_Individual variation._--Individual variation is small. This is shown by
a coefficient of variability of only 3.85 in the length of the shaft in
a series of 12 specimens of _E. umbrinus umbrinus_ from Paradise Park,
21 mi. W and 15 mi. N Vernal, 10,050 ft., Uintah County, Utah.
_Variation with age._--In the chipmunks the baculum varies but little
with age. In the youngest specimens that I have taken, the M3 and m3
have not yet erupted and there is no wear on P4 and p4; nevertheless,
the baculum in these specimens has nearly an adult configuration and
size. In juvenal _Eutamias minimus_ the tip of the baculum is longer in
relation to the length of the shaft than it is in adults; the tip is 18
to 28 per cent of the length of the shaft in adults, as opposed to 29 to
32 per cent in juveniles.
_Aberrations._--In a small percentage of specimens of _E. minimus_ and
_E. umbrinus_ the baculum is small and S-shaped, even in adults.
_Variations of taxonomic worth._--Variations in this category are
described in the section immediately following the key.