INTRODUCTORY. 



MAMMALIA are vertebrate * air-breathing animals, more or less 

 clothed externally with hair ; the females are provided with 

 mammary or milk-glands, and the young are brought forth 

 alive, with the exception of the Australian Ornithorhynchus 

 and Echidna, which are oviparous. Their limbs are usually four 

 in number, the hinder pair being, however, sometimes either mo- 

 dified into swimming-paddles or suppressed altogether, while the 

 anterior are in some cases developed into wings, and in others into 

 nippers. The tail may be quite rudimentary, as in Man and the 

 higher Apes ; long, simple, and forming an apparently useless 

 appendage, as in Cats ; prehensile f, as in the American Monkeys 

 and Opossums ; provided with a long tassel for driving away insects 

 from the skin, as in Elephants, Cattle, &c. ; or, finally, modified 

 into a swimming-organ, either by the development on it of broad 

 " flukes/' as in the Whales, or merely by being itself flattened 

 vertically as in the Beaver, or from side to side as in the Musk- 

 rat, Potamogale, and others. 



The heart of Mammalia consists of two completely separated 

 divisions, each with a ventricle and auricle. Their blood main- 

 tains a uniformly high temperature, with the exception of some of 

 the lowest forms^ as Echidna. 



The number of known kinds of Mammals at present existing 



* i. e. with a backbone. 



t i. e. with the power of curling round and grasping objects. 



B 



