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Show 1897.] MALAGASY GENUS BRACHYUROMYS. 711 Rh. sivalensis, speaking of specimens in the Indian Museum, Calcutta:-" The relatively wider molars and the larger size of the incisors of the existing forms as compared with those of the fossil seem to indicate that the specialization has tended to the production of these characters." l In a former paper2 it had been stated that " the fossil molars are in one specimen slightly smaller, and in the other slightly larger than those of the recent species" (i. e. Rh. sumatrensls). In the mandibulae preserved in the British Museum, the molars are wider and longer than those of the largest living species (Rh. sumatrensls), although the mandibula is shorter and lower. The elements constituting each of the molars in the recent forms appear to be more crowded together antero-posteriorly than in the fossil. In this last the molars are more complicated and remain so even in a worn condition, cusp 1 not becoming fused with 4, an occurrence which we meet wdth only in very young stages of some of the species of living Rhizomys. Besides, as has been already pointed out by Lydekker, in the fossil molars the external fold has a greater depth than in recent species. To sum up. The molars of Rh. sivalensis preserve the main characters of the molars of young specimens in later age, whilst in all the recent species the molars are very soon simplified, in some a little earlier, in others a little later. Comparison of the Molars of the Hesperomyinse with those of Brachyuromys and Nesomys. Amongst Eimerican Muridae the simplification of the pattern of molars has been attained in two ways : (1) by a more or less complete suppression of 1, 2, 3, accompanied by a predominance of the four cusps 4, 5, 6, 7, notably the two former; so that the crown of m. 2 remains essentially composed of four cusps ; (2) by a more or less complete suppression of 2 and 3, whilst 4, 5, 6, 7 are equally somewhat reduced in dimensions, whereas 1 is rather increased ; atanyrate it has obtained quite or almost the dimensions of 4 and 5. Of both molars Hensel and Winge have figured examples. To the latter belong such genera as Sigmodon (his-pidus), Holochllus (vulplnus), and the Neotomlnce. Whilst in the former there exists a notable difference in form and size between m. 1 and m. 2, as in the Hesperomyince with more complicated molars 3, in the latter m. 1 tends to become more similar to m. 2. Of the first type, viz. brachyodont, bunodont molars, with essentially only four tubercles, I have found no parallel amongst living Malagasy Eodents; but I discovered in the lower deposits of the " Children's Cave," near Sirabe (Central Madagascar), some jaws belonging to this type ; these will be dealt with on a future occasion. To the second type belongs, amongst Malagasy Eodents, Brachyuromys ; and from other parts of the Old World, 1 Pal. Ind. I. c. p. 108. 2 Rec. Geol. Surv Ind. xi. 1878, p. 101. 3 Of course, as was shown so long ago as 1873 by Hensel, there are all possible transitions to be found between the simple and the more complicated molars. |