26 COMPARATIVE ANATOMY 



articulates with the atlas vertebra by means of a single occipital condyle. 

 Arterial and venous blood are mixed in the dorsal aorta. 



Living reptiles are divided into Rhynchocephalia, Lacertilia, Ophidia, 

 Chelonia, and Crocodilia. Among fossil orders, the Theromorpha are 



SPHENODON 



Fig. 20. — Sphenodon has been characterized as a "Uving fossil." As a "primi- 

 tive" type of reptile it interests the student of phylogenesis. It belongs to the Order 

 Rhynchocephalia. 



important, since, especially in their dentition, they resemble mammals, 

 and the Dinosaurs because they are the ancestors of the Birds. 



Class Aves 



Birds differ from reptiles in having both feathers and scales, and in 

 having the anterior appendages modified as wings. The heart is four- 

 chambered, and the single aortic arch on the right. Teeth are wanting 

 in modern forms. The body temperature is higher than in other animals. 



Two large divisions are recognized, the flying birds or Carinatae with 

 a keeled sternum, and the running birds or Ratitae which have no keel 

 on the sternum. 



Class Mammalia 



Mammals are vertebrates with hairs and mammary glands. A few, 

 the monotremes, lay eggs, but all the rest bring forth their young well 

 developed. Mammals have a pair of occipital condyles, a muscular 

 diaphragm, and a chain of three ear bones. The heart is four-chambered, 

 and the aortic arch is on the left. The jaw articulates between the 

 dentary and squamosal bones. 



Two major divisions are recognized, placentals, the embryos of which 

 are attached to the mother by a vascular placenta; and the non-placentals, 

 the monotremes and marsupials, most of which lack a placenta. 



Sub-Class Monotremes (Prototheria) 



The monotremes or ornithodelphia are egg-laying mammals with a 

 cloaca. Teats are lacking. 



