Most of the described forms in the subgenus were initially named as distinct full species. More recently, however, in accordance with the ideas now prevalent in systematic work, many of the named kinds were reduced to the rank of subspecies. Tate first made a geographic arrangement (1935:399-400) and later (1939:177-178) provisionally synonymised several named kinds of _Proechimys_ with _Proechimys_ "_cayennensis cayennensis_." A similar tendency was clearly displayed by Ellerman (1940:115-122) who allocated 29 names, out of 33 (in the subgenus, as here understood), to the species _Proechimys guyannensis_ and gave full specific rank to four other named kinds. Osgood (1944) also had the same viewpoint; that is to say, he appeared to have the idea that there were only two full species in the subgenus in Brazil--admitting this orally--and consequently he synonymised some full species where two or more occurred in the same place, thinking that he was dealing with individual, rather than specific, differences. Evidently the number of species in the subgenus cannot be great because the known kinds show few patterns worthy of specific designation and therefore the majority of the existing names should be suspected of having no more than subspecific value. Nevertheless none of the above writers presented real evidence in support of his arrangement. Criteria for the recognition of full species are most easily recognized where two or more different species live together. In the literature, _P. goeldii_ and _P. "oris"_ were mentioned by Thomas (1912:89) as having been collected in the same place; _P. mincae_ and _P. canicollis_, by H. H. Smith (in Allen, 1904:440); _P._ "_leucomystax_," from Utiarití, by Miranda Ribeiro (1914:42) and _P._ "_longicaudatus_," from the same place, by Allen (1916:569) were other examples. In these, and other alleged instances of two or more kinds occurring together, detailed study of the specimens concerned was necessary to learn the true facts. Also with the opportunity to compare collections from several different places, new facts emerged. _P. longicaudatus_, as it was conceived of by Allen, was a composite species, but in one locality, Utiarití, Ribeiro and Allen actually were dealing with two distinct species.