_Pelage._--_Aristiforms on middorsal region_: Whitish basally, gradually blackening toward tip; total length, 19 to 22 mm; maximum width, 1.1 mm. _Setiforms on middorsal region_: Gray basally, gradually blackening toward tip but interrupted by dark (15'j) Ochraceous-Tawny, subapical zone 3 mm long; some completely blackish on distal parts; total length, 20 to 23 mm; maximum width, 0.07. _Setiforms on outer thighs_: Whitish basally, gradually blackening toward tip but interrupted by Ochraceous-Buff or Light Ochraceous-Buff, subapical zone; total length, 20 to 23 mm; maximum width, 0.07 mm. _Skull._--Size medium; nasals pointed posteriorly; supraorbital ridges prominent and slightly extended caudad onto anterior half of parietals; groove for transmission of nerve in infraorbital foramen weakly developed; zygomatic arches strong; postorbital process of zygoma involving mostly squamosal; incisive foramen approximately 5.5 × 2.7 mm, widest in anterior third and constricted posteriorly, with posterior margin about 1 mm anterior to plane of premolars; vomerine sheath complete, with premaxillary part expanded and maxillary part notably slender and sometimes lacking; mesopterygoid fossa in some specimens extending forward as far as middle parts of second molars; bullae large and inflated. _Teeth._--Upper molariform teeth with three counterfolds each. Lower premolar with three counterfolds; molars with only two. _Comparisons._--From _P. g. warreni_, _arabupu_ differs in: Narrower aristiforms; narrower incisive foramen; lower premolar with three instead of four counterfolds; lower molars with two, instead of three, counterfolds. From _P. g. oris_, _arabupu_ differs in: Aristiforms wider; posterior margin of incisive foramen farther from plane of premolars; upper parts dark (15'_j_) Ochraceous-Tawny, instead of Ochraceous-Orange. _Remarks._--The sample is fairly uniform. _Specimens examined._--Total number, 6 (AMNH), from Brazil, Territorio do Rio Branco, Bõa Vista, Mount Roraima, Arabupu; approximately 1540 m. altitude. Subgenus =TRINOMYS= Thomas _Genotype._--_Echimys albispinus_ Is. Geoffroy Saint-Hilaire, 1838; by original designation. _Trinomys_ Thomas, July 1921, Ann. Mag. Nat. Hist., 8 (ser. 9):140 (orig. descr.); Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):401; Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:115. [Illustration: FIG. 99. Map showing the geographic ranges of the subspecies of three species of the subgenus _Trinomys_.]