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 manubrium of malleus. | approximately 90 degrees. | approximately 60 degrees. |
| Position of keel on tip of baculum. | dorsal. | ventral. |
| Relation of hypohyal and ceratohyal bones of hyoid apparatus. | fused in adults. | never fused. |
| Appearance in cross section of conjoining tendon of anterior and posterior digastric muscles. | flattened. | rounded. |
| Presence or absence of P3. | present. | absent. |
| Projection of anterior root of P4 in relation to masseteric knob. | buccal. | lingual. |
| Length of tail in relation to total length. | more than 40 per cent. | less than 38 per cent. |
| Width of longitudinal stripes. | subequal. | median pair of light stripes twice as wide as others. |
| Length of lateral longitudinal light stripes. | outermost pair short. | both pairs 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 Neotamias | subgenus Eutamias | genus Tamias |
| Shape of infraorbital foramen. | subovate to rounded. | always rounded. | always rounded. |
| Relative width of the postorbital process at base. | narrow to broad. | broad. | broad. |
| Position of supraorbital notch in relation to posterior notch of zygomatic plate. | even with or slightly posterior. | anterior. | anterior. |
| Convergence, posteriorly, of upper tooth-rows. | not always. | not always. | always. |
| Degree of constriction of interorbital region. | slight to marked. | marked. | marked. |
| Shape of pinna. | long and pointed. | broad and rounded. | broad and 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.
Genera and Subgenera
Genus Eutamias Trouessart