226 S. W. Williston — Worth American Plesiosaurs. 



not all, motility was confined. The total length of the neck 

 in this species in life, allowing six millimeters only for the 

 thickness of the interarticular cartilages between adjacent 

 vertebrae, was exactly twenty-three feet. A close approxima- 

 tion to the length of the trunk is nine feet ; of the tail, eight 

 feet. The length of the head, using E. snowii for comparison 

 (and the remains of the type preserved show that there must 

 have been great resemblance between these two forms in the 

 skull), was less than two feet. The entire length of the 

 animal in life, then, was a little over forty-two feet, an esti- 

 mate somewhat less than that reached by Cope. Other speci- 

 mens referred to this genus exceeded these dimensions very 

 materially, indicating, if their proportions were alike, an 

 extreme length of not less than sixty feet. The elasmosaurs 

 doubtless were the longest, if not the largest, of all known 

 marine reptiles. 



In the extreme elongation of the neck, Elasmosaurus 

 exceeded all other vertebrated animals of the past or present, 

 and was, if we assume a diphyletic origin of the short-necked 

 forms, the most specialized of all plesiosaurs, since in no other 

 genus do we know of any species having as many as fifty cer- 

 vical vertebrae. But I am rather of the opinion that the 

 short-necked types were descendants of earlier and longer- 

 necked forms. Unless this is the case, we know of no early 

 plesiosaurs which could have been the progenitors of such 

 forms as Polycotylus with twenty-six cervical vertebrae, or 

 Brachauchenius with as few as thirteen. In their paddles, 

 the elasmosaurs were very generalized, in that the epipodials 

 were scarcely broader than long, and their number is but two. 

 In the clavicular arch, Elasmosaurus was specialized, while in 

 the coracoids it seems to have retained generalized characters. 



As to the habits of these long-necked plesiosaurs in life, I 

 am satisfied that they were in general scavengers, living 

 largely in shallow waters, as well as often out at sea. Numer- 

 ous remains were found the past season in Wyoming, in the 

 Upper Cretaceous, associated with longirostral and brevirostral 

 amphicoelian crocodiles, dinosaurs, and lepidosteal fishes, as 

 well as with turtles of marsh or fresh-water habit. And 

 especially noticeable was the large number of immature or 

 quite young animals found in these deposits. 



It was with a specimen of an elasmosaur (E. snowii) 

 that Mudge first noticed the occurrence of the peculiar 

 siliceous pebbles which he described ; and it was also with 

 another, a large species yet unnamed,' from the Benton Cre- 

 taceous, that the like specimens were found described by me 

 in 1892. That this habit was not confined to this type of 

 plesiosaur, however, is certain, since I have also observed it 



