_Diagnosis._--Upper parts Mars Orange on back, grading to Ochraceous-Tawny on sides; zygomatic breadth narrow; nasals short; incisive foramen narrow and short; vomerine sheath complete and thick; upper molars usually with four counterfolds. _Pelage._--_Aristiforms on middorsal region_: Grayish basally, gradually blackening toward tip; total length, 16 to 19 mm; maximum width, 0.5 mm. _Setiforms on middorsal region_: Grayish on basal third, gradually blackening toward tip but interrupted by a Mars Orange, subapical zone 1.5 mm long; total length, 16 to 19 mm; maximum width, 0.06 mm. _Setiforms on outer thighs_: Whitish basally, gradually blackening toward tip but interrupted by Orange Rufous or Ochraceous-Tawny, subapical zone; total length, 14 to 16 mm; maximum width, 0.05 mm. [Illustration: FIGS. 25, 27. _Proechimys goeldii steerei_, sex ?, "Hyutanaham," USNM no. 105537. × 1.] [Illustration: FIGS. 26, 28. _Proechimys goeldii goeldii_, female, AMNH no. 37488. × 1.] [Illustration: FIGS. 29, 30. _Proechimys goeldii steerei_, sex ?, USNM no. 105537, "Hyutanaham." × 1.] [Illustration: FIGS. 31, 32. _Proechimys goeldii goeldii_, female, AMNH no. 37488, Fazenda Paraiso. × 1.] _Skull._--Large and strong; rostrum rather pointed posteriorly; supraorbital ridges not much expanded and extending across anterior half of parietals; infraorbital foramen without groove for transmission of nerve, or groove obsolete; zygomatic arches slender; postorbital process of zygoma involving mostly squamosal; incisive foramen short and narrow (4.5 × 2.5 mm) with margins almost parallel or tapering gradually caudad and extending toward palate as ridges; posterior margin of incisive foramen approximately 2.5 mm anterior to premolars; vomerine sheath complete, with both elements well-developed; mesopterygoid fossa never extending anterior to middle of M3; bullae large, well inflated and with shallow grooves. _Teeth._--Upper molariform teeth: P4 with three counterfolds; upper molars with four counterfolds each or, less commonly, three. Lower premolars with four counterfolds; lower molars with three each. _Comparisons._--From _P. g. goeldii_, _steerei_ differs in: Back and sides with more reddish; narrower interorbitally and across zygomata; palatilar length less and nasals shorter; maxillary part of vomerine sheath thicker; usually four instead of three counterfolds in M3. _Remarks._--This subspecies is clearly related to _P. goeldii_. One skull from Porto Velho, Rio Madeira, _Guaporé_, Brazil (CNHM no. 21558) may belong to an unnamed subspecies but is provisionally included here.