8o 



WATER REPTILES OF THE PAST AND PRESENT 



Often the vertebrae are short throughout the vertebral column; 

 sometimes the posterior cervicals and the dorsals are elongated 

 and very robust. The trunk or body proper was never much 

 elongated in the plesiosaurs, having only from twenty-five to thirty 

 vertebrae. The taij was always shorter than the trunk, and it 

 tapered rapidly to the extremity; in some specimens it has been 

 observed to turn up slightly near the extremity, as though for the 

 support of a small terminal fin. 



The ribs in the cervical region are short, but so locked together 

 posteriorly as not to^ permit much lateral motion. They are 



«z^x 



Fig. 34. — Cen-ical vertebrae, from the side and behind, and dorsal vertebra from 

 in front of Polycotyliis, a Cretaceous plesiosaur: az, anterior zygapophysis; pz, pos- 

 terior zygapophysis, r, r, r, cervical ribs; d, articulation of dorsal rib. 



sometimes double-headed in the neck, sometimes single-headed, but 

 both heads when present articulate or are attached to the body of 

 the vertebrae, distinguishing them at once from those of other 

 animals, except the ichthyosaurs. In the dorsal region the ribs 

 are attached high on the arch to the extremity of the stout trans- 

 verse processes by a single head, very much as they are in some 

 cetaceans, and quite unlike the condition in any other known 

 reptile. They end freely below, having no attachment to a 

 breast bone or other bony parts. Because of their shape and 

 position as frequently found, the body in life must have been fat- 

 tened from above downward, and broad; indeed, this shape is 



