The main fold involves both the occlusal face of the tooth and the side wall. The counterfolds, which are smaller counterparts of the main fold, in most instances also implicate the wall of the tooth opposite to that marked by the main fold, but are to be seen mostly on only the occlusal face of the tooth. Unerupted teeth with the crowns unworn and other teeth which had barely broken through the gums were ground down to permit the making of drawings of the surfaces at different levels. This study revealed that the main fold is deepest in the wall of the tooth. The development of the main fold varies in two different ways: in all samples from southeastern and eastern Brazil it is strongly developed, deeply grooves the tooth through its crown and, in younger individuals, completely divides the occlusal surface of the tooth. As use wears down the crown, the main fold soon becomes separated from the opposite wall and then gradually shortens toward its basal portion. In the other type, common to animals of all the remaining part of the range of the genus, the main fold is rather short, never reaching the opposite wall. In this case, however, one of the counterfolds usually appears almost opposite the main fold in such a way that in non-erupted or just-erupted teeth the main fold and one counterfold may be connected by a shallow groove that may give the impression of extension of the main fold and, therefore, lead to false interpretations. Closer examination shows that the counterfold which apparently meets the main fold is really situated anteriorly or posteriorly to it, in upper or lower teeth respectively. One subspecies in the subgenus _Trinomys_ differs from the general characteristics of the subgenus in sometimes showing a small main fold in P4 whereas it is large in all other cheekteeth. The structural differences here mentioned in the main fold were never before recognized. Therefore, the meaning of "quadrilaminate" pattern, "three outer folds" or "three inner folds" (of authors) is not consistent insofar as the two groups are concerned.