Page:Popular Science Monthly Volume 21.djvu/500

486 the incisors, or front teeth, and the molars, or grinders; while the front teeth themselves only exist in the upper jaw. The incisors grow from "permanent pulps," and hence they increase during the whole life of the animal, or nearly so. A large pair of tusks may weigh from one hundred and fifty to two hundred pounds, and as regards structure



they are found to consist of dentine, or "ivory," and of "cement"; while the enamel, which forms such a characteristic feature of ordinary teeth, may or may not be represented. The tusks vary, according to Darwin, "in the different species or races according to sex, nearly as do the horns of ruminants. In India and Malacca, the males alone are provided with well-developed tusks. The elephant of Ceylon," adds Mr. Darwin, "is considered by most naturalists as a distinct race; there, 'not one in a hundred is found with tusks, the few that possess them being exclusively males.' The African elephant is undoubtedly distinct, and the female has large, well-developed tusks, though not so large as those of the male." The molars, or grinding-teeth, exhibit an equally curious structure. In the life-time of an elephant twenty-four molar teeth are developed in all, six on each side of each jaw. But, at any one time in the life of the animal, not more than two of these teeth are to be seen in each side of the jaw. A curious succession of these molars takes place in the elephants, for they are found to move from behind forward, the teeth in use being gradually ousted from their place by their successors as the former are worn away. Thus the whole set of molars in due time moves forward in the jaw and each successive tooth is, as a rule, larger than its predecessor. In structure, the molars of the elephant are highly peculiar, each exhibiting the appearance rather of a compound than of a single tooth. Each tooth is built up of a series of plates set perpendicularly in the tooth, and consisting of ivory, or "dentine," covered by enamel, while "cement" fills up the interspaces between the plates. As the tooth wears in its work, the enamel comes to project above the surface of the tooth,