130 MR. O. THOMAS ON A NEW GENUS OF UVRIDJE. [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 Milne-EJwards I have been 

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

 Paris Museum for some years, where it has borne the unpublished 

 name of " Malacomys ferrugineus," a name by 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 Mus. PoUex with a narrow nail. 

 Hind feet elongate. 



■ Infraorbital foramen triangular, not narrowed below, its external 

 ■plate 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, 

 arranged 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-iuternal cusp 

 not pushed forwards in 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 fawn- 

 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 very large, oval, rounded ; 



^ Seu), 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 lamince of mi A, B, and 0, and their respective cusps 1, 2, and 3, 

 counting from outside inwards. Thus the cusps just quoted would be A 3 and 

 B 2 respectively, while one would say of Bcomys that the cusp-formula of its 

 ml was A 1, 2 ; B 1, 2, 3 ; C 1, 2, since it is without the A 3 present in Mus, 

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

 equally applicable to m2 or any other tooth. The reason for numbering the 

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

 been added on the inner side, and therefore that the two cusps of Cricetus are 

 homologous with tlie two outer cusps of Mits. 



