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 counterfold. | |||
| 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 | ||
| 7. | (a) | No clavate aristiforms, tail with white tip, | ||
| P. setosus, | p. 384 | |||
| (b) | Clavate aristiforms among the ordinary ones, tail without white tip, | |||
| P. albispinus, | p. 388 | |||
Fig. 22. Map showing the distribution of the two subgenera of the genus Proechimys.