Proechimys longicaudatus [Thomas], 1901, Ann. Mag. Nat. Hist., 8 (ser. 7):532; [Thomas], 1904, Proc. Zool. Soc. London, p. 240; [Allen], 1916, Bull. Amer. Mus. Nat. Hist., 35(30):569; [Tate], 1935, Bull. Amer. Mus. Nat. Hist., 68(5):400.
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.