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Show 460 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, through the region of the 1st molar at two stages, show (that there is present on the outer side of the enamel-organ a conspicuous outgrowth of its cells extending down into the gum at right angles to the swollen portion of the dental lamina. This mass takes its origin from the dental lamina nearer to the surface of the gum than the point of origin of the molar tooth, which condition is apparently just the reverse to that which we have seen in the incisor region, where, if there was a rudiment of a second set of teeth, it was situated internal to, and deeper than, the functional one, and represented a replacing dentition. Here in the molar region, on the other hand, we can only interpret these appearances on the grounds that the functional molars belong to the 2nd or replacing dentition, and that this downgrowth on their outer sides represents the rudimentary, or rather vestigial, milk or 1st dentition. I may perhaps mention here that I have found what appears to be a similiar condition in Putorius and Cavia, while in Lepus and Talpa, where there is a suggestion of the reverse condition, we are in reality dealing with the dental lamina itself and not a downgrowth from it, as may be seen when the more posterior sections are examined. The second species of Petrogale, which was evidently the more advanced embryo of a smaller form, showed only two vestigial upper incisors, these having the relation of i2 and ib, and one only j3 in the lower jaw, viz. \; it may be thaty were already reabsorbed. MACROPUS BRACHTURTJS. Two specimens examined, measuring respectively 100 mm. and 135 mm. These specimens, although so small, were very much older than the Petrogales described above, Macropus brachyurus being one of the smallest Wallabies ; the teeth were in consequence much more highly developed. Only two vestigial teeth were present in each premaxilla, corresponding in position with i2 and i5; the former was of considerable size and so close to the surface of the gum that it is just possible that it is shed in this form ; the latter was, on the other hand, very small and already partially absorbed. The three functional incisors (i1, i4, and ie) all exhibit slight but distinct downgrowths from the inner side of their enamel-organs representing rudiments of the successional teeth, according to Kiikenthal's theory, and are therefore, as in all Macropids, referable to the 1st dentition. In the lower jaw there are present 2 pairs of incisors representing ix and ir The first being a vestigial tooth but of fair size and well calcified; the second is the large incisor of the adult and shows an epitheloid cord on its inner side representing the successional tooth. No trace of a canine was visible in either jaw of this specimen, the anterior premolar, which is here very large, coming close behind |
