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Show 1897.] MR. C. S. TOMES ON THE TEETH OF NOTORYCTES. 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 wrork. In Notoryctes only about one third, even of the worn 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 m y 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. Fig. 2. A portion of dentine and enamel from tbe anterior tooth of Notori/ctes, more highly magnified. The dentine lies to the left in the figure. In the Macropodldce the tubes pass across in very great abundance, there being a marked dilatation at the junction of the enamel 27* |