[Illustration: 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 1-1/2 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_. [Illustration: 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.]