MAMMALS. 



from their relations to the temporary series of milk-teeth, to divide them into four 

 distinct groups. Taking one side of the upper jaw, as that of the dog, of which tlie 

 teeth are shown in the tig-ure, we hnd the front bone, or premaxilla, carrying a small 

 number (in this instance tliree) of simple cutting teeth, tenned incisore. Behind 

 these teeth, from which, as in the figure, it is generally separated by a longer or 

 shorter gap, there is a tooth with a simple and often conical crown, which, like the 

 incisoi-s, is insei-ted in the jaw by a single root. Thi.s tooth, wliicli is usuallj' larger 

 than the incisors, is termed the tusk, or canine tooth, and in the wild boar and 

 most Carnivorous Mammals attains a very large size. It can always be distin- 

 guished from the incisoi-s by the fact that it is implanted in the maxilla, or second 

 bone of the jaw, or at least on the line of junction between that bone and the 

 premaxilla. Behind the canine wc have a series of teeth, which may be as many as 

 seven, although only six in the figure, with more complicated crowns, and, except the 

 first, inserted in the jaw by two or more roots. This series may be collectivelj- known 

 as the cheek-teeth ; but they may be di%-ided into two minor groups according as 

 to whether they are preceded by milk-teeth or not. In the dog the four teeth 



immediately behind 

 the canine, with the 

 exception of the fii-st, 

 ai-e the vertical stic- 

 cessoi-s of milk-teeth, 

 and are known as 

 premolai-s : while the 

 two hindmost teeth, 

 which have no such 

 temporary predeces- 

 sor, are known as 

 true molai's,or molai-s. 

 In the lower jaw the 

 tooth, usually larger 

 than the others, which bites in front of the upper canine is the lower canine. 

 In advance of this tooth are the incisoi-s, and behind it the premolai-s and molare, 

 distinguished from one another in the same manner as are the con-esponding teeth 

 of the upper jaw. 



With the exception of tlie Pouched Mammals, with which we shall not have 

 to deal till we come to the middle of the third volume, there are, in practically 

 all the Mammals with teeth of different kinds, never more than three incisors, one 

 canine, four premolars, and three molai-s on either side of each jaw : so that the 

 total number of teeth on both sides of the two jaws is not more than forty- 

 four. In the figured upper jaw of the dog the number falls short of this full 

 complement, o^^-ing to tlie circumstance that there are only two in place of three 

 molars. 



Dental As it would be exceedingly inconvenient always to have to 



Formute. describe the number of teeth in any given Mammal by writing 



them down at length, a graphic formula has been invented bj- which the number 



of teeth of each species can be shortly and clearly expressed. Thus, taking 



OUTER VIEW OF THE EIGHT MILK AND PER.MA^•E^T UPPER TEETH OF THE DOO. 



The lower row are the milk-teeth, and the u]>]wr the permanent 

 teeth.— After Sir W. H. Flower. 



