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Show 710 DR. C I. FORSYTH MAJOR ON THE [June 1, As regards the pattern of the Rhizomys molars, it results from a comparison with those of Brachyuromys aud Tachyoryctes, that in Rhizomys cusp 2 maintains its independence, whereas in the others, as shown by B. betsileoensis, this cusp is not even in youn<r specimens strongly developed, and becomes soon fused with 4. The homologies of the four outer cusps of Rhizomys are therefore to be expressed by the following figures:-1; 4; 2; 54-3 (see Pl. X L . figs. 1 a-7 a). Of the lower molars of Rhizomys (Pl. X L . figs. 1 6-8 h) m. 1 is constantly longer and more complicated than either of the two posterior molars, which last agree almost absolutely with each other in size and pattern. Internally some of the species show, when young, four cusps, the two anterior of which are not strongly separated, so that very soon the only remainder of the original separation is a small enamel islet, which too tends to disappear. In Rh. badius, 1 and 4 appear already fused from the beginning, I. e. in the youngest available stages. The homologies of the four cusps, as compared with Brachyuromys, are therefore as follows : 1 ; 4 (or 1 + 4 ) ; 2 + 5; 3. I feel justified in considering the third cusp to be, as in Brachyuromys, a compound of two (2 + 5), from what is visible in unworn posterior molars as compared with m. 1 (Pl. X L . fig. 16, 2 6, 4 6, 5 6, 6 6). M. 1 is besides distinguished by a surplus on the antero-internal side : not only is cusp 1 separated from 4, but, like m. 1 of Rh. betsileoensis, there is an antero-external cusp in addition to what obtains in m. 2 and m. 3. The molars of Rhizomys therefore, besides being less hypselodont than those of Tachyoryctes, are also more complicated than the molars both of the latter genus and of Brachyuromys, and thus approach more to the brachyodont amongst Malagasy Eodents ; and further on to some members (Trechomys, Theridomys) of a more primitive group, Winge's Anomalurldce. The molars of Tachyoryctes and Brachyuromys, on their side, show a remarkable likeness in pattern to some other members of the same group, viz. Protechimys l, Archceomys. The molars of the former genus, which are much more brachyodont than those of the latter, arrive at the more simplified pattern of Archceomys, only in a somewhat advanced stage of wear. In comparing the molars of these two genera with each other and with Brachguromgs and the Rhlzomyes, it becomes evident, beyond doubt, that the simplified pattern of the molars is the outcome of a complicated one. This is further confirmed by the little we know of a Pliocene Rhizomys, viz.:- Rhizomys sivalensis, Lydekker2, less specialized still than the existing Rhizomys.-I have reproduced the enlarged molars (Pl. X L. fig. 9 6) from a right mandibular ramus in the British Museum, No. 15925, mentioned in the Catalogue. Lydekker says of the 1 Cf. " Theridomys blainvillei, Gerv," in Gervais, Zool. et Pal. franc. pl. 47. fig. 17.-M. Schlosser, " Die Nager des europaischen Tertiars," Pabxon-tographica. xxxi. 1884, pp. 63 (45)-68 (50), pl. ix. (v). 2 Records Geol. Surv. India, xi. 1878, pp. 100, 101, xii. 1879, p. 41 ; id. Pal. Ind. x., iii. 1884, p. 107-108 ; id. Catalogue of the Fossil Mammalia in the British Museum (Natural History), i. London, 1885, p. 233. |