_Diagnosis._--Upper parts Tawny and blackish without marked longitudinal band on back; underparts buffy or white; aristiforms narrow; skull slender; incisive foramen wide; vomerine sheath complete; molariform teeth with three counterfolds, except p4 with four and m3 with only two.
_Pelage._--_Aristiforms on middorsal region_: Blackish basally, gradually blackening toward tip which is extended as a long filament; total length, 18 to 20 mm; maximum width, 0.65 mm. _Setiforms on middorsal region_: Gray basally, gradually blackening toward tip but interrupted by Tawny, subapical zone 1.2 mm long; total length, 19 to 21 mm; maximum width, 0.1 mm. _Setiforms on outer thighs_: Whitish basally, gradually blackening toward tip but interrupted by Ochraceous-Tawny, subapical zone 3 mm long.
_Skull._--Slender but not elongated; nasals tapering posteriorly; interparietals wide; supraorbital ridges not much extended and faintly shown across parietals; jugals dorso-ventrally "wide" (3.5 mm); postorbital process of zygoma weakly developed; incisive foramen 5.5 x 3 mm, oval, with posterior borders raised to form ridges which extend toward premolars; vomerine sheath complete, with maxillary part laterally compressed and extended toward palate as ridge; mesopterygoid fossa extending forward as far as third molars; bullae large and well inflated.
_Teeth._--Molariform teeth with three counterfolds, except p4 which has four and m3 which has only two counterfolds.
_Comparisons._--From _P. l. longicaudatus_, _brevicauda_ differs in: upper parts Tawny instead of Ochraceous-Buff; lower premolar with four instead of three counterfolds; m3 only, instead of both m1 and m3, with two counterfolds. From _P. l. boimensis_, _brevicauda_ differs in: upper parts Tawny instead of Ochraceous-Orange; aristiforms wider; m3 with two instead of three counterfolds.
_Specimens examined._--Total number, 3 (DZ), from Brazil, Amazonas, _João Pessoa_, Rio Juruá.
=Proechimys longicaudatus boimensis= J. A. Allen
_Proechimys boimensis_ Allen, 24 July, 1916, Bull. Amer. Mus. Nat. Hist., 35(27):523; Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):400; Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:119.