In Marmota the baculum is greatly enlarged at the posterior end and forms a shieldlike surface. The ventral surface of the base is flattened and the ventral surface of the shaft curves slightly ventrally then dorsally to the tip. The dorsal region of the base culminates in a point, from which there is a ridge that extends anteriorly and that tapers rapidly into the shaft near the tip. The tip, dorsally, has a slight depression surrounded by knobs, which are more or less well defined, and which resemble, topographically, the spines described for Spermophilus (see fig. [8]).
In Sciurus the baculum is semispoonshaped and asymmetrical. There is a winglike process on one side and a spine, which projects lateroventrally, on the other side of the tip. The base of the baculum is broad but not so broad as in most species of Spermophilus. Extending posteriorly from the region of the tip, at which point a spine projects lateroventrally, there is a ridge, which is often partly ossified and that extends to a point near the base (see fig. [4]).
In Tamiasciurus the baculum is absent or vestigial (Layne, 1952:457-459).
In Eutamias the baculum is broad at the base and the shaft tapers distally to the junction of the shaft and tip, or the base is only slightly wider than any part of the shaft. The tip often forms an abrupt angle with the shaft and there is a keel on the dorsal surface of the tip (see figs. [5, 6]).
The baculum in Glaucomys is the most distinctive of that of any American sciurid. According to Pocock (1923:243-244), “The baculum [of G. volans] is exceedingly long and slender, slightly sinuous in its proximal third, and inclined slightly upwards distally. The extreme apex is bifid, the lower process being rounded, the upper more pointed. On the left side there is a long crest running from the summit of the upper terminal process and ending abruptly behind the left side about one-third of the distance from the 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.