The fact that a much larger number of subspecies occupies the larger geographic range of the subgenus _Proechimys_ would not be sufficient to prove that this subgenus, _Proechimys_, is nearer to the primitive group than _Trinomys_, the subgenus occupying the smaller range with fewer subspecies. The paleontological evolution of the rodents, however, consistently points to teeth with a larger number of counterfolds (as seen in _Proechimys_) as the primitive condition. The extension of the main fold, tending to set apart one lamina in each upper molariform tooth, seems to be a specialization; reduction in the size of the head and body, increase in length of tail and decreasing size of molars posteriorly also may be specializations. The main point, however, is to establish if _Trinomys_ is a relic group rather than a "differentiated" one. If an intermediate form were known which connected _Trinomys_ with one species of _Proechimys_ more than with another or even if _Trinomys_ itself more closely resembled one of the groups of species of the subgenus _Proechimys_ than it did another, we would assume that divergence and selection accounted for the subgeneric variation. The lack of any such connecting link favors the first idea, namely that _Trinomys_ differentiated rapidly with the aid of geographic variation. If _Trinomys_ is, as I am inclined to consider it, the result of "differentiation," its subgeneric features are to be admitted as "new" and therefore the most primitive species in the genus should be found in the subgenus _Proechimys_. It is a matter of common sense to admit the two groups considered above as subgenera rather than genera. Since the two structural plans were established they would, and do, act as different sources of variation. On the other hand, the morphological differences do not give the two groups an amount of morphological differences that would justify full generic rank for each. SPECIFIC VARIATION IN THE SUBGENUS PROECHIMYS