The aristiforms are narrow and soft in _P. dimidiatus_ and in the other species are wide and stiff, and on the outer thighs and rump some are light-colored. _P. albispinus_ has the maximum number of light-colored aristiforms; they are present over the sides and back. This species has, however, a type of aristiforms unique in the genus--the clavate type. The tail is longer in _P. iheringi_ and _P. setosus_ than in _P. dimidiatus_ and _P. albispinus_; the longer type is associated with a penicillate tip suggesting an adaptation to arboreal habit. The skull and nasals are longer in _P. dimidiatus_ and _P. iheringi_ than in _P. setosus_ and _P. albispinus_. In the latter two species the longitudinal dorsal outline of the skull is conspicuously convex as opposed to slightly convex in the other two species. The palate is longest in _P. dimidiatus_ and _P. iheringi_ extending posteriorly to the level of the second molars; it is slightly shorter in _P. setosus_ and shortest in _P. albispinus_ where it does not extend behind the level of the first molars. The incisors are opisthodont in _P. dimidiatus_ and _P. iheringi_ and orthodont in _P. setosus_ and _P. albispinus_ and even proodont in one part of the last species. The molariform teeth have a large number of counterfolds in both _P. dimidiatus_ and _P. iheringi_, although the number varies but little in the first species and much in the second. The variation in _P. iheringi_ decreases in populations of increasingly more northern geographic distribution; in both _P. setosus_ and _P. albispinus_ the number of counterfolds is greatly reduced; there is only one in most specimens of _P. albispinus_. The incisive foramen is small and nearly round in _P. dimidiatus_, larger and elongate in _P. iheringi_, very narrow and fissurelike in both _P. setosus_ and _P. albispinus_. The characters of _Trinomys_, as briefly outlined above, seem to be the result of one original species having split first into four species which provide a gradient for certain characters. Subsequently one of these four species, _P. iheringi_, split into six subspecies and another gradient, parallel to the first, and involving the same characters, is to be seen. The interrelationship among the species is evident, not only because they have the same subgeneric characters, but because the full species themselves provide successive steps in a stairway of increasing specialization from _P. dimidiatus_ to _P. albispinus_.