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WILLIAM HERBERT HOB BS ºf * 3%

University of Michigan l 1915 Zº. 3
Ann Arbor, Mich, U. S. A. $C#NC; , ;
U. V. OF MICH,
Ll BRARY || 4
THE
AMERICAN NATURALIST
A MONTHLY JOURNAL DEVOTED TO THE
NATURAL SCIENCES IN THEIR
WIDEST SENSE

—- !
REPRINT g-z- *.
FROM
Vol. XLI, No. 486. JUNE, 1907
BOSTON
GINN & COMPANY, PUBLISHERs
Čhe 3themwum #ress
1907
Science Library
3/.
667
, If 7?
A Preliminary Note on the Variation of Scutellation in the Garter
Snakes.— Three years ago the writer began an investigation into the
relationships of the different races of garter snakes (Thannophis) in
an attempt to determine the laws involved in the differentiation of
the genus. The results of this work are being included in a mono-
graph of the genus, but as it will be several months before this work
can be completed it has been thought best to publish a brief outline
of some of the conclusions.
In the progress of this investigation it was seen very early that before
a serious attempt could be made to determine the affinities of the
different races, the significance of the variations in scale arrangement
or scutellation must be determined. This was attempted with the
following results:
(1) The number of dorsal scale rows on an individual snake
decreases posteriorly by the elimination of certain rows, and
the series eliminated are always the same for snakes with the
same number of rows, as for example T. sirtalis and T. saurita.
(2) The rows dropped posteriorly in individual snakes are those
which have entirely disappeared in races with a fewer number
of scale rows.
(3) The reduction in the number of dorsal scale rows is generally
accompanied by a reduction in the number of labial, ventral,
and Subcaudal scales (gastrosteges and urosteges).
(4) There is considerable evidence that the reduction in scutella-
tion is directly or indirectly associated with a diminution in size.
The general reduction in scutellation described above is exhibited
by each of the several (natural 7) groups into which the genus can be
divided. These groups all occur together only in northern Mexico,
which may be considered the center of origin for the genus. The
races that occur in this region all exhibit the maximum scutellation
for their respective groups, the dwarfing in size and scutellation
taking place at points away from the center of origin. The dis-
covery of these methods of variation in the different series of scales
has been an indispensable aid in determining the affinities of the dif-
*
ferent races. +4 °
ALEXANDER G. RUTHVEN
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