10 



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 the 

 teeth are shown in the figure, we find the front bone, or premaxilla, carrying a small 

 number (in this instance three) of simple cutting teeth, termed incisors. 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 

 incisors, is inserted in the jaw by a single root. This tooth, which is usually 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 incisors 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 we 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 collectively known 

 as the cheek-teeth ; but they may be divided 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 first, 

 are the vertical suc- 

 cessors of milk-teeth, 

 and are known as 

 premolars; while the 

 two hindmost teeth, 

 which have no such 

 temporary predeces- 

 sors, are known as 

 true molars, or molars. 

 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 incisors, and behind it the premolars and molars, 

 distinguished from one another in the same manner as are the corresponding teeth 

 of the upper jaw. 



With the exception of the 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 molars 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- 

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

 complement, owing to the circumstance that there are only two in place of three 

 molars. 



Cental As it would be exceedingly inconvenient always to have to 



describe the number of teeth in any given Mammal by writing 



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



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



OUTER VIEW OF THE RIGHT MILK AND PERMANENT UPPER TEETH OF THE DOG. 



The lower row are the milk-teeth, and the upper the permanent 

 teeth. After Sir W. H. Flower. 



