IO 



MAMMALS 



during life. The first of the two includes the milk- or baby-teeth, which are 

 generally shed at a comparatively early age, are of small size and few in number, 

 and are finally succeeded by the larger and more numerous permanent set, which 

 remain during the rest of life, unless previously worn. 



In 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 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. Be- 

 hind 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 accord- 

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



teeth immediately be- 

 hind 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 pred- 

 ecessors, are known 

 as true molars, or 

 molars. In the lower 

 jaw the tooth, usu- 

 ally 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 correspond- 

 ing 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-four. In the 



OUTER VIEW OF THE RIGHT MIUC 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. 



