Page:Popular Science Monthly Volume 17.djvu/244

232 the bone. The loss of an incisor results in the abnormal lengthening of the opposing one, which may finally interfere with mastication and cause starvation. Ruminants have no incisors in the upper jaw; the elephant has none in the lower jaw, but the tusks are upper incisors. An elephantine mammal of former geological time, the dinotherium, had incisor tusks in the lower jaw, pointing down and backward; while the extinct mastodon had tusks in both jaws.

The canines are intended for seizing and tearing prey and especially characterize carnivorous mammals. They are lacking in rodents and most herbivores, and are never more than four in number. In the apes they are very prominent; those of the gorilla nearly equaling a lion's in size. The tusks of the walrus are upper canines, as also are the terrible tusks of the wild hog. In the Malayan hog the upper



canines, instead of pointing downward, as would seem proper, grow upward through the integuments of the skull and curve backward, sometimes reaching the skull again. Their purpose is obscure. Although possessed in such degree by the male alone, the form precludes their efficient use as weapons. The lower canines also grow to enormous length and are directed outward, forming weapons which make the beast a formidable antagonist.

Herbivorous mammals have the molars flat on the grinding surface, and the enamel and cement disposed in plates and folds perpendicular to this face. Thus by the unequal wear of the tissues the acting surfaces are ridged and admirably adapted for grinding. They form an actual grist-mill, the stones of which never need any "picking." In fruit-eaters the crowns of the teeth are rounded. Insectivorous mammals have the teeth conical and fitting in opposing depressions. Those living on a mixed diet, as man, have the tubercles or cusps somewhat blunt, and suited either for crushing or cutting. In purely carnivorous mammals the molars have sharp edges fitted for cutting meat. They act like chopping-knives, or more accurately like scissors. Quite the only motion of the jaw is up and down, as flesh can not well be ground. Here we have a genuine "hash-mill." The backward and forward