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Show 464 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, iEPYPRYMNUS RUFESCENS. Two specimens, measuring 125 mm. and 195 mm. long respectively. In this Kangaroo-Rat no trace of the reduced incisors was observed, the three functional ones being well developed, the first and the last showing traces of an internal downgrowth from the dental lanima similar to that seen in the true Kangaroos. The jaw is here very short, so that the last incisor, the canine, and the anterior end of the first premolar appear in the same section. The last-mentioned tooth is very large and extends through a great number of sections; towards its posterior end, and just as the 4th premolar is appearing on its external side (fig. 24), a mass of cells is noticed lying above it; this mass of cells is connected with a long epitheloid cord which is applied to the inner side of the dental sac of the 3rd premolar1, the two having very much the same relation as that seen between the dental sac of a milk-tooth and its so-called permanent successor. The condition seen in the lower jaw is very similar to that in tbe upper, except that the swollen portion of the germ of the so-called successional tooth is situated rather farther back, as in most other Macropids, owing to the backward extension of the large incisor. If one was able to study the development of the functional successional tooth in this form only, one could not help concluding that it was developed side by side with the 3rd premolar, and that it really represented the successor of that tooth and not that of the 4th premolar as has generally been believed hitherto. BETTONGIA LESUEURI. One embryo, about 180 mm. long. The teeth of this foetus were in an advanced stage of calcification and no traces of the supposed rudimentary successional teeth were to be observed. There were four pairs of incisors present in the upper jaw, viz. i1, i4, i5, and ie; of these the first two and the last were very large, while is was minute, somewhat irregular, and undergoing absorption. All traces of r and i3 had disappeared. No trace of additional incisors was observed in the lower jaw. The upper canine in these Kangaroo-Rats persists in the adult as a large tooth, and in conformity with this w e find that in the embryo this tooth is larger than in the true Kangaroos and well calcified. The 3rd premolar is very large, and already shows its characteristic shape, it overlaps the canine in front and nearly reaches the 4th premolar behind. The one successional tooth is of considerable size and slightly calcified; its enamel-organ is connected by a long strand of epithelium with the extreme posterior 1 In the figure the cord of epithelium connected with the developing premolar (ppm) is represented as coming into too intimate connection with the enamel-organ of the 3rd premolar; it should in reality be only applied to the side of that structure, and not fused with it as appears from the drawing. |