e rows of upper
cheek-teeth are nearly parallel to each other, although in the specimens
that I have seen of _E. townsendii_, the upper rows of cheek-teeth
converge posteriorly.

_Degree of constriction of the interorbital region._--The interorbital
region is more constricted in most species of the subgenus _Neotamias_
than in the subgenus _Eutamias_ and the genus _Tamias_. In specimens of
_E. t. townsendii_ of the subgenus _Neotamias_, however, the degree of
constriction of the interorbital region is approximately the same as in
the subgenus _Eutamias_ and the genus _Tamias_.

_Shape of the pinna._--The pinna is narrower and more pointed in the
subgenus _Neotamias_ than in the subgenus _Eutamias_ and the genus
_Tamias_.

STRUCTURAL FEATURES THAT ARE TOO WEAKLY EXPRESSED TO BE OF TAXONOMIC USE

The following alleged characters have been mentioned in the literature.
Since the degree of expression of these features is so slight, or
since there is marked variation within one or more natural groups of
chipmunks, no reliance is here placed on these features. They are as
follows: (1) Degree of the posterior projection of the palate; (2)
relative size of the auditory bullae; (3) position, in relation to P4,
of the notch in the posterior edge of the zygomatic plate; (4) size of
m3 in relation to m2; (5) degree of development of the mesoconid and
ectolophid of the lower molars; (6) shape and length of the rostrum;
(7) degree of distinctness of minute longitudinal grooves on the upper
incisors.

A variation that does not readily fall in any one of the three
categories mentioned above is the degree of development of the
lambdoidal crest. The crest is least developed in the subgenus
_Neotamias_ and most developed in the genus _Tamias_. The larger the
skull, the more the lambdoidal crest is developed; seemingly, therefore,
the degree of development is an expression of size of the skull and may
be determined by heterogonic growth.

DISCUSSION

As shown in table 1, there are ten characters by means of which
_Eutamias_ and _Tamias_ can be separated consistently. The subgenus
_Eutamias_ occurs on the Asiatic side and the subgenus _Neotamias_
occurs on the North American side of Bering Strait, yet the two
subgenera agree in the ten features referred to. Although the subgenus
_Neotamias_ and the genus _Tamias_ occur together in parts of the United
States and Canada, they differ in the ten features, indicating that the
subgen