252 PALEONTOLOGY. 



more normal. Specific characters are afforded by the propor- 

 tions of the vertebral centra, by the relative size of the cervical 

 ribs, by the relative position, shape and prominence of the 

 costal articular surfaces ; by the flatness or concavity 

 of the terminal articular surfaces ; by the relative length of 

 the neck relating usually to a larger or smaller size of head ; 

 by the structure and relative size of the fore and hind paddles. 

 More than twenty species of Plesiosaurus have been described 

 by, or are known to, the writer ; their remains occur in the 

 oolitic, Wealden, and cretaceous formations, ranging from the 

 lias upwards to the chalk, inclusive. 



Genus Pliosaurus, Ow. — M. von Meyer regards the num- 

 ber of cervical vertebrae and the length of neck as characters 

 of prime importance in the classification of Reptilia, and 

 founds thereon his order called Macrotrachelen, in which he 

 includes Simosaurus, Pistosaurus, and NotJiosaitrus, with 

 Plesiosaurus* No doubt the number of vertebrae in the same 

 skeleton bears a certain relation to ordinal groups : the Ophidm 

 find a common character therein ; yet it is not their essential 

 character, for the snake-like form, dependent on multiplied 

 vertebrae, characterizes equally certain Batrachians (Ccecilia) 

 and fishes (Murwna). Certain regions of the vertebral column 

 are the seats of great varieties in the same natural group of 

 Reptilia. We have long-tailed and short-tailed lizards ; but 

 do not therefore separate those with numerous caudal verte- 

 brae, as " Macroura," from those with few or more. The 

 extinct Dolichosaurus of the Kentish chalk, with its proccelian 

 vertebrae, cannot be ordinarily separated, by reason of its 

 more numerous cervical vertebrae, from other shorter-necked 

 proccelian lizards. As little can we separate the short-necked 

 and big-headed amphicoelian Pliosaur from the Macrotrache- 

 lians with which it has its most intimate and true affinities. 



There is much reason, indeed, to suspect that some of the 



* "Die Saurier des Musclielkalkes," fol. 1847-55. 



