MAMMALS. 



353 



gradually, or, as in the case of many inhabitants of colder 

 climates, before and after the winter season. In the case of 

 some arctic species this molting is accompanied by color changes, 

 the winter pelage being white. 



Frequently the epidermal layers of the skin becomes greatly 

 thickened and cornified, producing callosities, or thickened, hair- 

 less surfaces like those found on the soles of the feet. Cornifi- 

 cation of the epidermis also results in the formation of horn, 

 such as that found in the cavicornia (cattle, etc.), and in the 

 rhinoceros, as well as nails, claws, and hoofs. 



Some of the relations between the epidermis of the ap- 

 pendages and the claws are interesting. In the human finger 



FIG. 345. Diagrams of the relations of nails, claws, and hoofs, after Boas. 

 A, in man; J3, in unguiculates; C, in ungulates. S, unmodified epidermis; SU, 

 subungual epidermis (Sohlenhorn); U, nail. 



(Fig. 345 A), there exists beneath the nail a peculiarly modified 

 epidermis, su, the subungual portion, while the usual epidermis, s, 

 covers the ball of the finger. In the unguiculates (^) the 

 subungual portion is more developed, and forms the lower sur- 

 face, su, of the claw. In the ungulates (C), the unguis becomes 

 much wider, and is rolled into a hoof, on the lower surface of 

 which is still to be recognized the subungual epidermis, forming, 

 for instance, in the case of the horse, the sole, into which there 

 projects behind the frog of the foot, which, in reality, corre- 

 sponds to the ball of the finger in man. 



Scales are not frequent in the mammals. They occur upon 

 the tails of certain rodents, and upon the feet of these and some 



