1897.] 



>tR. C. S. TOMES ON THE TEETH OP XOTOBTCTES. 



411 



Insectivora — usually, however, iu specimens hidden away in store, 

 as not good for display in the cases of a museum, so that, until 

 I specially looked up this point, I was under the impression that 

 great wear of the molars was more uncommon than it really is. 

 The cusps of the teeth in young specimens are, of course, covered 

 by enamel. 



It seems probable, at all events in the case of Notoryctes, that 

 the wearing-down of the teeth is due to sand, in which the 

 animal is perpetually burrowing, being taken in with the food, 

 rather than to the hardness of the food itself ; for the feeble 

 implantation of the teeth militates against the idea that they are 

 put to very hard work. 



In Notoryctes only about one third, even of the woi'n tooth, is 

 implanted in a socket, whereas most Insectivora have very long 

 and firmly implanted roots to their teeth ; as, however, the sections 

 are not exactly in the axis of the middle of the teeth, the roots 

 may be a little, though not very much, longer than they appear. 



The occurrence of obsolete vascular canals near the worn surface of 

 the left-hand tooth (as seen in fig. 1, p. 410) is an indication, so far 

 as it goes, that these teeth come into wear before their growth is 

 complete, and that the roots are only formed late ; such obliterated 

 vascular canals are very common in the axis of the worn teeth of 

 rodents, especially of their incisors, in which the dentine forms a 

 large share of the masticating surface, and the enamel only fulfils 

 the function of keeping the edge sharp. 



In 1849 my father described (Phil. Trans.) the penetration of 

 the enamel by the dentinal tubes as a character common to all 

 Marsupials, with the exception of the Wombat, though the extent 

 to which it takes place varies much in different members of the 

 group. This character is not peculiar to the Marsupials, as it 

 occurs in a good many Insectivora, and occasionally, though rarely, 

 in other Placental mammals, as for example in the Hyrax. 



rig. 2. 



A portion of dentine and enamel from the anterior tooth of Notoryctes, more 

 highly magnified. The dentine lies to the left in the figure. 



In the Macropodidce the tubes pass across in very great abund- 

 ance, there being a marked dilatation at the junction of the enamel 



27* 



