called enamel, coats the tooth, and penetrates into it in some; whilst in a few a third substance is added, called crusta petrosa, or cement.
A tooth consists of two parts, a crown, or external portion, and a root, which is fixed in the socket. The root is either real or fictitious, in the latter case being merely a continuation of the crown, and continuing to grow for an indefinite period, as in the tusk of the Elephant, and the quasi-incisors of Rodents. Certain teeth are deciduous, and fall or are pushed out (after a longer or shorter period) by their successors. Others are permanent, and from their first appearance are never succeeded by others. The first are often called milk-teeth, but in some animals they are shed before birth, and in others not till a late period of life. They are never (or very rarely) renewed more than once, in this differing essentially from the teeth of Reptiles and Fishes, which are being continually shed and renewed.
Teeth vary much in number among Mammals, and are entirely wanting in a very few. In the more perfect animals they are of four kinds; viz., Incisors, Canines, Præmolars or false molars, and Molars. Incisors are situated in the front of each jaw, in the intermaxillary bones, and corresponding portion of the lower jaw. They never exceed six above and six below, except in Marsupial animals. They are sometimes wanting in the upper jaw, as in Ruminating animals, and entirely in Ant-eaters and Armadillos. They vary much in form and size, and in many Bats are unequal in number in the upper and lower jaws.
The Canine teeth are one on each side of the incisors, both above and below, and are fixed in the maxillary bone. They usually, except in Man and one or two others, succeed the incisors after more or less interval. They are absent in several animals, especially among the Ruminants, but are present in some of that order. In some they occur only in the males, in others they are larger in that sex, and in a few are very largely developed. The præmolars are those next the canines, and, like them and the incisors, are deciduous, and succeeded by others. They vary greatly in form and size, being unicuspid in some, bicuspid in others. They are not found amongst Rodents, but in these animals they are present
