156 WATER REPTILES OF THE PAST AND PRESENT 



concave in front and convex behind, like those of most modem 

 lizards and all modern snakes and crocodiles, but quite unlike the 

 biconcave vertebrae of all other aquatic reptiles. This kind of 

 articulation of the backbones gave greater firmness and strength 

 to the spinal column, but decreased the flexibiUty, and its posses- 

 sion by these animals was doubtless due to their descent from 

 land lizards which had already acquired it. The loss of flexibihty, 

 however, was partly compensated by the loss of the additional 

 articulating surfaces of the tail. 



As in all other aquatic reptiles, it is in the limbs that the most 

 striking characteristics of these water lizards ,or "sea-serpents" 

 are found. The legs were so completely adapted to an aquatic 



mode of living that the ani- 

 mals must have been practi- 

 cally helpless upon land, able 

 perhaps to move about in a 

 serpentine way when acci- 

 dentally stranded upon the 

 beaches, but probably never 

 seeking the land voluntarily. 

 The front limbs, like those of 

 all other swimming animals 

 having a powerful propelling 

 tail, were larger than the 

 hind ones, though not very much so. The bones of the first 

 two segments, that is, the arm, forearm, and thigh and leg 

 bones, were all short and broad, resembling those of the ichthyo- 

 saurs more than those of any other reptiles, save perhaps the 

 thalattosaurs, discussed below. The articular surfaces of all the 

 the limb bones, as in other aquatic animals, were restricted in extent, 

 indicating Umited motion between the joints, though doubtless 

 having great flexibility. In the most specialized tj^es, such as 

 Tylosaurus, the wrist and ankle bones were almost wholly carti- 

 laginous, just as they are in the water salamanders, and in whales 

 and porpoises. This tendency of the ends of long bones, the wrists 

 and ankles as well as other bones of the skeleton, to become more 

 cartilaginous, or less well ossified, in animals purely aquatic in 



Fig. 73. — Platecarpus; occipital view of 

 skull: bo, basioccipital; eo, exoccipital; pf, 

 postfrontal; st, stapes; pt, pterygoid; g, 

 quadrate. 



