DENTITION. 645 



enlargement of one set and a consequent abstraction of growth-energy 

 and material from the underlying sets. The specialisation would not 

 appear in the first generation of teeth, which must necessarily be of 

 small size from its early development and consequent adaptation to the 

 small jaw of the young animal, and which would, moreover, be required 

 for temporary use, while the larger and more complicated dentition was 

 developing. The increased size of the second set of teeth might well 

 abstract the growth-energy from the succeeding sets and retard them." 

 The diet of milk w^ould, however, do away with the function of the 

 first teeth, while the third gradually gained in prominence. 

 The following series, after Leche, is of interest — 



(i) In toothed Cetaceans, the persisting dentition is wholly of the 

 milk set (Odontoceti). 



(2) In Marsupials, the persisting dentition is of the first set, except 



the third (or fourth) premolar. 



(3) In the hedgehog, the persisting dentition is mixed, thus the 



incisors, canines, and premolars are partly of the milk set 

 and partly replacing teeth. 



(4) In the great majority of Mammals the persisting dentition consists 



of replacing teeth, excepting in most the first premolar, and 

 also excepting (according to many) the molars. 



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 occur as one set, having no suc- 

 cessors. They are therefore distinguished as true molars, 

 from the four more anterior, and often simpler premolars, 

 which occur in two sets, the milk set being replaced by a 

 permanent set, except in most cases the first. 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. 



