Part 3

FIG. 9. Glaucomys sabrinus bangsi, No. 15079; from 10 mi. NE Pinedale, 8,000 ft., Sublette Co., Wyoming.

FIG. 10. Spermophilus armatus, No. 14888; from W end Half Moon Lake, 7,900 ft., Sublette Co., Wyoming.]

In the subgenus Eutamias, the baculum "tapers gradually from base to tip, the distal portion upturned in an even curve and slightly flattened ..." (op. cit.:26). Microscopic examination reveals that there is a faint keel on the dorsal surface of the tip.

Eutamias, like Callosciurus, Menetes, Dremomys, Lariscus, Rhinosciurus, and Nannosciurus, has a keel on the dorsal surface of the tip of the baculum (compare figures 5 and 6 with the descriptions and figures in Pocock, 1923:217-225).

In Tamias the baculum is "a slender bone 4.5-5 millimeters in length, nearly straight, upturned at the tip and slightly expanded into the shape of a narrow spoon or scoop, with a slight median ridge on the under surface." (Howell op. cit.:13.) The "median ridge" is a keel on the ventral surface. In having a keel on the ventral surface of the tip, the baculum of Tamias is comparable to that of Spermophilus.

Examination of series of bacula of the subgenus Neotamias and the genus Tamias indicates, as in the case of the mallei, that there is slight individual variation and slight variation with age. In the subgenus Neotamias interspecific variation in the baculum is considerable, but the general plan of structure remains constant. From this study of variation of the baculum in American chipmunks, it can be extrapolated that the baculum in the Asiatic Eutamias would show little individual variation in structure. I have seen only two bacula of the Asiatic Eutamias.

FIG. 11. Tamias striatus venustus, No. 11072 female; from Winslow, Washington Co., Arkansas.

FIG. 12. Eutamias minimus operarius, No. 5376 male; from 14 mi. N El Rito, Rio Arriba Co., New Mexico.]

Structure of the Hyoid Apparatus.--The hyoid apparatus in the chipmunks is made up of an arched basihyal with a thyrohyal attached to each limb of this "arch." To each junction between the "arch" and the thyrohyals, a hypohyal is attached by ligaments to a flat articular surface. A ceratohyal then is attached posteriorly to the hypohyal and a stylohyal ligament is attached to each ceratohyal posteriorly. The stylohyal is loosely attached along its sides to the tympanic bulla and finally attached, at the posterior end, to the bulla at a point slightly ventral and posterior to the auditory meatus.

In the genus Eutamias the hypohyal and ceratohyal are completely fused in adults, the suture between these two bones being visible in juvenal specimens (see fig. 12).

In the genus Tamias the hypohyal and ceratohyal remain distinct throughout life. The hypohyal may frequently be divided into two parts, a variation which is also present in Marmota.

The musculature associated with the hyoid apparatus in Eutamias and Tamias is as described by Bryant (1945:310, 316) for the Nearctic squirrels. However, the conjoining tendon of the anterior and posterior pairs of digastric muscles is ribbonlike in Eutamias and rodlike (rounded in cross section) in Tamias.

The presence or absence of P3 and the projection of the anterior root of P4 in relation to the masseteric knob.--Only rarely is P3 absent in Eutamias or present in Tamias. P3 in specimens of old adult Eutamias, shows wear, thus suggesting that P3 is functional in older chipmunks. In Eutamias, which normally has a P3, the anterior root of P4 projects to the outside of the masseteric knob, whereas in Tamias, which normally lacks a P3, the anterior root of P4 projects directly to the masseteric knob or to the lingual side of this structure. The projection of the anterior root of P4 seems to be correlated with the presence or absence of P3. However, in a specimen of Tamias striatus rufescens (No. 11117 KU), the left P3 is present, yet the anterior root of P4 still projects to the lingual side of the masseteric knob.

Ellerman (1940:48) and Bryant (1945:368-369, 372) think that the presence or absence of P3 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.