CHELONIA 



231 



Extinct members of the family are known from scanty remains 

 in Cenozoic and late Cretaceous rocks. From the earlier Cretaceous 

 deposits of the plains more primitive allied forms occur, often 

 classed in distinct families of which Toxochelys (Fig. 120) and 

 Desmatochelys are the more noteworthy. The latter genus, espe- 

 cially, might well have 

 been an ancestor of all 

 the modern forms. 

 About three feet in 

 length, it had all the 

 essential characteristics 

 of the sea-turtles, in its 

 thin form, roofed-over 

 skull, reduced carapace, 

 loose plastron, and 

 flipper-like limbs. The 

 single known speci- 

 men, preserved in the 

 museum of the Uni- 

 versity of Kansas, came 

 from the lower rocks of 

 the Upper Cretaceous 

 of Nebraska. Yet 

 earlier, at the close of 

 the Jurassic, there were 

 shore turtles of con- 

 siderable size that had begun to develop a fondness for the open 

 seas; to acquire a depressed form and lightened shell, the limbs 

 still retaining, however, more of the terrestrial or crawling form. 

 They are grouped as a separate family, the Thalassemydae, and 

 include the first of the Chelonia to depart from the marsh and 

 fresh-water habits which for long ages, perhaps, had limited the 

 activities and evolution of the turtles. 



ANCIENT SEA-TURTLES. PROTOSTEGIDAE 



Forty-four years ago the late Professor E. D. Cope, one of the 

 greatest naturalists America has ever produced, in almost the 



Fig. 120. — Carapace of Toxochelys baiiri, an 

 Upper Cretaceous sea-turtle: ep, epineural. (After 

 Wieland.) 



