THE CKOCODILIA. 217 



between rougH sutural surfaces furnished by the neural arch 

 above and the centrum below. 



The first caudal vertebra is biconvex, but all the others are 

 procoelous ; those of the anterior moiety of the tail have long 

 ribs fixed in between the neural arches and centra, as in the 

 sacrum, and becoming anchylosed in that position. Chevron- 

 bones are attached to the posterior edges of the centra of the 

 vertebrae, except that of the first, and those of the posterior 

 part of the tail. 



From seven to nine of the anterior dorsal ribs are united 

 with the sternum by sternal ribs, the form of which varies a 

 good deal in different GrocodlUa^ being sometimes narrow, 

 sometimes broad and flattened. An elongated plate of carti- 

 lage, which may be partially converted into cartilage-bone, is 

 attached to the hinder margin of several of the most anterior 

 ribs, above the junction between the ossified and the cartilagi- 

 nous part of the vertebral rib. (Fig. 5, P.u.) These are the 

 so-called " uncinate processes," which also exist in Hatteria^ 

 and reappear in Birds. 



The sternum consists of a rhomboidal plate of cartilage- 

 bone, with the posterolateral edges of which two pairs of 

 sternal ribs articulate. The posterior angle of the plate is con- 

 tinued into a median prolongation, which, at length, divides 

 into two curved divergent cornua. From five to seven pairs 

 of sternal ribs are united with the prolongation and its cornua. 

 A long and slender interclavicle lies in a groove of the middle 

 of the ventral face of the rhomboidal part of the sternum. 



In the ventral wall of the abdomen, superficial to the recti 

 muscles, lie seven transverse series of membrane-bones, which 

 are termed " abdominal ribs ; " though it must be recollected 

 that they are quite distinct from true ribs, and rather corre- 

 spond with the dermal ossicles of the Lahyrinthodonta. Each 

 series is composed of four elongated and more or less curved 

 ossicles, pointed at each end, and so disposed that inner ends 

 of the inner pair meet at an angle, open backward in the 

 middle line, while their outer ends overlap the inner ends of 

 the outer pair. The most posterior of these ossicles are 

 stronger than the others, and are closely connected with the 

 pubic cartilages. 



In the Crocodilian skull the following are the chief pecu- 

 liarities which are worthy of especial notice : 



1. There is an interorbital septum, and the presphenoidal 

 and orbitosphenoidal regions remain cartilaginous, or verT 

 incompletely ossified. 

 10 



