296 FOSSIL TURTLES OF NORTH AMERICA. 



This genus is known principally from the Bridger Eocene, but also from the Wasatch and 

 the Uinta. 



In the absence of definite knowledge regarding many parts of the anatomy of the Eocene 

 Emydidae they have been usually, tho sometimes provisionally, referred to the genus Emys. 

 More recently they have been supposed to belong to the North American genera Chrysemys 

 or Clemmys. The writer has exprest himself to this effect in 1902 (Bibliog. and Cat. Foss. 

 Vert. N. A., p. 447) and again in 1905 (Amer. Geologist, xxxv., p. 332). Recently there has 

 come into the possession of the American Museum of Natural History a considerable number 

 of shells of Emydidae, especially from the Bridger deposits. Most of these specimens are some- 

 what damaged, but the majority of them are in condition to furnish abundant information. 

 Unfortunately no skulls accompany these remains. A single dentary bone only was found in 

 company with a good plastron and some fragments of the carapace. A study of these and 

 other accessible materials has demonstrated that most of the Bridger species of Emydidae 

 belong to a hitherto unrecognized genus, Echmatemys (Hay, Bull. Amer. Mus. Nat. Hist., 

 xxn, 1906, p. 27), the essential characters of which are the strongly developt buttresses and 

 the feebly developt crushing surfaces of the jaws. In the existing North American genera of 

 Emydidaa the buttresses rise little, if at all, above the lower borders of the costals with which 

 they come into contact. In Asia there are, according to Boulenger, four genera which possess 

 greatly developt axillary and inguinal buttresses, the latter articulating with the fifth and sixth 

 costals. All of these genera differ, however, from Echmatemys in having the triturating surfaces 

 of the jaws broad, and those of the maxillae furnisht with one 'or two longitudinal ridges. 

 The Eocene genus appears therefore to be well separated from both the American and the 

 Asiatic forms. 



The degree of development of the buttresses of Echmatemys appears to differ among the 

 various species. It is doubtful whether in any of them the buttresses ascend so high within the 

 carapace as they do in the Asiatic species. In E. wyomingensis the bases of the inguinal but- 

 tresses begin to rise perceptibly from the upper surface of the plastron at a point less than half- 

 way from the free border of the hinder lobe to the midline. In E. arethusa they spring from the 

 general level of the plastron at points about half-way between the border mentioned and the 

 midline. In other species they rise still nearer the midline. There appears to be less variation 

 in the width of the bases of the anterior buttresses. In E. wyomingensis these buttresses 

 ascend but little above the lower border of the first costals. In other species they ascend half- 

 way to the neural border of the costals. We find the same differences of development in the case 

 of the inguinal buttresses, the height to which they ascend varying from half-way to two- 

 thirds or more of the distance from the lower to the upper border of the costals concerned. 



The presence of these buttresses results in the production of a deep sternal chamber on 

 each side. In a specimen of//, septaria, the width of whose carapace is 225 mm., the width 

 of the space between the inguinal buttresses is only about 80 mm. 



The existence of these buttresses in E. septaria was recognized by Professor E. D. Cope 

 (Vert. Tert. Form. West, p. 130). In the case of his Emys shaughnessiana they were regarded 

 as less well developt. On the contrary, the present writer finds that they were quite as broad 

 and high as in E. septaria. 



The determination of the species of this genus offers great difficulties. There is among 

 them a great sameness in structure and general appearance. Few of them are markt by 

 trenchant characters. Yet there exist differences that we can not overlook. Had we the 

 skulls belonging to the shells in our possession, we might be relieved of some of our embarrass- 

 ment; but for the present we must be content with the knowledge and the conclusions that we 

 can derive from the carapaces and the plastra. 



Another difficulty arises from the fragmentary character of the materials on which some of 

 Dr. Leidy's species from the Bridger were establisht. If it is difficult to identify species when 

 we have practically whole shells, it is far more difficult when we have only a fragment for 

 instance, the epiplastral lip or a fragment of the carapace. 



In studying the materials in his hands the present writer has been compelled to recognize 

 several new species. It is not unlikely that in some cases he has erred; but it has appeared to 

 him that, better than the lumping together of forms which seem to be distinct, is their separa- 

 tion under special names, with careful descriptions, leaving to future writers, in the possession 



