_Range._--Probably most of the region on south bank of Amazon River, between Tocantins (west) and Gurupí River (south). _Diagnosis._--Aristiforms narrow but somewhat stiff; color on upper parts Ochraceous-Orange; incisive foramen long and wide, conspicuously constricted posteriorly; posterior margin of incisive foramen close to plane of premolars; vomerine sheath complete but maxillary part threadlike; upper molariform teeth and lower premolar with three counterfolds; lower molars with only two counterfolds. _Pelage._--_Aristiforms on middorsal region_: Gray basally, gradually blackening toward tip, which is extended as a filament; total length, 16 to 17 mm; maximum width, 0.6 to 0.7 mm. _Setiforms on middorsal region_: Gray basally, gradually becoming blackish toward tip but interrupted by Ochraceous-Orange, subapical zone 2 to 4 mm long; total length, 18 to 20 mm; maximum width 0.06 mm. _Setiforms on outer thighs_: Whitish basally, gradually becoming blackish toward tip but interrupted by Ochraceous-Buff, subapical zone; total length, 15 to 16 mm; maximum width, 0.04 mm. _Skull._--Of medium size; supraorbital ridges well developed and extending across anterior fifth of parietals; zygomatic arches strong; jugal with masseteric fossa deep and with well-developed posterior process; postorbital zygomatic process involving mostly squamosal; incisive foramen long (6 to 7 mm), widest anteriorly (2.5 to 3.2 mm); but narrowing posteriorly to less than 1 mm and extending caudad almost to plane of premolars; vomerine sheath complete but maxillary part delicate and threadlike; mesopterygoid fossa extending forward as far as third molars or posterior parts of second molars; bullae large and inflated. _Teeth._--Each upper molariform tooth with three counterfolds. In lower jaw, premolar with three, and molars with only two, counterfolds. _Comparisons._--Differences from the subspecies with adjoining ranges are given in the accounts of those subspecies. _Remarks._--Thomas (1912:89) extended the known range of the subspecies to Faro, on the Jamundá River, on the left bank of the Amazon, and to Boim, on the Tapajoz River, as well as to Benevides, E. F. Braganca, near Belem. It seems to me that the specimens from Faro should be referred provisionally to _Proechimys guyannensis hyleae_; the specimens from Boim are "more brightly rufous" (Thomas, _loc. cit._) and could be referred to Allen's _P. boimensis_, described in 1914, but _P. guyannensis hyleae_ probably lives in the same place and only an examination of the specimens, which I have not seen, would permit of certainly allocating the specimens to their correct species. The specimens from Benevides are more certainly _P. g. oris_.