igo COMPARATIVE ANATOMY 



tusks of the upper pair only, which also are unrooted. The narwhal, 

 for a Hke purpose, uses one incisor, its mate growing indefinitely in size, 

 and has no other teeth. 



The carnivora make the canines, especially the upper pair, into long 

 curved daggers, which reach their extreme development in the extinct 

 saber-toothed tiger of the Pliocene but are noteworthy even in the domestic 

 cat. With each canine, in the flesh eaters, goes a "carnassial" tooth, 

 especially developed in the cats, a premolar above and a molar below. 

 Other cheek teeth, especially in the cats, tend to be reduced almost to 

 rudiments. 



Moreover, the carnivora, though uniform as to incisors and canines, 

 differ somewhat widely in the cheek teeth. Thus, while the dog is 



^'^''^'^ the cat is reduced to - — - — The lynx is made a separate genus 



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



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



the 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 AustraHan 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. 



Probosidians. 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 life, 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 



