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Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 465 end oipm3, the tooth itself being situated above the anterior end of pm4, but is in no way connected with the latter. The specimen was, however, too old to show the true origin of this tooth. The molars w7ere too advanced to show any indication of their lost predecessors or otherwise. Of course the most posterior molar in the jaw was still very young, but as a matter of fact no traces either of predecessors or of successors have been observed to any but the 1st molar, the rest seemingly being too much modified. III.-GENERAL CONSIDERATIONS. We have seen from the above that many of the Macropodidae possess vestiges of the five upper incisors of the Polyprotodonts, and that in two cases, viz. Petrogcde penicillata and Macropus giganteus, there are traces of no less than six of these teeth, the full upper incisor formula being, 1.2.3.4.5.6. That the three adult incisors are the 1st, 4th, and 6th; this conclusion is at variance with that of Oldfield Thomas (11, pp. 454 and 457), who shows, in a diagram illustrating the relations between the teeth of the Polyprotodonts and Diprotodonts, that he believes the reduction in the number of the incisors in the latter to have been brought about by a suppression of the two posterior teeth of the former. This interpretation I have shown, by the discovery of vestigial teeth, to be erroneous, the teeth which disappear being incisors nos. 2, 3, and 5. The discovery of 6 pairs of incisors, although an absolute fact, is iu many respects an unfortunate one, as we know of no adult Mammal with so many, and even amongst Reptiles many Lizards and Crocodiles have the number of teeth in each premaxilla restricted to five. I can only suggest in explanation that in Petrogale, where the 3 adult incisors are so slight, and where there is in consequence more room in the premaxilla, the additional incisor, which is only apparently lost in the Polyprotodonts (see ante, p. 456), has reappeared as a calcified tooth owing to the greater amount of room in the jaw and the lesser abstraction of growth-energy on account both of the smaller size of the adult teeth and of the very late development of the most posterior incisor. In Macropus giganteus only the late calcification of the functional teeth can be supposed to account for the presence of so many vestiges, and as a fact w e notice, directly these adult teeth begin to calcify, the vestigial ones become reabsorbed. The incisor which I regard as wanting in the Polyprotodonts is the 5th incisor of Petrogcde, for I have found what appears to be an undeveloped enamel-organ in Perameles, between incisors 4 and 5 of that form, and corresponding in position with the large diastema of the adult. On one side of the upper jaw of an adult Perameles in the Teaching Collection of the Royal College of Science there is a curiously elongated tooth, occupying a position intermediate between incisors 4 and 5, both of which are wanting |