292 CLASSIFICATION OF VERTEBRATES. 



characteristic of the sauropsida. The skin is remarkably defi- 

 cient in glands, these, when present, usually occurring upon the 

 legs or upon the tail. The characteristic scales are comifica- 

 tions of the epidermis, and are occasionally re-enforced by bony 

 plates developed in the dermis. The single occipital condyle is 

 situated on the basioccipital, the exoccipitals contributing to its 



pao 



Fig. 291. Base of skull of alligator, showing the single occipital condyle, bo, 

 basioccipital ; bs, basisphenoid ; eo, exoccipital ; el, opening of Eustachian tube ; fm, 

 foramen magnum ; pao, paroccipital ; //, pterygoid ; q, quadrate ; qj, quadratojugal ; 

 j^, squamosal ; /r, transversum. 



formation to a varying extent. The mandible consists of a single 

 cartilage bone, the articulare, and at most five membrane bones, 

 — dentary, splenial, coronoid, angulare, and surangulare. The 

 cer\'ical ribs are usually well developed, the neck passing insen- 

 sibly into the thorax. The ovarian ducts have their inner ends 

 entire as in the ichthyopsida. 



The sauropsida contains the Reptilia and the Aves. 



SUB-CLASS I. REPTILIA. 



Cold-blooded amniotes ; the external surface of the body 

 (except in a few fossil forms) covered with horny epidermal 

 scales or bony dermal plates ; anterior appendages, when present, 

 ambulatory (except in pterodactyls), the carpals and meta- 

 carpals numerous ; sacral vertebrae usually two ; pubic and is- 

 chiadic bones united by symphysis, except in some dinosaurs ; 

 persisting right and left aortic arches. 



