Considered by itself the variation in the tooth pattern can lead to
erroneous conclusions as to differentiation of species, because the
number of folds on the occlusal face of a tooth and the depth of certain
folds may be subject to great individual variation as shown by
examination of more than one large series of specimens of the same kind,
age and sex from a single locality. Also there are geographic gradients
or clines, in number of folds. Nevertheless the variation in number of
folds, when measured at sufficient intervals along a cline, may provide
quantitative characters useful in differentiating subspecies.
[Illustration: FIGS. 2-17. Second left upper molar of the two subgenera
_Proechimys_ and _Trinomys_. All × 8. Anterior border of tooth is at the
top of each figure (Nos. 9 and 17 excepted). Note especially that main
fold is short in _Proechimys_ and long in _Trinomys_.
FIGS. 2-9. _Proechimys_ (_Proechimys_) _semispinosus liminalis_,
female, MN no. 6243, Rio Quichito. Fig. 2, unworn crown. Figs. 3-8,
cross sections at 0.5 mm. intervals, showing changes in the main fold
and counterfolds at increasing depths as the tooth was ground down.
Fig. 8 is 3 mm. below surface shown in fig. 2. Fig. 9, posterior view
with proximal end of the tooth open showing basal ends of folds.
Later in life the proximal end closes and three roots are formed.
FIGS. 10-17. _Proechimys_ (_Trinomys_) _iheringi denigratus_, female,
SEPFA no. 17060, Mata do Ribeirão da Fortuna. Figs. 10-16
corresponding to figs. 2-8. Fig. 17, posterioventral view with
proximal end of the tooth open and part of walls cut away, showing
basal ends of folds. Later in life, as in _Proechimys_, the proximal
end closes and three roots are formed. ]
The main fold involves both the occlusal face of the tooth and the side
wall. The counterfolds, which are smaller counterparts of the main fold,
in most instances also implicate the wall of the tooth opposite to that
marked by the main fold, but are to be seen mostly on only the occlusal
face of the tooth. Unerupted teeth with the crowns unworn and other
teeth which had barely broken through the gums were ground down to
permit the making of drawings of the surfaces at different levels. This
study revealed that the main fold is deepest in the wall of the tooth.
The development of the main fold varies in two different ways: in all
samples from southeastern and eastern Brazil it is strongly developed,
deeply grooves the tooth through its crown and, in younger individuals,
completely divides the occlusal surface of the tooth. As use wears down
the crown, the main fold soon becomes separated from the opposite wall
and then gradually shortens toward its basal portion. In the other type,
common to animals of all the remaining part of the range of the genus,
the main fold is rather short, never reaching the opposite wall. In this
case, however, one of the counterfolds usually appears almost opposite
the main fold in such a way that in non-erupted or just-erupted teeth
the main fold and one counterfold may be connected by a shallow groove
that may give the impression of extension of the main fold and,
therefore, lead to false interpretations. Closer examination shows that
the counterfold which apparently meets the main fold is really situated
anteriorly or posteriorly to it, in upper or lower teeth respectively.
One subspecies in the subgenus _Trinomys_ differs from the general
characteristics of the subgenus in sometimes showing a small main fold
in P4 whereas it is large in all other cheekteeth. The structural
differences here mentioned in the main fold were never before
recognized. Therefore, the meaning of "quadrilaminate" pattern, "three
outer folds" or "three inner folds" (of authors) is not consistent
insofar as the two groups are concerned.
Writers have more or less tacitly admitted three as the usual number of
counterfolds present in the upper molariform teeth. Ellerman, for
example (1940:117), states: "Upper cheekteeth normally with three outer
and one inner folds each, these soon becoming isolated as islands. A few
species, which will be discussed below, vary slightly in pattern."
Thomas (1921:140) erected the subgenus _Trinomys_ on the basis of the
upper molariform teeth having only three laminae. Actually a meticulous
study of widely varying samples shows that the number of counterfolds
may vary from _one_ to _five_, the usual number being three or four. One
of the most important facts to record on this subject is that young
specimens with slightly worn molariform teeth are more apt than either
adult or younger specimens to show the maximum number of counterfolds.
Usually nonworn teeth show rounded crests and valleys of different
depth. For example, it is common to see one continuous groove giving the
impression of a main fold crossing the occlusal surface transversely.
The slightest wear of the occlusal surface, however, shows that really
there are two valleys instead of one. The two are the main fold and one
counterfold. In this case, it would be easy to confuse the two types of
teeth, one with the main fold short and the other with the main fold
extending almost all the way across the occlusal surface.
Some of the counterfolds are especially shallow and tend to disappear in
an early stage of wear, and adult individuals may have these folds
completely worn away. Advanced wear usually develops a cuplike occlusal
surface with only the remains of the main fold and also remains of one
or more counterfolds represented by small enamel islands (Figs. 2 to
17). In the form _Proechimys iheringi iheringi_, for example, every
tooth shows three counterfolds in the upper molariform teeth of
individuals in which the wear is not advanced. This number, however, is
less in all or part of the molariform teeth of older individuals.
Adjacent counterfolds may appear to be coalesced in many instances.
Coalescence is more likely to be seen in species where a wider variation
in the number of the counterfolds is involved and it appears as a
gradient in the reduction of the number of counterfolds.
Of great importance, as a general feature of molariform teeth, is the
relative size as related to the geographical distribution, showing,
again, a natural division in the genus. In all forms of southeastern
Brazil the premolars are larger than the first molars, the first molars
are larger than the second molars, and the second molars are larger than
the third molars. The forms from central and northern Bahia, Brazil,
have the molariform teeth more or less the same size. The forms from the
remaining part of the area occupied by the genus have premolars smaller
than the first molars, the first molars smaller than the second molars,
but the second molars larger than third ones.