Remarks.—Proechimys leucomystax was described mainly on the basis of the Utiarití specimen, here designated lectotype of the species. The specimen from the Juina River is younger, as stated by [Ribeiro] in his description. [Ribeiro] mentions the skin of the specimen from Utiarití as "em muito mao estado" and I presume it was discarded as it has not been found in the collection of the Museu Nacional, Brazil.
Specimens examined.—Total number, 6, from Brazil, Mato Grosso: Cáceres, Salto Sepotube, 2 (MN); Cáceres, Tapirapoã, Rio Sepotuba, 2 (AMNH); Diamantino, Utiarití, Rio Papagaio, 1 skull (MN); Diamantino, Rio Juina, 1 (MN).
Proechimys longicaudatus roberti [Thomas]
Proechimys roberti [Thomas], December, 1901, Ann. Mag. Nat. Hist., 8 (ser. 7):531 (orig. descr.); [Thomas], 1904, Ann. Mag. Nat. Hist., 14 (ser. 7):195; [Thomas], 1921, Ann. Mag. Nat. Hist., 8 (ser. 9):140; [Tate], 1935, Bull. Amer. Mus. Nat. Hist., 68(5):400.
Proechimys cayennensis roberti [Ellerman], 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:121.
Type locality.—Rio Jordão, Araguarí, Minas Gerais, Brazil; 960 meters alt. Type: British Museum (Nat. Hist.), no. 1.11.3.62, old male, collected 8 August, 1902, by A. Robert; original number, 705.
Range.—Western Minas Gerais and southern Goiaz.
Diagnosis.—Color almost uniformly Ochraceous-Orange on upper parts; setiforms long and narrow; incisive foramen long and wide; vomerine sheath usually complete; upper molariform teeth and lower premolar with three counterfolds; m3 with two, and m1 and m2 with two or three, counterfolds.
Pelage.—Aristiforms on middorsal region: Whitish basally, gradually blackening toward tip; total length, 21 to 23 mm; maximum width, 0.6 mm. Setiforms on middorsal region: Whitish on basal half, gradually blackening toward tip but interrupted by an Ochraceous-Orange, subapical zone 6 to 7 mm long; total length 25 to 30 mm; maximum width, 0.05 mm. Setiforms on outer thighs: Whitish on basal half, gradually becoming gray and then blackish toward tip but interrupted by wide, Ochraceous-Buff, subapical zone, the tip being brownish or blackish; total length, 22 to 23 mm; maximum width, 0.04 mm.
Skull.—Slender; supraorbital ridges bent outward and making sharp angle at point of frontosquamosal suture, then continuing backward parallel to each other and extending across parietals; squamosal taking small part in supraorbital ridges; bullae large (11 x 8 mm), inflated, with shallow depressions; incisive foramen not especially long but wide (5 x 3 to 3.5 mm), widest in anterior third and constricted posteriorly; vomerine sheath usually complete, with maxillary part reduced to slender threadlike process or, less commonly, missing; mesopterygoid fossa extending forward as far as centers of third molars; zygomatic arches strong with jugals of medium dorsoventral width (approximately 3.5 mm); postorbital process of zygoma weakly developed and involving only squamosal; posterior palatine foramina on plane of posterior margins of first molars or slightly anterior thereto.