In this order the special feature about the teeth is that the molars bristle with sharp points, specially fitted tocrush the hard coverings of the insects on which they feed. Only one movement-an up- and-down one-is required for this work. Another remarkable modification is seen here.
Certain of the bats live on fruits in preference to insects. In these the molarsare true grinders, with the ordinary, flat, roughened surfaces, and no sharp points, while the jaw has the double movement for mill-work.
In the order of Rodents we find the incisors specially developed. There are never more than two in each jaw, but they are long and broad, and have sharp, chisel-shaped, cutting edges, specially fitted for gnawing purposes. These animals live on vegetable food, which they gnaw ornibble with their chisel-shaped incisors. The remarkable provision about these teeth is that they never lose their sharpness-the very work of gnawing tends to sharpen them.
Like all other teeth, they are composed of the two substances, dentine and enamel. The front of the teeth is hard enamel, the rest is the softer material dentine. The gnawing work of the animal wears away both substances, of course; but as the dentine wears faster than the enamel, the edge is always left sharp.
There is one other remarkable fact in connection with these teeth. From the nature of their work they wear away faster than the teeth of any other animal; indeed, they would soon wear away completely unless some provision were made to prevent it. Look at Nature"s provision in this case. The incisors of all these gnawing animals are entirely without roots, such as other teeth have, and they grow from below as quickly as they are worn away at their edges.
Just one thought more before leaving this order. There are no canine teeth, but there is a considerable space between the incisors and the molars, where the canineteeth should be. The molars themselves, too, are deser ving of notice. Instead of irregular projections, these teeth have a number of parallelridges running crosswise. The movements of the jaw, moreover, are most remarkable. In addition to the up- and-down and the sideway movements, it has a peculiar movement of its own: backwards and forwards. This helps the molars to do their work by a sort of rasping process.
The Ruminants, or cud-chewers, are distinguished, as regards their teeth, by one characteristic-the want of incisors in the upper jaw, their place beingsupplied by a pad. One member of the order, however, the camel, is a departure from the rule, as hehas incisors in both jaws. The whole order, being exclusivelyvegetarians, either have very small canine teeth, or are entirely devoid of them. The molars are always large and well developed.
The teeth of the horse present the remarkable peculiarity of a space between the canines and molars. This is known as the bar.
The bit of the bridle fits into it. The canines, as might be expected, are very small ; the molars are largely developed, and are twenty-four in number, and there are six incisors in each jaw.
In the elephant and wild boar the canine teeth aredeveloped to an enormous size and form tusks, which serve as weapons of attack and defence.