_Range._--Known only from the type locality.
_Diagnosis._--_Aristiforms wide and stiff_; incisive foramen small and narrow; vomerine sheath complete and thick; p4 with four counterfolds; remaining molariform teeth with three counterfolds.
_Pelage._--_Aristiforms on middorsal region_: Whitish basally, gradually blackening toward tip; total length, 19 to 22 mm; maximum width, 0.8 mm. _Setiforms on middorsal region_: Whitish on basal half, gradually blackening toward tip but interrupted by subapical zone probably of some tint of ochraceous; total length, 22 to 24 mm; maximum width, 0.06 mm. _Setiforms on outer thighs_: Whitish basally, gradually blackening toward tip but interrupted by probably light ochraceous, subapical zone; total length 14 to 16 mm; maximum width, 0.03 mm.
_Skull._--Slender; supraorbital ridges low; bullae ovate with shallow grooves; postorbital process of zygoma almost obsolete and involving mostly jugal; incisive foramen short and narrow (4 × 2 mm), constricted posteriorly and with posterior margins raised; vomerine sheath complete and thick; mesopterygoid fossa extending forward as far as posterior faces of second molars; posterior palatine foramina on plane with centers of second molars.
_Teeth._--Upper molariform teeth with three counterfolds. Lower molariform teeth: p4 with four counterfolds; molars with three counterfolds which sometimes are fused.
_Comparisons._--From _P. g. bolivianus_, _ribeiroi_ differs in: aristiforms wider; incisive foramen shorter and narrower; vomerine sheath complete and thick, instead of complete or incomplete and not thick; p4 always with four, instead of usually only three, counterfolds and lower molars always with three, instead of sometimes with only two, counterfolds in m3. Differences from _P. g. villicauda_ are given in the account of that subspecies.
_Remarks._--The name _ribeiroi_ is given in honor of the late Professor Alipio Miranda Ribeiro, in recognition of his important work in Brazilian vertebrate zoology.
_Specimens examined._--Total number, 2 (MN), from Brazil, Mato Grosso, _Mato Grosso_, Rio 12 de Outubro; altitude, 414 meters.
=Proechimys guyannensis hyleae= subspecies nova
_Type locality._--Tauarí, Rio Tapajoz, _Porto de Moz_, Pará, Brazil; approximately 87 kilometers south of Santarem. _Type_: Museum of Comparative Zoology at Harvard College, no. 30887, adult male; collected on 19 January, 1934, by A. M. Olalla; original number 7288.