132 



THE EXTINCT BATHACHIA, REPTILIA 



Eurystcrnum differs from Chelydra as Osteopygis does, in a greater eoossification of 

 the discal and marginal bones anteriorly and posteriorly. It (lifters from the latter in the 

 stout ambulatory foot like that of the Chelydra, and in the apparent absence of tin; 

 temporal osseous roof, which I suspect Osteopygis to possess. The vertebral dermal 

 scuta are wider than in any species of our genera, but this is but a specific character. 



In Idiochelys the marginal bones arc; more distinct from the disc, entirely so poste- 

 riorly; there arc numerous omissions of the vertebral bones of the carapace; this docs 

 not occur among our species so far as known. 



Ilvdropelta is in general much like Osteopygis, but in it the hyo and hyposternal 

 bones, have an articulation by gomphosis with the third and tenth marginals respectively, 

 a character certainly wanting to the American genera. There is also no median sternal 

 fontanelle, which is present in our types. 



Platychelys has a still greater union of carapacial disc and margin; only three ribs 

 have free extremities on each side. 



Our forms then appear to differ from those; of the Jurassic of Europe, up to the 

 present time. It is important to observe, that instead of being of marine habit as lias 

 been supposed, they are representatives of modern fresh water species, and were probably 

 inhabitants of brackish estuaries of our coast. 



* 



OSTEOPYGIS, Cope. 

 Proceed. Acad. Nat. Sciences, I'liila., 1808, 147. 



This genus has been characterized above, in part. It has the usual twenty-three 

 marginal bones with ten costal bones on each side, nine of which send their free extremi- 

 ties for gomphosis with the marginals. Though in sutural union with the second mar- 

 ginals, each sends a costal process into a, corresponding pit of the latter. The inter- 

 costal union is prolonged, and the vertebral scuta are rather narrow, and do not therefore 

 extend far on the former. The four posterior marginals arc prolonged considerably within 

 the groove marking the suture of the costal and marginal scuta. The posterior marginal 

 plates of all the species an? very Hat and expanded, the lateral on the contrary trigonal 

 or subtrigonal in section, with a distinct inferior plane. 



The accompanying cut gives the characters of the genus as exhibited in the carapace 

 of the typical species. The dermal sutures are omitted from the left side, so as to show 

 more clearly those of the skeleton. 



The plastron of Osteopygis is more like that of Chelydra than any other known 

 genus. The hyo-,hypo, and xiphisterhals are united on the median line by a coarse open 

 suture as in that genus, and are not separated as in Cbelone. They arc much more united 

 than in Trionyx. 



