372 REPTILIA. 



through the water, presenting a striking contrast to the organi- 

 sation which so admirably fits the Ichthyosaurus to cut through 

 the waves." As its respiratory organs were such that it must 

 of necessity have required to obtain air frequently, we may 

 conclude " that it swam upon or near the surface, arching 

 back its long neck like a swan, and occasionally darting it 

 down at the fish which happened to float within its reach. It 

 may perhaps have lurked in shoal water along the coast, con- 

 cealed amongst the sea-weed ; and raising its nostrils to a 

 level with the surface from a considerable depth, may have 

 found a secure retreat from the assaults of powerful enemies ; 

 while the length and flexibility of its neck may have com- 

 pensated for the want of strength in its jaws, and its incapacity 

 for swift motion through the water." 



The geological range of the Plesiosaurus is from the Lias to 

 the Chalk inclusive, and specimens have been found indicat- 

 ing a length of from eighteen to twenty feet. 



About twenty species of Plesiosaurus have been described 

 in all. Of the remaining genera of the Sauropterygia, Notho- 

 saurus, Simosaurus, Placodus, Pistosaurus, and Conchiosaurus 

 are Triassic. In Nothosaurus the neck is long, composed of 

 at least twenty vertebrae. The dorsal vertebrae are biconcave, 

 and the limbs are converted into swimming-paddles. The 

 teeth are numerous and conical, and are implanted into dis- 

 tinct sockets. Several species are known, all Triassic, and 

 especially characteristic of the Muschelkalk. Shnosaurus had 

 a large head with enormous orbits, and teeth sunk into dis- 

 tinct sockets. This genus is exclusively confined to the Mus- 

 chelkalk. In Placodus (fig. 322), the teeth are in distinct 

 sockets, and resemble those of 

 many fishes in being round- 

 ed and obtuse, forming broad 

 crushing plates adapted for the 

 comminution of shell-fish. The 

 upper jaw contains a double 

 series of these teeth, an outer 

 or maxillary series, and an in- 

 ternal or palatal series ; but 

 the under jaw has only a sin- 

 gle row of teeth. 



Lastly, in Pliosaurus we 



have a hu e e re P. tile ' allied to 



the Plesiosaurus in its fin-like 

 paddles, but having an enormous head supported upon a short 

 neck. The teeth are simple and conical, and in large speci- 



