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CHAPTER Y. 



THE CLASSIFICATION AND THE OSTEOLOGY OF THE 

 EEPTILIA. 



The province Sauropsida is divisible into the two classes. 

 Beptilia and Aves. 



All Eeptilia, so far as their organization is known to us. 

 are distinguished from Aves by the following characters : — 



1. The exoskeleton is composed of homy plates (scales). 

 or bony plates (scutes), never of feathers. 



2. The centra of the vertebrae may be amphiccelous, pro- 

 ccelous, opisthocoelous, or may have nearly flat articular 

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

 cylindi'oidal, even in the cervical region.* 



3. When reptiles possess a sacrum, the sacral vertebra; 

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

 articulate. 



4. The sternum is rhomboidal ; and, when many ribs lu-e 

 connected with it, the hindermost of these are attached to 

 a single, or double, median backward prolongation (except, 

 perhaps, in the Pterosauria). The sternum may be con- 

 verted into cartilage bone, but (with the possible exception 

 of the Pterosauria) is never replaced by membrane bon^-. 

 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 (? D'mu- 

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



* The articular faces of the are very much elongated traiis- 

 vertebrse of some Pterosauria versely. 



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