Specimens examined.—Total number, 3, from Brazil, Maranhão, as follows: Vitoria do Alto Parnaiba, Fazenda Inhuma (below Santa Filomena), 2 (CNHM); Alto da Alegria, Turí-assú, 1 (MN).
Proechimys guyannensis riparum subspecies nova
Type locality.—Manaus, Manaus, Amazonas, Brazil. Type: American Museum of Natural History, no. 143018, adult female; collected 6 March, 1943.
Range.—Known only from type locality but probably extending northward and eastward.
Diagnosis.—Aristiforms wide and stiff; upper parts Ochraceous-Tawny; incisive foramen short, wide, and oval; vomerine sheath incomplete; upper molariform teeth and lower premolar with three counterfolds; lower molars with only two counterfolds.
Pelage.—Aristiforms on middorsal region: Gray basally gradually blackening toward tip; total length, 18 to 20 mm; maximum width, 0.9 mm. Setiforms on middorsal region: Whitish basally, gradually blackening toward tip but interrupted by 2 mm long, Ochraceous-Tawny, subapical zone; total length 20 to 22 mm; maximum width, 0.04 mm. Some are whitish basally and gradually become black toward tip with no subapical zone. Setiforms on outer thighs: Whitish basally, gradually becoming gray and then blackish toward tip but interrupted by Ochraceous-Buff, subapical zone 3 to 4 mm long; tip not conspicuously dark; total length, 17 to 19 mm; maximum width, 0.03 mm.
Skull.—Of medium size and slender; rostrum slender; nasals rounded posteriorly; supraorbital ridges well developed and barely extended onto anteriormost part of parietals; zygomatic arches slender; infraorbital foramen with well-developed groove for nerve transmission; postorbital process of zygoma almost obsolete and involving mostly squamosal; incisive foramen short and wide (4.5 × 3.3 mm), oval and with posterior margins raised to form ridges which extend toward premolars; posterior margin of incisive foramen approximately 2.5 mm anterior to premolars; vomerine sheath incomplete, with only short, premaxillary part; mesopterygoid fossa extending forward as far as anterior parts of third molars; bullae large and inflated, with more or less triangular outline.
Teeth.—Crown length of well worn P4-M3, 6.8 mm; upper molariform teeth with three counterfolds each. Lower premolar with three counterfolds; lower molars with two counterfolds.
