286 FOSSIL TURTLES OF NORTH AMERICA. 
In the Pliocene deposits of the Siwalik Hills, India, there have been found five genera of 
Emydide. Why these did not effect an entrance into Africa in company with the various 
ruminants, hog-like ungulates, antelopes, monkeys and apes, is not now to be explained. 
We have no certain evidence of the existence of Emydide before the beginning of the 
Tertiary. Indeed, not until the Wasatch epoch in America and the epoch of the London Clay 
in Europe, do we find undoubted Emydida. When they do appear they are in many respects 
as advanct as are some of the living forms. We can not, therefore, doubt that they had a much 
earlier beginning and we may await with confidence the discovery of their predecessors in 
Cretaceous deposits. The genus Gyremys, found in the Judith River beds, is provisionally 
assigned to the family. 
As regards the evolution of the family not much can be said. In the Eocene forms the shell 
resembles greatly that of the modern genera. It is composed of the same pieces and these are 
arranged in the same way. Most of the forms from the Wasatch and the Bridger possess 
strongly developt axillary and inguinal buttresses, such as we find to-day in a number of 
Asiatic genera. There appear to have been other genera which did not possess such buttresses, 
and from such as these have probably descended most modern forms. 
We are, unfortunately, without sufficient materials to enable us to determine the structure 
of the rest of the skeleton. The limbs of the Bridger turtles, so far as represented, were not 
different from those of living aquatic emyds. Only a single Bridger emyd skull is known, and 
this was not associated with a shell. This skull resembles considerably that of our genus 
Clemmys, the triturating surfaces of the jaws being narrow and with only a rudimentary 
longitudinal ridge. In many modern genera of this family the jaws have become broadened 
and the maxillary, and sometimes the mandibular, triturating surfaces have developt one or 
more longitudinal ridges. The horny plates which cover these surfaces are often rough with 
processes that resemble and function as teeth. The living genus Terrapene, as befits its 
terrestrial mode of life, has simulated many of the features of Testudo. The shell has become 
inflated, the digits shortened, and the diet a vegetable one. Besides these modifications, the 
plastron has become movable on the carapace and a hinge has developt between the hyo- 
plastron and the hypoplastron. Even in the Lower Oligocene the genus Ptychogaster had 
developt somewhat similar structures. 
Mr. G. A. Boulenger (Catalogue of Chelonians, p. 49) has exprest the opinion that the 
genus Enmrys is, in many respects, the least specialized of this family and might be placed at the 
base of the family, with two series of genera culminating, on the one hand, in the Batagurs 
and, on the other, in the land-tortoises. The present writer can not agree in this opinion. He 
holds first of all that the land-tortoises form a distinct family which diverged from the Emydidze 
even as early as the Cretaceous. Again, Emys displays a rather high grade of differentiation 
in the ridged triturating surfaces of the jaws, and especially in the secondary loosening of the 
articulation between the carapace and the plastron, and in the formation of a hinge between 
the hyoplastron and the hypoplastron. Such a form as Clemmys appears to represent better 
the simple emyd structure. In this the jaws are narrow and smooth, the plastron is immovably 
articulated with the carapace, there is no hinge in the plastron, and the plastral buttresses are 
feeble. From such a form there might be produced the Batagurs, with their broad corrugated 
jaws and their powerful buttresses, and Emys and Terrapene, with their narrow jaws, their 
feeble buttresses, and their hinged plastra. 
From what forms the Emydide sprang we know not as yet. We shall probably find that 
the limbs of the earliest Cryptodira were not greatly different from those of our aquatic Emy- 
dida. The shells of the family ancestral to the Emydidz need not have differed greatly from 
that of Chrysemys, for example; for even the early Amphichelydia have the carapace and the 
plastron closely articulated. The buttresses of the ancestors of the early emyds were probably 
little developt. The neck was probably short and the head not wholly retractile within the 
shell. The skull was probably considerably like that of Chelydra, but with a more completely 
developt temporal roof. Perhaps the skull of Platysternum resembles it more. Such a family 
would not be greatly removed from the Thalassemydide. 
In a paper publisht in the Bulletin of the American Museum of Natural History, vol. 
XX1, p. 167, the author indicated his belief that the Emydide had sprung from the Chelydridz. 
He is now inclined to the opinion that they had their origin from some of the more primitive 
