_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 posterior end, to the bulla at a point slightly ventral and posterior to the auditory meatus. In the genus _Eutamias_ the hypohyal and ceratohyal are completely fused in adults, the suture between these two bones being visible in juvenal specimens (see fig. 12). In the genus _Tamias_ the hypohyal and ceratohyal remain distinct throughout life. The hypohyal may frequently be divided into two parts, a variation which is also present in _Marmota_. The musculature associated with the hyoid apparatus in _Eutamias_ and _Tamias_ is as described by Bryant (1945:310, 316) for the Nearctic squirrels. However, the conjoining tendon of the anterior and posterior pairs of digastric muscles is ribbonlike in _Eutamias_ and rodlike (rounded in cross section) in _Tamias_. _The presence or absence of P3 and the projection of the anterior root of P4 in relation to the masseteric knob._--Only rarely is P3 absent in _Eutamias_ or present in _Tamias_. P3 in specimens of old adult _Eutamias_, shows wear, thus suggesting that P3 is functional in older chipmunks. In _Eutamias_, which normally has a P3, the anterior root of P4 projects to the outside of the masseteric knob, whereas in _Tamias_, which normally lacks a P3, the anterior root of P4 projects directly to the masseteric knob or to the lingual side of this structure. The projection of the anterior root of P4 seems to be correlated with the presence or absence of P3. However, in a specimen of _Tamias striatus rufescens_ (No. 11117 KU), the left P3 is present, yet the anterior root of P4 still projects to the lingual side of the masseteric knob. Ellerman (1940:48) and Bryant (1945:368-369, 372) think that the presence or absence of P3 is not of generic significance in chipmunks, since P3 is vestigial and probably is in the process of being lost, and since this character is rarely used as a generic character in other sciurids. I think that the presence or absence of P3, together with the projection of the anterior root of P4 in relation to the masseteric knob, is of generic significance, for, squirrels in general have retained the dentition and dental formula of a primitive rodent, and any change in the pattern of the teeth or in dental formula is, in my opinion, of a fundamental nature.