Page:Popular Science Monthly Volume 17.djvu/243

Rh in embryo, but a single tooth developed. This, however, is remarkable as being the longest in the animal kingdom. It is the left upper canine, and except in rare cases possessed by the male alone. Instances are recorded where both teeth were developed. This ivory tusk points directly forward in line with the body, perfectly straight, and sometimes attains the length of ten feet. The rough surface is spirally grooved as if the tusk were twisted. Various unsatisfactory conjectures have been offered regarding the object of this strange development; but, beyond its evident use as a weapon, its purpose is still a mystery.

The whole group of ant-eaters, and the sloths and armadillos, are quite destitute of teeth, on which account they are called edentates. Many of them have no teeth whatever; and when teeth are present



they are limited to the back part of the jaws, without enamel and rootless—all of which features help to rank the edentates very low among mammals. In place of teeth, the duck-bill—the lowest of mammals—has four horny plates, two in each jaw.

The average number of mammalian teeth is thirty-two—possessed by man, apes, and ruminants. But, the hog is the happy possessor of the typical number, forty-four; which honor is also shared by the opossum and mole. Man is the only living animal with an unbroken succession of teeth, and having the canines of the same height as the others.

The incisors of rodents are very interesting. They are the only prehensile organs of the gnawing mammals, and are exposed to severe wear. But, as the enamel is thicker and the dentine harder in front, the abrasion constantly produces a chisel-edge admirably fitted for gnawing hard substances. They must, however, be kept of a certain length; and to supply the loss by abrasion they are continually growing out from the base, being supplied by a permanent pulp. Their length in the jaw is great, to insure solidity without actual union with