THE JURASSIC PERIOD. 



89 



dependent on the paddles, though a fin-like adaptation of the tail is some- 

 times observed. Their movements were hence probably slow. The 



Fig. 366. — Outline and skeleton of Ichthyosaurus quadriscissus. (After Jaekel.) 



elongation of the neck was variable, some even being short, while 

 the more typical forms were very long. The vertebrae of the neck 

 ranged from 13 to 76, the last being more than any other animal, living 

 or extinct, is known to have possessed (Williston). The neck appears 

 not to have been as flexible as familiar illustrations have represented 

 it, nor were the jaws separable and extensible as in the case of snakes. 



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Fig. 367. — Skeleton of Plesiosaurus dolichodeirus Conyb. (Restored by Conybeare.) 



This implies that they either lived on small prey, or tore their food 

 to pieces before swallowing. They were doubtless formidable foes of 

 the smaller sea life, but probably not of the larger. Like the ichthyo- 

 saurs, they were covered with smooth skins unprotected by scales or 

 scutes. They ranged from 8 to 40 or more feet in length. They had 

 the singular habit of swallowing and retaining in the stomach, 

 small stones, " gizzard stones," the purpose of which has given rise 

 to much speculation and discussion. As some of these stones must 

 apparently have been picked up far from the final resting-place of 

 the skeleton, it is inferred that the plesiosaurs were wide rovers of 

 the seas. Williston regards them as solitary in habit, while he thinks 

 the ichthyosaurs were gregarious, somewhat like the dolphin. The 

 distribution of the plesiosaurs seems to have been world-wide, and 

 the species were numerous. 



