CHARACTERISTICS OF CROCODILIANS. 483 



The skin bears epidermic scales, and underneath some of 

 these there are bony dermic scutes. 



The tail is laterally compressed and assists in swimming. 



Teeth occur in distinct sockets in the premaxillae, maxillae, 

 and dentaries. 



In all modern Crocodilians, almost all the vertebrae are 

 procoelous. 



The skull has many characteristic features, such as the 

 union of maxillae, palatines, and pterygoids in the middle 

 line on the roof of the mouth, and the consequent position 

 of the posterior nares at the very back of the mouth. 



Some of the ribs have double articulating heads, and bear 

 uncinate processes somewhat like those of birds ; transverse 

 ossifications associated with the sub-cutaneous fibrous tissue 

 of the abdomen form so-called abdominal 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. 



Some of the Characteristic Features in the Skeletal System oj 

 Crocodilians. 



(These notes on the skeleton are in great part taken from 

 Huxley's Manual.) 



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 ossified. 

 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 procoelous except the first two cervicals, 

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

 Crocodilians, however, the vertebrae were amphicoelous. The centra of 

 the vertebrce are united by fibro-cartilages, and the sutures between the 

 neural arch and the centrum persist at least for a long time. Chevron 

 bones are formed beneath the centra of many of the caudal vertebrje. 



Many of the ribs have two heads — capitulum and tubercle — by which 

 they articulate with the vertebrae. From seven to nine of the anterior 

 dorsal ribs are connected with the sternum by sternal ribs, and from 

 several of these anterior ribs cartil^nous or partially ossified uncinate 

 processes project backwards. The so-called abdominal ribs have nothing 



