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_.
In northern South America, north of the Amazon River, the subspecies
with the greatest number of counterfolds is _P. guyannensis warreni_
(known from only the Demerara River area); p4 has four counterfolds and
the lower molars have three each. The number decreases in all the
adjacent populations: _P. g. guyannensis_, in the Guianas, _P. g.
trinitatis_, and _P. g. urichi_ (going westward from the Guianas to
Venezuela) have the counterfolds reduced to three in p4, but the lower
molars still have the same number of counterfolds, namely, three,
although there is a tendency for them to coalesce; farther west, on the
coast, the number decreases to three counterfolds in p4 and to only two
in the lower molars as in _P. g. guairae_ and _P. g. mincae_. Subspecies
south of the coast show the same reduction of counterfolds, _P. g.
cherriei_ and _P. g. o'connelli_ being examples; _P. g. ochraceus_ and
_P. g. poliopus_ have the reduction carried to the upper molars, M3
having usually only two counterfolds; _P. g. chrysaeolus_ in the valley
between the Madalena and the Cauca rivers seems to be somewhat isolated
and shows reversion to three counterfolds in the lower molariform teeth;
directly southward of the range of _P. g. warreni_ the number of
counterfolds decreases to three in all lower cheekteeth (population at
Ayan-Tepuy, southern Venezuela), and then to three in p4 and to two in
the lower molars, as in _P. g. arabupu_ on the Brazilian side of Mount
Roraima, and the reduction is extended to the upper molars in _P. g.
vacillator_.
On the north bank of the Amazon, the only population of _P. g. hyleae_
known to me (from Obidos) has four counterfolds in p4 and three in the
lower molars; _P. g. riparum_, from Manaus, also on the north bank of
the Amazon, has three counterfolds in p4 and two counterfolds in the
lower molars. _P. g. hyleae_ occurs also on the south bank of the
Amazon. _P. g. riparum_, therefore, may be the northern part of the
southern cline, instead of the southern end of the northern cline.
The whole picture, as outlined above, may be explained by assuming that
the species _P. guyannensis_ differentiated somewhere on the Central
Plateau of South America, with three counterfolds in each upper
molariform tooth, four counterfolds in the lower premolar and three
counterfolds in the lower molars. The species might have extended its
range to the Guianas and then all the biotypes with reduced number of
counterfolds might have slowly developed by natural selection. The
gradient is, broadly, from subspecies with greater number of
counterfolds in more humid areas, to a gradually lessening number of
counterfolds in less humid areas.
_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.