THE JURASSIC PERIOD. 251 



the sea. Some of the larger specimens of Ichthyosaurus which 

 have been discovered in the Lias indicate an animal of from 

 20 to nearly 40 feet in length ; and many species are known to 

 have existed, whilst fragmentary remains of their skeletons are 

 very abundant in some localities. We may therefore safely 

 conclude that these colossal Reptiles were amongst the most 

 formidable of the many tyrants of the Jurassic seas. 



The Plesiosaurus (fig. 177) is another famous Oolitic 

 Reptile, and, like the preceding, must have lived mainly or 

 exclusively in the sea. It agrees with the Ichthyosaur in some 

 important features of its organization, especially in the fact 

 that both pairs of limbs are converted into " flippers " or 

 swimming-paddles, whilst the skin seems to have been equally 

 destitute of any scaly or bony investiture. Unlike the Ichthy- 



Fig. Ill . Plesiosaurus dolichodeirus , restored. Lias. 



osaur, however, the Plesiosaur had the paddles placed far back, 

 the tail being extremely short, and the neck greatly lengthened 

 out, and composed of from twenty to forty vertebrae. The 

 bodies of the vertebrae, also, are not deeply biconcave, but are 

 flat, or only slightly cupped. The head is of relatively small 

 size, with smaller orbits than those of the Ichthyosaur, and with 

 a snout less elongated. The jaws, however, were armed with 

 numerous conical teeth, inserted in distinct sockets. As re- 

 gards the habits of the Plesiosaur, Dr. Conybeare arrives at the 

 following conclusions : " That it was aquatic is evident from 

 the form of its paddles; that it was marine is almost equally 

 so from the remains with which it is universally associated; 



