Fig. 99. Map showing the geographic ranges of the subspecies of three species of the subgenus Trinomys.
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.
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.