=Proechimys longicaudatus brevicauda= (Günther) _Echimys brevicauda_ Günther, 1 April 1877, Proc. Zool. Soc. London for 1876, (49):748, fig. 9. _Proechimys brevicauda_ Ihering, 1904, Rev. Mus. Paulista, S. Paulo, 6:422; Osgood, 1914, Zool. Ser. Field Mus. Nat. Hist., 10(12):168; Thomas, 1924, Ann. Mag. Nat. Hist., 13 (ser. 9):534; Thomas, 1927, Ann. Mag. Nat. Hist., 19 (ser. 9):553; Thomas, 1927, Ann. Mag. Nat. Hist., 20 (ser. 9):604; Thomas, 1928, Ann. Mag. Nat. Hist., 2 (ser. 10):262; Thomas, 1928, Ann. Mag. Nat. Hist., 2 (ser. 10):292; Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68 (5):399; Osgood, 1944, Zool. Ser. Field Mus. Nat. Hist., 29 (13):201. _Proechimys cayennensis brevicauda_ Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:120. _Type locality._--Chamicuros, Rio Huallaga, Peru. _Type_: British Museum (Nat. Hist.), no. 69.3.31.7,[F]; _Lectoparatype_: British Museum (Nat. Hist.), no. 66.1.29.8, from Upper Amazons (E. Barttet), selected by Thomas (1900:301). _Range._--Region of the headwaters and upper courses of the Juruá and Ucaiali rivers, eastern Peru and northwestern Brazil. _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.