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

 of the dentition. Thus we use " dental formute," 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 : — 



3—3 I— I 4—4 3—3 II— II 



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



3—3 I— I 4—4 3—3 II— II 



or using initial letters : — 



. 3—3 I— I 4—4 3—3 



1. , c. , pm. , m. — 44. 



3—3 I— I 4—4 3—3 



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



and omitting the initial letters : — ^^ ■ 



3143 



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

 sentative mammals : — 



5134 4134 3024 1014 3143 H33 



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



4134 3134 1024 10T4 " 3143 3123 



o°33 3143^ =033 3131 3142 3142 3141 



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



3>33 3133 1023 3121 3143 J,.,, 2141 



3133 2132 2133 2123 



Hedgehog , Marmcset , New World Monkej , Old \\'orld Monkey 



= 123 2132 2133 ,jj3 



2123 

 Man . 



2123 



It is more interesting, however, to try to assuriate diflerent kinds of 

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



or, 



