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
subgenera _Eutamias_ and _Neotamias_ are more closely related to each
other than either is to _Tamias_.
TABLE 1.--CHARACTERS BY MEANS OF WHICH THE GENERA EUTAMIAS AND TAMIAS
CAN BE DISTINGUISHED
=======================================================================
Character | _Eutamias_ | _Tamias_
-------------------------------------+----------------+----------------
Shape of head of malleus. | not elongated. | elongated.
-------------------------------------+----------------+----------------
Angle formed by planes of lamina and | approximately | approximately
manubrium of malleus. | 90 degrees. | 60 degrees.
-------------------------------------+----------------+----------------
Position of keel on tip of baculum. | dorsal. | ventral.
-------------------------------------+----------------+----------------
Relation of hypohyal and ceratohyal | fused in | never fused.
bones of hyoid apparatus. | adults. |
-------------------------------------+----------------+----------------
Appearance in cross section of | flattened. | rounded.
conjoining tendon of anterior and | |
posterior digastric muscles. | |
-------------------------------------+----------------+----------------
Presence or absence of P3. | present. | absent.
-------------------------------------+----------------+----------------
Projection of anterior root of P4 in | buccal. | lingual.
relation to masseteric knob. | |
-------------------------------------+----------------+----------------
Length of tail in relation to total | more than | less than
length. | 40 per cent. | 38 per cent.
-------------------------------------+----------------+----------------
Width of longitudinal stripes. | subequal. | median pair of
| | light stripes
| | twice as wide
| | as others.
-------------------------------------+----------------+----------------
Length of lateral longitudinal light | outermost | both pairs
stripes. | pair short. | short.
-----------------------------------------------------------------------
It must be pointed out here that the subgenus _Neotamias_ always differs
from both the subgenus _Eutamias_ and the genus _Tamias_ in pointed
_versus_ rounded pinna of ear (see table 2) and in the supraorbital
notch being slightly posterior to or even with, instead of distinctly
anterior to, the posterior notch of the zygomatic plate. The relative
position of these two notches, however, seems to be a matter of relative
(heterogonic) growth. Further, the base of the postorbital process of
the frontal usually is narrower (relative to the length of the process)
in the subgenus _Neotamias_ but there is a gradation in this feature in
_Neotamias_ culminating in the species _E. townsendii_ in which the
bases of the processes are relatively as broad as in the subgenus
_Eutamias_ and the genus _Tamias_. The same condition obtains in the
shape of the infraorbital foramen which is subovate to rounded in the
subgenus _Neotamias_ and always rounded in the other chipmunks.
TABLE 2.--CHARACTERS BY MEANS OF WHICH THE SUBGENUS EUTAMIAS AND THE
GENUS TAMIAS MAY BE DISTINGUISHED FROM THE SUBGENUS NEOTAMIAS
========================================================================
Character | subgenus | subgenus | genus
| _Neotamias_ | _Eutamias_ | _Tamias_
-------------------------------+--------------+-------------+-----------
Shape of infraorbital foramen. | subovate to | always | always
| rounded. | rounded. | rounded.
-------------------------------+--------------+-------------+-----------
Relative width of the | narrow to | |
postorbital process at base. | broad. | broad. | broad.
-------------------------------+--------------+-------------+-----------
Position of supraorbital notch | even with or | |
in relation to posterior | slightly | anterior. | anterior.
notch of zygomatic plate. | posterior. | |
-------------------------------+--------------+-------------+-----------
Convergence, posteriorly, | not always. | not always. | always.
of upper tooth-rows. | | |
-------------------------------+--------------+-------------+-----------
Degree of constriction of | slight | marked. | marked.
interorbital region. | to marked. | |
-------------------------------+--------------+-------------+-----------
Shape of pinna. | long and | broad and | broad and
| pointed. | rounded. | rounded.
------------------------------------------------------------------------
These differences of _Neotamias_ are so slight in comparison with the
similarities (ten features mentioned above) that _Neotamias_ here is
accorded only subgeneric rank under the genus _Eutamias_, instead of
generic rank.
Howell's (1929) arrangement of the genera and subgenera of chipmunks is
judged to be correct as indicated by the following arrangement that I
propose.