THE PLESIOSAUEIA. 215 



vertically elongated bone, narrower below than above, 

 wbere it becomes connected with the sacral ribs. In- 

 feriorly, it nrdtes with the pnbis and with the ischium, to 

 form the acetabxdum. The pnbes are very broad qxiadrate 

 bones, much larger than the ischia, and they meet in a 

 median symphysis. The ischia, triangular and expanded, 

 also unite in a ventral symphysis. The femur resembles the 

 humerus in its general form, although both its sides are 

 straighter, and the other bones of the hind limb are so like 

 those of the fore limb, as to need no special description. 



There can be little doubt that aU the bones of the limbs 

 were, like those of the Cetacea, enclosed within a common 

 sheath of integument, so as to form a paddle. 



Such is the general organization of the skeleton of the 

 Plesiosauria, which are long extinct animals, entirely con- 

 fined to the Mesozoic Rocks, from the Trias to the Chalk, 

 inclusive. They may be divided into two groups, according 

 as they are Triassic, or Post-Triassic, in age. 



The Post-Triassic group contaias the genera Plesiosaums 

 and PUosaurus, the different species of which appear to 

 differ in little more than the proportions of the head to the 

 trunk, and the relative length and degree of excavation of 

 the centra of the vertebra?. In the species which have been 

 named PUosaurus, the vertebrae are wide in proportion to 

 their length, and deeply excavated in front and behind. 

 PUosaurus attained gigantic dimensions, paddles of some 

 individuals reaching a length of not less than six feet. 



The Triassic genera, Nothosaurus, Simosatvrus, Pisto- 

 sa^irus (for a knowledge of the organization of which we are 

 chiefly indebted to the labours of Hermann Von Meyer), 

 appear to have differed from Plesiosaurus principally in the 

 following respects : — 



The connection of the neural arches with the centra of 

 the vertebrae seems to have been looser. The supra-tem- 

 poral fossae in the skull appear to have been larger in 

 proportion. In these animals, the under-surface of the 

 skull has the same structure as in Plesiosaurus, but appa- 

 rently lacks the posterior fossae; while there is no doubt 



