ette 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.]

_Structure of the Hyoid Apparatus._--The hyoid apparatus in the
chipmunks is made up of an arched basihyal with a thyrohyal attached to
each limb of this "arch." To each junction between the "arch" and the
thyrohyals, a hypohyal is attached by ligaments to a flat articular
surface. A ceratohyal then is attached posteriorly to the hypohyal and
a stylohyal ligament is attached to each ceratohyal posteriorly. The
stylohyal is loosely attached along its sides to the tympanic bulla and
finally attached, at the post