from the Delta of the Parana. 99 



n general canangement was published last year *. It is 

 probably most nearly allied to Akodon itselt'j but the remark- 

 able narrowness of the skull, the small zygomatic spread, the 

 reduction of the zygomatic plate, and the simplitication o£ 

 tlie teeth, notably of m^, all indicate sufficient peculiarity to 

 prevent the species being included in that genus. 



As a ready means of identification the fact that the zygo- 

 matic spread is less instead of more than half the condylo- 

 incisive length will enable Deltamys to be distinguished from 

 all the genera mentioned in my Icey. 



The teeth seem to be somewhat simpler in structure than in 

 the known forms, but more specimens will be needed before 

 the value and constancy of this character can be determined, 

 for only one of the four specimens of J), kempi has the teeth 

 unworn enough to show the structure. But certainly this 

 specimen exhibits a very distinctive difference in the reduc- 

 tion, practically to nil, of the small cross-ridge at the front 

 outer corner of m^ — a ridge present, and generally far more 

 conspicuous than in Akodon, in most S. -American Muridte. 



The following is a description of the species : — 



Size rather smaller than in Akodon arenicola, form de- 

 cidedly more slender. Fur of normal character and medium 

 length, liairs of back about 8-9 mm. long. General colour 

 above blackish brown, inconspicuously washed on head, fore- 

 back, and sides with drabby or olivaceous, nearly unmixed 

 black on the median part of the hind back. One specimen, 

 however, is more olivaceous throughout, and so far as body- 

 colour is concerned might be taken for a rather dark-coloured 

 individual of A. arenicola. Under surface dull brownish grey, 

 the hairs slaty basally, their tips brownish on belly, greyer on 

 the chest ; a marked wholly white patch on the chin. Eyes 

 rather small, without lighter ring round them. Ears short, 

 Avell-haired, blackish brown. Hands and feet dark brown ; 

 claws normal in length, the poUex with a nail. Tail rather 

 shorter than head and body, thinly haired, finely scaled (15 

 rings to the cm.), dark brown throughout. 



Skull slender, remarkably narrow, with but slightly ex- 

 panded zygomata. Interorbilal region smooth, convex, 

 unridged, its edges scarcely squared. Brain-case narrow, 

 smooth, and rounded. Interparietal much reduced, scarcely 

 perceptible at all in three of the specimens, and a mere narrow 

 strip in the fourth. Zygomatic plate narrow, scarcely pro- 

 jected forwards. Palatal foramina long, well open, not 



* Ann. & Mag. Nat. Hist. (8) xviii. p. 336. 



7* 



