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.
Skull.—Strong; supraorbital ridges raised and extending across anterior fourth of parietals; nasals elongate; bullae rounded, inflated, with shallow grooves; postorbital process of zygoma weakly developed and constructed entirely of jugal; incisive foramen elongate and narrow (5.5 x 2.5 mm), posteriorly constricted with posterior margins elevated above surface of bones; vomerine sheath incomplete or complete but, when complete, with maxillary part filiform and delicate; mesopterygoid fossa extending forward as far as middle, or even anterior, parts of third molars; posterior palatine foramina on plane of centers, or even anterior faces, of second molars.