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 
flippers. 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 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 MTales, 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 
