SAURO PTERYGIA 



89 



those of the Kansas chalk, or the Jurassic of Wyoming, which 

 probably reached a length of nearly or quite fifty feet, of which the 

 neck formed about one-half. Some of them had paddles more than 

 six feet in length. The head of the largest was about five feet in 

 length, or about the size of that of the largest known ichthyosaurs 

 and mosasaurs. The smallest known adult plesiosaurs were nearly 

 ten feet in length. The teeth of the largest and most carnivorous 

 plesiosaurs sometimes measure four inches in length. 



As is the case with both the ichthyosaurs and mosasaurs, 

 skeletons of plesiosaurs have been discovered with nearly all their 



Fig. 42. — Restoration of Trinacromerum, a Cretaceous plesiosaur; length about 

 ten feet. 



bones in their relative positions, and with impressions of skin and 

 outlines of body made before decomposition. Though our knowl- 

 edge of the external appearance of the plesiosaurs when alive is 

 perhaps not as full as we could wish, it is sufficient to give us a fairly 

 good conception of what the animals really were. The skin was 

 smooth and bare, without scales or plates of any kind, and Dames 

 has described a terminal or nearly terminal fleshy dilatation of the 

 tail, forming a sort of caudal fin, which may have aided as a steering 

 apparatus. Mounted skeletons are preserved in a few museums, 

 notably the British Museum, the American Museum of New York 

 City, and the museum of the University of Kansas. Many nearly 



