The ^Mammals, 113 



'"Those Mammals in which the permanent teeth 

 differ from one another in form in different regions of the 

 jaw, we are enabled from their position, and also from 

 their relations to the temporary series of milk-teeth, to 

 divide into four distinct groups. Taking one side 

 of the upper jaw of the dog we find the front 

 bone, or premazilla. carrying a small number of simple 

 cutting teeth, termed incisors. Behind these teeth, from 

 which 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 in- 

 cisors, 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 distinguished 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, with more 

 complicated crowns, and except the first, inserted in li:e 

 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 pre- 

 ceded by milk-teeth or not. In the dog the four teeth im- 

 mediately behind the canine, with the exception of the 

 first, are the vertical successors of milk-teeth, and are 

 known as premolars ; while the two hindmost teeth which 

 have no such temporary predecessors, 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 pre-molars and molars, dis- 

 tinguished from one another in the same mannei* as are 

 the corresponding teeth of the upper jaw. 



''With the exception of the Pouched Mammals, 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 eithei side of each jaw; 

 80 that the total number of teeth on both sides of the two 

 jaws is not more than forty-four. 



"In regard to the external covering, hairs are always 

 present on some portion of the body during some period 



