136 CHORDATE ANATOMY 



the cats because it has lost the minute first premolar of the upper jaw and 



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brought its dentition down to On the other hand, some of the 



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whales have gone back to primitive conical teeth used only for holding, 



are virtually or quite homodont, and have fifty or more pegs in each jaw. 



Characteristic of rodents is the complete absence of canines, and the 

 reduction of the incisors to one functional pair in each jaw. The single 

 pair, however, is a remarkable tool. Each tooth grows from a permanent 

 germ that is set far back in the jaw, so that each passes under all the 

 cheek teeth before it emerges at the front of the mouth. Enamel coats 

 the front surface only, so that as the tooth wears, the dentine wears most, 

 and the thin plate of enamel remains always sharp. Since these teeth 

 grow throughout life, if they are not worn away by gnawing they become 

 too long and the animal cannot feed. 



No rodent has more than six cheek teeth, many have only four, and 

 an Australian mouse so far depends on its incisors, that it has brought its 



dentition down to But the hares and rabbits, and some other 



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rodents, as if to exhibit their affinities with other mammals, have two 



more incisors, very minute, behind the large pair in the upper jaw. 



Proboscidians. The most specialized of all teeth are those of elephants. 

 Incisors and canines are completely lacking in the lower jaw. In the 

 upper jaw, one pair only of incisors become the tusks, but the other two 

 pairs have so completely vanished that it is not known certainly which 

 pair remains. The tusks are rootless, and grow from far up in the skull. 

 They elongate throughout fife, growing faster than they wear away, until 

 in some instances they have reached a length of eight feet and a weight 

 of more than 150 pounds each. Certain extinct elephants had tusks even 

 larger, up to twelve feet and two hundred pounds. In the Indian ele- 

 phant, only the males have tusks. But the larger African species, which 

 uses the tusks for digging roots, has them in both sexes. The famous 

 African elephant Jumbo, in a fit of rage, broke off both tusks inside his 

 cheeks. When they grew out again, they made new holes through the 

 flesh, but the original holes remained for the rest of the animal's life. 



Two extinct proboscidians, Tetrabelodon and Dinotherium, had tusks 

 on the lower jaw also, those of Tetrabelodon nearly parallel with the upper 

 pair, those of the Dinotherium turned downward like those of a walrus. 



Curiously, the small milk tusks of the young elephants, which are 

 shed early, are rooted like ordinary teeth — another illustration of Von 

 Baer's law that the young in a speciaUzed group tend to resemble general- 

 ized ancestors. 



The cheek teeth of proboscidians, less conspicuous than the tusks, 

 are even more remarkable. There are six in each half jaw, i.e., twenty- 



