_General characters._--Size large; tail averaging 80 per cent of head and body; aristiforms narrow and soft (0.4 to 0.5 mm wide), imparting a non-spiny character to the pelage; general color of upper parts Ochraceous-Buff, finely lined with blackish brown, gradually becoming lighter on sides; ventral surface of body and inner sides of legs white; feet dorsally white but with a sepia-colored stripe along outer margin; tail brownish-black above and white below, but white sometimes extended to upper side in distal part; skull broad with no conspicuous ridges; jugals deep with transverse ridge usually conspicuous; postorbital process of zygoma involving only squamosal; incisive foramen short and wide posteriorly; vomerine sheath complete in 95 per cent of specimens and with maxillary part thick; posterior palatine foramina at plane of first molars or slightly anterior to them; bullae moderately developed; in juvenal specimens, each upper molariform tooth with three counterfolds, but posteriormost counterfold small; in adult specimens, posteriormost counterfold disappearing in 50 per cent of fourth premolars and first molars, in 20 per cent of second molars, and in 15 per cent of third molars; lower molariform teeth with two counterfolds in almost every juvenal specimen, this number, in adult animals, decreasing in m3 to one in 20 per cent of specimens but rarely being reduced in other teeth. _General characters._--Aristiforms soft and narrow, ranging from 15 to 19 mm in total length and 0.4 to 0.5 mm in maximum width; pelage generally non-spiny and not harsh; length of tail ranging from 20 per cent shorter than head and body to as long as, or slightly longer than, head and body; ears rather small (23 to 25 mm). [Illustration: FIGS. 100-103. _Proechimys dimidiatus_, male, MN no. 5452, Tijuca. × 1.]