_Skull._--Of medium size and slender; rostrum slender; nasals rounded posteriorly; supraorbital ridges well developed and barely extended onto anteriormost part of parietals; zygomatic arches slender; infraorbital foramen with well-developed groove for nerve transmission; postorbital process of zygoma almost obsolete and involving mostly squamosal; incisive foramen short and wide (4.5 × 3.3 mm), oval and with posterior margins raised to form ridges which extend toward premolars; posterior margin of incisive foramen approximately 2.5 mm anterior to premolars; vomerine sheath incomplete, with only short, premaxillary part; mesopterygoid fossa extending forward as far as anterior parts of third molars; bullae large and inflated, with more or less triangular outline. _Teeth._--Crown length of well worn P4-M3, 6.8 mm; upper molariform teeth with three counterfolds each. Lower premolar with three counterfolds; lower molars with two counterfolds. [Illustration: FIG. 98. Map showing the geographic ranges of the subspecies of _Proechimys guyannensis_ in Brazil.] _Comparisons._--From _P. g. oris_ and _P. g. hyleae_, _riparum_ differs in: Shorter and wider incisive foramen; vomerine sheath incomplete, instead of sometimes incomplete. From _P. g. oris_, _riparum_ differs in: Upper parts Ochraceous-Tawny instead of Ochraceous-Orange; aristiforms conspicuously wider. From _P. g. hyleae_, _riparum_ differs in: Aristiforms narrower; upper parts Ochraceous-Tawny instead of Tawny; lower premolars with three, instead of four, counterfolds; lower molars with two, instead of three, counterfolds. _Specimens examined._--Type only. =Proechimys guyannensis arabupu= subspecies nova _Type locality._--Arabupu, Mount Roraima, _Bõa Vista_, Territ. Rio Branco; about 1540 meters altitude. _Type_: American Museum of Natural History, no. 75816, adult male; collected by Dr. G. H. H. Tate on 30 December, 1927; original number, 4716. _Range._--Known only from the type locality. _Diagnosis._--Aristiforms conspicuously wide and stiff; color on upper parts dark, near (15'_j_) Ochraceous-Tawny; incisive foramen widest in anterior third; vomerine sheath complete, sometimes incomplete; upper molariform teeth and lower premolar with three counterfolds; lower molars with two counterfolds.