130 MR. O. THOMAS ON A NEW GKNtIS OF MURIDiE. [Feb. 21, 



4. On a new and interesting Annectant Genus of Muridce, 

 with Remarks on the Relations of the Old- and New- 

 World Members of the Family. By Oldfield Thomas^ 

 Natural History Museum. 



[Eeceived February 10, 1888.] 

 (Plate V.) 



By the kindness of Prof. Alphonse Mihie-EJwards I have been 

 entrusteJ with the description of a specimen which has been in the 

 Paris Museum for some years, where it has borne the unpubhshed 

 name of " Malacomys ferruginens'' a name liy which it has been 

 incidentally referred to in print, and which therefore, so far as the 

 species is concerned, I now retain in order to avoid confusion. 



The genus may be termed 



Deomys\ g. n. 



General external form as in Miis. PoUex with a narrow nail. 

 Hind feet elongate. 



Infraorbital foramen triangular, not narrowed below, its external 

 jilate slender, not produced forwards. Upper incisors each with 

 two minute, almost microscopic, grooves ; lower incisors smooth. 

 Anterior upper molars with seven distinct and prominent cusps, 

 arianged 2-3-2, the extra one on the middle lamina" placed quite 

 internal to the general series. Second molars not placed obliquely ; 

 with five cusps arranged 3-2, as in Mus, but the antero-internal cusp 

 not pushed forwards iu front of the others. Lower molars with the 

 cusps biserially arranged as usual. All the cusps above and below 

 unusually high and distiuct, connected with one another by quite 

 low and inconspicuous enamel ridges. 



Deomys ferrugineus, sp. n. 



General colour of head and body a clear pale red or reddish fiiwn- 

 colour, thickly grizzled along the centre of the back with black, but 

 the reddish colour of the cheeks, shoulders, sides, and hips quite 

 clear and unmixed. Face rather duller in general tone ; area round 

 eyes black, not sharply defined. Ears ver}' large, oval, rounded ; 



' ^6w, I link. 

 . ^ To avoid the too frequent use of such terms as the "anterior internal" 

 cusp, or " central cusp of the middle lamina," it would be useful to have a 

 simple formula for the naming of each cusp. This might be done by calling 

 the three laminte of m"^ A, B, and 0, and their respective cusps 1, 2, and '6, 

 counting from outside inwards. Thus the cusps just quoted would be A3 and 

 B 2 i-espectively, wliilo one would say of Bcomys that the cusp-formula of its 

 m' was A 1, 2 ; B 1, 2, 3 ; C 1, 2, since it is without the A 3 present in 3/ws, 

 and possesses the B 3 absent in the Criccti. The same formula is of course 

 equally applicable to "J^^ or any other tooth. The reason for numbering the 

 cusps from the outside inwards is that Bcumys shows that the third cusp has 

 Ijeen added on the inner side, and therefore that the two cusps of Crice/iis are 

 homologous with the two outer cusps of Mns. 



