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).
Figs. 100-103. Proechimys dimidiatus, male, MN no. 5452, Tijuca. × 1.
Color.—General color of back and sides results from uniform mixture of black distal parts of aristiforms with Ochraceous-Buff of subapical zone of setiforms. Dorsally, from nose caudad to rump, mixture appears brownish-black, lined with Ochraceous-Buff; toward sides, amount of Ochraceous-Buff gradually increases and resultant color is much lighter brown than on back. On outer parts of arms and legs, color turns gradually to sepia toward distal parts and finally to uniform sepia on wrists and ankles, this color extending to outer dorsal parts of hands and feet; on ankles, sepia forms complete ring, as usual in the genus. Tail blackish-brown on upper parts, this stripe gradually tapering toward tip where dark brown hairs form small pencil; white of under side of tail sometimes seen also entirely around distal part, short of tip which remains dark brown. Ventral surfaces wholly white, from upper lips caudad including inner surfaces of legs.
Hairs.—Aristiforms on middorsal region: Gray basally, gradually blackening toward tip that has long, fine filament; total length 16 to 19 mm; maximum width 0.5 mm. On outer thigh whitish basally, gradually blackening toward tip; some with Ochraceous-Buff, subapical zone; total length 13 to 15 mm; maximum width 0.25 mm. Setiforms on middorsal region: Whitish on basal half, gradually blackening toward tip, but interrupted by Ochraceous-Buff, subapical zone; some with Light Ochraceous-Buff, subapical zone and short, blackish zone on tip; total length 12 to 14 mm; maximum width 0.02 mm. Setiforms on outer thighs: Whitish on basal half, then gradually becoming gray on middle part and finally Light Ochraceous-Buff on distal third, or with tip blackish and Ochraceous-Buff, subapical zone.
Skull.—Elongate and broad with no conspicuous crests; rostrum rather stout; jugals deep with transverse crest usually well-developed; zygomatic postorbital process conspicuous and formed entirely of squamosal; incisive foramen short and wide posteriorly; vomerine sheath complete in great majority of specimens, its maxillary part wide and strong; posterior palatine foramina on plane with front of M1 or slightly farther forward; bullae rather small and elongate.
Teeth.—P4 with three secondary folds in all juvenal specimens, but posteriormost fold small and disappearing in 50 per cent of adult specimens; M1 with 3 outer folds in juveniles and also disappearing in 50 per cent of adults; M2 with three outer folds in juveniles, but only 20 per cent remaining in adults; M3 with 3 outer folds in 50 per cent of juveniles, decreasing to 15 per cent in adults. Lower molariform teeth: p4 with 2 secondary folds; m1 with 2 secondary folds in 90 per cent of adults and in all juveniles; m2 with 2 secondary folds in 98 per cent of adults and in all juveniles; m3 with 2 secondary folds in 81 per cent of adults, remaining percentage with only one counterfold, and with 2 secondary folds in all juveniles.
Remarks.—Samples studied of P. dimidiatus are notably uniform throughout the geographic range of the species. The few biotypes detected seemed unworthy of subspecific rank.