_Pelage._--_Aristiforms on middorsal region_: Lanceolate aristiforms whitish basally, gradually blackening toward tip; total length, 23 to 27 mm; maximum width, 1.3 mm. Clavate aristiforms, and some lanceolate ones, whitish basally, gradually blackening toward tip but interrupted by Ochraceous-Tawny, subapical zone. Some clavate aristiforms without subapical zone but blackened in distal part; total length, 23 to 24 mm; maximum width, 0.7 mm. _Aristiforms on outer thighs_: Whitish basally, gradually blackening toward tip but interrupted by Light Ochraceous-Buff, subapical zone; tip slightly darker; some whitish basally, grayish in middle and light yellowish toward tip; total length, 20 to 22 mm; maximum width, 0.9 mm. _Setiforms on middorsal region_: Whitish basal part succeeded by grayish, then by long, light, yellowish band, which becomes Light Ochraceous-Buff, and blackish tip; total length, 26 to 29 mm; maximum width, 0.15 mm. _Setiforms on outer thighs_: Whitish basal part succeeded by grayish, then Light Ochraceous-Buff, subapical zone and blackish tip; total length, 18 to 20 mm; maximum width, 0.13 mm. _Skull._--Broad; bullae small and smooth; jugals dorso-ventrally "wide," with conspicuous transverse ridge; postorbital process of zygoma well-developed and formed by both jugal and squamosal; incisive foramen narrow and short; vomerine sheath incomplete or complete but premaxillary part on a lower level than maxillary part (when skull is viewed from ventral face); mesopterygoid fossa extending forward as far as anterior faces of second molars. _Teeth._--Incisors proodont; molariform teeth with only one counterfold. _Comparisons._--Differences from _P. a. albispinus_ are given in the account of that subspecies. _Remarks._--Localities where samples were collected are typical "caatinga" forest, a climax of mainly deciduous trees; cacti are also common in the region. _Specimens examined._--Total number, 10, from Brazil, Bahia, as follows: _Ituiutaba_, Lamarão, 4 (1 DZ, 1 CNHM, 1 MCZ, 1 USNM); _Bonfim_, Bonfim, 6 (5 DZ, 1 MN). INCERTA SEDIS =Proechimys myosuros= (Lichtenstein) _Loncheres myosuros_ Lichtenstein, 1818, Das zoologische Museum der Universität zu Berlin, (2):18 (_nomen nudum_); Lichtenstein, 1820, Abhandl. K. Akad. Wissensch., Berlin (1818-1819):192, pl. 1, fig. 2 (orig. descr.); Wied, 1826, Beiträge zur Naturgeschichte von Brasilien, 2:445. _Mus leptosoma_ Brants, 1827, Het geslacht der Muizen door Linnaeus opgesteld ..., Berlyn, p. 150; Lichtenstein, 1830, Darstellung neuer oder wenig bekannter Säugethiere, Berlin, Heft 7, pl. 36, fig. and text pages.