_Remarks._--Thomas (1921:140-143) summarized the available information on the forms from southeastern Brazil and synonymised _myosuros_ with _setosus_. The description of _leptosoma_ by Brants applies to this same species except in a few features. However, neither Brants nor Lichtenstein described the tail of _myosuros_ as having a white tip or even as having a heavily pencilled tip, although Wagner (1843:342) in redescribing Lichtenstein's species indicated that the tail had a white pencil. He gave also measurements of the head and body, and tail, which do not agree with the original measurements given by either Lichtenstein or Brants. Lichtenstein, in the original description, gave the type locality of _myosuros_ as Bahia, and stated that it was collected by Freireiss. Brants also gave Bahia as the type locality for _leptosoma_. The names _leptosoma_ Brants and _cinnamoneus_ Lichtenstein are evidently no more than duplicate names for _myosuros_, as pointed out by Thomas (1921:141) and subsequent writers. Specimens referred to any of these forms, therefore, could take the earlier name _myosuros_ until identified otherwise. Lichtenstein later (1830, text for plate 36, fig. 2) added São Paulo State to the known range of the species, mentioning specimens collected there by Sello. Probably, therefore, Wagner redescribed _leptosoma_ using a composite sample; the white tip on the tail could occur in any race of _P. iheringi_ from São Paulo. Considering the short hind feet and the wide aristiforms, _Proechimys myosuros_ probably will eventually prove to be related to _albispinus_; perhaps it will prove to be a synonym of _albispinus_. CONCLUSIONS 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_.