One of the species of _Trinomys_, _Proechimys iheringi_, is here
subdivided into six subspecies which show a clinal variation. _P. i.
iheringi_, in the southernmost part of the range of the species (Ilha de
São Sebastião), has three counterfolds in the upper cheekteeth of almost
every young specimen but one of these counterfolds, since it is small,
very shallow, and disappears after little wear, is probably in the
process of disappearance; all lower cheekteeth have two counterfolds or,
rarely, m3 has only one. _P. i. bonafidei_ is the next subspecies
northward, where it was collected at 850 m altitude (Fazenda Bõa Fé).
This subspecies still has two counterfolds in all the upper cheekteeth;
only 3 out of 16 specimens fail to have these counterfolds coalesced in
one or more of the teeth. In the lower cheekteeth the coalescence is
evident in 18 per cent of the specimens. _P. i. gratiosus_, from
Floresta da Caixa Dagua (alt. 750 m), geographically is well removed from
_bonafidei_ (more than two degrees north), and no samples were obtained
from the intervening area. It shows such great reduction in the
counterfolds that the existence of intermediate populations is clearly
suggested. Every upper cheektooth of this subspecies has the two
counterfolds coalesced and in 40 per cent of the specimens M3 has only
one counterfold; in the lower cheekteeth 60 per cent of the specimens
have only one counterfold in m3. _P. i. panema_, occurring approximately
100 kilometers to the northward of _P. i. gratiosus_ (lowland form), has
one counterfold in M3 in only 20 per cent of the specimens but the lower
third molar has only one counterfold in 80 per cent of the specimens. In
_P. i. denigratus_, from about 3 degrees north of the range of _P. i.
panema_, the reduction is proportionately greater: P4 now is the only
upper cheektooth with two counterfolds in every specimen; all molars
tend to have only one; p4 has also two counterfolds but all lower molars
have only one.
The relative size of the tail also varies in a cline from south to
north. Its length is approximately 87 per cent of the length of the head
and body in _P. iheringi_; 88 per cent in _bonafidei_; 99 per cent in
_gratiosus_; 100 per cent in _panema_; and 103 in _denigratus_.
One of the subspecies, _P. i. paratus_, however, seems to be completely
out of the dental cline. It was collected in the near proximity of the
type locality of _P. i. gratiosus_, at an elevation of 120 m lower. This
subspecies has two counterfolds in all molariform teeth and only one of
the two specimens known shows these counterfolds coalesced in P4 and M1.
The sample, 2 specimens, is too small to be trustworthy; hence it is
impossible satisfactorily to account for the break in the clinal
variation. Conceivably two full species are involved, but I prefer at
present to defer decision on this problem until such time as more
evidence is accumulated.
_P. setosus_ is poorly represented, both of the available skins being
faded. Furthermore, no type locality is known for the subspecies _P. s.
setosus_.
_P. albispinus_ has only two known subspecies: _P. a. albispinus_,
living in a region of higher humidity, is slightly the darker and has
subapical zones of the setiforms on the sides Ochraceous-Tawny; _P. a.
sertonius_, living in a much drier region, has the same subapical zone
Ochraceous-Buff. The number of specimens of _P. a. sertonius_ is so few
that no gradient can be detected, even if one exists.