78 WATER REPTILES OF THE PAST AND PRESENT 



nine cervical vertebrae. The long neck of birds is due both to an 

 increase in the length of the individual vertebrae and to an increase 

 in their number, to as many as twenty-one. But the elongation of 

 the neck among plesiosaurs was very variable indeed; sometimes 

 it was ten or twelve times the length of the head, at other times it 

 was even shorter than the head. And the number of bones com- 

 posing it was also extremely variable, scarcely any two species 

 having the same, the known extremes being seventy-six and 

 thirteen. In Elasmosaurus platyuriis, for instance, the longest- 

 necked plesiosaur known, the head was two feet in length, the 

 neck twenty-three, the body nine, and the tail about seven; on the 

 other hand, in the shortest-necked plesiosaur known, Brachaii- 



YiG. 32. — Skeleton of Triuacromcnim osbonii, a Cretaceous plesiosaur, as mounted 



in llu' UniNiTsily of Kansas Museum. 



chcnius Lucasi. the head was two and one-half feet in length, the 

 neck less than two feet, and the bod)- al)out five; the length of the 

 tail is unknown. 



Not only was the number of vertebrae so extraordinarily 

 increased in many plesiosaurs. but in the longest necks the verte- 

 brae themselves, as in birds, were more or less elongated, especially 

 the posterior ones, which may be six or seven times the length of 

 the anterior ones. Not only was the neck of such great length 

 in many plesiosaurs. but it also tapered very much toward the 



head. 



The vertebrae are always biconcave, but the cavities are shallow, 

 saucer-like, sometimes almost flat at each end, and ver\- different 

 from the conical fish-like cavities of ichthvosaurian vertebrae. 



