_Proechimys cayennensis longicaudatus_ Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:121; Osgood, 1944, Zool. Ser. Field Mus. Nat. Hist, 29(13):198. _Type locality._--Northern Paraguay ("unter dem ein und zwansigsten Breitengrade"). _Type_: Apparently no type specimen was preserved. _Range._--Western Mato Grosso, Brazil, and northern Paraguay. _Diagnosis._--Upper parts almost uniformly Ochraceous-Buff; incisive foramen widest posteriorly; vomerine sheath complete; p4 and m2 with three counterfolds; m1 and m3 usually with two counterfolds. _Pelage._--_Aristiforms on middorsal region_: Dark gray, gradually blackening toward tip that has long filament; total length 20 to 22 mm; maximum width 0.6 mm. _Setiforms on middorsal region_: Whitish basally, gradually blackening toward tip but interrupted by Ochraceous-Buff, subapical zone; blackish tip extended (3.5 mm) and thin; total length 19 to 20 mm; maximum width 0.06 mm. _Setiforms on outer thighs_: Whitish basally, progressively grayish or blackish toward tip but interrupted by Light Ochraceous-Buff or Ochraceous-Buff, subapical zone; total length 18 to 20 mm; maximum width 0.06 mm. _Skull._--Slender; supraorbital ridge notably raised; bullae large, elongate, smooth and inflated; jugals narrow; postorbital process of zygoma of medium size and constructed entirely of jugal; incisive foramen wide and large (5.5 x 3 mm), being wider posteriorly than anteriorly and with posterior margins raised; vomerine sheath complete, maxillary part slender and laterally flattened; mesopterygoid fossa extending forward as far as centers, or anterior margins, of third molars; posterior palatine foramina on plane with posterior faces, or centers, of second molars. _Teeth._--Upper molariform teeth always with three counterfolds. Lower molariform teeth: p4 and m2 always with three counterfolds, sometimes the two anterior ones coalesced in m2; m1 with three counterfolds in one specimen (33 per cent); m3 with only two counterfolds in all specimens. _Comparisons._--Differences from _P. l. leucomystax_ and _P. l. roberti_ are given in the accounts of those subspecies. _Specimens examined._--Total number, 3 (2 AMNH, 1 CNHM), from Brazil, Mato Grosso, _Corumbá_, Urucum. =Proechimys longicaudatus leucomystax= Ribeiro _Proechimys leucomystax_ Ribeiro, May, 1914, Commissão de linhas telegraphicas estrategicas de Matto Grosso ao Amazonas, Annexo no. 5. Hit. Nat., Zool., Mammiferos, p. 42, pl. 24 (orig. descr.); Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):400; Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:119.