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Show 458 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, so that one can only suggest by a comparison with the 1st of these teeth that it is to be referred to the 1st dentition. The Canines. O w e n (p. 291) and Flower have already pointed out the existence, in the mammary foetus and the adults of some Kangaroos, of vestigial upper canines, so that their presence here in all the Macropodidse (save M. brachyurus) which I have examined calls for no special comment. The dental sac of this tooth is of considerable size and situated close to the premaxillo-maxillary suture, a considerable space intervening between it and the premolars behind. In the dried skull (7 cm. long) of an advanced mammary foetus of Petrogale xanthopus the canine was present on either side as an exceedingly minute pointed tooth in a distinct alveolus (fig. 29, c)l. It seems probable that this tooth is shed, not reabsorbed, being pushed out of the g u m by the development of the enormous 3rd incisor which is situated immediately internal to it. There is no trace of a lower canine in any Macropid which I have examined. The Premolars. The dental lamina is more or less continuous between the canine and the most anterior upper premolar (pm3), but at the same time it is very irregular, showing several curious swellings (fig. 12); these, however, are so irregular that I hesitate in ascribing any importance to them, but it is just possible they may represent the missing 1st and 2nd premolars. The 3rd premolar (fig. 1, pm3) shows as a fair-sized dental sac with a well-developed enamel-organ and pulp, but at present there is no trace of calcification, its enamel-organ remains attached to the small dental lamina whose free end projects slightly into the surrounding tissue (fig. 15); this structure, which might possibly be interpreted as a rudimentary successional tooth, disappears as we trace this tooth backwards in the sections, and soon also does the swollen portion of the enamel-organ of the 3rd premolar itself, but in its place, and directly continuous with it, w e find a strongly developed club-shaped mass of cells which run in form of a lamina between the 3rd and 4th premolars, connecting their enamel-organs and beaiing a definite swelling in the middle (fig. 19, ppm). A section through this region shows (fig. 16) that we are dealing with an incipient enamel-organ, the centre of which is already differentiated into the typical stellate tissue. The 4th premolar, which is very large though not calcified, is still connected by the dental lamina with the gum, but shows no trace of a successional tooth unless the small process of the enamel-organ (fig. 18, p>) be interpreted as such. In a slightly older embryo the same condition was observed, but owing to the development of the 3rd and 4th premolars, especially 1 In fig. 29 the canine is repreaented as lying in the suture between the premaxilla and the maxilla, in reality it is surrounded by a thin layer of the latter, |
