OF THE CLASS MAMMALIA. 227 



colour or character of the hair ; in other animals it is different. 

 The common squirrel, for example (Fig. 100), whose robe is of 

 a deep red in summer, becomes of a fine greyish-blue tint in 

 winter. It is at this season that the hair of mammals be- 



Fig. 173. The Pangolin. 



comes much thicker, having underneath the fur in much larger 

 quantity. Climate also plays its part in modifying the fur 

 or hairy covering of the skin ; hence we seek in cold climates 

 chiefly for the skins of animals supplying the valuable furs of 

 commerce : Siberia and North America are the true fur coun- 

 tries. 



When the hairs grow extremely close together, they seem 

 to become matted, and to form those horny plates covering 

 the bodies of some mammals, as the pangolins (Fig. 173) and 

 the cuirass of the armadillo. Anatomists also consider nails 

 and horn as having the same origin. 



391. The Skeleton. The general form of the body is 

 determined by the skeleton, but this must not be extended 

 to the outline in every instance ; the hump of the camel, and 

 the dorsal fins of whales, are not supported internally by any 

 portion of the skeleton (Fig. 174). The skeleton pi'esents 

 always the greatest analogy with the human, which we have 

 already studied ( 269, &c.) The differences may be reduced, 

 1st. To the absence of the abdominal limbs in the fish -shaped 

 mammals called cetacea (Fig. 170). 2nd. In the diminution 

 of the number of the fingers and toes, and the absence of collar 

 bones in those in whom the limbs serve merely as instruments 

 of progression and support. 3rd. To some varieties in the 

 number of the vertebrae, especially of the caudal. 4th. In- 

 equalities in the relative size of various bones. 



392. Conformation of the Head. In the form of the 



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