286 FOSSIL TURTLES OF NORTH AMERICA. 



In the Pliocene deposits of the Siwalik Hills, India, there have been found five genera of 

 Emydidae. 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. 



W e have no certain evidence of the existence of Emydidae 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 Emydidae. 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 Emys 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 Emydidae 

 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 Emydidae 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- 

 didae. The shells of the family ancestral to the Emydidae 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 Thalassemydidae. 



In a paper publisht in the Bulletin of the American Museum of Natural History, vol. 

 XXI, p. 167, the author indicated his belief that the Emydidae had sprung from the Chelydridae. 

 He is now inclined to the opinion that they had their origin from some of the more primitive 



