Speaking generally, the teeth of most mammals are arranged on similar lines to those of man; that is to say, we find, as a rule, the same kinds of teeth-incisors, canines, and molars. Every order of the class, however, is distinguished by the special development of one or other of these, to suit the habits of the individuals. Let us examine them one by one.
In the Carnivora, or flesh-eating mammals, the special development is in the direction of the four canine teeth, which are always remarkable for their size, sharpness, and prominence. The main object of these teeth is to seize and hold the prey and to tear their flesh. There is very little work for the incisors, hence they are always small, like those of the cat. The molars are well developed, but their surfaces are specially furnished with sharp, jagged ridges, which work against each other at every movement of the jaw, like the sharp edges of a pair of scissors. The peculiar development of the teeth in these animals is accompanied by a corresponding modification in the movement of the lower jaw. The flesh-food, on which they live, cannot be ground up as other food can. The molars,instead of doing the grinding work of a mill, become mere cutting or chopping instruments; in consequence, the jaw requires only a simple up-and-down movement.
A remarkable divergence may be seen in the case ofsome of the so-called Carnivora, such as the bears and the domestic dog, which vary their flesh diet with more or less vegetable food. In all these the sharpcutting ridges of the exclusively flesh-eating animals beginto disappear, and give place
to the ordinary, roughened surface of the grinders, while, more remarkable than this,the jaw begins to take the double movement-sideways as well as up and down-which is wanting in the true flesh- eaters.
The greater the mixture of food, the greater the modification, both in the teeth themselves and in the movement of the jaw. In the dogs, for instance, only the two back molars are changed; in most of the bears three or four are affected, the bears being further advanced as vegetarians than the dogs. The teethof the aquatic flesh-eaters, such as seals and otters, aredeserving of notice. Their prey consists of the slimy, slippery inhabitants of thewater, and not only are their canine teeth well developed, but all the teeth are furnished with sharp, saw-like edges, to serve the double purpose of holding the victim, and cutting through its flesh.
In the walrus, one of the family, the canines of the upper jaw are very largely developed, and form great tusks, often measuring 2 feet in length, and serving as weapons of attack and defence. It has neither canines nor incisors in the lower jaw.
The Insectivora include the shrews, hedgehogs, and moles, in addition to the bat family. The former find their prey on and in the earth, the latter feed mainly on the insects that people the air, and this explains why they are flying mammals.