344 



COMPARATIVE ZOOLOGY 



have the structure of teeth, while the scutes, or plates, of 

 the crocodiles, turtles, and armadillos are of true bone. 



The scales of snakes and lizards are horny epidermal 

 plates covering the overlapping folds of the true skin. 



FIG. 299. Diagrammatic Section of the Skin of a Fish (Carp) : a, derm, showing lam- 

 inated structure with vertical fibers, b; c, gristly layer; e, laminated layer, with 

 calcareous granules; d, superficial portion developing into scales ;./, scale pit. 



In some turtles these plates are of great size, and are 

 called " tortoise shell " ; they cover the scutes. The 

 scales on the legs of birds, and on the tail of the beaver 

 and rat, have the same structure. Nails are flattened 



horny plates de- 

 veloped from the 

 upper surface of 

 the fingers and 

 toes. Claws are 

 sharp conical 

 nails, being de- 

 veloped from the 

 sides as well as 

 upper surface; 

 and hoofs are 

 blunt cylindrical 

 claws. Hollow 



FIG. 300. Vertical Section of the Forefoot of the Horse homS, aS of the 



(middle digit): i, 2, 4, proximal, middle, and distal, , ,., _. 



or ungual, phalanges; 3, sesamoid, or nut bone; 5, 6, 7, OX, may DC llKCned 



tendons ; 9 elastic tissue ; 8, 10, internal and external ^ Q daws sheathing 

 floor ol the hoof; n, 12, internal and external walls. 



a bony core. The 



horn of the rhinoceros is a solid mass of epidermal 

 fibers. " Whalebone," the rattle of the rattlesnake, and 

 the beaks of turtles and birds, are likewise epidermal. 



