1. The genus _Proechimys_ is divisible into two subgenera. In all
Brazil there are four full species of each subgenus, or 8 species
in all. All but one of these are divisible into subspecies of which
there are 29, making a total of 30 kinds in Brazil; 14 of these are
here newly named.
2. It is new information, I think, that: (1) One main fold
extending entirely across the worn crown of the molariform tooth
is peculiar to _Trinomys_; in the subgenus _Proechimys_, apparent
complete division of the crown surface is accomplished by a short
main fold meeting a counterfold originating on the opposite side
of the tooth; (2) progressive decrease in size of molariform teeth
from P4 to M3 is peculiar to the subgenus _Trinomys_; in the
subgenus _Proechimys_, M2 is largest and the teeth are
progressively smaller anteriorly.
3. In the one species, _Proechimys albispinus_, which has the
widest distribution of aristiforms on the body of any species in
the genus, some of the aristiforms are clavate. Clavate
aristiforms occur in the most spiny species of the related genus
_Echimys_.
4. In subspecies of any one full species the incisive foramen is
larger in animals which inhabit arid areas than in those which
inhabit humid areas. Possibly increased area of moist mucosa
associated with Jacobson's organ is required in arid areas for
maintenance of the necessary keenness of smell.
5. The number of counterfolds in the molariform teeth vary in
clinal fashion. Their variation is in response to humidity.
Increasing humidity is correlated with increasing number of folds,
and decreasing humidity is correlated with decreasing number of
folds.
6. Clinal variation correlated with increasing humidity is to be
seen also in longer tail and darker color of pelage.
7. The primitive _Proechimys_ probably was large, with a short
tail, narrow aristiforms, strongly built skull, and five
counterfolds in each molariform tooth.
8. Geographic isolation appears to have been a factor in the
establishment of the two subgenera; the arid belt along the São
Francisco River and northward to Ceará appears to be uninhabited
by _Proechimys_ and constitutes a barrier separating the two
subgenera, _Proechimys_ and _Trinomys_.
9. This arid belt probably developed relatively early, since in
deposits of late Pleistocene age, remains of the subgenus
_Trinomys_ have been found in the area where the subgenus still
occurs.
10. The most primitive types occur at the periphery of the range
of the genus.
11. Populations from small islands tend to be more primitive than
populations on the mainland. Insular populations develop a
homozygous condition with resultant disappearance of secondary
biotypes.
12. Insular animals ordinarily are larger than their mainland
counterpart.