698 MAMMALIA. 



having no milk predecessors. They are therefore dis- 

 tinguished as true molars, from the four more anterior and 

 often simpler premolars, which usually occur in two sets, 

 the milk set being replaced by a permanent set. In many 

 cases, however, the first premolar seems to be only once 

 represented. 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. Here 

 the lower canine is defined as the tooth which bites in front of the 

 upper, and the incisors as the teeth in front of this tooth. 



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

 mammalian characteristic is more useful in diagnosis than the dentition. 



o 



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 

 lower jaw. The typical mammalian dentition already referred to may 

 be expressed as follows : 



Incisors ^ ?, canines T - 1 , premolars 4 - - 4 , molars 3ZI3 = - = total 44 ; 



33 i i 44 3-3 ii TI 



or, using initial letters : 



3=3, c . ?=2, pm. 4-4, m. 3=3 = 44 ; 

 3-3 i 1 44 33 



or, recognising that the right and left side are almost invariably identical, 



and omitting the initial letters : ^ 4 - 3 -. 



3143 



The formulae for the adult dentition of some representative Mammals 

 are the followin : 



Opossum 5I 34 ) Thylacine 1134, Kangaroo 3I24 , Wombat IHii, Pig 3I - 4 - 3 , Camel II33 . 

 4134 3134 1024 1014 3143 3123 



Sheep 2E23, Horse 3 -^ 3 , Rabbit 2 ^ 33 , Cat 5I 3 - 1 , Dog ^il 2 , Bear IM- 2 , Seal 3J4i 

 3133 3143 1023 3121 3143 3143 2141' 



Hedgehog O3, Marmoset 5i2?, New World Monkey ^i 33 , Old World Monkey 



2123 2132 2133 212 



Man ii 3 . 

 2123 



Useful as these formula: are, they are often deceptive in practice, 

 and should be regarded as merely a description of the dentition of the 



