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).

[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., Subl