TEETH 189 



successive sets might fuse with one another. The factual foundations 

 of the theory, however, are weak. 



TEETH OF MAMMALS 



Teeth of mammals are especially important for the paleontologist, 

 partly because they are hard and therefore likely to be preserved, but 

 more because mammalian teeth are closely correlated with feeding habits. 

 But feeding habits, in their turn, are correlated with the entire bodily 

 structure, so that teeth are a key to the whole organism. Moreover, 

 mammalian teeth are so highly specialized and so diverse in size and 

 structure, that a single one is often sufficient to identify a species. 



In general, the tendency has been to reduce the number and to do 

 away with the division into two sets, and at the same time to specialize 

 and elaborate individual teeth. An ideally complete set for a placental 

 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, 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 aberrant, 

 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: i-l, c\, pi, ml, the httle 

 hyrax or cony allied to the elephants 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 incisorform teeth from the upper jaw. Their 



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, and 

 are kept sharp by whetting against one another. The walrus makes 



