EMYDID^. 



Dermatemydidae. The latter family was abundantly represented during the Upper Cretaceous 

 and only slight modifications would be required to transform them into Emydidae. 



In Europe the earliest Emydidae appear to be those found in the London Clay, which 

 is of about the same age as the American Wasatch beds. Two species are found there; and 

 these have been referred by Mr. Lydekker to Chrysernys, a living North American genus. 

 Doubtless when the skulls shall have been found these species will have to be relegated to some 

 other genus. In the Oligocene of Hordwell, England, two species occur which Lydekker 

 refers to Ocadia, a genus existing to-day in China. It resembles closely the genus Echmatemys, 

 but the latter probably did not have a longitudinal ridge on the maxillary triturating surface, 

 and the plastron is less contracted than it is in Ocadia. No other species of Emydidae have 

 been discovered in England, so far as known to the writer. 



On the continent of Europe, as recorded by Maack (Palaeontographica, xvm, 1869, p. 193) 

 and by Zittel (Handbuch der Palaeontologie, in, p. 537), many species of Emydidae have been 

 described and the greater part of them referred to Emys, Cistudo, and Ptychogaster. Only 

 those called Cistudo are really members of Emys as now understood. This genus thus appears 

 to extend back to the Oligocene. Those placed by Maack and Zittel in the genus Emys are 

 probably more closely related to Clemmys or Ocadia. Ptychogaster is an Oligocene and Lower 

 Miocene genus, the carapace of which resembles in many respects that of Testudo, while the 

 plastron is hinged like that of Ttrraferu, 



The following table shows the distribution of the North American fossil species of the 

 Emydidae: 



KEY TO GENERA OF EMYDIDJE. 



A. Insufficiently known genera. 



a 1 . Carapace broad as long; the outer surface sculptured somewhat as in Trionychidae . .Gyremys 



a 2 . Small Bridger species, with dorsal keel and thick bones Palieotheca 



a 3 . A hemispherical boss situated on each interperipheral suture . . . Hybemys 



A A. Better known genera. 



a 1 . Axillary and inguinal buttresses strongly developt; the latter projecting well into 



the cavity of the shell and rising high above the costo-peripheral suture Echmatemys 



a 2 . Buttresses rising but little above the costo-peripheral sutures. 

 4 1 . Plastron without either lateral or transverse hinge. 



c. Crushing-surface of upper jaw with feeble or no longitudinal ridge. 



</'. Humero-pectoral sulcus crossing the entoplastron Clemmys 



J 2 . Humero-pectoral sulcus crossing behind the entoplastron. 



