_Proechimys longicaudatus_ is limited in the south to the headwaters of the Parana River drainage, where the subspecies _P. l. roberti_ and _P. l. longicaudatus_ are found. The species ranges northward through the Tapajoz drainage, with _P. l. leucomystax_ in the headwaters and _P. l. boimensis_ in the lower course. To the northwest and west the species is represented in Bolivia by _P. l. securus_; _P. l. elassopus_, _P. l. simonsi_, _P. l. pachita_, and _P. l. hendeei_ occur in Peru and _P. l. brevicauda_ in Peru and Brazil; and _P. l. nigrofulvus_ occurs in Ecuador. Again in _P. longicaudatus_ it seems that the number of counterfolds follows a gradient from more humid areas with four counterfolds in p4, as seen in _nigrofulvus_, _pachita_, _simonsi_, _elassopus_ and _brevicauda_, decreasing to three or four in _securus_, to three in _longicaudatus_, but with m3 having only two counterfolds in _leucomystax_ and _roberti_. _P. l. boimensis_, widely separated in the lower Tapajoz (no samples being known from the intervening range) may be the end of a cline started by _leucomystax_ with only 2 counterfolds in m3 and ending to the northward with four counterfolds in m3. Over the same area the counterfolds in p4 increase from 3 to 4. Of _Proechimys goeldii_ I have had inadequate material but there seems to be a similar gradient in it which may be traced from _P. g. steerei_ to _P. g. goeldii_. _P. g. steerei_ has four counterfolds in more upper molars than occurs in the other subspecies. _Proechimys semispinosus_ has its wide range in the mountainous, western area of South America, the headwaters of the Amazon drainage and northward in Central America and the nearby Pacific Islands. In these populations a gradient may exist in the number of counterfolds which is varied in every population. The highest number seems to occur in the populations from northern Peru and Ecuador, decreasing from there in all directions, except in the Panamanian and Columbian islands. In gross examination, it seems that the size of the animals increases to the northwards. SPECIFIC VARIATION IN THE SUBGENUS TRINOMYS Some specific characters are duplicated in each of the two subgenera; that is to say, there are some parallel developments and they give the common generic stock its biotypical variability. Among these parallel developments are the width of the aristiforms, the amount of pigment in the agouti-colored setiforms, and the shape of the nasal bones. Other characters, however, appear in one subgeneric group and not in the other. The specific variation will be discussed separately for each subgenus.