_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. _Proechimys cayennensis_ Osgood, 1944, Zool. Ser. Field Mus. Nat. Hist., 29(13):199. _Type locality._--Boim, Rio Tapajoz, Santarem, Pará, Brazil. _Type_: American Museum of Natural History, no. 37486, adult male; "October 10, 1911 (ex Museu Goeldi)." _Range._--Along lower course of Tapajoz River. _Diagnosis._--Upper parts Ochraceous-Orange; incisive foramen posteriorly constricted; mesopterygoid fossa sharply pointed anteriorly; p4 with four counterfolds, remaining molariform teeth with three counterfolds. _Pelage._--_Aristiforms on middorsal region_: Gray basally, gradually blackening toward tip which is extended as a long filament; total length, 16 to 18 mm; maximum width, 0.5 mm. _Setiforms on middorsal region_: Gray basally, gradually blackening toward tip, but interrupted by short (1.5 mm), Ochraceous-Orange, subapical zone; total length 18 to 20 mm; maximum width 0.09 mm. _Setiforms on outer thighs_: White basally, gradually blackening toward tip but interrupted by Ochraceous-Orange, or Ochraceous-Buff, subapical zone 3.5 mm long; total length 15 to 17 mm; maximum width, 0.06 mm. _Skull._--Elongate and slender; rostrum slender and long; nasals evenly pointed posteriorly; supraorbital ridges weak and barely reaching anteriormost part of parietals; zygomatic arches slender; infraorbital foramen with groove for nerve-transmission obsolete; postorbital process of zygoma weak and involving mostly squamosal; incisive foramen 5 mm long and 2.5 to 3 mm wide, oval, with margins not much extended caudad as ridges; posterior margin of incisive foramen approximately 2 mm anterior to premolars; vomerine sheath complete with maxillary part slender; mesopterygoid fossa sharply pointed anteriorly and extending forward as far as anterior part of third molar; bullae of medium size, smooth and more or less triangular in contour.