636 



REPTILIA. 



N.S. 



P. A. 



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 cer- 

 vical, dorsal, lumbar, 

 sacral, and caudal verte- 

 brae, all precocious except 

 the first two cervicals, 

 the two sacrals, and the 

 first caudal. In most 

 of the pre-cretaceous 

 Crocodilians, however, 

 the vertebrae were amphi- 

 coelous. The centra of 

 the vertebrae are united 

 by fibro-cartilages, and 

 the sutures between the 

 neural arch and the cen- 

 trum 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 par- 

 tially 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 pterygoids meet in the middle line of the roof of 



FIG. 349. Cervical vertebra of crocodile. 



N.S., Neural spine ; P. A., posterior articular pro- 

 cess ; A.A., anterior articular process; C./?., 

 cervical rib ; C., procoelous centrum. 



