1 8 CHORDATE ANATOMY 



bone is attached to the cranium as in mammals; the zygomatic arch of the 

 skull resembles that of mammals. 



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 vas- 

 cular placenta; and the Non-Placen- 

 tals, the monotremes and marsupials, 

 most of which lack a placenta. 



ORNITHORHYNCHUS. 



Fig. 20. — Ornithorhynchus is a repre- 

 sentative of the most primitive group of 

 mammals, the monotremes. As an 

 egg-laying mammal it bridges over the 

 gulf separating reptiles and mammals. 



Fig. 21. — Opossum, the typical genus of 

 didelphians. (Redrawn after Newman.) 



Sub -Class Monotremata (Prototheria) 



The monotremes or ornithodelphians are egg-laying mammals with a 

 cloaca. Teats are lacking. 



Ornithorhynchus, the duck-bill of Australia, is the best-known genus; 

 and there are two species of the spiny anteater. Echidna. There seem to 

 be no more than a half-dozen species surviving for the entire sub-class.- 



