REPTILES, BIRDS, AND MAMMALS 



443 



feet long, which lived in the Cretaceous seas. They had short, paddlelike 

 limbs, a powerful flattened swimming tail, and an immense mouth with 

 many sharp recurved teeth for seizing such slippery prey as fish. During 

 the Jurassic a group of crocodiles went to sea and developed paddlelike 

 legs and a fishlike tail, and other short-lived attempts at marine adapta- 

 tion were made by various reptiles; but the ichthyosaurs, plesiosaurs, 

 mosasaurs, and sea turtles were the most successful of the sea farers. 



The scaly reptiles. The lizards and snakes are the most abundant of 

 modern reptiles, represented in the tropical and warm temperate regions 

 by a host of genera and species. Although they are diapsids, they have 

 lost the lower of the two skull openings characteristic of the group; but in 



Fig. 28.7. Marine reptiles of the Jurassic. The long-necked plesiosaurs reached a length of 

 21 feet. The ichthyosaurs (right) were the most completely adapted to aquatic life of any 

 reptiles. (Drawing by Charles R. Knight, courtesy Chicago Natural History Museum.) 



other respects they are relatively primitive. The lizards (Fig. B.32) 

 retain the sprawling gait of the cotylosaurs, though some can run rapidly. 

 Most of them are small and lightly built, but some are large and heavy, 

 the Komodo monitor being the largest living species, with a length of 12 

 feet. The lizards first appeared in the Jurassic. During the Cretaceous 

 they gave rise to the marine mosasaurs described above and to the snakes. 

 In the latter the body has become much elongated and the legs have been 

 lost; 1 progression is accomplished by a sinuous twisting of the trunk aided 

 by the backwardly directed points of the scales. Some of the true lizards 

 have also lost their legs and become snakelike; but the snakes have a 

 further peculiarity in that the structure of the skull and the loosely 

 attached jaws permit very large prey to be swallowed without chewing. 

 Snakes are probably more numerous and varied today than at any time 

 in the past. The earliest were large, heavy snakes similar to their de- 



1 Except for rudiments of the hind legs in pythons and boas. 



