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.