| OCR Text |
Show 1897.] MALAGASY GENUS BRACHYUROMYS. 709 the former are perfectly similar to the following tooth and to of B. ramlrohltra, even in the oblique direction of the transverse ridges ; whereas in Rhizomys these are placed at right angles to the long axis of the skull. As in B. ramlrohltra, m. 3 sup. of Tachyoryctes is longer than the two anterior, this being a consequence of a fuller development of its posterior portion ; that is, 3 remains longer independent from 5. The different proportional size between the molars of Tachyoryctes and those of Rhizomys is apparently to a great extent due to the influence exercised by the incisors. In old specimens of Tachyoryctes (Plate X X X I X . fig. 6 a) the upper molars increase in size from before backwards. The upper incisors in this genus form a smaller segment of a circle than in Rhizomys, so that their posterior end interferes wdth the development of m. 1. In the latter genus the segment is a larger one ; as a consequence, the first molar is partly or totally (according to the different species) withdrawn from the influence exercised by the incisor, which interferes wdth the posterior molars. In Rh. badius the hinder end of the incisor reaches farther backwards than in any other species ; m. 1, which is from the beginning the largest of the three (Plate X L . figs. 1 a, 2 a), remains such to an advanced stage of wear (Plate X L . fig. 3 a); the two posterior molars soon diminish in size. This decrease in size, from ra. 1 to m. 3, is more intensified with age.-In Rh. sumatrensls, the incisor does not seem to affect the molars, their proportional size remaining the same in youth and advanced age (Plate X L . figs. 5 a, 6 a) ; m. 3 sup. is very small from the beginning.-In Rh. pruinosus and Rh. sinensis (Plate X L . fig. 7 a) the incisor forms a smaller segment of a circle than in sumatrensis, and a much smaller one than in badius. I have not all the stages of wear of Rh. sinensis, but there seems to be no essential difference from Rh. prulnosus. In this latter the unworn m. 1 sup. is considerably elongate (Plate X L . fig. 4 a); very soon it becomes reduced to the size of m.2, and in very old dentitions it is even smaller than the latter. To recapitulate the foregoing with regard to the two extremes, Rhizomys badius and Tachyoryctes. In both, m. 1 sup. is, before wear, slightly larger and especially longer than m.2. In old specimens of Rh. badius both teeth, unaffected by the incisor, maintain their relative proportions, whereas both become considerably reduced in Tachyoryctes. The latter's m. 1, which is the most interfered with by the incisor, is eventually reduced to the size of m. 2, and in old individuals becomes even somewhat smaller ; m. 1 becomes equally much reduced in size. In Tachyoryctes the greater vertical extension of the molars is a further reason for their being very soon interfered with by the incisor. M. 3, which from the beginning is somewhat atrophied in Rh. badius, becomes more so with advance of wear; whilst in Tachyoryctes, where the posterior part of m. 3 is on the contrary somewhat produced, as compared with its anterior molars, and with m. 3 of Rh. badius, this tooth maintains its proportions even in old individuals. |
