_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. _Range._--Region of lower Tapajoz River and banks of Amazon up to the Jamundá River. _Diagnosis._--Aristiforms conspicuously wide and stiff; general color on upper parts Tawny; incisive foramen long and oval; vomerine sheath complete but with maxillary part slender or, sometimes, incomplete; p4 with four counterfolds, rarely three; remaining molariform teeth with three counterfolds.