from the Delta of the Parana. a 
a general arrangement was published last year*. It is 
probably most nearly allied to Akodon itself, but the remark- 
able narrowness of the skull, the small zygomatic spread, the 
reduction of the zygomatic plate, and the simplification of 
the 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 key. 
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). kempz 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 
_coyspicuous than in Akodon, in most S.-American Muride. 
f 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, hairs 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 arather 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. Lars short, 
well-haired, blackish brown. Hands and feet dark brown ; 
claws normal in leneth, the pollex with a nail, Tail rather 
shorter than head and body, thinly haired, finely scaled (15 
rings to the em.), dark brown throughout. 
Skull slender, remarkably narrow, with but slightly ex- 
panded zygomata. Interorbital 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* 
