DENTITION OF MAMMALS. 537 



teeth. Mammals with these two sets of teeth are called 

 diphyodont. The milk-teeth may be shed, as in seals, 

 before or shortly after birth, or they may remain, as in Un- 

 gulates, for a long time, being gradually replaced by the 

 permanent set. A milk-tooth generally gives origin to a 

 permanent tooth, but this is not invariably the case ; more- 

 over, the back teeth or molars are permanent teeth which 

 have no direct predecessors in the milk-set. 



In a few mammals, for instance in the dolphins, the teeth 

 are very uniform, almost all alike from beginning to end. 

 Such a dentition is called homodont, in contrast to the 

 common heterodont dentition, in which the teeth are more 

 or less markedly different in form and function. It is 

 necessary now to consider these differences. 



In the typical dentition of Mammals, there are forty-four 

 permanent teeth, eleven on each side above and below. 

 The eleven on each of the upper jaws, may be divided into 

 four sets. Most anteriorly, associated with the premaxilla, 

 are three simple, single-rooted teeth, usually adapted for 

 cutting or seizing. These are called incisors. Posteriorly 

 there are crushing or grinding teeth, whose crowns bear 

 cusps or cones, or are variously ridged, and which have two 

 or more roots associated with the maxilla. But of these 

 grinders, the last three have no milk predecessors and are 

 therefore distinguished as true molars, from the four more 

 anterior, and often simpler premolars, which in diphyodont 

 mammals have predecessors in the milk-set. Finally, the 

 tooth just behind the incisors, that is to say, immediately 

 posterior to the suture between premaxilla and maxilla, is 

 distinguished as the canine, and is often long and sharp. 



This classification of teeth is in great part one of convenience ; thus, 

 the distinction between incisors and grinding teeth is anatomical, that 

 between molars and premolars refers to the history of these teeth ; the 

 connection between the teeth and the subjacent bones is a secondary 

 matter ; there is often little to differentiate canine from premolar. 

 Moreover, the teeth of the lower jaw, which is a single bone on each 

 side, cannot be so certainly classified as those of the upper jaw. 



No part of a Vertebrate is more distinctive than the skull, and no 

 mammalian characteristic is more useful in diagnosis than the dentition. 

 It is convenient, therefore, to have some notation expressing the nature 

 of the dentition. Thus we use " dental formulae," in which the incisors, 

 canines, premolars, and molars are enumerated in order, and in which 

 the teeth of the upper jaw are ranked above the analogous teeth of the 



