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Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 457 incisors 4 and 5, in the position of the rudimentary incisor 5 of Petrogale and of the large diastema in the adult Perameles, and this possibly represents a 6th incisor. From this I conclude that the Marsupials originally possessed 6 pairs of incisors in the upper jaw, the first of which to disappear being no. 5 in the Polyprotodonts, while in the Macropodidse nos. 2, 3, and 5 are wanting; here, however, no. 5 is less reduced than nos. 2 and 3, this being due to the fact that the large functional incisors nos. 1 and 4 develop early and consequently cause a corresponding early absorption of nos. 2 and 3, while owing to the late development of no. 6 the small no. 5 persists for a considerable period. The Lower Incisors. In the adult Petrogale and in all the Macropodidse there is but one incisor in either half of the lower jaw, which tooth according to Thomas is to be identified either with no. 1 or 2 of the Polyprotodonts. In the young Petrogale there are, however, no less than three teeth situated close on either side of the mandibular symphysis. Fig. 1 shows one set of these teeth in situ: it will be seen that two of them (i^ and i3) are very small although fully calcified, consisting even at this early period of both enamel and dentine. The remaining one is very large and may be shown to be the developing single incisor of the adult. The position of the two minute teeth close the tip of the large incisor (i2) shows the former to be undoubted incisors. The examination of a series of sections through the mandible showed that the most anterior of these three teeth is the minute tooth tx, fig. 9, and w e m a y call this tooth provisionally the first lower incisor. This tooth is so well calcified that, in spite of its small size, w e must conclude that it is fully formed, and from an examination of a series of stages I have come to the conclusion that it does not cut the g u m but is probably reabsorbed. One of the most interesting points in connection with this tooth is the fact that from the posterior or internal side of its enamel-organ there arises a long cord-like downgrowth with an enlargement on its deep-seated extremity (figs. 9 and 10, ix), which at a later stage becomes slightly indented below, and has all the relations of an undeveloped permanent tooth. This small vestigial incisor (ij is thus proved to belong to the 1st dentition ; and this fact further clears up any doubt which might exist as to its relation to the functional incisor (i2), to which it is very closely approximated, but to which it is in no w?ay related. Judging from the analogy of the other incisors, I should conclude that the functional incisor also belongs to the 1st dentition, but I have not been able to trace any very definite downgrowth from the inner side of its enamel-organ in this species. The 2nd small tooth (is) (? morphological 3rd incisor) is more vestigial than the first, and shows no trace of a successionalt ooth, |
