204 ANIMAL LIFE PAST AND PRESENT. 



cheek-teeth, the peculiar extinct Keptile from South 

 Africa, of which the skull is shown in Fig. 62, agrees 

 with Mammals, although it differs in that each cheek- 

 tooth is fixed in the jaw by a simple undivided root, 

 whereas, in all mammals (with the exception men- 

 tioned), these teeth are implanted by two or more 

 roots ; the number of these roots being greatest in 

 the hindmost teeth, as all those who have had the 

 misfortune to part with a "back-grinder under the hands 

 of the dentist know only too well. 



Again, there is another very important feature by 

 which the teeth of the great majority of Mammals are 

 distinguished from those of Reptiles. As we have 

 already mentioned, the old teeth of most Reptiles are 

 being continuously and irregularly replaced by new 

 ones throughout the life of their owners; but in 

 Mammals there is only one such replacement during 

 the whole of life, and that only of a certain definite 

 number of the teeth. Thus in Chapter XII. it has 

 been shown that among the Pouched Mammals there is 

 only one tooth on either side of each jaw which is so 

 changed, that tootli corresponding to the eighth tooth 

 from the front in Fig. 64 In the case, however, of a 

 Mammal with a dentition like that represented in the 

 figure last mentioned, all the teeth in advance of the 

 last two, with the exception of the small one immedi- 

 ately behind the tusk, have been preceded by baby-, 

 or milk-teeth, that is to say, with teeth corresponding 

 to such of those of the human child as are gradually 

 replaced by larger successors with advancing age. The 



