1897.] . MALAGASX GENUS BEACHYUROMYS, 713 



" Nectomyn squamipes," ' " Hesperomys dorsalis," - " Hesperomys 

 tumidus." ^ As in the Hesperomyince just luentioued, cusps 2 

 aod 3 are well developed in Nesomys , and had 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 HesperomyincB, is a reduction in size o£ the cusps 

 4 and 5, and a corresponding increase in size and greater indepen- 

 dence of the adjoining parts, of what I consider to be the 

 homologue of the " intermediate " cusps of Sciuroides, Pseudosciuru^, 

 and of many, especially the 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 Murince. 



Another distinguishing feature of the upper molars m. 1 and 

 m. 2 of Nesomys — in which, by the way, they approach somewhat 

 to the Murince, 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 the 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 inter- 

 space 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 ai-rived at in much 

 worn molars of Nesomys ; whilst in this respect BracJiyuromys het- 

 sileoensis agrees with the Hesperomyince. 



Obviously, the formation of transverse ridges in molars by 

 means of a fusion of the outer with the inner cusps (loj^hodonty) 

 is brought about with the participation of the " intermediate " 

 cusps, which, e. g. in Pseudosciurus and Sciuroides amongst Eodents, 

 are as yet independent. This holds good with regard both to 

 upper and lower molurs.* These " intermediate " or " secondary " 

 cusps, Osborn's " protoconule " and " metaconule," are generally con- 

 sidered to be of secondary importance, viz. Liter additions to the 

 crown, because as a rule they are inferior in size to the outer and 

 inner cusps. Wlien this is not the case, they are liable not to be 

 recognized, and such appears to have been the case with regard to 

 the Murince, amongst others. Winge derives the Murine molar 

 from the Hesperomyine type, by supposing that in the form.er 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 



' Winge, ' Gnavere Lagoa Santa,' pi. iii. fig. 12 a. 



■^ Hensel, I. c. pi. ii. figs. 16, 26. 



^ Heusel, I. o. pi. iii. i:gs. 20, 30. 



* Besides, these intermediate cusps connect the two principal transverse 

 ridges in the molars of Brachyuromys bctsileoejisis (not so in B. r amir o hit ra), 

 as a result of which connection we obtain the interspace (fold) on the inner 

 side of upper, and on the outer side of lower molars, a widespread feature in 

 Mammalian molars. 



