_Skull._--Of medium size; rostrum short; nasals pointed posteriorly; postorbital ridges extending caudad across anterior fifth of parietals; zygomatic arches strong; jugal with process in posterior part of masseteric fossa; postorbital process of zygoma involving mostly squamosal; incisive foramen elongate, narrow (5 x 2.3 mm), and parallel sided; posterior margin of incisive foramen approximately 3 mm anterior to premolars; margins of foramen raised to form ridges; vomerine sheath complete, of almost uniform width and set deeply in foramen; mesopterygoid fossa extending forward as far as centers of third molars; bullae of medium size and inflated. _Teeth._--Upper molariform teeth with three counterfolds; p4 with four counterfolds; m1-3 with three counterfolds. _Comparison._--_From P. g. oris_, _nesiotes_ differs in: Aristiforms conspicuously wider; incisive foramen shorter and narrower, with borders parallel instead of posteriorly constricted; posterior margin of incisive foramen farther from premolars; p4 with four, instead of three, counterfolds; lower molars with three instead of two counterfolds. From _P. g. leioprimna_, _nesiotes_ differs in: Incisive foramen with parallel borders instead of oval; p4 with four instead of three counterfolds; m3 always with three instead of two counterfolds. _Remarks._--Dr. E. Snethlage mentions the type as having been collected at night in the forest. _Specimens examined._--Total number, 8 (MCZ, CNHM, MN), from Brazil, Pará, Tocantins River, Ilha de Manapirí. =Proechimys guyannensis leioprimna= subspecies nova _Type locality._--Cametá, left bank of Tocantins River, near its mouth, Cametá, Pará, Brazil. _Type_: Chicago Natural History Museum, no. 19503, adult female; collected on 21 January, 1911, by Dr. Emilia Snethlage; original number, 35. _Range._--Known only from type locality but probably extending westward toward Xingú River. _Diagnosis._--Aristiforms wide and stiff; general color on upper parts Ochraceous-Orange, incisive foramen moderately long; oval; vomerine sheath complete; all molariform teeth with three counterfolds, except lower, third molar which has only two.