RMYNCEOCEPHALIA 183 



nessed the extinction of the great dinosaurs and plesiosaurs, but 

 the advent also of the early placental mammals. They Hved mil- 

 lions of years after the proganosaurs became extinct, and, similar 

 as they are in form, there is no relation between them. Moreover, 

 in all probability they did not migrate to the eastern continent over 

 the same route. 



The structure of the head and teeth of the Choristodera clearly 

 indicates a fish-eating habit, or at least a diet of soft-bodied, free- 

 swimming invertebrates. The legs and ribs, as also the armor of 

 ventral ribs, like those of the plesiosaurs, point very insistently 

 toward a bottom-crawhng habit while in the water. 



