The Classification of Animals. 207 



great division with the reptiles, which is then called Sauropsida. 

 They agree with them in the single condyle of the skull, and 

 in the fact that the ankle-joint is in the middle of the tarsus, 

 and not between the tarsus and the tibia and fibula, as in other 

 Vertebrates. Birds, however, differ from reptiles in the peculiar 

 modification of the fore-limb to form the wing (see above), in 

 the presence of feathers, which is itself sufficient to define 

 birds. The blood is hot ; the heart is four-chambered. The 

 bones and viscera are permeated with air from the air-sacs, 

 which are prolongations of the lungs, and are developed to an 

 extent never found in reptiles, with the possible exception of 

 the extinct Dinosaurs (cf. p. 205). 



Sub-class 6. Mammalia. — The Mammals are separated from 

 all other Vertebrates by a number of important points. They 

 differ from all in the presence of mammary glands, with which 

 the young are suckled, and by the hairy covering which 

 is rarely nearly or quite absent, as in the whales. In all 

 Mammals a complete muscular septum, the diaphragm, sepa- 

 rates a cavity containing the heart and lungs from the cavity 

 in which the intestine, liver, and the rest of the viscera, lie. 

 These three characters — or, indeed, any one of them alone — 

 is amply sufficient to define a Mammal. 



The Mammalia are commonly divided in the first place into 

 three different divisions : the Frofofheria, Metatkeria, and 

 Eutheria. 



To the Prototheria belong only the Platypus and the Echidna., 

 both Australian, and distinguished by the fact that they lay 

 large-yolked eggs, that they possess a well-developed coracoid, 

 and a cloaca. The mammary glands open on to the skin, and 

 are enclosed by a pouch, which really represents the teats, not 

 drawn in into a teat-like form. 



The Metathena are the marsupials, the kangaroos, wallabies, 

 native bear, etc., which have, for the greater part, a well- 

 developed pouch, into which the separate teats open, and in 

 which the young are carried. The young of the marsupials 

 are born in a very imperfect condition. The egg, however, is 

 minute, like that of the Eutheria. There is just a vestige of the 

 cloaca remaining. 



