| OCR Text |
Show 1897.] MALAGASY GENUS BRACHYUROMYS. 715 part is the antero-internal cusp 6, which, together with 7, appears always shifted forwards in Murlnce, as was assumed by Winge '. In Hesperomyince cusp 6 stands opposite the outer cusp 4; it is the median of the three internal cusps of m.l; the anterior one is the homologue of the intermediate cusp, above mentioned, of Murince, which in Hesperomyince occupies a more internal position. In Nesomys (text-fig. 3) this intermediate cusp occupies the same position as in Murince, but it is much less developed than either in Murince or in Hesperomyince 2. Owing to the smaller size of this cusp and to its position on the anterior side of the tooth, the formation of an anterior fold, I. e. " interspace," between this intermediate cusp anteriorly and cusp 6 posteriorly, which we have in Hesperomyince, is not arrived at in Nesomylnce. The antero-internal cusp, 6, occupies in the latter about the same position as in the former; it is much less shifted forwards than in Murlnce. Lower Molars of Nesomys.-With regard to the lower molars in Hesperomyince and Nesomys, there cau be no doubt as to which are the two principal internal cusps. The posterior internal cusp, 3, is sometimes rather strongly developed in Hesperomyince, just as in Brachyuromys ; in other members of the group it is reduced as compared with 4 and 5, just as in Nesomys. In the Hesperomyince the median crest, corresponding to 2, either remains rather independent, reaching the inner side, but still with the tendency to cling to the antero-internal cusp ; or it is more or less intimately connected with the postero-internal cusp, 5. In the latter case we find an enamel islet as remainder of the original separation 3. In Nesomys the median crest becomes fused, near the inner side, with the posterior part of the antero-internal cusp, 4 ; a configuration to which there is an approach in those Hesperomyince, " H. rattlceps," 4 " H. longicaudatus," a " H. nasutus," 6 H. suhterraneus," 7 in which the usually independent crest leans against the antero-internal cusp. In both Hesperomyince and Nesomys the principal outer cusps, 6 and 7, are internally connected by a small longitudinal crest, from which the median transverse crest, 2, starts inwards at right angles. In Brachyuromys betsileoensis the aforesaid small longitudinal crest is likewdse present, which explains the formation in this species of two creeks, an outer and an inner one, correspondiug to the transverse valley in B. ramlrohltra. In unworn teeth of Br. betsileoensis the median transverse ridge, 2, starting at right angles from the longitudinal crest, is equally to be seen; it 1 Considering the backward inclination of the intermediate and external cusps of Murince, whilst the inner cusps remain upright, it might be questioned whether the latter have shifted their place forwards, or not rather the former backwards. 2 This is one of the reasons for the great similarity between m.l and m.2 in this as well as in the other Murince, as compared to what we find in the two subfamilies just mentioned. 3 See the figures in Hensel, I. c. 4 Hensel, I.e. pl. i. fig. 25 b. 3 lb. pl. ii. fig. 28b. 6 lb. pl. iii. fig. 29 b. 7 lb. pl. iii. fig. 31 b. |
