296 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



which has its skin greatly thickened. The claws of beasts of 

 prey, the hoofs of hoofed mammals, and the outer horny 

 sheaths of the hollow-horned ruminants are all epidermal 

 structures. 



The bones of mammals are firmer than those of other ver- 

 tebrates, containing a larger proportion of salts of lime. 

 Among the different forms the spinal column varies largely in 

 the number of vertebras, this variation being chiefly due to 

 differences in length of tail. Apart from the caudal vertebrae 

 their usual number is about thirty. The mammalian skull 

 is very firm and rigid, all the bones composing it, excepting 

 the lower jaw, the tiny auditory ossicles, and the slender 

 bones of the hyoid arch, being immovably articulated. The 

 correspondence between the bones of the two sets of limbs is 

 very apparent. The number of digits varies in different mam- 

 mals, and also in the fore and hind limbs of a single species. 

 Among the Ungulates, or hoofed animals, the reduction in the 

 number of digits is especially noticeable; the forefoot of a pig 

 has four digits, that of the cow two, and that of the horse one. 

 The two short "splint" bones in the horse are remnants of 

 lost digits. The teeth are important structures in mammals, 

 being used not only for tearing and masticating food, but as 

 weapons of offense and defense. A tooth consists of an inner 

 soft pulp (in old teeth the pulp may become converted into 

 bone-like material) surrounded by hard white dentine, or ivory, 

 which is covered by a thin layer of enamel, the hardest tissue 

 known in the animal body. A hard cement sometimes covers 

 as a thin layer the outer surface of the root, and may also 

 cover the enamel of the crown. The teeth in most forms are 

 of three groups : (a) the incisors, with sharp cutting edges and 

 simple roots, situated in the center of the jaw; (b) the canines, 

 often conical and sharp-pointed, next to the incisors; (c) 

 next the molars, broad and flat-topped for grinding, and 

 divided into premolars and true molars. There is great variety 

 in the character and arrangement of the teeth in mam- 

 mals, their variations being much used in classification. 



The mouth is bounded by fleshy lips. On the floor of the 

 mouth is the tongue, which bears the taste-buds or papillae, 



