238 YERTEBRATA. 



Class V. MAMMALIA. 



Vertebrate, warm-blooded, viviparous animals, more or less 

 covered with hair, breathing by lungs, the females having mam- 

 mary glands (rudimentary in the males). A corpus callosum. 



Mammals are the only animals that suckle their young, and 

 which, before birth, are nourished by a placenta. There are two 

 types of placentas : in the one the uterus produces a " decidua," 

 a modification of its mucous membrane, between which and the 

 chorion a placenta is gradually formed ; in the other there is no 

 decidua, the uterus simply becoming more vascular and forming 

 a union with the chorion, which is dissolved at parturition. 



It is only in the brain of mammals that we find a greater or 

 anterior commissure [corpus callosum] uniting the two hemi- 

 spheres of the cerebrum, and a lesser commissure [pons Varolii] 

 those of the cerebellum ; but they are small or wanting in the 

 Monotremata and Marsupialia. 



The skull is articulated to the atlas by double condyles, and the 

 lower jaw to the temporal bone without the intermediation of an 

 os quadratum, as in birds and reptiles. 



The teeth are fixed in distinct sockets, and " usually consist of 

 hard unvascular dentine, defended at the crown by an invest- 

 ment of enamel, and everywhere surrounded by a coat of cement." 

 The typical number is forty-four. In some mammals the teeth 

 are permanent [monophyodont], in others the first-developed 

 teeth are succeeded by another set [diphyodont]. 



The heart has two auricles and two ventricles. The abdo- 

 minal are separated from the thoracic viscera by the midrib, or 

 diaphragm. Unlike birds and reptiles, the kidneys are com- 

 posed of two parts cortical and medullary ; one kidney is always 

 placed higher than the other. There are two ovaries, but in 

 the Monotremata the right one is rudimentary. A membrana 

 nictitans is present in nearly all mammals except man and 

 monkeys, but in whom it is represented by the plica semilunaris. 

 In the mole and Spalax typhlus the eyes are obsolete, but in the 

 former they are well developed in the embryo. Most mammals 

 have an external ear (concha). The young are often born blind ; 

 they acquire their sight in from eight to fifty days ; others see 

 at once, and are able to accompany their mother a few hours 

 after birth. 



Among the many systems of classification of the Mammalia, 



