proximal end of the bone. It lies over a well-marked groove, and there is a second shallower groove on the right side of the bone.” The baculum of G. sabrinus is markedly wider, more flattened and shorter than in G. volans. The crest, which is also present in G. volans, starts from the upper terminal process and extends to the base of the baculum on the left side. There is a knoblike process on the crest at a point three fourths the length of the baculum from its base. The distal one third of the baculum curves sharply but smoothly upwards (see fig. 9).

Keeping in mind that the baculum in the North American sciurids can be classified into six structural groups, as given above, the baculum in each of the subgenera Eutamias and Neotamias and in the genus Tamias is briefly described.

In the subgenus Neotamias the baculum resembles a leg and foot of man, with a narrow ridge (keel) in the center of the “instep” of the foot (Howell 1929:27). The tip (=foot) curves dorsally at the distal end (see figs. 5, 6).

Figs. 4-10. Lateral views of right side (except left-lateral view in fig. 9) of baculum.

Fig. 4. Sciurus aureogaster aureogaster, No. 37000; from 70 km. S C. Victoria (by highway), and 6 km. W of highway, Tamaulipas.

Fig. 5. Eutamias quadrimaculatus, No. 95780 BS; from Mountains near Quincy, Plumas Co., California.

Fig. 6. Eutamias sibiricus asiaticus, No. 199632 NM; from 120 mi. up the Yalu River, Korea.

Fig. 7. Tamias striatus lysteri, No. 193493 NM; from Locust Grove, New York.

Fig. 8. Marmota flaviventer dacota, No. 41641; from 1½ mi. E Buckhorn, 6,150 ft., Weston Co., Wyoming.

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.

Figs. 11-12. Ventral views of the hyoid apparatus in Tamias and 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