_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.