366 A MANUAL OF DENTAL ANATOMY. 



all other Rodents in having enamel all round them, although 

 it is very thin at the back. I have not been able to satisfy 

 myself that the thin clear layer at the back of the tooth is 

 enamel, and am disposed to regard it as cementum, the more 

 so as it seems to be continued a little way upon the enamel, 

 and in very young teeth the enamel organ is confined to the 

 anterior surface. 



When a rodent incisor has been exposed to wear, the 

 anterior layer of enamel is left projecting beyond the level 

 of the dentine, and this arrangement results in a very sharp 

 .edge being constantly maintained. The dentine also is 

 harder near to the front of the tooth than towards the back 

 of the tooth. 



A thin external coat of cement is found upon the back of 

 the tooth, but is not continued far over the face of the 

 enamel. In the marsupial wombat this layer of cement is 

 continued over the whole anterior surface of the scalpriform 

 incisors. 



The molar teeth are not very numerous; the mouse 



2 



family have usually r ; the porcupines have constantly 

 o 



j n 



-, and the hares - ; the Australian water-rat (Hy- 



4r 



2 

 dromys) is altogether exceptional in having so few as X 



Observation has established that the last three of these 

 teeth are always true molars, and that when there are more 

 than three, the rest are premolars, and have had deciduous 

 predecessors. 



But the extent to which the milk teeth are developed 

 varies much. Mr. Waterhouse (Nat. Hist, of Mammalia 

 Rodents, p. 4), has found the milk molar still in place in 

 the skull of a half-grown beaver, while in the hares they 

 are shed about the eighteenth day after birth, and in the 

 guinea-pig disappear before birth. Deciduous incisors have 



