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Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 455 If I am right in concluding that these three teeth belong to the same dentition, then the 2nd functional incisor of tbe adult is in reality the 4th incisor, this small calcified tooth is the 5th, and the 3rd adult incisor must consequently represent a 6th incisor. Unfortunately, although possessing several specimens of this interesting form they were all too young to show the further development of these three last incisors, and in order to completely settle the question as to the relation between these teeth I was obliged to refer to other Macropids of which I possessed older examples. In both Macropus giganteus and M. brachyurus the second adult incisor shows a fairly well-marked rudiment of the replacing tooth in the form of a cellular downgrowth from the inner side of the neck of its enamel-organ, thus showing that this tooth itself belongs to the 1st dentition. A similar condition is observable in the 3rd functional incisor of M. giganteus and of M. eugenii (figs. 5 and 6, i6'), which must also be referred to the 1st dentition, and not, as Eose believes, to the 2nd. In both these forms the small calcified incisor (i°) is present and situated between the two larger ones; in M. giganteus^ it is extremely vestigial and only to be recognized with difficulty; but in M. eugenii it is, on the contrary, very large and well developed. In no case, however, did it show any indication of a replacing tooth as seen in the 1st rudimentary incisor of Petrogale; so that it is impossible to say for certain to which dentition it wras to be referred, but, judging from its analogy with the other vestigial teeth, it should belong to the 1st dentition. W e find therefore in the upper jaw of Petrogale penicillata 6 incisors, three of wmich are very small and obviously disappearing, and three which w e recognize a3 the incisors of the adult: these teeth are all referable to the 1st (or milk) dentition, the 1st, 2nd, 4th, and 6th all possessing at some period in their development, either in Petrogale or in some other allied Macropid, rudiments of the 2nd or replacing teeth. The adult incisors are the 1st, 4th, and 6th, the 2nd, 3rd, and 5th being in a vestigial condition and completely reabsorbed almost before the former become calcified. Rose's suggestion concerning the relation of the 3rd adult incisor (i6) is not borne out by the study of its development, fig. 5 distinctly showing that in M. giganteus this tooth belongs to the 1st dentition, and not to the 2nd as he stated for that species. His conclusions were based solely on the fact that the 3rd incisor cuts the gum so much later in life than the anterior teeth. The explanation of this arrangement becomes self-evident when one compares the young skull (fig. 29) with that of the adult: it will be seen at once that the premaxilla is very small in the former in proportion to the teeth, and is consequently far too short to accommodate the three large incisors of the adult at the same time, the teeth being extra large for a foetus owing to the fact that they are the adult teeth and are not replaced by a second set as in most 1 I have since cut a younger M. giganteus, and find this tooth is very large; BO that in the specimen above mentioned it must have been largely reabsorbed. 31* |
