_General characters._--Size small; tail of same length as head and body or slightly less; feet small; ears of medium size; color of upper parts Ochraceous-Tawny gradually changing to Ochraceous-Buff on sides; differentiated, light-colored aristiforms on back, sides, rump and at base of tail; clavate aristiforms on back with Ochraceous-Tawny or Ochraceous-Buff, subapical zone; underparts of body and inner sides of legs white; tail blackish above, white below, with no white tip; hands and feet white on dorsal parts and some specimens darker on outer margins of feet; skull short and smooth, somewhat flattened in frontal region; jugal dorso-ventrally wide and with moderately conspicuous transverse ridge; postorbital process of zygoma well developed and involving both jugal and squamosal; bullae large and smooth; incisive foramen short and narrow; vomerine sheath incomplete or complete; molariform teeth with only one counterfold; incisors orthodont or proodont. _Remarks._--A good series from Macaco Seco, Andaraí, Bahia, agrees closely with the form first described (_albispinus_) from the Island Madre de Deus, in Todos os Santos Bay, Bahia. Compared with topotypes of _P. albispinus sertonius_, the animal from Macaco Seco in general color is more Ochraceous-Tawny and has a narrower skull with orthodont incisors. Specimens from Bonfim, northeastern Bahia, on the other hand, agree with Thomas' _albispinus sertonius_, from Lamarão, being browner and having broader skulls than _P. a. albispinus_ and having proodont, instead of orthodont, incisors. The range of each of the two subspecies is, therefore, fairly extensive. The insular form extends to the less rainy, continental area and the form from Lamarão ranges northward (NNW) in the same type of highly deciduous forest, the "caatinga." The species _albispinus_ is certainly the most specialized form of the entire genus for drier habitats. In addition to the general adaptations described above, it is noteworthy for having both lanceolate and clavate aristiforms. The latter type has a wide basal part and an abruptly narrowed, distal part. The same development is seen in the genus _Echimys_, where highly spinous forms, like _Echimys paleacea_ (Lichtenstein), show the same two types of aristiforms. [Illustration: FIGS. 133, 135. _Proechimys albispinus albispinus_, male, CNHM no. 20409, Macaco Seco. × 1.] [Illustration: FIGS. 134, 136. _Proechimys albispinus sertonius_, male, MN no. 6454, Bonfim. × 1.] [Illustration: FIGS. 137, 138. _Proechimys albispinus albispinus_, male, CNHM no. 20409, Macaco Seco. × 1.] [Illustration: FIGS. 139, 140. _Proechimys albispinus sertonius_, male, MN no. 6454, Bonfim. × 1.] =Proechimys albispinus albispinus= (Is. Geoffroy)