1. The genus Proechimys is divisible into two subgenera. In all Brazil there are four full species of each subgenus, or 8 species in all. All but one of these are divisible into subspecies of which there are 29, making a total of 30 kinds in Brazil; 14 of these are here newly named.

2. It is new information, I think, that: (1) One main fold extending entirely across the worn crown of the molariform tooth is peculiar to Trinomys; in the subgenus Proechimys, apparent complete division of the crown surface is accomplished by a short main fold meeting a counterfold originating on the opposite side of the tooth; (2) progressive decrease in size of molariform teeth from P4 to M3 is peculiar to the subgenus Trinomys; in the subgenus Proechimys, M2 is largest and the teeth are progressively smaller anteriorly.

3. In the one species, Proechimys albispinus, which has the widest distribution of aristiforms on the body of any species in the genus, some of the aristiforms are clavate. Clavate aristiforms occur in the most spiny species of the related genus Echimys.

4. In subspecies of any one full species the incisive foramen is larger in animals which inhabit arid areas than in those which inhabit humid areas. Possibly increased area of moist mucosa associated with Jacobson's organ is required in arid areas for maintenance of the necessary keenness of smell.

5. The number of counterfolds in the molariform teeth vary in clinal fashion. Their variation is in response to humidity. Increasing humidity is correlated with increasing number of folds, and decreasing humidity is correlated with decreasing number of folds.

6. Clinal variation correlated with increasing humidity is to be seen also in longer tail and darker color of pelage.

7. The primitive Proechimys probably was large, with a short tail, narrow aristiforms, strongly built skull, and five counterfolds in each molariform tooth.

8. Geographic isolation appears to have been a factor in the establishment of the two subgenera; the arid belt along the SÃo Francisco River and northward to CearÁ appears to be uninhabited by Proechimys and constitutes a barrier separating the two subgenera, Proechimys and Trinomys.

9. This arid belt probably developed relatively early, since in deposits of late Pleistocene age, remains of the subgenus Trinomys have been found in the area where the subgenus still occurs.

10. The most primitive types occur at the periphery of the range of the genus.

11. Populations from small islands tend to be more primitive than populations on the mainland. Insular populations develop a homozygous condition with resultant disappearance of secondary biotypes.

12. Insular animals ordinarily are larger than their mainland counterpart.