Writers have more or less tacitly admitted three as the usual number of counterfolds present in the upper molariform teeth. Ellerman, for example (1940:117), states: "Upper cheekteeth normally with three outer and one inner folds each, these soon becoming isolated as islands. A few species, which will be discussed below, vary slightly in pattern." Thomas (1921:140) erected the subgenus _Trinomys_ on the basis of the upper molariform teeth having only three laminae. Actually a meticulous study of widely varying samples shows that the number of counterfolds may vary from _one_ to _five_, the usual number being three or four. One of the most important facts to record on this subject is that young specimens with slightly worn molariform teeth are more apt than either adult or younger specimens to show the maximum number of counterfolds. Usually nonworn teeth show rounded crests and valleys of different depth. For example, it is common to see one continuous groove giving the impression of a main fold crossing the occlusal surface transversely. The slightest wear of the occlusal surface, however, shows that really there are two valleys instead of one. The two are the main fold and one counterfold. In this case, it would be easy to confuse the two types of teeth, one with the main fold short and the other with the main fold extending almost all the way across the occlusal surface. Some of the counterfolds are especially shallow and tend to disappear in an early stage of wear, and adult individuals may have these folds completely worn away. Advanced wear usually develops a cuplike occlusal surface with only the remains of the main fold and also remains of one or more counterfolds represented by small enamel islands (Figs. 2 to 17). In the form _Proechimys iheringi iheringi_, for example, every tooth shows three counterfolds in the upper molariform teeth of individuals in which the wear is not advanced. This number, however, is less in all or part of the molariform teeth of older individuals. Adjacent counterfolds may appear to be coalesced in many instances. Coalescence is more likely to be seen in species where a wider variation in the number of the counterfolds is involved and it appears as a gradient in the reduction of the number of counterfolds. Of great importance, as a general feature of molariform teeth, is the relative size as related to the geographical distribution, showing, again, a natural division in the genus. In all forms of southeastern Brazil the premolars are larger than the first molars, the first molars are larger than the second molars, and the second molars are larger than the third molars. The forms from central and northern Bahia, Brazil, have the molariform teeth more or less the same size. The forms from the remaining part of the area occupied by the genus have premolars smaller than the first molars, the first molars smaller than the second molars, but the second molars larger than third ones. HABITS