Comparisons.—From P. g. bolivianus, ribeiroi differs in: aristiforms wider; incisive foramen shorter and narrower; vomerine sheath complete and thick, instead of complete or incomplete and not thick; p4 always with four, instead of usually only three, counterfolds and lower molars always with three, instead of sometimes with only two, counterfolds in m3. Differences from P. g. villicauda are given in the account of that subspecies.
Remarks.—The name ribeiroi is given in honor of the late Professor Alipio Miranda [Ribeiro], in recognition of his important work in Brazilian vertebrate zoology.
Specimens examined.—Total number, 2 (MN), from Brazil, Mato Grosso, Mato Grosso, Rio 12 de Outubro; altitude, 414 meters.
Proechimys guyannensis hyleae subspecies nova
Type locality.—Tauarí, Rio Tapajoz, Porto de Moz, Pará, Brazil; approximately 87 kilometers south of Santarem. Type: Museum of Comparative Zoology at Harvard College, no. 30887, adult male; collected on 19 January, 1934, by A. M. Olalla; original number 7288.
Range.—Region of lower Tapajoz River and banks of Amazon up to the Jamundá River.
Diagnosis.—Aristiforms conspicuously wide and stiff; general color on upper parts Tawny; incisive foramen long and oval; vomerine sheath complete but with maxillary part slender or, sometimes, incomplete; p4 with four counterfolds, rarely three; remaining molariform teeth with three counterfolds.
Pelage.—Aristiforms on middorsal region: Whitish basally, gradually becoming blackish toward tip; total length, 19 to 21 mm; maximum width, 1.1 mm. Setiforms on middorsal region: a. Gray basally, gradually blackening toward tip but interrupted by wide (5 to 6 mm) Tawny, subapical zone; some are whitish basally and gradually become sooty brown toward tip except for same type of subapical zone (tip only slightly darker than subapical zone); b. With the same type described above and some completely blackish, with the base gray; total length 22 to 25 mm; maximum width, 0.1 mm. Setiforms on outer thighs: Whitish basally, gradually becoming gray and then blackish toward tip but interrupted by long, Ochraceous-Tawny, subapical zone; tip, itself, blackish brown; sometimes this type appears with some lighter ones and sometimes with completely blackish setiforms; total length 16 to 18 mm; maximum width 0.3 mm.
Skull.—Medium in size and slender; cranium narrow and not increasing much in breadth posteriorly; rostrum stout, laterally thick, with masseteric crest well-developed; nasals pointed posteriorly; supraorbital ridges broad but barely extended across parietals; zygomatic arches strong; postorbital process of zygoma involving mostly squamosal; incisive foramen long and narrow (5.5 to 6 × 2.5 mm), oval and extending posteriorly to point only 2 mm anterior to premolars; vomerine sheath complete with maxillary part usually slender; mesopterygoid fossa extending forward as far as centers of third molars; bulla of medium size, well inflated and with shallow grooves on surface.
Teeth.—Upper molariform teeth with three counterfolds. Lower premolar with four counterfolds or, sometimes (20 per cent of 15 specimens), with only three; lower molars with three counterfolds.