OCR Text |
Show 1897.] MALAGASY GENUS BRACHYUROMYS. 719 The light which the investigation of the Malagasy Eodentia has thrown on their relatives outside the Island has been somewhat unexpected to me. Erom what Peters had noted concerning the affinities of one of the genera l and from other considerations, I was prepared to meet with the nearest and perhaps the only close affinities amongst the American Hesperomyince. These affinities certainly exist, and I have endeavoured to put them iu their true light. However, other affinities, apart from those just mentioned, are very remarkable. The genusBrachytarsomys, which, as stated on a former occasion2, stands somewhat apart from tbe other Malagasy Eodents, proves to be a forerunner of the Microtince. It is, however, certainly not a member of the genus Microtus, nor of any of the other genera included in the subfamily; it cannot even, in m y opinion, be placed within this subfamily, for it lacks the specializations which characterize the latter. Apart from the molars being not only rooted, but even perfectly brachyodont, neither the last upper nor the first lower molar show any additional increase to the normal Muridine form ; the skull, too, differs from the Microtine cranium in all the characters, which in these are the direct outcome of the increased vertical size of the molars and the adaptation to a subterranean life. But otherwise the teeth as well as the cranium (size and shape of the jugal, form of the rostrum, of the outer wall of the infraorbital foramen and of the foramen itself, general conformation of the upper region of the skull and its crests) are precisely such as w e might expect them to have been in the forerunners of the Microtince. Next as to the genus Nesomys. The large size and breadth of the foramina incisiva, and, what is still more to the point, the large size of the infraorbital foramen, and the strong development of the jugal-which characters this genus shares with most of the other Malagasy Eodents-show it to be a very low member of the Muriclce, approaching the Dlpodldce. The two anterior molars, agreeing in size and general form with each other, tell the same tale. The intimate structure of the molars, as compared with the Hesperomyince and the Murlnce, might induce us to consider Nesomys as a connecting-link between these two groups. But the relationship to them will be more rightly expressed by considering it to be ancestral to both ; especially if w e bear in mind that the characters of both the cranium and teeth are less specialized than in the two subfamilies mentioned. The present paper deals chiefly with a third genus, Brachyuromys. Its affinities with some fossorial Eodents, viz. Tachyoryctes from Abyssinia, Rhizomys from the Oriental Eegion, Spalax and Siphneus from the Palaearctic, have been fully discussed, and as one of the results these four genera are classed amongst the lowest Muriclce. Eetirement under the earth and adaptation to fossorial habits have done for these four genera what isolation has done for 1 Sitzungsber. Ges. naturf. Freunde Berlin, Oct. 18, 1870, pp. 54, also P. Z. S. 1896, p. 978. 8 P. Z. S. 1896, p. 979. 47* |