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
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
[Illustration: FIG. 22. Map showing the distribution of the two
subgenera of the genus _Proechimys_.]
[Illustration: FIG. 23. Map showing the geographic ranges of four
species of the genus _Proechimys_.]
[Illustration: FIG. 24. Map showing the geographic ranges of four
species of the genus _Proechimys_.]