_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. In discussing the type locality of the species, Thomas (1921:141) states: "We know that its donor did obtain a number of specimens from Rio Janeiro, and the skull agrees so closely with those of two examples from Itatiaia, near to the Rio-Minas frontier, collected and presented by Prof. J. P. Hill, that I have no hesitation in referring the latter to Günther's species." _Specimens examined._--Total number, 211 (MN), from Brazil as follows: Rio de Janeiro; _Parati_, Pedra Branca (400 m.), 113; _Mangaratiba_, Fazenda do Rubião (750 m.), 3; Fazenda do Tenente (700 m.), 4; Fazenda da Lapa (450 m.), 13; _Teresópolis_, Fazenda Guinle (960 m.), 61; _Nova Iguassú_, Barro Branco (20 m.), 16; Distrito Federal, Tijuca, 1. _Additional records._--Rio de Janeiro, Itatiaia (Thomas, 1921:141); Rio de Janeiro, Zona da mata, Mont-Serrat, Serra do Itatiaia (Ribeiro, 1905:187). =Proechimys iheringi= Thomas _General characters._--Size large; tail long; aristiforms generally wide and stiff; general color on upper parts and sides a combination of blackish from tips of aristiforms with cinnamon ground color from subapical zones of setiforms; darker band on middorsal line; differentiated light-colored aristiforms conspicuous on outer sides of thighs and rump; usually rufous tint on neck and postauricular region; underparts white; tail with white tip, usually accentuated by white brush; feet white on dorsal surface; hind feet slightly darker on outer sides; skull elongate and smooth; jugals wide dorso-ventrally; incisive foramen elongate; upper molariform teeth usually with one to five counterfolds, number varying with subspecies; lower premolar always with two counterfolds and lower molars always with one or two counterfolds.