARIDA). 
 
 This order forms a transition between the Phyllopoda 
 and the Malacostraca; in the Palaeozoic formations it is 
 represented by numerous forms, but only three genera are 
 now living, the most important being Nebalia. They are 
 provided with a large carapace which is usually bivalved 
 and covers the thoracic and some of the abdominal seg- 
 ments; in front it possesses a movable rostrum. There 
 are twenty-one segments in the body five in the head, 
 eight in the thorax, eight in the abdomen, the last 
 segment being forked or spined. There are nineteen 
 pairs of appendages, as in the Malacostraca: the head 
 bears two pairs of antennae, one of mandibles, two of 
 maxillae ; on the thorax there are eight pairs of leaf-like 
 feet similar to those of the Phyllopods ; the abdomen has 
 six pairs of appendages, the first four being large, the last 
 two small. The two posterior segments are without 
 appendages. The eyes are compound and stalked.
 
 CRUSTACEA. PHYLLOCARIDA. 191 
 
 Hymenocaris. Carapace semi-oval, smooth, not bi- 
 valved. Abdomen formed of eight segments and four or 
 five caudal spines. Appendages unknown. Lingula Flags. 
 
 Ceratiocaris. Carapace bivalved, often marked with 
 striae, oval, truncated behind and with a lanceolate rostrum 
 in front. Body formed of fourteen or more segments, the 
 first six or seven being covered by the carapace ; the last 
 segment is produced into a strong spine and provided 
 with two smaller spines. Ordovician to Carboniferous. 
 
 Distribution of the Leptostraca. 
 
 The three living genera are all marine. The earliest 
 representative is Ceratiocaris which occurs in the Tre- 
 macloc Beds and ranges on to the Devonian. Hymenocaris 
 is found in the Lingula Flags; Echinocaris in the De- 
 vonian; and Dithyrocaris in the Carboniferous. 
 
 ORDER. THORACOSTRACA. 
 
 In this order there is a cephalothorax formed by the 
 fusion of the head with some or all of the segments of the 
 thorax, and this is covered dorsally by a shield or carapace. 
 There are thirteen segments in the cephalothorax, six 
 in the abdomen and a terminal telson. The eyes are 
 compound and are usually placed on movable stalks. 
 The Thoracostraca includes four sub-orders, (1) Cumacea, 
 (2) Stomatopoda, (3) Schizopoda, (4) Decapoda. The 
 first and third are unknown in the fossil state ; and only a 
 few forms of the Stomatopoda have been found. 
 
 SUB-ORDER. DECAPODA. 
 
 The Decapods include the lobsters, crayfishes, and 
 the crabs. The head and in almost all cases all the 
 segments of the thorax are covered by the large and
 
 192 CRUSTACEA. DECAPODA. 
 
 usually strong cephalothoracic shield ; this is frequently 
 marked out into anterior and posterior portions by a 
 groove, the cervical suture, the anterior belonging to the 
 head, the posterior to the thorax. Anteriorly the shield 
 is produced into a spine, the rostrum. The gills are placed 
 in a chamber formed by the downward prolongation of 
 the cephalothoracic shield. The cephalothorax bears 
 thirteen pairs of appendages, five belonging to the head, 
 eight to the thorax. On the head there are (1) antennules, 
 (2) antennae, (3) mandibles, (4 and 5) maxilla, the last 
 three pairs serving as jaws. On the thorax the three first 
 pairs (maxillipedes) serve in locomotion and also in masti- 
 cation ; the posterior five pairs are the ambulatory limbs, 
 the first pair being the large chelae. The abdomen bears 
 six or fewer pairs of small appendages. 
 
 There are two sections, (1) Macrura, (2) Brachyura. 
 
 Section 1. Macrura. 
 
 This includes the lobsters and crayfishes. The abdo- 
 men is long, well developed, and provided with four or 
 five pairs of appendages. There is a large caudal fin. 
 
 Meyeria. Cephalothorax laterally compressed, with 
 a sharp rostrum, and a deep V-shaped cervical suture; 
 the posterior part of the cephalothorax covered with sharp 
 granules. Abdomen semicylindrical, longer than the 
 cephalothorax, and ornamented with rows of granules. 
 Lower Greensand. 
 
 Section 2. Brachyura. 
 
 This includes the crabs. The abdomen is short and 
 more or less rudimentary ; it is curved up underneath the 
 thorax, and bears from one to four pairs of appendages. 
 There is no caudal fin.
 
 CRUSTACEA. DECAPODA. 193 
 
 Xanthopsis. Cephalothorax rounded, convex, surface 
 punctate, the posterior portion with rounded elevations; 
 the frontal border with four, and the anterior-laterals 
 with one to three, tooth-like processes. Orbital cavities 
 deep, without fissures. Chelae unequal. Abdomen of the 
 male narrow and formed of five segments, owing to the 
 fusion of the third, fourth, and fifth segments. Abdomen 
 of female broad, composed of seven segments. London 
 Clay. 
 
 Distribution of the Decapoda. 
 
 All the Brachyura, and most of the Macrura, are 
 marine. The latter are the first to appear in the geo- 
 logical series, a few genera being found in the Upper 
 Palaeozoic ; the earliest is Palceopalcemon from the Upper 
 Devonian; in the Carboniferous Anthrapalcemon and 
 others occur. The section becomes much more abundant 
 in the Mesozoic, attaining its maximum in the Jurassic, 
 where we get, amongst others, the genera Glyphcea, Eryon, 
 jEger, and Eryma. In the Cretaceous, Enoploclytia, 
 Hoploparia, and Meyeria occur. Only a few forms are 
 found in the Tertiary. The Brachyura are represented 
 by one or two genera only in the Jurassic ; in the Creta- 
 ceous by Paloeocorystes and several others; the Eocene 
 forms are much more numerous, Xanthopsis being the 
 commonest genus ; but the section attains its maximum 
 at the present day. 
 
 ORDER, ARTHROSTRACA. 
 
 There is no cephalothoracic shield ; there are usually 
 
 seven (sometimes fewer) segments in the thorax ; and 
 
 usually six in the abdomen, and a telson, but the abdomen 
 
 is sometimes rudimentary. The head carries two pairs of 
 
 w. P. 13
 
 194 CRUSTACEA. AMPHIPODA. 
 
 antennas, one of mandibles, two of maxillae, and one of 
 maxillipedes. The thorax has seven, and the abdomen 
 six, pairs of appendages. The eyes are sessile. There 
 are two sub-orders, (1) Amphipoda, (2) Isopoda. 
 
 SUB-ORDER. AMPHIPODA. 
 
 The Amphipods (e.g. Gammarus, Talitrus) are usually 
 of small size and the body is laterally compressed. There 
 are seven segments in the thorax, the appendages of which 
 bear the gills. The abdomen is elongated and bears six 
 pairs of appendages, the three anterior serve for swimming, 
 the three posterior for jumping. Some of the Amphipods 
 are marine, others fresh water ; the fossil forms are rare, a 
 few doubtful examples occur in the Palaeozoic ; those found 
 in the Tertiary are mainly fresh water genera. 
 
 SUB-ORDER. ISOPODA. 
 
 The body is flattened dorso-ventrally. In the thorax 
 there are six or seven segments, each with a pair of 
 ambulatory appendages. The abdomen is often short, 
 and is formed of six segments (frequently fused) and a 
 telson ; some of the appendages function as gills. Many 
 Isopods are marine, others fresh water, whilst a few live 
 on land ; the fossil forms are rare. A specimen, named 
 Prearcturus, from the Old Red Sandstone of Herefordshire, 
 has been referred to this group, but two undoubted 
 examples occur in the Solenhofen Slates ; in England the 
 earliest form (with the possible exception of Prearcturus) 
 is Archceoniscus from the Purbeck Beds.
 
 ARTHROPODA. TRILOBITA. 
 
 195 
 
 GROUPS OF DOUBTFUL POSITION. 
 OEDEE. TEILOBITA. 
 
 The trilobites derive their name from the fact that the 
 body is divided into three parts, by means of two furrows, 
 which extend from the anterior to the posterior extremi- 
 ties; in a few genera, however (Homalonotus, Illcenus), 
 
 ..k 
 
 FIG. 53. Calymene blumenbachi, from the Wenlock Limestone. A, head, 
 B, thorax, C, pygidium. a, glabella; a', axal furrow; b, one of the 
 glabella furrows; b', neck furrow; d, facial suture; e, eye;/, free 
 cheek; g, fixed cheek; h, genal angle; i, axis of thorax; k, pleura. 
 Natural size. 
 
 this trilobation is only very feebly shown, or almost absent. 
 In a longitudinal direction, three regions can be distin- 
 guished in the body, the head (fig. 53 A), the thorax (B), 
 
 132
 
 196 ARTHROPODA. TRILOBITA. 
 
 and the pygidium 1 ((7) ; each of these three regions is 
 covered dorsally by a calcareous shell. That covering the 
 head, and known as the head-shield or cephalic shield, is 
 semicircular or triangular in shape and is not segmented ; 
 in it may be distinguished a median and two lateral 
 portions ; the former is the more convex and is termed 
 the glabella (a), the latter are the cheeks. The glabella is 
 marked off from the cheeks by means of a furrow on each 
 side, known as the axal furrow (a'). The form of the 
 glabella varies in different genera, in some it extends 
 quite to the anterior margin of the head- shield, in others 
 only a part of the way; sometimes it is wider behind than 
 in front, at others vice versa, or it may be uniform 
 throughout. Generally it is divided at the sides into lobes 
 by means of transverse furrows (b), usually three on each 
 side ; in some cases the furrows from the two sides meet 
 in the middle of the glabella. On the posterior part of 
 the glabella there is another furrow, which extends quite 
 across, this is the neck-furrow (&'). The cheeks are each 
 more or less triangular in shape and less convex than the 
 glabella, they are frequently bordered by a flattened 
 margin. The posterior angles of the cheeks, often spoken 
 of as the genal angles (h), may be rounded (e.g. Calymene} 
 but are often pointed or produced into spines (e.g. Tri- 
 nucleus}. Each cheek is divided into two portions by 
 means of a suture (the facial suture, d). The inner part, 
 that is, that between the facial suture and the glabella, is 
 termed the fixed cheek (g) and is immovable ; the outer 
 part, known as the free cheek (f), is movable on the fixed 
 cheek. The course of the facial suture varies in different 
 forms ; it may commence at the genal angle (h), or 
 
 1 These are by some authors regarded as cephalothorax, abdomen, 
 and post-abdomen respectively.
 
 ARTHROPODA. TRILOBITA. 197 
 
 on the posterior border inside the genal angle, or on 
 the external border just in front of the genal angle ; it 
 passes anteriorly and may be continuous with the suture 
 of the other cheek in front of the glabella, or it may cut 
 the anterior margin of the head-shield, in which case it is 
 often united with the other suture on the inferior surface 
 of the head. When the sutures are continuous in front of 
 the glabella, it is obvious that the cheeks will also be 
 continuous. Tn some genera (Agnostus, Trinucleus) the 
 facial suture is absent. The eyes (e) are placed on the 
 upper surface of the head, one on each free cheek near the 
 facial suture, and generally near the middle of the cheek ; 
 they are compound, each consisting of a number of lenses ; 
 in a few genera (Agnostus, Ampyx) eyes are absent. The 
 head-shield is continued on the under surface of the head 
 forming a marginal rim (fig. 54 a) ; attached to this in the 
 median line is a plate, usually oval in shape, situated just 
 in front of the mouth and known as the hypostome (6). 
 
 The thorax (fig. 53 B) is formed of a series of segments, 
 which vary in number, from two to twenty-six ; these are 
 movable upon one another, in some cases sufficiently to 
 enable the animal to roll itself up like a wood-louse. 
 Each segment is divided into a central and two lateral 
 parts by means of two furrows. The central part is more 
 convex than the lateral and forms the axis (i), the lateral 
 parts being known as the pleurae (k). The anterior part 
 of the axis of each segment is not visible when the animal 
 is unrolled, since it is overlapped by the preceding seg- 
 ment, with which it forms an articulating surface. The 
 pleurae in some genera possess a longitudinal ridge, in 
 others a groove ; a few forms have smooth pleurae. Each 
 pleura at some distance from the axis is curved down- 
 wards and usually also backwards, the point where this
 
 198 ARTHROPODA. TRILOBITA. 
 
 curvature occurs is known as the fulcrum ; generally each 
 pleura overlaps the anterior part of the succeeding one, 
 and often the fulcrum is rounded so as to form an articu- 
 lating facet. The terminations of the pleurae are in some 
 cases rounded, in others produced into spines. 
 
 The pygidium (fig. 53 (7) is usually triangular or semi- 
 circular in shape, it is formed of a variable number of 
 segments, which differ from those of the thorax in being 
 fused together and immovable ; the segmentation being 
 shown by grooves. Just as in the thorax, the pygidium 
 possesses a central part or axis, and lateral portions ; the 
 axis may reach quite to the posterior extremity or only 
 part of the way. The margin of the pygidium may be 
 even or entire, or it may be provided with a posterior 
 spine or with lateral spines. 
 
 The appendages of the trilobites were for a long time 
 unknown, since only a very few specimens showing the 
 ventral surface have been found ; the most satisfactory of 
 these is Asaphus megistos (fig. 54) from the Ordovician 
 Beds of Ohio. In this each segment of the thorax and 
 pygidium was found to bear a pair of jointed appendages. 
 Further information on this subject has been obtained by 
 Mr C. D. Walcott, by means of cutting thin sections of 
 rolled up specimens of Galymene and Cheirurus. In this 
 way he found that on the ventral surface of the body there 
 was a thin membrane, which was strengthened by trans- 
 verse arches to which the appendages of the thorax and 
 pygidium were attached ; these each consisted of six or 
 seven joints, and at the base a spiral gill was attached. 
 The head also carried four pairs of jointed appendages. 
 
 The Trilobites are more closely related to the Xipho- 
 surans than to any other Arthropods, and since these are 
 now regarded by many zoologists as belonging to the
 
 ARTHROPODA. TRILOBITA. 
 
 199 
 
 Arachnida, rather than to the Crustacea, it is possible that 
 the Trilobites will also have to be placed in that class. 
 
 FIG. 54. Under surface of Asaphus megistos, from the Ordovician rocks 
 of Ohio, showing a pair of appendages attached to each segment. 
 The shaded part represents the actual specimen, a, infolded margin 
 of the head-shield; b, hypostome; c, branchial filaments? (After 
 Walcott.) 
 
 They agree with Limulus in the trilobation of the body 
 and in the possession of a similar shield covering the 
 anterior part of the body; with the Palaeozoic Xiphosurans
 
 200 ARTHROPODA. TRILOBITA. 
 
 the resemblance is more marked, since some of these 
 possess facial sutures and have the segments of the thorax 
 and abdomen free. The larval form of Limulus presents 
 a great resemblance to some trilobites, in this stage the 
 caudal spine is absent and the segmentation is better 
 marked. The trilobites differ from the Xiphosurans in 
 the absence of ocelli, in the possession of a hypostome, and 
 in the form and arrangement of the appendages. 
 
 Agnostus. Body small, head-shield and pygidium 
 similar in form and size ; eyes and facial suture absent. 
 Thorax formed of two segments, pleurae grooved. Harlech 
 to Bala Beds. 
 
 Microdiscus. Similar to Agnostus but with four 
 segments in the thorax. Harlech to Lingula Flags. 
 
 Trinucleus. Head -shield large, with long genal 
 spines, and a broad punctate border; glabella inflated, 
 pyriform, furrows absent, or indistinct. Eyes generally 
 absent. No facial suture. Thorax formed of six seg- 
 ments, pleurae grooved, straight, but slightly curved near 
 their extremities. Pygidium short, triangular, margin 
 entire. Arenig to Bala Beds. 
 
 Ampyx. Similar to Trinucleus, but without a punc- 
 tate border to the head-shield, and with a long straight 
 spine given off from the front of the glabella, and also 
 with discontinuous facial sutures. Tremadoc to Bala 
 Beds. 
 
 Olenus. Body oval ; head-shield larger than the 
 pygidium, with a narrow border, and genal spines; glabella 
 usually a little contracted in front, separated from the 
 anterior border by a flat space ; facial sutures discontinu- 
 ous in front; eyes well-developed, placed forward and
 
 ARTHROPODA. TRILOBITA. 201 
 
 united to the front of the glabella by a straight ridge. 
 Thorax of from twelve to fifteen segments, pleurae 
 pointed. Pygidium small. Lingula Flags to Tremadoc 
 Beds. 
 
 Paradoxides. Body large, elongated, narrowed pos- 
 teriorly. Head-shield broad, semicircular, with a border, 
 and long genal spines ; glabella elongated in front, with 
 two to four furrows on each side. Facial sutures extend 
 from the posterior to the anterior border. Eyes well 
 developed. Thorax long, formed of sixteen to twenty 
 segments; pleurae grooved and produced into long back- 
 wardly directed spines. Pygidium very small, axis of two 
 to eight segments. Harlech Beds to Lingula Flags. 
 
 Olenellus. Similar to Paradoorides ; but differs in 
 the glabella not expanding in front, in the facial suture 
 being invisible, in having only thirteen or fourteen seg- 
 ments in the thorax, and in the absence of lateral lobes on 
 the pygidium. Lower Cambrian. 
 
 Conocoryphe (= Conocephalites). Head-shield semi- 
 circular, without , genal spines ; axal furrows deep, gla- 
 bella narrow in front, and with three or four backwardly 
 directed furrows; free cheeks small. Facial sutures not 
 continuous in front and ending near the genal angles. 
 Thorax with fourteen or fifteen segments. Pygidium 
 small, margin entire, axis with from two to eight segments. 
 Harlech to Tremadoc Beds. 
 
 Angelina. Body oval. Head-shield with long genal 
 spines, glabella without furrows; eyes small, near the 
 middle of the cheeks. Thorax with fourteen or fifteen 
 segments, pleurae facetted. Pygidium short, margin pro- 
 vided with two teeth, axis formed of four or five segments. 
 Tremadoc Beds.
 
 202 ARTHROPODA. TRILOBITA. 
 
 Calymene. Head-shield semicircular, genal angles 
 rounded, occasionally pointed ; glabella greatly inflated, 
 broadest behind, with three pairs of lateral furrows, form- 
 ing three globular lobes on each side. Eyes prominent. 
 Facial suture extending from the genal angle to the 
 anterior border. Thorax formed of thirteen segments, 
 axis prominent, pleurae grooved and facetted. Pygidium 
 with six to eleven segments, margin entire. Arenig to 
 Upper Ludlow. 
 
 Homalonotus. Body large, elongated, trilobation 
 indistinct. Head-shield broad, genal angles rounded, 
 furrows on the glabella indistinct or absent. Eyes small. 
 Facial suture passing from the genal angles to the front 
 margin, but often continuous in front. Thorax with 
 thirteen segments ; axis wide, not well-marked. Pygidium 
 rather small, triangular, axis with ten to fourteen seg- 
 ments. Arenig to Devonian. 
 
 Ogygia. Body large, nearly flat. Head-shield large, 
 semicircular; glabella distinct, wider in front, with four 
 lateral furrows. Eyes large. Hypostorne not notched. 
 Facial suture starting from the posterior border, sometimes 
 cutting the front margin, but generally continuous. Tho- 
 rax consisting of eight segments, axis narrow, distinct; 
 pleurae grooved. Pygidium large, semicircular, margin 
 entire, axis of numerous segments. Tremadoc to Llan- 
 deilo Beds. 
 
 Asaphus. Body oval, surface smooth or with striae. 
 Head-shield semicircular, with a flattened border, genal 
 angles rounded or spinose ; glabella wide in front and 
 usually without lateral furrows. Eyes large. Hypostome 
 notched posteriorly. Thorax formed of eight segments, 
 pleurae obliquely grooved, with rounded extremities.
 
 ARTHROPODA. TRILOBITA. 203 
 
 Pygidium of about the same size as the head, rounded, 
 margin entire ; formed of numerous segments which are 
 usually visible on the axis only. Tremadoc to Bala Beds. 
 
 Illaenus. Body oval, convex. Head-shield large, 
 semicircular ; glabella indistinctly marked, without furrows 
 externally, but within the test there are four pairs. Eyes 
 remote from one another. Facial sutures commence on 
 the posterior border, cut the anterior border in front of 
 eye and unite on the inferior surface. Thorax with 
 usually ten segments, pleurae not grooved or ridged. 
 Pygidium large, semicircular, axis rudimentary, segments 
 not visible externally. Arenig to Wenlock. 
 
 JEglina. Head-shield large; glabella large, convex, 
 projecting beyond the margin in front. Cheeks narrow ; 
 eyes very large. Facial suture discontinuous, nearly 
 parallel to the axis of the body. Thorax with five or 
 six segments, pleurae grooved. Pygidium rounded, axis 
 reduced. Arenig to Bala Beds. 
 
 Bronteus. Head-shield large, semicircular, genal 
 angles pointed. Glabella very wide in front, three lateral 
 furrows in some species, none in others. Facial sutures 
 discontinuous in front, eyes sickle- shaped, placed near 
 the posterior border. Thorax with ten segments, axis 
 narrow, pleurae ridged. Pygidium very large, fan-shaped, 
 axis short, lateral lobes large with radiating grooves. 
 Bala Beds to Devonian. 
 
 Phacops. Head-shield nearly semicircular, glabella 
 prominent, broadest in front, with three or four furrows ; 
 facial sutures commencing on the lateral border of the 
 cheek in front of the genal angle, and continuous in front 
 of the glabella. Eyes generally large, granulated, formed
 
 204 ARTHROPODA. TRILOBITA. 
 
 of large facets. Thorax with eleven segments, pleurse 
 grooved. Pygidium variable. Ordovician to Devonian. 
 
 Cheirurus. Head-shield semicircular, genal angles 
 pointed ; glabella convex, with three pairs of furrows, the 
 last uniting with the neck-furrow. Facial sutures con- 
 tinuous in front and ending on the external margin. 
 Eyes prominent. Thorax with usually eleven segments, 
 pleura grooved, and produced into spines. Pygidium 
 with four segments, lateral lobes with backwardly-directed 
 spines. Tremadoc to Devonian. 
 
 Encrinurus. Head -shield covered with tubercles ; 
 glabella pyriform, eyes small, placed on short peduncles. 
 Facial sutures not continuous, ending on the lateral 
 margin. Thorax with eleven segments, pleuras ridged. 
 Pygidium narrow, triangular, pleurae bent backwards. 
 Bala to Upper Ludlow. 
 
 Acidaspis. Head-shield broad, trilobation not well- 
 marked, genal angles spined ; glabella with a pair of 
 longitudinal furrows parallel to the axal furrows, and with 
 two or three lateral furrows. Eyes connected with the 
 glabella by a ridge. Facial sutures continuous in front 
 and ending on the lateral margins. Thorax with nine 
 or ten segments, pleurae produced into long spines. 
 Pygidium small, with long spines. Llandeilo Beds to 
 Devonian. 
 
 Phillipsia. Body oval ; glabella with nearly parallel 
 sides, with two or three lateral furrows, the posterior one 
 being directed backwards and united to the deep neck- 
 furrow, thus cutting off a basal lobe. Eyes large, reniform. 
 Facial sutures not continuous in front ending on posterior 
 border. Thorax with seven segments, pleurse grooved.
 
 ARTHROPODA. TRILOBITA. 205 
 
 Pygidium semicircular, formed of from twelve to eighteen 
 segments, margin entire. Devonian to Permian. 
 
 Griffithides. Body oval ; glabella pyriform without 
 lateral furrows, with inflated basal lobes ; eyes small. 
 Thorax with nine segments. Pygidium rounded, formed 
 of about thirteen segments. Carboniferous. 
 
 Distribution of the Trilobita. 
 
 The Trilobites form one of the most important and 
 striking features in the Palaeozoic faunas, they occur first 
 in the Lower Cambrian Beds, and reach their maximum 
 in the Ordovician ; they are still abundant in the Silurian, 
 but become less important in the Devonian, and in the 
 Carboniferous are represented by four genera only. In 
 Europe they do not extend beyond the Carboniferous 
 Limestone, but in North America one species of Phillipsia 
 has been found in the Permian. 
 
 The most important genera are : 
 
 Cambrian. Agnostus, Microdiscus, Paradoxides, Olenellus, Olenus, 
 Conocoryphe, Niobe, Asaphus, Angelina, 
 
 Ordovician. Ogygia, Ampyx, TrinucUus, Illcmus, Asaphus, ^glina, 
 Calymene. 
 
 Silurian. Phacops, Homalonotus, Calymene, Acidaspis, Cheirurus, 
 Proetus, Illcenus, Lichas. 
 
 Devonian. Phacops, Homalonotus, Cheirurus, Bronteus, 
 
 Carboniferous. Phillipsia, Griffithides, Brachymetopus. 
 
 ORDER. XIPHOSURA. 
 
 The only living representative of the Xiphosura is 
 Limulus, the king-crab, found on the eastern shores of 
 North America and Asia, and in the Malay Archipelago. 
 The body is protected by a chitinous shell, which shows a 
 fairly well marked trilobation ; the cephalothorax is
 
 206 ARTHROPODA. XIPHOSURA. 
 
 covered dorsally by a large horse- shoe shaped shield, 
 which carries on its upper surface two pairs of eyes, one 
 compound, near the middle of the lateral parts, the other 
 simple and placed close together near the anterior margin. 
 Behind the cephalothorax, and movably articulated with 
 it, comes the abdominal shield ; this is composed of six 
 fused segments, the segmentation being shown by grooves ; 
 at the sides it bears movable spines, and posteriorly it is 
 provided with a long caudal spine. The cephalothorax 
 bears six pairs of appendages on its under surface, the 
 anterior pair only being placed in front of the mouth ; 
 the basal joints of the five posterior pairs function in 
 mastication. The abdomen carries six pairs of appendages, 
 the anterior pair are plate-like and serve as an operculum 
 to cover the remaining five pairs, these bear the gills 
 which consist of a large number of thin plates. 
 
 The earliest Xiphosurans (Hemiaspis, Neolimulus) are 
 found in the Silurian ; in the Carboniferous, Prestwichia, 
 Belinurus and others occur. Limulus appears first in the 
 Trias. The Palaeozoic genera differ in some respects from 
 the later ones; the head-shield is not fused with the 
 thoracic shield, and the former sometimes possesses a 
 facial suture ; generally also, some or all of the segments 
 of the thorax and abdomen remain free and movable. 
 Thus in Hemiaspis there are six segments in the thorax 
 and three in the abdomen, all of which are free: in 
 Belinurus there are five free thoracic segments and three 
 fused abdominal ones ; in Prestwichia all the segments of 
 both thorax and abdomen are fused. 
 
 ORDER. EURYPTERIDA. 
 
 The Eurypterids are quite extinct and form a very 
 remarkable group of the Arthropoda ; they are usually of
 
 ARTHROPODA. EURYPTERIDA. 
 
 207 
 
 large size, some having a length of several feet ; trilobation 
 is either absent or only very faintly shown. The body is 
 protected by a chitinous skeleton (fig. 55), the anterior 
 part (cephalothorax) consisting of a large quadrate or 
 semicircular shield ; this is followed posteriorly by thirteen 
 
 FIG. 55. Dorsal surface of Pterygotus osiliensis, from the Upper Silurian, 
 Rootzikull. c, first pair of appendages ; d, compound eyes ; e, ocelli ; 
 g, telson; 5', fifth pair of appendages; 16, segments of the 
 abdomen; 712, segments of the post-abdomen. Reduced. (After 
 Schmidt.)
 
 208 
 
 ARTHROPODA. EURYPTERIDA. 
 
 free and movable segments which taper towards the 
 posterior extremity. The first six free segments (1 6) 
 are regarded by some as the abdomen, by others as the 
 thorax, and the remaining seven (7 12, #) as either the 
 abdomen or post-abdomen ; the last segment is usually an 
 oval pointed plate, the telson (g). The cephalothoracic 
 
 FIG. 56. Ventral surface of Pterygotus osiliensis, from the Upper Silurian, 
 Rootzikiill. a, epistome ; b, metastoma ; c, first pair of appendages ; 
 d, compound eyes ; /, operculum ; g, telson ; 1' 5', second to sixth 
 pairs of appendages; I. V., ventral plates of the abdomen ; 7 12, 
 segments of the post-abdomen. Reduced. (After Schmidt.)
 
 ARTHROPODA. EURYPTERIDA. 209 
 
 shield carries on its upper surface two pairs of eyes; a 
 large compound pair (d) placed laterally or at the anterior 
 angles, and a pair of simple ocelli (e) placed close together 
 near the centre of the shield. The mouth is on the under 
 surface of the cephalothorax (fig. 56); in front of the 
 mouth there is one pair of appendages only (c), they are 
 jointed and often end in chelae, and are considered to 
 represent the antennae ; the other four or five pairs of 
 appendages (!' 5') consist of seven or eight joints each 
 and are sometimes chelate, they function both in mastica- 
 tion and swimming, the posterior pair (5') is often very 
 much larger than the others. Placed just behind the 
 mouth, in the median line, is an oval or heart-shaped plate 
 the metastoma (b). The six segments of the abdomen are 
 not continued round on the ventral surface as would at 
 first sight appear. But there are instead five plate-like 
 appendages which cover the gills (I. V.) ; the first, known 
 as the operculum, is often divided into two parts by a 
 median process, and corresponds with that of Limulus. The 
 segments of the post-abdomen (7 12) are continued on 
 the under surface and bear no appendages. 
 
 The Eurypterids agree with the Xiphosura in many 
 points, but differ in the abdominal segments being always 
 free, in the possession of a metastoma, and in the absence 
 of trilobation. 
 
 Eurypterus. Cephalothoracic shield quadrate; the 
 compound eyes are not at the margin, but a little in front 
 of the median lateral point on each side. The telson is 
 greatly elongated. The pre-oral appendages are small 
 and not chelate ; of the five other pairs of appendages the 
 posterior is very much the larger. The metastoma is 
 oval. Lower Ludlow to Carboniferous. 
 
 w. P. 14
 
 210 ARTHROPODA. EURYFfERIDA. 
 
 Pterygotus. Cephalothoracic shield rounded in front; 
 the compound eyes are marginal, at the anterior angles. 
 The telson is oval with a pointed extremity. The pre- 
 oral appendages are long and chelate. Metastoma oval. 
 The examples of this genus are usually of enormous size, 
 P. anglicus sometimes reaching a length of nearly six feet. 
 Lower Ludlow to Old Red Sandstone. 
 
 Slimonia. Cephalothoracic shield quadrate ; the com- 
 pound eyes at the anterior angles. The telson is oval, 
 ending in a point. Metastoma heart-shaped. The pre- 
 oral appendages short and not chelate. Upper Ludlow 
 and Passage Beds. 
 
 Distribution of the Eurypterida. 
 
 The order ranges from the Ordovician to the Permian, 
 and is most abundant in the Upper Silurian and Old Red 
 Sandstone. The chief genera are Eurypterus, Pterygotus, 
 Slimonia, and Stylonurus.
 
 LIST OF SOME IMPORTANT PAL^EONTOLOGICAL 
 WORKS. 
 
 GENERAL. 
 
 K. A. von Zittel. Handbuch der Palseontologie. 4 vols. 1876- . 
 
 (There is a French translation by Barrois.) 
 G. Steinmann and L. Doderlein. Elemente der Palaontologie. 
 
 1888-1890. 
 H. A. Nicholson and R. Lydekker. Manual of Palaeontology. Third 
 
 edition. 2 vols. 1889. 
 J. Morris. A Catalogue of British Fossils. Second edition. 1854. 
 
 PROTOZOA. 
 
 W. B. Carpenter. Introduction to the Study of the Foraminifera. 
 
 1862. (Ray Society.) 
 H. B. Brady. Report on the Foraminifera. (Challenger Report) 
 
 1884. 
 T. R. Jones, W. K. Parker, and H. B. Brady. A Monograph of the 
 
 Foraminifera of the Crag. 1866. (Palseont. Soc.) 
 H. B. Brady. A Monograph of Carboniferous and Permian Fora- 
 minifera, 1876. (Palaeont. Soc.) 
 
 E. Haeckel. Report on the Radiolaria. (Challenger Report) 1887. 
 G. J. Hinde. Notes on Radiolaria from the Lower Palaeozoic Rocks 
 
 (Llandeilo-Caradoc) of the South of Scotland. Ann. Mag. Nat. 
 
 Hist. ser. 6, Vol. vi. (1890), p. 40. 
 Rust. Beitrage zur Kenntniss der fossilien Radiolarien aus Gesteineu 
 
 des Jura. Palseontographica. Band xxxi. 1885. 
 
 142
 
 212 PAL^ONTOLOGICAL WORKS. 
 
 PORIFERA. 
 
 G. J. Hinde. Catalogue of the Sponges in the Geological Depart- 
 ment of the British Museum (Natural History). 1883. 
 
 G. J. Hinde. A Monograph of British Fossil Sponges. 1887- . 
 (Palseont. Soc.) 
 
 W. J. Sollas. On the structure and affinities of the Genus 
 Siphonia. Quart. Journ. Geol. Soc. Vol. xxxm. (1877), p. 790. 
 
 HYDROZOA. 
 
 C. Lapworth. On the geological distribution of the Khabdophora. 
 
 Ann. Mag. Nat. Hist. ser. 5, Vol. in. (1879), pp. 245, 449 ; 
 
 Vol. iv. (1879), pp. 333, 423 ; Vol. v. (1880), pp. 45, 273, 358 ; 
 
 Vol. vi. (1881), pp. 16, 185. 
 C. Lapworth. On Scottish Monograptidae. Geol. Mag. dec. 2, 
 
 Vol. II. (1876), pp. 308, 350, 499, 544. 
 J. Hopkinson and C. Lapworth. Descriptions of the Graptolites of 
 
 the Arenig and Llandeilo Rocks of St David's. Quart. Journ. 
 
 Geol. Soc. Vol. xxxi. (1875), p. 631. 
 H. A. Nicholson. A Monograph of the British Graptolitidse. Part I. 
 
 General Introduction. 1872. 
 
 J. Hall. Graptolites of the Quebec group. (Figures and descrip- 
 tions of Canadian Organic Remains. Dec. n.). 1865. 
 H. A. Nicholson. A Monograph of the British Stromatoporoidea. 
 
 1886-1892. (Palseont. Soc.) 
 
 ACTINOZOA. 
 
 H. M. Edwards and J. Haime. A Monograph of the British Fossil 
 Corals. 1850-54. (Palseont. Soc.) 
 
 P. M. Duncan. A Monograph of the British Fossil Corals. (Sup- 
 plement to the preceding.) 1866-72. (Palseont. Soc.) 
 
 P. M. Duncan. A Revision of the families and genera of the Sclero- 
 dermic Zoantharia, or Madreporaria. Journ. Linn. Soc. (Zool.) 
 Vol. xvin. (1885), pp. 1-204. 
 
 H. A. Nicholson. On the structure and affinities of the "Tabulate 
 Corals " of the Palaeozoic Period. 1879.
 
 PAL.EONTOLOGICAL WORKS. 213 
 
 //. A. Nicholson. On the structure and affinities of the Genus 
 Monticulipora and its sub-genera. 1881. 
 
 ECHINODERMATA. 
 
 T. Wright. Monograph on the British Fossil Echinodermata from 
 
 the Cretaceous Formations. Vol. I. Echinoidea. 1864-1882. 
 
 (Palaeont. Soc.) 
 W. P. Sladen. A Monograph of the British Fossil Echinodermata. 
 
 Vol. n. Asteroidea. 1891- . (Palseont. Soc.) 
 T. Wright. A Monograph on the British Fossil Echinodermata of 
 
 the Oolitic Formations. Vol. i. Echinoidea. (1857-1878.) 
 
 Vol. ii. Asteroidea and Ophiuroidea (1863-1880). (Palseont. 
 
 Soc.) 
 P. M. Duncan. A Revision of the Genera and. great Groups of the 
 
 Echinoidea. Journ. Linn. Soc. (Zool.), Vol. xxm. (1889), p. 1. 
 
 E. Forbes. Monograph of the Echinodermata of the British 
 Tertiaries. 1852. (Palseont. Soc.) 
 
 J. W. Gregory. A Revision of the British Fossil Cainozoic 
 Echinoidea. Proc. Geol. Assoc., Vol. xn. (1891), p. 16. 
 
 F. A. Bather. British Fossil Crinoids. Ann. Mag. Nat. Hist. 
 
 ser. 6, Vol. v. (1890), pp. 306, 373, 485 ; Vol. vi. (1890), p. 222 ; 
 
 Vol. vn. (1891), pp. 35, 389 ; Vol. IX. (1892), pp. 189-202. 
 R. Etheridge and P. H. Carpenter. Catalogue of the Blastoidea in 
 
 the Geological Department of the British Museum (Natural 
 
 History). 1886. 
 E. Forbes. On the Cystidese of the Silurian Rocks of the British 
 
 Islands. Mem. Geol. Survey, Vol. 11., Part n. 1848. 
 J. Barrande. Systeme Silurien du centre de la Bohgme. (Cys- 
 
 tide"es par W. Waagen.) 1888. 
 
 VERMES. 
 
 G. J. Hinde. On Annelid Jaws from the Cambro-Silurian, Silurian, 
 
 and Devonian Formations in Canada, and from the Lower 
 Carboniferous in Scotland. Quart. Journ. Geol. Soc., Vol. xxxv. 
 (1879), p. 370. 
 
 G. J. Hinde. On Annelid Jaws from the Wenlock and Ludlow 
 Formations of the West of England. Quart. Journ. Geol. Soc., 
 Vol. xxxvi. (1880), p. 368.
 
 214 PAI^EONTOLOGICAL WORKS. 
 
 POLYZOA. 
 
 G. Busk. Report on the Polyzoa. (Challenger Report) 1886. 
 
 G. Busk. A Monograph of the Fossil Polyzoa of the Crag. 1859. 
 
 (Palaeont. Soc.) 
 
 T. Hincks. History of the British Marine Polyzoa. 1880. 
 G. W. Shrubsole. A Review of the British Carboniferous Fenestel- 
 
 lidae. Quart. Journ. Geol. Soc., Vol. xxxv. (1879), p. 275. 
 G. W. Shrubsole. A Review and Description of the various species 
 
 of British Upper Silurian Fenestellidae. Quart. Journ. Geol. 
 
 Soc., Vol. XXXVL (1880), p. 241. 
 G. R. Vine. Reports on Fossil Polyzoa. Rep. Brit. Assoc. 
 
 1880-92. 
 
 BRACHIOPODA. 
 
 P. Fischer. Manuel de Conchyliologie. 1887. [Brachiopods, by 
 
 D. P. CEhlert.] 
 T. Davidson. British Fossil Brachiopoda. Six vols. 1851-1886. 
 
 (Palseont. Soc.) 
 J. Hall and J. M. Clarke. An Introduction to the Study of the 
 
 Genera of the Palaeozoic Brachiopoda. (Geol. Survey New 
 
 York, Palaeontology. VoL VHL) 1892. 
 
 MOLLUSCA. 
 
 S. P. Woodward. Manual of the Mollusca. Fourth edition by 
 R. Tate. 1880. 
 
 P. Fischer. Manuel de Conchyliologie. 1887. 
 
 J. Sowerby. Mineral Conchology of Great Britain. 1812-29. 
 
 W. Carpenter. Report on the Microscopic Structure of Shells. 
 Rep. Brit. Assoc. for 1844 (1845), p. 24, and for 1847 (1848), 
 p. 93. 
 
 J. Lycett. British Fossil Trigoniaj. 1872-79. (Palaeont. Soc.) 
 
 S. V. Wood. A Monograph of Eocene Bivalves of England. 
 1861-71. (Palseont. Soc.) 
 
 S. V. Wood. A Monograph of the Crag Mollusca. Vol. I. Uni- 
 valves, 1848. Vol. II. Bivalves 1850, and supplements. 
 (Palaeont. Soc.)
 
 PAI^EONTOLOGICAL WORKS. 215 
 
 J. Morris and J. Lycett. Mollusca from the Great Oolite, with 
 
 supplement, 1850-63. (Palseont. Soc.) 
 W. H. Hudleston. British Jurassic Gasteropoda. 1887- . (Palseont. 
 
 Soc.) 
 A. H. Foord. Catalogue of the British Fossil Cephalopoda in the 
 
 British Museum. Parts I. and n. 1888-91. 
 T. H. Huxley. On the structure of the Belemnitidse. (Mem. Geol. 
 
 Survey.) 1864. 
 J. Phillips. A Monograph of the British Belemnitidee. 1865. 
 
 (Palseont. Soc.) 
 S. S. Buckman. A Monograph of Inferior Oolite Ammonites of the 
 
 British Islands. 1887- . (Palaeont. Soc.) 
 T. Wright. Monograph of the Lias Ammonites. 1878-86. 
 
 (Palseont. Soc.) 
 
 J. F. Blake. British Fossil Cephalopoda. Part i. 1882. 
 D. Sharpe. Mollusca in the Chalk of England. Parts i. and n., 
 
 Cephalopoda. 1853-4. (Palseont. Soc.) 
 
 ARTHROPODA. 
 
 H. Woodward. Catalogue of British Fossil Crustacea. 1877. 
 
 T. R. Jones. A Monograph of the Fossil Estheriaa. 1862. (Palseont. 
 
 Soc.) 
 T. R. Jones. A Monograph of the Tertiary Entomostraca of 
 
 England. 1856. (Palaeont. Soc.) 
 T. R. Jones, J. W. Kirkby and G. S. Brady. A Monograph of the 
 
 British Fossil Bivalved Entomostraca from the Carboniferous 
 
 Formations. 1874-84. (Palaeont. Soc.) 
 
 T. R. Jones. A Monograph of the Entomostraca of the Cre- 
 taceous Formations of England. 1849. (Palseont. Soc.) 
 C. Darwin. A Monograph on the Fossil Lepadidse. 1851. (Palseont. 
 
 Soc.) 
 C. Darwin. A Monograph on the Fossil Balanidse. 1854. (Palseont. 
 
 Soc.) 
 T. R. Jones and H. Woodward. A Monograph of the British 
 
 Palaeozoic Phyllocarida. 1888-92. (Palseont. Soc.) 
 T. Bell. A Monograph of the Fossil Malacostracous Crustacea. 
 
 1857-62. (Palasont. Soc.)
 
 216 PALJEONTOLOGICAL WORKS. 
 
 J. Carter. On the Decapod Crustacea of the Oxford Clay. Quart. 
 Journ. Geol. Soc., Vol. XLII. (1886), p. 542. 
 
 B. N. Peach. On some new Crustaceans from the Lower Carbonifer- 
 
 ous Rocks of Eskdale and Liddesdale. Trans. Roy. Soc. Edin., 
 Vol. xxv. (1880-83), pp. 73, 511. 
 
 J. W. Salter. A Monograph of the British Trilobites from the 
 Cambrian, Silurian, and Devonian Formations. 1864-1883. 
 (Palseont. Soc.) 
 
 H. Woodward. A Monograph of the British Carboniferous Trilo- 
 bites. 1883-4. (Palaeont. Soc.) 
 
 J. Barrande. Systeme Silurien du centre de la BohSme. Trilo- 
 bites. 1852. 
 
 C. Lapworth. On Olenellus callavei and its geological relationships. 
 
 GeoL Mag. dec. 3, Vol. vin. (1891), p. 529, plates xiv. and xv. 
 C. D. Walcott. The Fauna of the Lower Cambrian or Olenellus 
 
 zone. 1890. (Ann. Rep. U. S. Geol. Survey.) 
 C. D. Walcott. The Trilobite : New and Old Evidence relating to 
 
 its Organization. (Bull. Mus. Comp. Zool., Vol. vin.), 1881. 
 H. Woodward. A Monograph of the British Fossil Crustacea 
 
 belonging to the order Merostomata. 1866-78. (Palseont. 
 
 Soc.) 
 T. H. Huxley and J. W. Salter. On Pterygotus. 1859. (Mem. 
 
 Geol. Survey, Organic Remains.)
 
 INDEX. 
 
 Acervularia, 56 
 Acidaspis, 204 
 Actinaria, 51 
 Actinoceras, 174 
 Actinocrinus, 89 
 Actinozoa, 50 
 jEglina, 203 
 Agnostus, 200 
 Alcyonaria, 59 
 Ammonites, 176 
 Ammonoidea, 175 
 Amphipoda, 194 
 Ampyx, 200 
 Ananchytes, 81 
 Ancyloceras, 177 
 Angelina, 201 
 Annelida, 99 
 Anthracosia, 142 
 Antipatharia, 51 
 Apiocrinus, 90 
 Aporosa, 55 
 Aptychus, 176 
 Area, 140 
 Archaeoniscus, 194 
 Archseoscyphia, 32 
 Arcomya, 147 
 Artemis, 146 
 
 Arthropoda, 183 
 Arthrostraca, 193 
 Articulata, 116 
 Asaphus, 202 
 Asiphonida, 136 
 Astarte, 143 
 Asteractinella, 34 
 Asteroidea, 65 
 Astraeospongia, 34 
 Astropecten, 69 
 Astylospongia, 32 
 Atrypa, 119 
 Aturia, 175 
 Avicula, 138 
 Aviculopecten, 137 
 
 Bacnlites, 177 
 Balanus, 189 
 Belemnitella, 180 
 Belemnites, 179 
 Belinurus, 206 
 Bellerophon, 156 
 Beyrichia, 188 
 Blastoidea, 94 
 Bothriocidaris, 83 
 Brachiopoda, 106 
 Brachyura, 192
 
 218 
 
 INDEX. 
 
 Bronteus, 203 
 Bryograptus, 48 
 Buccinum, 160 
 Bulla, 162 
 
 Calceola, 58 
 Calcispongise, 34 
 Calymene, 202 
 Calyptrsea, 158 
 Cardinia, 142 
 Cardiola, 140 
 Cardita, 143 
 Cardium, 145 
 Cephalopoda, 168 
 Ceratiocaris, 191 
 Ceratites, 176 
 Ceratospongise, 31 
 Cerithium, 159 
 Chsetopoda, 99 
 Chama, 144 
 Cheilostomata, 105 
 Cheirurus, 204 
 Chonetes, 117 
 Cidaris, 79 
 Cirripedia, 188 
 Clathrodictyon, 50 
 Climacograptus, 48 
 Clymenia, 175 
 Ccelenterata, 38 
 Ccenograptus, 47 
 Columnaria, 61 
 Conocoryphe, 201 
 Conularia, 163 
 Conus, 161 
 Corbula, 148 
 Crania, 115 
 Crassatella, 143 
 Crinoidea, 85 
 Crioceras, 177 
 Crotalocrinus, 89 
 Crustacea, 185 
 
 Cucullasa, 140 
 Cyathophyllum, 56 
 Cyclostomata, 105 
 Cyprsa, 160 
 Cypridea, 188 
 Cyprina, 145 
 Cypris, 188 
 Cyrena, 145 
 Cyrtoceras, 175 
 Cyrtograptus, 46 
 Cystidea, 92 
 Cytherea, 146 
 
 Decapoda (Cephalopoda), 179 
 Decapoda (Crustacea), 191 
 Dendrograptus, 39 
 Dentalium, 168 
 Dibranchiata, 177 
 Dicellograptus, 47 
 Dicranograptus, 47 
 Dictyonema, 39 
 Dictyophyton, 33 
 Didymograptus, 47 
 Diplograptus, 47 
 Discina, 115 
 Dosinia, 146 
 
 Echinobrissus, 81 
 Echinoconus, 80 
 Echinocorys, 81 
 Echinodermata, 63 
 Echinoidea, 73 
 Echinosphaerites, 92 
 Ectoprocta, 104 
 Emarginula, 156 
 Encrinasterias, 69 
 Encrinurus, 204 
 Entomostraca, 186 
 Eozoon, 21 
 Estheria, 187 
 Euasteroidea, 69
 
 INDEX. 
 
 219 
 
 Euechinoidea, 79 
 Euomphalus, 157 
 Eurypterida, 207 
 Eurypterus, 209 
 Exogyra, 136 
 
 Fascicularia, 105 
 Favosites, 59 
 Fenestella, 105 
 Foraminifera, 15 
 Fungida, 58 
 Fusus, 160 
 
 Geodites, 31 
 Gervillia, 138 
 Globigerina, 20 
 Glycimeris, 147 
 Glyptocrinus, 91 
 Glyptosphserites, 92 
 Gomphoceras, 174 
 Goniatites, 176 
 Goniomya, 147 
 Granatocrinus, 97 
 Graptolitoidea, 40 
 Griffithides, 205 
 Gryphsea, 136 
 Gymnolsema, 105 
 
 Halysites, 60 
 Hamites, 177 
 Heliolites, 59 
 Helix, 162 
 Hemiaspis, 206 
 Hemicidaris, 80 
 Heteractinellidae, 34 
 Heteromyaria, 138 
 Heteropoda, 162 
 Hexactinellidse, 33 
 Hippurites, 144 
 Holaster, 81 
 Holectypus, 80 
 
 Holocystis, 56 
 Holothuroidea, 84 
 Homalonotus, 202 
 Homomyaria, 140 
 Hyalostelia, 33 
 Hydractinia, 39 
 Hydromedusas, 39 
 Hydrozoa, 38 
 Hymenocaris, 191 
 Hyolithes, 163 
 
 lUaenus, 203 
 Inarticulata, 114 
 Inoceranms, 138 
 IntegripaUiata, 143 
 Isastraea, 55 
 Ischadites, 33 
 Isopoda, 194 
 
 Labechia, 50 
 Lagena, 19 
 
 Lamellibranchiata, 126 
 Leda, 141 
 Lepadocrinus, 93 
 Lepas, 189 
 Leperditia, 187 
 Leptama, 117 
 Leptostraca, 190 
 Lima, 137 
 Limnsea, 162 
 Limulus, 205 
 Lingula, 114 
 LinguleUa, 115 
 Litharsea, 59 
 Litbistidse, 31 
 Lithodomus, 139 
 Lithostrotion, 57 
 Littorina, 158 
 Lucina, 145 
 
 Macrura, 192
 
 INDEX. 
 
 Mactra, 148 
 Madreporaria, 52 
 Malacostraca, 190 
 Marsupites, 90 
 Melania, 159 
 Membranipora, 106 
 Meretrix, 146 
 Meyeria, 192 
 Micraster, 82 
 Microdiscus, 200 
 Miliola, 19 
 Mitra, 161 
 Modiola, 139 
 Mollusca, 125 
 Molluscoidea, 101 
 Monactinellidae, 31 
 Monograptus, 46 
 Monomyaria, 136 
 Monticuliporoidea, 51 
 Montlivaltia, 55 
 Murchisonia, 156 
 Murex, 161 
 Myacites, 147 
 Mytilus, 139 
 Myxospongiae, 29 
 
 Nassa, 160 
 Natica, 158 
 Nautiloidea, 174 
 Nautilus, 175 
 Neocrinoidea, 90 
 Nerinea, 159 
 Nerita, 157 
 Nodosaria, 19 
 Nucleolites, 81 
 Nucula, 141 
 Nummulites, 20 
 
 Octactinellidse, 34 
 Octopoda, 180 
 Ogygia, 202 
 
 Olenellus, 201 
 Olenus, 200 
 Omphyma, 57 
 Ophioderma, 72 
 Ophiuroidea, 70 
 Opis, 143 
 
 Opisthobranchiata, 162 
 Orthis, 118 
 Orthoceras, 174 
 Ostracoda, 187 
 Ostrea, 136 
 
 Palseaster, 69 
 Palffiechinoidea, 78 
 Palffiechinus, 78 
 Palaeocrinoidea, 89 
 Paludina, 158 
 Panopea, 147 
 Paradoxides, 201 
 Patella, 156 
 Pecten, 137 
 Pectunculus, 141 
 Pelecypoda, 126 
 Peltastes, 79 
 Pentamerus, 121 
 Pentremites, 94, 95 
 Perforata, 58 
 Perna, 139 
 Peronella, 35 
 Phacops, 203 
 Phasianella, 156 
 Phillipsia, 204 
 Pholadomya, 147 
 Pholas, 148 
 Phormosella, 33 
 Phragmoceras, 174 
 PhylactolaBma, 105 
 Phyllocarida, 190 
 Phyllograptus, 48 
 Phyllopoda, 186 
 Pinna, 139
 
 INDEX. 
 
 221 
 
 Planorbis, 162 
 Pleurotoma, 161 
 Pleurotomaria, 156 
 Plicatula, 137 
 Polychaeta, 99 
 Polyplacophora, 155 
 Polyzoa, 101 
 Porifera, 26 
 Prearcturus, 194 
 Prestwichia, 206 
 Primitia, 188 
 Productus, 116 
 Prosobranchiata, 155 
 Protaster, 73 
 Protocystites, 94 
 Protospongia, 33 
 Protozoa, 14 
 Pseudocrinites, 93 
 Pseudornelania, 158 
 Pterinea, 138 
 Pteropoda, 163 
 Pterygotus, 210 
 Puluionata, 162 
 Purpura, 160 
 Pygaster, 81 
 
 Radiolaria, 25 
 Rastrites, 46 
 Receptaculites, 33 
 Reniera, 31 
 Retiolites, 48 
 Rhizopoda, 15 
 Rhynchonella, 120 
 Rostellaria, 159 
 Rotalia, 20 
 Rugosa, 56 
 
 Saccamina, 19 
 Scalaria, 157 
 Scaphites, 177 
 Scaphopoda, 168 
 
 Scyphomedusse, 40 
 Sea-cucumbers, 84 
 Sea-lilies, 85 
 Sea-urchins, 73 
 Serpula, 100 
 Sinupalliata, 146 
 Siphonia, 32 
 Siphonida, 142 
 Siphonophora, 40 
 Slimonia, 210 
 Solarium, 157 
 Solen, 147 
 Sphaerospongia, 33 
 Spirifer, 118 
 Spirorbis, 100 
 Sponges, 26 
 Spongilla, 31 
 Starfishes, 65 
 Stringocephalus, 122 
 Stromatopora, 50 
 Stromatoporoidea, 49 
 Strombus, 159 
 Strophomena, 117 
 Syringopora, 59 
 
 Tellina, 146 
 Tentaculites, 163 
 Terebratula, 121 
 Teredo, 148 
 Tetrabranchiata, 171 
 Tetractinellid, 31 
 Tetragraptus, 47 
 Textularia, 19 
 Thamnastrsea, 58 
 Theca, 163 
 Thecosmilia, 56 
 Tholiasterella, 34 
 Thoracostraca, 191 
 Thracia, 148 
 Trigonia, 141 
 Trilobita, 195
 
 222 INDEX. 
 
 Trinucleus, 200 Vermes, 98 
 
 Trochus, 157 Voluta, 161 
 Turbo, 157 
 
 Turrilites, 177 Waldheimia, 122 
 
 Turritella, 158 Worms, 98 
 
 Uncites, 119 Xanthopsis, 193 
 
 Unio, 142 Xiphosura, 205 
 
 Ventriculites, 34 Zaphrentis, 57 
 
 Venus, 146 Zoantharia, 51 
 
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