EXTINCT OEDEES OF REPTILES. 



219 



and were inserted into distinct sockets, and not lodged in a 

 common groove. 



The most familiar and typical member of the Baiiro'pterygia 

 is the genus Plesiosaurus (fig. 567), comprising gigantic 



Fig. .567. — Plesiosaiinis dolicliodciriis. Lias. 



marine reptiles, chiefly characteristic of the Lias and Oolites. 

 In its general structure, there are various points in which 

 Plesiosaurus makes a near approach to Ichthyosaurus, while, 

 on the other hand, there are equally striking- 

 points of difference between the two genera. 

 Thus, in both, the pelvic and pectoral limbs 

 have the digits enveloped in the integument, 

 and the fore-arm and arm much shortened, 

 these organs thus being reduced to the 

 condition of efficient swimming-paddles or 

 " flippers." In both the skin is naked, 

 and the snout is prolonged, the jaws being 

 furnished with numerous teeth. In both 

 there is an absence of a sternum and of 

 sternal ribs, but the walls of the abdomen 

 are strengthened by supplementary ossifi- 

 cations, each consisting of a central and of 

 lateral overlapping pieces. On the other 

 hand, the head in Plesiosaurus is com- 

 paratively small, and the neck is quite disproportionately 

 elongated, while the tail is short. The orbits are not ex- 





Fig. 568.— Left fore- 

 paddle of Plesiosaurus. 

 a, Humerus ; &, Radius ; 

 c, Ulua. 



