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.