232 PALEONTOLOGY 



muschelkalk predecessors of theliassic Plesiosawri were better 



organized for occasional progression on dry land. 



More than twenty species of Plesiosaurus have been de- 

 scribed 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. A comparison 

 of remains of various Plcsiosauri has led to a conviction, that 

 specific distinctions are accompanied with well-marked dif- 

 ferences in the structure and proportions of answerable verte- 

 brae, but are not shown in small differences of number in the 

 cervical, dorsal, or caudal vertebrae. 



When any region of the vertebral column presents an 

 unusual excess of development in a genus, such region is more 

 liable to variation, within certain limits, than in genera where 

 its proportions are more normal. The differences of the 

 number of cervical and dorsal vertebrae, ranging between 29 

 and 31 in the Plesiosaurus Hawkinsii, e.g. — as noted in the 

 description of that species in the writer's Report on British 

 Fossil Reptiles, 1839 — indicate the range of variety observed 

 in the only species of which, at that time, the vertebral column 

 of different individuals could be compared. 



Genus Pliosaueus, 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 Macrotrachelm, in which he 

 includes Simosaurus, Pistosaurus, and Nothosaurus, with 

 Plesiosaurus. No doubt the number of vertebras in the same 

 skeleton bears a certain relation to ordinal groups : the Oph idia 

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

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

 vertebr;e, characterizes equally certain Batrachians {CcrcHin) 

 and fishes {Mv/rcenciS. Certain regions of the vertebral column 

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

 ReptUia. We have Long-tailed and short-tailed lizards ; but 



