CROCODILIA. 583 



The skull has many characteristic features, such as the 

 itnion of maxilla, palatines, and pterygoids in the middle 

 line on the roof of the mouth, and the consequent 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 

 abdominal ribs. 



The heart is four-chambered ; a muscular diaphragm par- 

 tially 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 cal- 

 careous 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 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 amphiccelous. The centra of 

 the vertebra? 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 vertebrae. 



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 cartilaginous or partially ossified uncinate 

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

 to do with ribs, but are ossifications in the fibrous tissue which lies 

 under the skin and above the muscles. They form seven transverse 

 series, each composed of several ossicles. 



As to the skull, there is an interorbital septum with large alisphenoids ; 

 the presphenoid and orbitosphenoids are at best incompletely ossified ; 

 all the bones are firmly united by persistent sutures ; both upper and 

 lower temporal arcades are completely ossified ; the maxillae, the 

 palatines, and the pterygoid meet in the middle line of the roof of 

 the mouth, covering the vomers, and determining the position of the 

 posterior nares — at the very back of the mouth ; an os transversum or 

 transpalatine extends between the maxilla and the junction of palatine 

 and pterygoid ; a postorbital rod (epipterygoid or columella) is 

 formed by a downward process of the postfrontal meeting an upward 

 process from the jugal ; the quadrate is large and immovable ; there 

 are large parotic processes ; the tympanic cavity is completely bounded 

 by bone ; the teeth, which are borne by premaxillas, maxillae, and 



