2462 TORTOISES, TURTLES, AND PLESIOSAURS 



to seize passing fishes in their jaws. In the groups mentioned the head was com- 

 paratively small, but in the huge pliosaurs {Pliosaurus) of the upper Oolitic strata 

 the skull was of enormous size, attaining in some instances a length of six feet, and 

 the neck proportionately short and thick. Their teeth had more or less triangular 

 crowns, and in some cases, inclusive of the root, measured quite a foot in length. 



As in the case with all the higher aquatic Vertebrates, there is evidence to 

 show that the plesiosaurs were originally derived from land animals; the representa- 

 tives of the group found in the earlier (Triassic) Secondary rocks having limbs de- 

 parting much less widely from the ordinary type, and bearing claws at the extremities 

 of their digits. In the small lariosaur, which measured about a yard in length, the 

 limbs appear to have been somewhat intermediate in structure between the clawless 

 paddles of the true plesiosaurs and those of more ordinary reptiles; and the 

 creatures were probably amphibious in their habits, spending part of their time on 

 laud, and part in the water. In the allied nothosaurs and simosaurs the limbs 

 were better adapted for walking, from which we may infer that their owners were 

 still more terrestrial in their habits. 



