The species, or subspecies belonging to different species, living together are: _goeldii_ and _hyleae_, at Fazenda Paraiso; _goeldii_ and _riparum_ in Manaus; _boimensis_ and _hyleae_ in Tauarí; _leucomystax_ and _villicauda_ in Utiarití; _mincae_ and _canicollis_ in Bonda; _gularis_ and _hendeei_ on the banks of Rio Napo ("same trap lines," according to P. Hershkovitz, _In Litt._). Study of samples of the above named pairs of kinds of _Proechimys_ showed the following specific differences: _goeldii_ is large with narrow aristiforms, has a large and strongly built skull, with four counterfolds in one or more upper molars: _hyleae_ is smaller, has wide aristiforms, smaller skull with less pronounced ridges, and never has more than three counterfolds in the upper molariform teeth; _riparum_ closely resembles _hyleae_; _boimensis_ has thin aristiforms, small skull and no more than three counterfolds in the upper molariform teeth in contrast to _hyleae_, already discussed; _leucomystax_ closely resembles _boimensis_; _villicauda_ closely resembles both _hyleae_ and _riparum_; _mincae_ is similar to _hyleae-riparum-villicauda_; _canicollis_ has the number of counterfolds in all molars reduced to two; _gularis_ is large, has a strongly built and ridged skull, some upper molariform teeth with four counterfolds and wide aristiforms; _hendeei_ closely resembles _leucomystax_ and _boimensis_. The evidence obtained from study of specimens where two or more species occurred together was applied to the remaining samples and the geographic distribution was worked out. As a result the arrangement below was made, including all valid kinds already named and those here newly named from Brazil. The names of kinds I do not consider as belonging to the subgenus (and genus) are excluded. These are _Echimys macrourus_ Jentink, not seen, and _Proechimys cayennensis hoplomyoides_ Tate (= genus _Hoplomys_). The application of names is tentative, however, because the types deposited in Europe have not been seen. An asterisk denotes the forms not seen by me. _Proechimys guyannensis: arabupu, arescens, bolivianus, cherriei, chrysaeolus, guairae, o'connelli, guyannensis*, hyleae, leioprimna, mincae, nesiotes, ochraceus, oris, poliopus, ribeiroi, riparum, trinitatis, urichi, vacillator*, villicauda, warreni._ _Proechimys longicaudatus: boimensis, brevicauda, elassopus, hendeei, leucomystax, longicaudatus, nigrofulvus, pachita, rattinus*, roberti, securus, simonsi._ _Proechimys semispinosus: amphichoricus, burrus, calidior, centralis, chiriquinus, colombianus, decumanus, goldmani*, gorgonae, gularis, hilda*, ignotus, kermiti, liminalis, panamensis, rosa*, rubellus, semispinosus._ _Proechimys goeldii: goeldii, steerei._ _Proechimys canicollis._