FIGS. 18-19. _Proechimys_ (_Proechimys_) _goeldii steerei_, sex ?, USNM no. 105537, "Hyutanaham." Upper teeth at left (fig. 18). FIGS. 20-21. _Proechimys_ (_Trinomys_) _dimidiatus_, male, MN no. 6256, Pedra Branca. ] _Teeth._--Incisors opisthodont, orthodont or proodont, not grooved; upper molariform teeth with a main internal fold and one to five external counterfolds which usually appear as enamel islands in worn teeth, these counterfolds barely implicating the lateral wall; lower molariform teeth with folds as in the upper molariform teeth except that they are reversed and the number of internal counterfolds is usually fewer in the molars. ARTIFICIAL KEY TO THE SUBGENERA AND SPECIES 1. (_a_) Tail less than 90 per cent of head and body; aristiforms not evident on outer thighs and rump; skull with ridges across parietals; size of upper cheekteeth increasing from P4 to M2; main fold small. subgenus PROECHIMYS, 2 (_b_) Tail 90 per cent or more of head and body; aristiforms evident on outer thighs and rump; skull with no ridges across parietals; size of upper cheekteeth decreasing from P4 to M3; main fold large. subgenus TRINOMYS, 5 2. (_a_) One or more upper molars with four counterfolds 3 (_b_) Upper molars with no more than three counterfolds, 4 3. (_a_) Aristiforms wide (more than 0.7 mm). _P. semispinosus_, p. 342 (_b_) Aristiforms narrow (less than 0.7 mm) _P. goeldii_, p. 338 4. (_a_) Aristiforms wide (0.9 mm or more), or narrow (0.6 to 0.7 mm) but then with only two counterfolds in each lower molar _P. guyannensis_, p. 355 (_b_) Aristiforms narrow (0.5 to 0.65 mm) but with one or more lower molars having three counterfolds. _P. longicaudatus_, p. 346 5. (_a_) Aristiforms narrow (0.5 mm) and limber; no differentiated light-colored aristiforms on outer thighs and rump; incisive foramen short and widest posteriorly _P. dimidiatus_, p. 371 (_b_) Aristiforms 0.6 mm or more and stiff; differentiated light-colored aristiforms on outer thighs and rump; incisive foramen elongated and constricted posteriorly 6 6. (_a_) Skull large, more than 50 mm in total length, incisors opisthodont _P. iheringi_, p. 373 (_b_) Skull small, less than 49 mm in total length, incisors orthodont or proodont 7