874 



MONOGRAPHS OF NORTH AMERICATJ «QDENTIA. 



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with tlie 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 van tridecemlineatus, but less marked. The 

 light stripes are generally grayish-white, sometimes suffused a little with 

 yellowish, aod 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 Platt« 

 Rivers, an especially pale and small phase characterizing the Mauvaises 

 Torres of the Upper Missouri region. ,. . . , . .^ > . t, ?j i 



GENERAL REMARKS UPON SPEivMOPHILUS TRIDECEMLINEATUS AND ITS VARIETIES. 



Differential characters and affinities. — In pattern of coloration, 

 Spcrmophilus tridecemlineatus quite strongly resembles S. inexicanus, 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 shewn. 



In general form, and especially in the form " the skull, S. tridecemline- 

 atus resembles /S. franklini, but it is much smaller, with a ii^uch 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 phatcs, 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 & bleached, 

 tlcpauperate, desert form of tridecemlineatus. In var. pallidus, the tints are 

 not only paler, but the light spots and stripes increase in breadth at the 

 expense of the darker ground-color. Var. jiallidus reaches its extreme phase 

 of differentiation in tiie driest portions of the plains and tlie desert regions 

 more to the westward ; var. tridecemlineatus 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 



