OCR Text |
Show 452 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, PETROGALE PENICILLATA. Examined 4 embryos, either by clarification or sections, or both. Specimens examined measured from snout to tip of tail respectively 8-5, 9-5, 11-5, 14-5 centims. D. f. of adult according to Thomas (12), I. -^ C. ~0, P. °A~[> iU-- 1.2.3.4- Fig. 1, Plate X X X V . , represents the entire jaw clarified with the teeth in situ. At this stage it will be seen that there are present in the upper jaw in all 10 teeth, 6 being in the premaxilla and 4 in the maxilla, while in the lower jaw there are only 6 teeth. In the upper jaw, only three out of the 10 were calcified, viz., i2, i3, i\ fig. 1, all being situated in the premaxilla; from their minute size and advanced calcification these are obviously vestigial structures, the three functional incisors of the adult being at present represented only by their enamel-organs, of which the first two are very large (i1 and i4). In the maxilla the germs of 4 teeth are present, the 1st, from its proximity to the premaxillo-maxillary suture and the large diastema which separates it from the remaining teeth, evidently represents the canine, while the others represent the 3rd and 4th premolars and the 1st molar. In the mandible w e find the large functional incisor (i2) oi the adult slightly calcified, and on either side of this a small rudimentary but well-calcified functionless tooth (^ and i3); behind these w e find the 3rd and 4th premolars and the germ of the 1st molar. The Incisors. The most striking feature in this Petrogale is the presence of three additional vestigial incisors in the upper jaw and two in the mandible,-a fact not hitherto recorded. The presence of these teeth naturally enables us to homologize the functional incisors of the adult with those of the Polyprotodont Marsupials. Here I would direct attention to a source of error which might creep in through the examination of clarified preparations of the jaw only, for it will be seen that the 1st rudimentary upper incisor is apparently situated in front of the 1st functional one; and it was only by means of a very carefully prepared series of sections taken at right angles to the premaxilla, starting parallel with the premaxillary suture and changing the plane of the sections as one passed outwardly, so that the plane of the last section was at right angles to that of the first, that one was enabled finally to determine which of these teeth was really the 1st incisor. Eig. 2 is a wax model constructed from a series of sections by His's method; this shows that the enamel-organ of the 1st large and at present undifferentiated tooth is connected with the gum nearer to the premaxillary suture than that of the small rudi- |