646 MAMMALIA. 



There is still much uncertainty in regard to the nature of the molar 

 teeth. Thus some regard them as resulting from a fusion of several 

 dentitions, others regard them as milk teeth which are not replaced, 

 others as belonging to the replacing set. There seems no doubt that 

 traces of both preceding and succeeding dental structures have been 

 seen associated with molar teeth ; the question is, which set has been 

 suppressed ? 



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 ^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 : 



33 i i 44 33 ii ii 



Incisors , canines , premolars , molars = = total 44. 



33 i i 44 33 ii ii 



or using initial letters : 



. 33 i i 44 33 



i. , c. , pm. , m. = 44. 



33 i i 44 33 



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



and omitting the initial letters : 3 -^ 3 



3143 



We may cite the formulae for the adult dentition of some repre- 

 sentative mammals : 



5134 4134 3024 1014 3143 1133 



Opossum , Thylacine , Kangaroo , Wombat , Pig , Camel , 



4134 3134 1024 1014 3143 3123 



oo33 3i43 2033 3131 3142 3142 3141 



Sheep , Horse , Rabbit , Cat , Dog , Bear , Seal 



3133 3133 1023 3121 3143 3143 2141 



3133 2132 2133 2123 



Hedgehog , Marmoset , New World Monkey , Old World Monkey 



2123 2132 2133 2123 



2123 



Man . 



2123 



It js more interesting, however, to try to associate different kinds of 

 dentition with different kinds of diet. Thus, dolphins, which feed on 



