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Show 1897.] MALAGASY GENUS BRACHYUROMYS. 713 " Nectomys squamipes," ' " Hesperomys dorsalls,'''2 " Hesperomys tumldus."3 As in the Hesperomyince just mentioned, cusps 2 and 3 are well developed in Nesomys, and bad younger stages been at hand, these cusps would have probably shown to be as independent from 4 and 5 as they are in Winge's fig. 12 a. A feature common to the upper and lower molars of Nesomys, as compared with Hesperomyince, is a reduction in size of the cusps 4 and 5, and a corresponding increase in size and greater independence of the adjoining parts, of what I consider to be the homologue of tbe " intermediate " cusps of Scluroldes, Pseudosclurus, and of many, especially tbe older Placentals. Of this more will be said below, when m. 1 sup. of Nesomys will be discussed as compared with the same tooth of Murlnce. Another distinguishing feature of the upper molars m. 1 and m. 2 of Nesomys-in which, by the way, they approach somewhat to the Murinae, as well as in the greater development of the intermediate cusps-as compared with the Hesperomyince, is the lesser degree of symmetry between the anterior and tbe posterior part of the molars (see m. 1). In the Hesperomyince the enamel-fold, penetrating from the inner side (in upper molars), is separated from the anterior valley (x in text-figs. 1 & 2, p. 707) by a ridge- apparently the homologue of the intermediate cusp-connecting the outer cusp 4 with the inner cusp 6. In Nesomys the interspace formed by the enamel-fold mentioned is confluent with the anterior valley, thus separating cusp 6 from connection with the outer cusp 4, or rather with the intervening intermediate cusp (y, fig. 3)-a state of things which is only arrived at in much worn molars of Nesomys ; whilst in this respect Brachyuromys betsileoensis agrees with the Hesperomyince. Obviously, tbe formation of transverse ridges in molars by means of a fusion of the outer with the inner cusps (lophodonty) is brought about with the participation of the " intermediate " cusps, which, e. g. in Pseudosclurus and Scluroldes amongst Bodents, are as yet independent. This holds good with regard both to upper aud lower molars.4 These " intermediate " or " secondary " cusps, Osborn's " protoconule " and " metaconule," are generally considered to be of secondary importance, viz. later additions to the crown, because as a rule they are inferior in size to the outer and inner cusps. When this is not the case, they are liable not to be recognized, and such appears to have been the case with regard to the Murlnce, amongst others. Winge derives the Murine molar from the Hesperomyine type, by supposing that in the former the outer cusps (of upper molars) have increased in size as compared with the latter, and that by so doing they have pushed the inner 1 Winge, ' Gnavere Lagoa Santa,' pl. iii. fig. 12 a. 2 Hensel, I. c. pl. ii. figs. 16, 26. 3 Hensel, /. c. pl. iii. figs. 20, 30. 4 Besides, these intermediate cusps connect tbe two principal transverse ridges in the molars of Brachyuromys betsileoensis (not so in B. ramirohitra), as a result of which connection we obtain tbe interspace (fold) on the inner side of upper, and on the outer side of lower molars, a widespread feature in Mammalian molars. |
