TEETH 135 



mammal would consist of three incisors, one canine, four premolars, and 

 four molars in each half-jaw. The distinction between premolars and 

 molars is that the premolars replace milk teeth, and are therefore of the 

 second set, like the teeth in front of them ; but the molars have no predeces- 

 sors, and are therefore really of the first set. Functionally, however, and 

 often in size and shape, there is little difference, and the two groups are 

 conveniently lumped together as cheek teeth. 



But while 3-1-4-4, 48 teeth in all, is ideal (conceptual), no placental 

 mammal conforms to it, 44 being the usual limit in any actual animal and 

 3-1-4-3 a common formula. Nevertheless, it is convenient to think of each 

 actual tooth as one particular member of the ideal set. Thus can the 

 history of each tooth be followed throughout all the placental mammals, 

 and each be identified wherever it occurs. But the marsupials are aber- 

 rant, opossums, for example, having four and five incisors, so that their 

 teeth cannot always be homologized with those of placentals. 



Starting, then, with the ideal dental formula: ^'3, c\, p\, m\, the little 

 hyrax or cony, allied to the ungulates, is one of the few mammals to retain 

 the full eight cheek teeth. But its incisors are reduced to one in each 

 half-jaw and it has no canines. On the other hand, the ungulates tend 

 to have the typical four front teeth, but to lack one molar and sometimes a 

 premolar also. They usually have the canine like the incisors and so 

 have practically, though not morphologically, four incisors. But all 

 hollow-horned ungulates lack upper canines, and many, like domesticated 

 sheep, have lost all four incisor-form teeth from the upper jaw. Their 



0-0-^-3 

 dental formula is, therefore, in brief form: 



The pigs, with forty-four teeth in all, are peculiar in having the canines 

 in both jaws grow throughout life as fast as they wear away. They are 

 kept sharp by whetting against one another. The walrus makes tusks of 

 the upper pair only, which also are unrooted. The narwhal, for a like pur- 

 pose, uses one incisor, its mate remaining rudimentary, 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, 



3-1-4-2 



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



3-1-4-3 



the cat is reduced to The lynx is made a separate genus from 



