SAUROPTERYGIA 87 



broad bones in place of the elongated forearm and leg bones of the 



land reptiles. Not only are these bones much broader than they 



are long, but there have been developed additional bones back of 



them in the same row — new bones which have no counterpart in 



any terrestrial reptiles. In the first of the three figures is shown 



a hind paddle of one of the earliest known plesiosaurs, Thauma- 



tosaurus, from the lower part of the Jurassic of Germany. It will 



be seen here that the tibia and fibula are much more elongated than 



in Trinacromerum, and much more like the leg bones of land reptiles. 



A still more primitive stage in the evolution of the swimming paddle 



of the plesiosaurs will be seen in Fig. 48 on p. 99, the possibly 



ancestral, amphibious nothosaur. Here the tibia and fibula, while 



relatively very much shorter than in any land reptile, still have, 



together with all the other 



bones of the leg, a terrestrial 



or amphibious type. In Fig. 



39 is seen the front paddles 



of the long-necked Elasmo- 



saurus, which, though one of 



the latest of all plesiosaurs in _ c1 „ , „, ., 



r tig. 40. — Skull of Elasmosaurus trom the 



geological history, has the s ide: p m> premaxilla; m, maxilla; po, post- 

 structure of its paddles some- orbital; j, jugal. 

 what intermediate between 



that of the earlier Plesiosaurus and the later Trinacromerum. 

 The skull of the long-necked plesiosaurs is surprisingly small 

 in comparison with the remainder of the skeleton, often very 

 snake-like in shape, though very un-snake-like in structure. The 

 short-necked plesiosaurs had often a relatively larger skull, mPlio- 

 saurus, for instance, more than five feet long, sometimes rather 

 broad and short, sometimes remarkably long and slender. The 

 external nostrils were situated far back, very near the eyes, and 

 were very small. The eyes, of considerable size, though by no 

 means so large as those of the ichthyosaurs, were directed laterally, 

 and were provided with a ring of bony sclerotic plates — rather 

 small and weak ones, however. The quadrate bones — bones pecu- 

 liar to the reptiles and birds — to which the lower jaws are articu- 

 lated, are, as in the ichthyosaurs and crocodiles, rigidly fixed and 



