THE CRETACEOUS PERIOD. 



181 



and the even greater Archelon. These were broad, flat forms, degene- 

 rate in having the carapace reduced to the ribs alone, and probably 

 covered with a soft skin, as are some living marine turtles. Archelon 



Fig. 413a. — Trinacromem m osborni Williston. A mounted skeleton of a typical fish- 

 eating plesiosaur, 10 feet long. The elongate head and the shortened neck (Com- 

 pare Fig. 3(37) represent specialization characteristic of the late plesiosaurs (Wil- 

 liston). From the Niobrara of Kansas. 



had a skull larger than that of a horse, and must have measured fully 

 twelve feet across the shell. 



Following the fashion of the day, the rhynchocephalians gave 



Fig. 414. — Champsosaurus, from the Laramie of Montana. 



feet. (After Brown.) 



Length, about six 



rise to a group of aquatic reptiles, by some considered of ordinal rank 

 (Choristodera) , represented in Europe and in North America by two 

 closely allied forms, Simoedosaurus and Champsosaurus (Fig. 414). 



