_Skull._--Medium in size, not elongated; nasals pointed posteriorly; supraorbital ridges strong and thick, extending caudad across anterior third of parietals; zygomatic arches strong; postorbital process of zygoma involving only squamosal; incisive foramen 5 by 2.7 mm, oval and extending caudad to plane approximately 2 mm anterior to premolars; posterior margins of incisive foramen not forming a ridge; vomerine sheath complete and with maxillary part slender and threadlike, or incomplete, in which event, maxillary part not extended enough to join premaxillary process; mesopterygoid fossa extending forward as far as centers of third molars; bulla large and more or less triangular in its peripheral outline. _Teeth._--Upper molariform teeth with three counterfolds each. Lower premolar with three counterfolds; lower molars with two counterfolds. _Comparisons._--From _P. g. oris_, _arescens_ differs in: Color of upper parts lighter and more uniform; incisive foramen oval instead of conspicuously constricted posteriorly; posterior margin of incisive foramen farther from premolars. _Remarks._--One specimen from Turí-assú (MN) has been identified by O. Thomas as "_P. oris_" (his own handwriting is on the label) and the subspecies is really closely related to _oris_. _Specimens examined._--Total number, 3, from Brazil, Maranhão, as follows: _Vitoria do Alto Parnaiba_, Fazenda Inhuma (below Santa Filomena), 2 (CNHM); Alto da Alegria, Turí-assú, 1 (MN). =Proechimys guyannensis riparum= subspecies nova _Type locality._--Manaus, _Manaus_, Amazonas, Brazil. _Type_: American Museum of Natural History, no. 143018, adult female; collected 6 March, 1943. _Range._--Known only from type locality but probably extending northward and eastward. _Diagnosis._--Aristiforms wide and stiff; upper parts Ochraceous-Tawny; incisive foramen short, wide, and oval; vomerine sheath incomplete; 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; total length, 18 to 20 mm; maximum width, 0.9 mm. _Setiforms on middorsal region_: Whitish basally, gradually blackening toward tip but interrupted by 2 mm long, Ochraceous-Tawny, subapical zone; total length 20 to 22 mm; maximum width, 0.04 mm. Some are whitish basally and gradually become black toward tip with no subapical zone. _Setiforms on outer thighs_: Whitish basally, gradually becoming gray and then blackish toward tip but interrupted by Ochraceous-Buff, subapical zone 3 to 4 mm long; tip not conspicuously dark; total length, 17 to 19 mm; maximum width, 0.03 mm.