OCR Text |
Show 580 MR. M. F. WOODWARD ON [May 5, tooth; at the horns of the crescents, i. e. at the anterior and posterior extremities of the outer border, and in the middle of this edge where the two crescents meet, slight additional cones are raised up ; these, however, appear very late (Plate X X V I . fig. 35, a & b, 1, 2, 3, 4). Though no less than four stages were examined, yet it was not quite possible to determine which cusp was the first to appear, for even in the earliest stage of m- 3 two slight prominences were already visible corresponding to the paracone and metacone. In the case of m- 1 and m. 2, three cusps were present in all stages, but of these the two external were alone conspicuous in the younger stages, the antero-external (paracone) being the largest, though in the adult it is smaller than the metacone; this, 1 think, shows that the paracone is the first to develop. The internal protocone (7) appears late as a low inward extension from the base of the paracone (fig. 32) and cannot possibly be regarded as the original axis of the tooth. The 4th cusp to appear is the small anterior external cusp, which is connected with the anterior slope of the paracone, the hypocone evidently appearing very late. In the lower molars the protoconid forms the main axis of the dentine germ, and develops long before any of the other cusps, the next in order being the metaconid, followed by the hypoconid and entoconid, and lastly the paraconid. The heel itself minus its two cusps is developed very early before even the metaconid. The paraconid is especially late in its development; consequently the molar tooth before this cusp appears presents a very curious shape, the entire antero-external region of the dentine germ being absent. Upper molars. Lower molars. 1. Paracone. 1. Protoconid. 2. Metacone. 2. Metaconid. 3. Protocone. 3. Hypoconid. 4. Small antero-external. 4. Entoconid. 5. Hypocone. 5. Paracouid. GENERAL COMPARISION OF RESULTS. The 4th Premolar. The homology of the 4th premolar of the Placentalia with the posterior premolar of the Marsupialia was first pointed out by Thomas, and there can be no doubt that this tooth in the two groups presents certain constant and striking features ; thus dpm. 4 is nearly always molariform, whereas ppm. 4 is often almost unique in its pattern, being a highly specialized tooth, which in those cases where it resembles any other tooth has a striking similarity to dpm. 3 (Hypsiprymnus, Canis, &c). Some time ago I pointed out that in Macropus the so-called ppm. 4 developed from the dental lamina between dpm. 3 and dpm. 4 (28, pl. 36, fig. 19), and was evidently serially homologous |