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.