Among the mammals the teeth display a very great variety of size and form in accordance with the manner in which they are used. Primarily, the function of the teeth is to seize and masticate food, and the kind of food habitually eaten by any animal is well indicated by the form of its teeth. The beasts of prey have teeth adapted for shearing flesh and crushing bones; plant-feeders have teeth fitted for cropping plants and triturating vegetable tissues; insect-eaters have teeth with numerous sharp-pointed cusps, or it may be, no teeth at all, swallowing without mastication the insects which they capture, etc. Among animals that have similar diet there is very great difference in the form and elaborateness of the grinding apparatus and it is often possible to follow out the steps of evolutionary change, by which, from simple beginnings, a high degree of complexity has been attained. In addition to the uses of the teeth as organs of mastication, they frequently serve as weapons of offence or defence. In the flesh-eaters which capture living prey they are formidable offensive weapons, and the fangs of the Lion or the Wolf are instances familiar to every one; but the tusks of the elephants or the hippopotamuses have nothing to do with the taking of prey. Several Old World deer, which have no antlers or very small ones, possess scimitar-like upper tusks, with which they are able to defend themselves very effectually. In the lower vertebrates, such as reptiles and fishes, the number of teeth is usually indefinite and they continue to be shed and replaced, as needed, throughout life; but in each species of mammal, aside from abnormalities, the number is fixed and constant. Mammalian teeth are very generally divisible into four categories: (1) the _incisors_, or front teeth, which in the upper jaw are inserted in the premaxillary bones, (2) the _canines_, or eye-teeth, which are never more than one on each side of each jaw, or four in all, (3) the _premolars_, called in Man the bicuspids, the anterior grinding teeth which have predecessors in the milk-series and (4) the _molars_, the posterior grinding teeth which have no such predecessors. [Illustration: FIG. 44.—Dentition of Wolf, left side. _i. 3_, third incisor. _C._, canine. _p. 1_, first premolar. _p. 4_, fourth premolar. _m. 1_, first molar.]