_Pelage._--_Aristiforms on middorsal region_: Gray basally, gradually blackening toward tip which is extended as a short filament; total length, 19 to 21 mm; maximum width, 0.8 to 1 mm. _Setiforms on middorsal region_: Whitish basally, gradually blackening toward tip but interrupted by Ochraceous-Orange, subapical zone 2 to 3 mm long; total length, 17 to 20 mm; maximum width 0.1 mm. _Setiforms on outer thighs_: Whitish basally, becoming gradually gray and then blackish toward tip but interrupted by Ochraceous-Buff, subapical zone; blackish tip short; total length 13 to 15 mm; maximum width 0.06 mm. _Skull._--Of medium size; rostrum relatively short; nasals with posterior borders rounded; postorbital ridges extending across anterior fourth of parietals; zygomatic arches moderately strong; postorbital process of zygoma involving both jugal and squamosal; incisive foramen of medium length (4 to 5 mm) and narrow (about 2.5 mm), oval and extending caudad to a plane approximately 2 mm anterior to premolars; vomerine sheath complete, with premaxillae forming approximately anterior 3/4 of sheath; maxillary part of sheath short but well-developed; mesopterygoid fossa extending forward as far as centers of third molars; bullae of medium size but well-inflated. _Teeth._--Upper molariform teeth with three counterfolds. Lower molariform teeth with three counterfolds, except third molar which has only two. _Comparison._--From _P. g. oris_, _leioprimna_ differs in: conspicuously wider aristiforms; shorter and narrower incisive foramen; lower molariform teeth with three counterfolds (except m3 with only two), instead of lower molars with only two counterfolds. Differences from _P. g. nesiotes_ are given in the account of that subspecies. _Remarks._--The paratype was collected in an "igarapé," depression usually invaded by the river waters; the paratype, collected on 18 January, 1911, had two large embryos. _Specimens examined._--Total number, 4 (2 CNHM, 2 AMNH), from Brazil, Pará, Cametá. =Proechimys guyannensis oris= Thomas _Proechimys oris_ Thomas, September, 1904, Ann. Mag. Nat. Hist., 14 (ser. 7): 195; Thomas, 1905, Ann. Mag. Nat. Hist., 15 (ser. 7):587; Thomas, 1912, Ann. Mag. Nat. Hist., 9 (ser. 8):89; Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68:400; Osgood, 1944, Zool. Ser. Field Mus. Nat. Hist., 29:199. _Proechimys cayennensis oris_ Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:121. _Type locality._--Igarapé-assú, E. F. B., near Belem, Igarapé-assú, Pará, Brazil. _Type_: British Museum (Nat. Hist.), no. 4.7.4.78, old male; collected on 6 March, 1904, by Alphonse Robert; original number, 1818.