CHAPTER V. 



XUE CLASSIFICATION AND THE OSTEOLOGY OF THE EEPTILIA. 



The province Sauropsida is divisible into the two classes, 

 Reptilia and Aves. 



All Reptilia, so far as tlieir organization is known to us, 

 are distinguished from Aves by the following characters : 



1. The exoskeleton is composed of horny plates (scales), 

 or bony plates (scutes), never of feathers. 



2. Tlie centra of the vertebrae may be amphicoelous, pro- 

 coelous, opisthocoelous, or may have nearly flat articular faces ; 

 but these faces are spheroidal or oval, and are never cylin 

 droidal, even in the cervical region.* 



3. When reptiles possess a sacrum, the sacral vertebra3 

 have large expanded ribs, with the ends of which the ilia 

 articulate. 



4. The sternum is rhomboidal ; and, when many ribs are 

 connected with it, the hindermost of these are attached to a 

 single, or double, median backward prolongation (except, per- 

 haps, in the Pterosaiiria). The sternum may be converted 

 into cartilage-bone, but (with the possible exception of the 

 Pterosauria) is never replaced by membrane-bone, and does 

 not ossify from two, or more, definite centres. 



5. When an interclavicle exists, it remains distinct from 

 the clavicles. 



6. The manus contains more than three digits (? Dino- 

 sauria), and the three radial digits, at fewest, have claws. 



7. In all existing reptiles, the ilia are prolonged farther 

 behind the acetabulum, than in front of it ; and the inner wall 

 of the acetabulum is wholly, or almost completely, ossified. 

 The pubes are directed downward and forward, and, like the 



* The articular faces of the vertebrse of some Pterosauria are very much 

 elongated transversely. 



