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 
Emydide they have been usually, or 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., - 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 Emydidz, especially from the Bridger deposits. Most of these specimens are some- 
what damaged, but the majority of them are incondition to furnish abundant information. 
Unfortunately no skulls accompany these remains. A single dentary bone only was found in 
company w ith 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 Emydidz 
belong to a hitherto unrecognized genus, Echmatemys (Hay, Bull. Amer. Mus. Nat. Hist., 
XXL, 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 
Emy didz the buttresses rise little, if at all, above the lower border of the costals ani 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 maxilla furnisht with one 'or two longitudinal ridges, 
The Eocene genus appears therefore to be well separated from both the American and ae 
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 £. 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 £. 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 fon 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 4. 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 houeiae sstana 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£. septarta. 
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 dificult 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 ce 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 
