yio 



PHYLUM CHORDATA : CLASS REPTILIA 



The skin hears epidermic scales, underneath some of which 

 there are dermic bones or scutes. 



The tail is laterally compressed, and assists in swimming. 

 Teeth occur in distinct sockets in the premaxillce, maxilla, 

 and dentaries. 



In modern Crocodilians, almost all the vertebrce are 

 procoelous. 



The skull has many character- 

 istic features, such as the union 

 of maxillce, palatines, and ptery- 

 goids in the middle line on the 

 roof of the mouth, and the con- 

 sequent shunting of the posterior 

 nares to the very back of the 

 mouth. 



Some of the ribs have double 

 articulating heads, and bear small 

 uncinate processes ; transverse 

 ossifications form so-called ab- 

 dominal ribs. 



The heart is four-chambered ; 

 a muscular diaphragm partially 

 separates the thoracic from the 

 abdominal cavity. 



The cloaca has a longitudinal 

 opening. The males have a 

 grooved penis. 



The Crocodilians are oviparous. 

 The eggs have firm calcareous 

 shells, and are laid in holes in 

 the ground. 



Skeletal system. — Numerous 

 transverse rows of sculptured bony 

 plates or scutes, ossified in the dermis, form a dorsal shield. On the 

 ventral surface the scutes are absent, except in some alligators, in 

 which they are partially t)ssified. But besides and above the scutes, 

 there are horny epidermic scales like those in other Reptiles. The hide 

 is often used as leather. 



The vertebral column consists of distinct cervical, dorsal, lumbar, 

 sacral, and caudal vertebrae all proccelous except the first two cervicals, 

 the two sacrals, and the first caudal. In most of the pre-cretaceous 

 Crocodilians, however, the vertebrae were amphiccelous. The centra 

 of the vertebrae are united by fibro-cartilages, and the sutures between 



Fig. 419. — Lower surface of 

 skull of a young crocodile. 



p.mx., Premaxilla ; mx., maxilla ; 

 pal., palatine ; o.t., os trans- 

 versa m ; />^., pterygoid ; j.,jiigal; 

 ()j.,quadrato-jugal ; ., quadrate ; 

 p.n., posterior nares ; c, condyle. 



