VERTEBRATA. 299 



resulting from the partial wearing down of the tooth, 

 which has at first a very thin crown of enamel ; and in 

 some rodents the molars as well as the incisors grow 

 from permanent pulps. The dentition of a Rat consists 



of incisors and molars only, the former ^j — =-, the latter 



g Q 



p o > with a long gap between them. The Hares and 



Rabbits have additional incisors and additional pre- 

 molars. The teeth of the Rat present the most highly 

 specialized instance of the rodent type. It will be 

 noticed that they are much reduced in number com- 

 pared with those of most mammals. It has been 

 suggested that the number of mammalian teeth is 

 primitively 44, this number becoming reduced in 

 highly specialized types of dentition. 



There are rodents among the marsupials. The 

 teeth of the Wombat are shown in fig. 12 ; the 

 incisors differ slightly in structure from those of the 

 true rodents, but are of similar type. There is also a 

 rodent type among the Lemurs, namely, the Aye-aye, 

 Cheiromys, of Madagascar. This, like the Australian 

 Wombat, supplies in its native country the place of 

 true rodents, which are not fouild there. 



