STRUCTURE OF REPTILES. 491 



boas, and Tortrices, the pelvis exists in a rudimentary state, 

 and attached to it is a pair of rudimentary hind legs ending 

 in claws ; in all other existing reptiles the limbs are directly 

 comparable with those of birds and mammals, the bones of 

 the legs being best developed in the Chelonians (turtles), 

 which have nine carpal bones and five digits in each foot. 

 Certain extinct saurians had paddle-like limbs, others bird- 

 like limbs, and still others approached the crocodilian type, 

 in which the carpal bones and phalanges become reduced in 

 number. In the hind limbs an intermedium (in birds only 

 present in the embryo) is united with the tibiale bone to 

 form an astragalus or heel-bone. 



The scales of reptiles are very characteristic, though scales 

 existed on the underside of the body of most Stegocephalous 

 Batrachia. The scales of lizards and snakes are developed 

 from the cutis. The large horny plates of Chelonians are 

 greatly developed and unite above with the "ribs" to form 

 the shell or carapace, while nine large plates below form 

 the plastron. 



The teeth are simple, conical, and while in the lizards 

 and snakes they may exist on the palatine and pterygoid 

 bones, in the crocodiles, where they are implanted in sockets 

 of the jaw-bones, they are, as in the mammals, confined to 

 the maxillary bones. The teeth are said to be acrodont when 

 situated on the edge, or pleurodont if on the side of the jaw, 

 or thecodont if inserted in sockets. There is a middle and 

 internal ear much as in birds. The New Zealand lizard, 

 Hatteria, is the only reptile which has the beginning of a spiral 

 turn indicated in its cochlea, which in other reptiles is, as in 

 birds, merely a flask-shaped cavity. (Rolleston.) The eyes of 

 reptiles approach those of birds, and in both there is an upper 

 and a lower movable eyelid besides a nictitating membrane.* 



True nostrils exist in reptiles for the first time among Ver- 

 tebrates, and may be closed like the ears by cutaneous valves. 



The tongue is either not extended out of the mouth, and 

 is broad, as in turtles and crocodiles and some lizards, or as 

 in most lizards and all snakes it is long, slender, forked, and 

 can be darted rapidly out of the mouth. 



* Iu many reptiles there is a median rudimentary, " pineal " eye. 



