CHORDATA — VERTEBRATA — REPTILES 



357 



Stomach- stones (gastroliths) nearly always accompany com- 

 plete skeletons of the plesiosaurs, at least those from the 



Fig. 151. — An imaginary view along the margins of the sea which covered a large 

 portion of central and western North America during the Cretaceous time. In 

 the background, flying, are the leathery^ winged pterodactyls ; one is clinging to a 

 cliff by the claws upon its wings; swimming or upon land are three plesiosaurs, 

 with snake-like neck and head. In the foreground to the left are two examples of 

 the three-foot high diving bird, — Hesperomis ; to the right a mosasaur reptile, — 

 one of the Squamata. (From Williston.) 



Cretaceous of western North America. Within one large speci- 

 men from this region were one half bushel of polished stones, 

 ranging in size up to four inches in diameter. I' is known by 

 the bones and shells found in the region of that portion of the 

 body where the stomach must have been located that these 

 plesiosaurs lived upon such invertebrates as Scaphites, and such 

 vertebrates as fish and pterodactyls; they hence probably 



