278 Mr. W. H. Osgood on 



tail long, hairy, and usually unicolour, and the ears clothed 

 with ochraceous hairs, in these respects agreeing with 

 Thamnomys. Moreover, their skulls, except for rather en- 

 larged audital bullae and deeper brain-cases, are much the 

 same as in Thamnomys, showing the same form of rostrum 

 and zygomata and the same incipient postorbital processes. 

 But their teeth can scarcely be distinguished from those of 

 Mus, the number and arrangement of cusps being very 

 similar. The third or postero-internal ousp of m 1 is un- 

 developed and the supplementary cusps of the lower molars 

 are small and rudimentary as in Mus. It therefore seems 

 best for the present to retain these species in the genus Mus. 



Thamnomys baliolus, sp. n. 



Type from Woodbush Hills, N.E. of Pietersburg, Trans- 

 vaal. No. 6. 4. 3. 51, British Museum. Adult female. 

 Collected December 13th, 1905, by C. H. B. Grant. Pre- 

 sented by C. D. Rudd, Esq. 



Characters. — Size medium, about as in Thamnomys sur- 

 daster ; pelage soft ; colour dark. Skull and teeth small ; 

 zygomatic plate narrow and deeply concave anteriorly. Most 

 similar to T. suralaster, but colour much darker and pelage 

 softer; upperparts dark russet finely and thickly mixed with 

 dusky; ears thinly haired and dark-coloured; postauricular 

 white spot conspicuously contrasted ; underparts pale creamy ; 

 hind feet white on the outer side and toes only, the inner side 

 and the tarsal joint pale ochraceous ; tail blackish all around, 

 slightly more intense terminally. jSkull similar in size to 

 that of T. surdaster, but brain-case relatively large ; audital 

 bullae larger ; rostrum and infraorbital region weak; zygo- 

 matic plate narrow. 



Measurements. — Type : total length 284 mm. ; head and 

 body 104 ; tail-vertebrae 180 ; hind foot (s. u.) 22 ; ear 18. 

 Skull of type : greatest length 28"6 ; basilar length 21*2; 

 zygomatic breadth 13"9 ; nasals 102 ; least interorbital width 

 4 - 6 ; posfpalatal length 9"6 ; diastema 7'2 ; palatine slits 7 ; 

 maxillary tooth-row 44. 



Remarks. — This species belongs to the group in which the 

 third or postero-internal cusp of m 1 is reduced to a narrow 

 ridge. It is readily distinguished from all others of the 

 group by its dark colour and the relatively weak anterior 

 part of the skull. Like the other members of the group, it 

 has the supplementary external cusps of the lower molars 

 more highly developed than in Mus. The group as at present 

 understood includes the following forms : — 



