' THEORY OF THE EARTH. 257 



vertebrae had more of the forms and articulations 

 of the lizards ; but what distinguished it from all 

 oviparous and viviparous quadrupeds, was a slen- 

 der neck as long as its body, composed of thirty 

 and odd vertebras, a number greater than that 

 of the neck of any other animal,, rising from the 

 trunk like the body of a serpent, and termina- 

 ting in a very small head, in which all the essen- 

 tial characters of that of the lizard family are ob- 

 served. 



If any thing could justify those hydras and 

 other monsters, the figures of which are so often 

 presented in the monuments of the middle ages, 

 it would incontestibly be this plesiosaurus. * 



Five species are already known, of which the 

 most generally distributed (P. dolichodeirus) at- 

 tains a length of more than twenty feet. 



A second species (P. recentior), found in more 

 modern strata, has the vertebras flatter. 



A third (P. carinatus) shews a ridge on the 

 under surface of its vertebrae. 



A fourth, and lastly a fifth (P. pentagonus 

 and P. trigonus], have the ribs marked with five 

 and three ridges, f 



These two genera are found everywhere in the 



* Researches, &c. vol. v. part if. p. 475, et seq* 

 t Researches, vol. v, part ii. p. 485 and 486. 



R 



