which are
morphologically close to the mallei of the subgenus _Neotamias_.

[Illustration: FIGS. 1-3. Dorsomedial views of left malleus.

FIG. 1. _Tamias striatus lysteri_, No. 11920 sex?;
from Carroll Co., New Hampshire.

FIG. 2. _Eutamias sibiricus asiaticus_, No. 199637 male NM;
from I-mien-po, N. Kirin, Manchuria.

FIG. 3. _Eutamias townsendii senex_, No. 165 male;
from Lake Tahoe, California.]

_Structure of the Baculum._--In discussing the baculum in _Eutamias_ and
_Tamias_, it seems desirable to do so in the light of the structure of
the baculum in other sciurids.

The bacula of North American sciurids are divisible into six distinct
types represented by those of the genera _Spermophilus_, _Marmota_,
_Sciurus_, _Tamiasciurus_, _Eutamias_, and _Glaucomys_.

The type of baculum in _Spermophilus_ is spoonshaped with a ventral
process that is spinelike or keellike. Also, spines usually are present
along the margin of the "spoon." The base (proximal end) of the baculum
is broad, and some species have a winglike process extending dorsally
and partly covering a longitudinal groove. The shaft is more or less
curved downward in the middle (see figs. 7, 10).

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