In northern South America, north of the Amazon River, the subspecies with the greatest number of counterfolds is _P. guyannensis warreni_ (known from only the Demerara River area); p4 has four counterfolds and the lower molars have three each. The number decreases in all the adjacent populations: _P. g. guyannensis_, in the Guianas, _P. g. trinitatis_, and _P. g. urichi_ (going westward from the Guianas to Venezuela) have the counterfolds reduced to three in p4, but the lower molars still have the same number of counterfolds, namely, three, although there is a tendency for them to coalesce; farther west, on the coast, the number decreases to three counterfolds in p4 and to only two in the lower molars as in _P. g. guairae_ and _P. g. mincae_. Subspecies south of the coast show the same reduction of counterfolds, _P. g. cherriei_ and _P. g. o'connelli_ being examples; _P. g. ochraceus_ and _P. g. poliopus_ have the reduction carried to the upper molars, M3 having usually only two counterfolds; _P. g. chrysaeolus_ in the valley between the Madalena and the Cauca rivers seems to be somewhat isolated and shows reversion to three counterfolds in the lower molariform teeth; directly southward of the range of _P. g. warreni_ the number of counterfolds decreases to three in all lower cheekteeth (population at Ayan-Tepuy, southern Venezuela), and then to three in p4 and to two in the lower molars, as in _P. g. arabupu_ on the Brazilian side of Mount Roraima, and the reduction is extended to the upper molars in _P. g. vacillator_. On the north bank of the Amazon, the only population of _P. g. hyleae_ known to me (from Obidos) has four counterfolds in p4 and three in the lower molars; _P. g. riparum_, from Manaus, also on the north bank of the Amazon, has three counterfolds in p4 and two counterfolds in the lower molars. _P. g. hyleae_ occurs also on the south bank of the Amazon. _P. g. riparum_, therefore, may be the northern part of the southern cline, instead of the southern end of the northern cline. The whole picture, as outlined above, may be explained by assuming that the species _P. guyannensis_ differentiated somewhere on the Central Plateau of South America, with three counterfolds in each upper molariform tooth, four counterfolds in the lower premolar and three counterfolds in the lower molars. The species might have extended its range to the Guianas and then all the biotypes with reduced number of counterfolds might have slowly developed by natural selection. The gradient is, broadly, from subspecies with greater number of counterfolds in more humid areas, to a gradually lessening number of counterfolds in less humid areas.