MAMMALIA 311 



CLASS VI. MAMMALIA 



Mammals are homoiothermal, bilaterally symmetric, air- 

 breathing, usually hairy chordates. They are, as a rule, 

 viviparous and suckle their young. They vary in size from the 

 tiny little harvest mouse, probably less than 5 inches long, to 

 the great sulphur whale, weighing many tons. Not all of them, 

 however, are widely distributed, but everywhere, save in a few 

 of the South Sea Islands, some of the several thousand species 

 are found. 



Covering. — No other characteristic is more rightfully called 

 a distinguishing characteristic than the hair of mammals. 

 True hair is found on no other vertebrate. It is general to find 

 a hairy covering among mammals. Even among the Cetacea 

 hairs are sometimes found upon the muzzle, and traces of hair 

 are sometimes found in the embryo. The skin of the whale is 

 underlaid by a layer of fat or blubber, while that of those un- 

 gulates that are sparsely covered with hair, as the rhinoceros, 

 is very thick. The hair may differ greatly both in length and 

 texture. It may be soft and kinky "wool" or very fine fur. 

 The coarse hairs may become long, hard bristles, like those of 

 the hog, or be differentiated into stiff " spines," like those of 

 the porcupine, or into a scale armor, like that of the armadillo. 



Sweat glands in the skin are also characteristic of mammals. 



The skeleton is the most nearly perfect of any of the chordates. 

 The skull (Fig. 254) is composed of fewer bones more firmly 

 united than in the lower chordates. The lower jaw is composed 

 of a single bone on each side, the dentary, and is articulated 

 directly with the squamosal. The skull is connected with the 

 first vertebra, the atlas, by two occipital condjdes instead of one, 

 as in the birds and reptiles. 



The spinal column (Fig. 255) consists of a varying number of 

 vertebra, the first two, the atlas and axis, being somewhat modi- 

 fied to support the head and to permit its various movements. 

 The vertebrse do not articulate with one another by cup and 

 ball (except in some ungulates), as in amphibians, reptiles, and 

 birds, since the intervertebral disks of fibrocartilage permit 

 lateral bending of the spine. The vertebrae are classified, ac- 

 cording to their location, as cervical, dorsal, lumbar, sacral, 



