874 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
with the light stripes whiter, and wider, nearly equalling the interspaces ; 
also much paler on the sides and whiter below. 
Specimens from different localities vary somewhat in color, the varia- 
tions being similar to those in var. tridecemlineatus, but less marked. The 
light stripes are generally grayish-white, sometimes suffused a little with 
yellowish, and the dark interspaces are generally pale chestnut, varying to 
more dusky. The light stripes are generally more than half the width of the 
dark interspaces, and sometimes equal them. Among the palest and smallest 
examples are the specimens from Fort Union and the Yellowstone and Platte 
Rivers, an especially pale and small phase characterizing the Mauvaises 
Terres of the Upper Missouri region. 
GENERAL REMARKS UPON SPERMOPHILUS TRIDECEMLINEATUS AND ITS VARIETIES. 
DIFFERENTIAL CHARACTERS AND AFFINITIES—In pattern of coloration, 
Spermophilus tridecemlineatus quite strongly resembles S. mexicanus, but 
differs from it in having the ground-color of the dorsal surface much darker, 
and in the possession of continuous light stripes, alternating with interrupted 
lines of detached light spots, S. mexicanus having merely longitudinal rows 
of disconnected white spots, which cease at the occiput, instead of continuing 
over the top of the head to the forehead. S. mexicanus is also smaller, and 
has a longer tail, and differs in the characters of the skull, as already shown. 
In general form, and especially in the form of the skull, S. ¢ridecemline- 
atus resembles S. franklini, but it is much smaller, with a much narrower 
tail and entirely different pattern of coloration. It hence has no very near 
American ally, and is still more distinct from any of the Old World Spermo- 
philes. 
The two varieties of S. tridecemlineatus, in their extreme phases, quite 
widely differ in respect to size and color, but agree in every detail of form, 
proportion, and pattern of coloration; var. pallidus being merely a bleached, 
depauperate, desert form of tridecemlineatus. In var. pailidus, the tints are 
not only paler, but the light spots and stripes increase in breadth at the 
expense of the darker ground-color. Var. pallidus reaches its extreme phase 
of differentiation in the driest. portions of the plains and the desert regions 
more to the westward ; var. ¢ridecemlineatus is most specialized over the fer- 
tile prairies of the Mississippi Valley and thence northward along the Red 
River and the region to the westward of Lake Winnipeg. Specimens from 
