_General characters._--Pelage of upper parts with lanceolate and, sometimes, clavate aristiforms extending over most of rump and onto thighs; tail 86 to 103 per cent of length of head and body; tail sometimes white-tipped and sometimes penicillate; skull small, with ridges moderately developed; supraorbital ridges involving no part of parietals; infraorbital foramen with no separate groove for transmission of nerve; mesopterygoid fossa extending forward to level of second or first molars; incisors opisthodont, orthodont or proodont; molariform teeth, in occlusal view, with main fold large and usually reaching opposite wall; no counterfold anterior to main fold in upper molariform teeth and usually no counterfold posterior to main fold in lower molariform teeth; premolars larger than first molars, first molars larger than second molars and second molars larger than third molars; four molariform teeth of nearly equal size in some animals. Thomas (1921:140) erected the subgenus _Trinomys_, including in it the species _albispinus_ and _setosus_ and stated that "the primary distinction between these [_Trinomys_ and _Proechimys_] lies in the number of laminae present in the cheekteeth--four in _Proechimys_, three in _Trinomys_." The distinction is valueless as a subgeneric character, not only because the character is not constant in the species in the subgenus but also because there is subspecific variation in number of laminae in the cheekteeth. _Proechimys albispinus_, however, shares with three other species common characters, as listed above, and the name _Trinomys_ will, therefore, apply to this group of species, since _Proechimys albispinus_ is the genotype. =Proechimys dimidiatus= (Günther) _Echimys dimidiatus_ Günther, 1 April 1877, Proc. Zool. Soc. London, 1876(4):747. _Proechimys dimidiatus_ Allen, 1899, Bull. Amer. Mus. Nat. Hist., 12(20):264; Ribeiro, 1905, Arch. Mus. Nac. Rio de Janeiro, 13:187; Thomas, 1921, Ann. Mag. Nat. Hist., 8 (ser. 9):141; Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):400; Ellerman, 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:122. _Type locality._--Unknown; probably southwestern Rio de Janeiro, Brazil (see Remarks). _Type_: British Museum (Nat. Hist.), no. 51.7.21.24; presented by Lord Derby. _Range._--Rio de Janeiro, from the southern limit of the state northward to and including the Distrito Federal.