_General characters._--Size large; tail short and hairy; aristiforms wide and stiff, especially well-developed on back; general color on upper parts some shade of ochraceous, usually much darker on back and forming a conspicuous dorsal band; feet dark; ventral surfaces and inner sides of legs white; skull elongate and strong with ridges well developed; incisive foramen long and narrow; bullae large; usually four counterfolds in M3 and M2; usually three but sometimes four counterfolds in M1 and even P4; lower premolar with four and lower molars with three counterfolds. [Illustration: FIGS. 39, 40. _Proechimys semispinosus liminalis_, female, MN no. 6253, Rio Quichito. Type. × 1.] [Illustration: FIGS. 41, 42. _Proechimys semispinosus amphichoricus_, male, AMNH no. 77020, Mount Duida. Type. × 1.] [Illustration: FIGS. 43, 44. _Proechimys semispinosus kermiti_, female, AMNH no. 37124, Lower Rio Solimões. Type. × 1.2 (from photograph).] =Proechimys semispinosus liminalis= subspecies nova _Type locality._--Rio Quichito, affluent from the south of the Javarí River, near _Benjamin Constant_, Benjamin Constant, Amazonas, Brazil. _Type_: Museu Nacional, no. 6253, adult female, collected in August, 1942, by E. Parko. _Range._--Known only from the type locality. _Diagnosis._--Color uniformly dark, setiforms marked with Ochraceous-Tawny; skull wide across zygomata; nasals short; prepalatilar part of skull long; incisive foramen long and narrow; vomerine sheath incomplete or complete; M2 and M3 almost always with four counterfolds; M1 more rarely with four counterfolds. _Pelage._--_Aristiforms on middorsal region_: Gray basally, gradually blackening toward tip which is generally extended as a filament; total length, 21 to 23 mm; maximum width, 0.9 to 1 mm. _Setiforms on middorsal region_: Gray basally, gradually blackening toward tip but interrupted by Ochraceous-Tawny, subapical zone 3 mm long; total length, 22 to 24 mm; maximum width, 0.06 mm. _Setiforms on outer thighs_: Whitish basally, gradually blackening toward tip but interrupted by Ochraceous-Buff, subapical zone 2.5 mm long; total length, 13 to 15 mm; maximum width, 0.08 mm; some with gray base, blackening gradually toward tip, without any subapical zone; some with Light Ochraceous-Tawny, subapical zone.