Another point which favors the idea that narrow aristiforms result from an increased number of genes is that, generally, the aristiforms are narrow in any species whose geographic range is extensive and relatively uniform. _Proechimys goeldii_ is the species which has the largest number of characters that are judged to be primitive, and it may be the oldest stock. _P. semispinosus_, _P. longicaudatus_ and _P. guyannensis_ may have been derived from an early splitting of the genus or they may have branched off the main stem at different times. _P. canicollis_, however, seems clearly to be an offshoot of _P. guyannensis_; _canicollis_ shows greater resemblance to _guyannensis_ than to any other species. _P. g. vacillator_ is another close relative of _P. guyannensis_ with the number of counterfolds almost as much reduced as in _P. canicollis_. Conceivably, _vacillator_ is a full species, but the reduction in number of counterfolds in the teeth more probably expresses only one extreme of a gradient, as will be discussed below. SUBSPECIFIC VARIATION IN THE SUBGENUS PROECHIMYS In spite of the lack of specimens from areas in which _Proechimys_ certainly occurs, it is evident that the genus has great plasticity and that the number of subspecies will be greatly increased as additional material is studied. Only perfunctory examination of samples from outside the area of Brazil shows me that there are several unnamed subspecies there. My impression is that Allen's _trinitatis_, of Trinidad, the genotype of _Proechimys_, will eventually be split. There are two main lines of subspeciation in _Proechimys guyannensis_. The one south of the Amazon River includes _P. g. bolivianus_, in Bolivia, _P. g. villicauda_, and _P. g. ribeiroi_ occurring on the divide of the headwaters of the Amazon and Parana rivers, in Brazil, and _P. g. hyleae_ in the lower Tapajoz and _P. g. nesiotes_ in the lower Tocantins. All six subspecies have a large number of counterfolds in the molariform teeth. In these six subspecies, p4 has four counterfolds and the lower molars have three each. Toward the northeastern coast the number of counterfolds decreases to three in p4 and to two in the lower molars, as in _P. g. arescens_, _P. g. leioprimna_ and _P. g. oris_.