_Skull._--Large and strongly built throughout; supraorbital ridges expanded and thick, extending, in old specimens, across parietals to anterior angles of interparietals; interparietal ridges always conspicuous; rostrum elongated; nasals blunt posteriorly; zygomatic arches strong; infraorbital foramen with weakly-developed groove for transmission of nerve; postorbital process of zygoma involving mostly squamosal; incisive foramen averaging 6 x 2.7 mm, widest in middle part and posteriorly constricted, with raised margins which do not extend across maxillae as ridges; posterior margin of incisive foramen approximately 1.5 mm anterior to plane of premolars; vomerine sheath incomplete or, sometimes, complete but always with maxillary part slender; mesopterygoid fossa not extending forward past centers of third molars; bullae moderately developed. _Teeth._--Upper molariform teeth: P4 always with three counterfolds; M1 with three counterfolds in 9 of 10 specimens and four counterfolds in remainder; M2 with four counterfolds in 7 specimens, three counterfolds in remainder; M3 with four counterfolds in 6 specimens, three counterfolds in remainder. Lower premolar always with four, and molars with three, counterfolds. _Comparisons._--From _P. s. semispinosus_, _liminalis_ differs in: darker color; wider aristiforms; greater percentage of upper molars with four counterfolds. From _P. s. amphichoricus_, _liminalis_ differs in: lighter upper parts of almost uniform color instead of with conspicuous, blackish, middorsal, longitudinal band; more strongly built skull; longer incisive foramen; vomerine sheath usually incomplete instead of always complete. _Specimens examined._--Total number, 10 (MN) from the type locality. =Proechimys semispinosus amphichoricus= subspecies nova _Type locality._--Mount Duida, Esmeralda, _Amazonas_, Venezuela; altitude 325 m. _Type_: American Museum of Natural History, no. 77020, adult male; collected 16 October 1920 by Olalla Bros. _Range._--Headwaters of Negro and Orinoco rivers, along boundary of Brazil and Venezuela. _Diagnosis._--Color dark, blackish on middorsal area; subapical zone of setiforms on back Buckthorn Brown, but many with distal parts black; skull broad across zygomata; nasals long; prepalatilar area of skull long; incisive foramen long and narrow; vomerine sheath complete; upper molars usually with four counterfolds but P4 usually with only three.