_Skull._--Narrow; bullae small and smooth; jugals dorso-ventrally wide with conspicuous transverse ridge; postorbital process of zygoma well-developed and formed by jugal and squamosal; posterior palatine foramina obsolete; incisive foramen narrow and short; vomerine sheath complete or incomplete but premaxillary part at a level lower than that of maxillary part (when skull is viewed from ventral face); mesopterygoid fossa extending forward as far as anterior borders of second molars. _Teeth._--Incisors orthodont; molariform teeth with only one counterfold, except that p4 rarely has two counterfolds. _Comparisons._--From _P. a. sertonius_, _albispinus_ differs in: sides of body darker; incisors orthodont as opposed to proodont; p4 rarely with two counterfolds instead of always with one counterfold. _Remarks._--The localities where _P. a. albispinus_ has been collected have a forest climax with a moderate percentage of deciduous trees. _Specimens examined._--Total number, 19 (18 CNHM, 1 MCZ), from Brazil, Bahia, _Andaraí_, Macaco Seco. =Proechimys albispinus sertonius= Thomas _Proechimys albispinus sertonius_ Thomas, July, 1921, Ann. Mag. Nat. Hist., 8 (ser. 9):142 (orig. descr.); Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):401; Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:122. _Type locality._--Lamarão ("about 70 miles north of Bahia City"), _Ituiutaba_, Bahia, Brazil; altitude 300 meters. _Type_: British Museum (Nat. Hist.), no. 3.9.5.86, adult male; collected on 16 June, 1903, by Alphonse Robert; original number, 1508. _Range._--Known from the type locality and Bonfim; probably occupies valleys of Jacuipe and the Itapicurú rivers and littoral between them. _Diagnosis._--Aristiforms wide; color of setiforms Ochraceous-Tawny on back, grading to Ochraceous-Buff on sides; incisors proodont; no molariform tooth with more than one counterfold.