_Mus guyannensis_ E. Geoffroy Saint-Hilaire, 1803, Catalogue des mammifères du Museum d'Histoire Naturelle, Paris, p. 194. _Echimys cayennensis_ Desmarest, 1817, Nouv. Dict. Hist. Nat., Paris, nouv. ed., 10:59. _Proechimys cayennensis_ Allen, 1899, Bull. Amer. Mus. Nat. Hist., 12(20):261, 264; Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):399; Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:120. _General characters._--Size medium to large; tail usually short; aristiforms narrow to wide; general color of setiforms on back ranging from Tawny to Ochraceous-Buff and becoming gradually lighter on sides; no conspicuous dark longitudinal band on back; upper parts of hands and feet white to light brown; underparts white, including inner sides of legs; skull elongate and not conspicuously ridged; vomerine sheath complete or incomplete; upper premolar with three counterfolds and molars with two or three; lower premolar with three or four counterfolds, and lower molars with two or three. =Proechimys guyannensis villicauda= subspecies nova _Type locality._--Tapirapoã, Rio Sepotuba, _Cáceres_, Mato Grosso, Brazil. _Type_: Museu Nacional, no. 1932, adult male (color faded); collected on 2 February, 1909, by Prof. A. Miranda Ribeiro; original number, 788 A. _Range._--Serra dos Parecís, headwaters of Paraguai and Tapajoz rivers. _Diagnosis._--Aristiforms wide and stiff; general color on upper parts Ochraceous-Orange; incisive foramen long; vomerine sheath incomplete or complete; lower premolar with four counterfolds, remaining molariform teeth with three counterfolds. _Pelage._--_Aristiforms on middorsal region_: Whitish basally, gradually becoming gray toward tip, with distal fourth blackish and ending as a long filament; total length, 22 to 23 mm; maximum width, 1 mm. _Setiforms on middorsal region_: Whitish on basal half, gradually blackening toward tip but interrupted by Ochraceous-Orange, subapical zone 5 mm long; total length, 26 to 30 mm; width, 0.04 to 0.12 mm. _Setiforms on outer thighs_: Whitish on basal half, gradually blackening toward tip but interrupted by Ochraceous-Buff to Ochraceous-Orange subapical zone; total length 20 to 23 mm; width, 0.03 to 0.18 mm.