is not of generic significance in chipmunks, since P3 is vestigial and probably is in the process of being lost, and since this character is rarely used as a generic character in other sciurids. I think that the presence or absence of P3, together with the projection of the anterior root of P4 in relation to the masseteric knob, is of generic significance, for, squirrels in general have retained the dentition and dental formula of a primitive rodent, and any change in the pattern of the teeth or in dental formula is, in my opinion, of a fundamental nature.

Length of tail in relation to total length.—The tail in Eutamias is more than 40 per cent of the total length, whereas in Tamias the tail is less than 38 per cent of the total length. In this respect Tamias resembles most ground squirrels of the genus Spermophilus.

Color pattern.—The chipmunks vary but little in color pattern, for, even in Eutamias dorsalis, which is one of the most aberrant of the chipmunks in color pattern, the pattern is characteristic of Eutamias.

The width of the longitudinal stripes is uniform in Eutamias whereas in Tamias the dorsal, longitudinal light stripes are more than twice as wide as the other stripes.

In Eutamias, only the two lateralmost dark stripes are short, whereas in Tamias all four of the lateral dark stripes are short; none extends to the rump or to the shoulder.

The dark median stripe is present in both Eutamias and Tamias as well as in other genera such as Callosciurus and Menetes (Ellerman 1940:390).

Characters in which the Subgenus Eutamias and the Genus Tamias Agree,
but Differ from the Subgenus Neotamias

Shape of the infraorbital foramen.—In the subgenus Eutamias and in the genus Tamias the infraorbital foramen is rounded, whereas in most species of the subgenus Neotamias the foramen is slitlike. In Eutamias townsendii, however, the infraorbital foramen is rounded as much as in the subgenus Eutamias and in the genus Tamias.

Width of the postorbital process at base.—The postorbital process is broader at the base in the subgenus Eutamias and in the genus Tamias than in most species of the subgenus Neotamias. In E. townsendii, however, this process is relatively as broad as in the subgenus Eutamias and in the genus Tamias.

Position of the supraorbital notch in relation to the posterior notch of the zygomatic plate.—In the subgenus Eutamias and in the genus Tamias the supraorbital notch is distinctly anterior to the posterior notch of the zygomatic plate, whereas in the subgenus Neotamias, the supraorbital notch is only slightly anterior to the posterior notch of the zygomatic plate. This difference may be correlated with differences in size, since specimens of the subgenus Eutamias and the genus Tamias are larger than specimens of the subgenus Neotamias.

Degree of convergence of the upper tooth-rows.—The rows of upper cheek-teeth converge posteriorly in the subgenus Eutamias and in the genus Tamias, except that in some specimens of E. sibiricus asiaticus the rows of upper cheek-teeth are nearly parallel to each other. In most species of the subgenus Neotamias the 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 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