OCR Text |
Show 108 MR. W. BATESON ON [Feb. 2, which is thus partially divided into two. Upon comparing this with the series on the left side, it appears that two of the six teeth stand level with this bigeminous tooth, and both bite between p~* and m1 of the lower series, though there is no crowding in the jaw owing to the large spaces which exist between the normal teeth of this species. There is, therefore, no doubt that the fourth premolar is on the left side bodily represented by two distinct teeth, each of Fig. 2. Phoca granlandica, having the fourth upper premolar on the right side bigeminous, and on the left side represented by two complete teeth (shaded). Right and left profiles; from a specimen in the Leyden Museum. which is perfect and complete, while on the right side the process division is incomplete. In the Paris Museum (A 2897) there is a specimen of P. grcen-landica in which the second upper right premolar is represented by two teeth, each of which has two roots ; but these two teeth stand at the same level in the arcade, one being external and the other internal. On the left side the second upper premolar is incompletely double, the crown being partially divided by an oblique constriction into an anterior and internal portion and a larger posterior and external part. The former has one root and the latter has two. It is thus seen that reduplication of teeth may occur in such a way that a tooth which is normally single may be represented by two teeth, and that the two teeth thus formed may either (1) both take places in the ordinary series, or (2) may stand externally and internally respectively. Substitution of two teeth for one, both being in series, was seen in Ommatophoca rossii (p1), Phoca grcenlandica (r/), Otaria ursina (p2), Brachyteles hemidactylus^(p1), Phalanger orientalis (£), ditto (P3), Myrmecobius fasciatus (i3), ditto (i*). Probably also cases of the presence of two similar small teeth in place of the first premolar in the Canidae and Felidae should be looked on as belonging to this |