THE AGE OF REPTILES 



55 



various museums. The dinosaurs include many genera of all three 

 suborders, varying in size from that of a cat to some of the largest 

 known land animals. Of other reptiles a very few jaws of a true 

 rhynchocephalian, a fragment of a wing bone of a pterodactyl, 

 numerous turtles, and crocodiles, only, are known. The beds are 

 predominantly black-clay shales, intercalated with sandstones, and 

 all are of fresh-water origin. 



From beds definitely known as Lower Cretaceous (Trinity) 

 in Oklahoma, a few bones of a sauropod dinosaur are known, and 



Fig. 30. — Restoration of Casea, a theromorph reptile from the Permian of Texas, 

 about four feet long. 



from nearly corresponding rocks in southern Kansas, plesiosaurs, 

 crocodiles, turtles, and carnivorous dinosaurs are known from 

 sparse remains. Doubtless the Potomac beds of Virginia, which 

 have yielded bones of various dinosaurs, are also of Lower 

 Cretaceous age. 



With the exception of a single vertebra of doubtful affinities 

 and the cast of a turtle-shell no vertebrate fossils have ever 

 been discovered in the extensive sandstones of Dakota age, the 

 lowermost of the Upper Cretaceous. From the next horizon above 

 the Dakota, the Benton Cretaceous, chiefly marine limestones, at 



