HOW ANIMALS EAT. 231 



in embryo, but a single tooth developed. This, however, is remarkable 

 as being the longest in the animal kingdom. It is the left upper ca- 

 nine, 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 some- 

 times attains the length of ten feet. The rough surface is spirally 

 grooved as if the tusk were twisted. Various unsatisfactory conjec- 

 tures have been offered regarding the object of this strange develop- 

 ment ; 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 



Fig. 9. Mastodon Giganteus. 



they are limited to the back part of the jaws, without enamel and root- 

 less 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. t 



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 grow- 

 ing out from the base, being supplied by a permanent pulp. Their 

 length in the jaw is great, to insure solidity without actual union with 



