614 Mr. O. Thomas on 



colour above dark, finely grizzled olivaceous, rather lighter 

 on sides. Under surface dull olivaceous grey, the iiaiis 

 slaty basally, with buffy or dull whitish tips, a tew hairs in 

 the inguinal region more strongly bufFy. Ciiin prominently 

 white, the hairs white to the roots, this being the only 

 external difference in colour from A. sylvanus. 



Skull of the usual Akodont general shape, hardly so slender 

 as in H. budini, and considerably larger than in the latter. 

 Anterior edge of zygomatic plate decidedly slanted, much 

 more so than in most Akodons, though, as it happens, sone 

 specimens of A. sylvanus have their plate more slanted than 

 is normal in the genu^. Palatal foramina large, well 0j)en, 

 reaching nearly to the level of the middle ot the large m^. 

 Bullse rather large, decidedly larger than in //. budini or 

 A. sylvanus. 



Incisors fairly orthodont. Molars, as in //. budini, very 

 hy[)Sodont, niuch larger than in that species. Front of m^ 

 without trace of anterior central groove. 



Dimensions of the type : — 



Head and body Hi mm. ; tail 86 ; hind foot 2i ; 

 ear 16*5. 



Skull: greatest length 30; condylo-inclsive length 27*7 ; 

 zygomatic breadth 15; nasals 11'3 ; interorbital breadth 4:'6 ; 

 breadth of brain-case 18*2 ; zygomatic plate 2*7 ; palatal 

 foramina 7*3; upper molar series 5. 



Hab. as above. 



Type. Adult, but not old, male. B.M. no. 21. 11. 1. 66. 

 Original number 1488. (Collected 9th June, 1921. 



This is a very interesting vole-mouse on account of its 

 striking resemblance to a norn.al Akodon, and especially to 

 A. sylvanus, whicli is found in Jujuy only some 100 km. to 

 the east, and may very likely occur in the close neighbourhood 

 of the present animal. Externally it is practically impossible 

 to distinguish the two, except by the presence in the Hypsimys 

 of a prominent white chin-spot — though even in the Akodon 

 there is a slight lightening in this region. 



The teeth, however, are distinctly those of Hypsimys, and 

 confirm my distinction of the genus, while the slanting front 

 edge of the zygomatic plate is a further chaiacter common 

 to both species oi Hypsimys. From Jl. biidini, tl . deceptor is, 

 readily distinguishable by its larger size and longer tooth-row. 



" Dark Akodon, with a wliite chin, which I do not re- 

 member to have sent. Lives in holes, very open and visible, 

 at the foot of the trees in thick damp woods." — E. B. 



