230 University of California Publications in Zoology [Vol. 12 



Riverside Mountain, Palo Verde, opposite Cibola, twenty miles above 

 Picacho. eight miles below Picacho, Potholes, five miles above Yuma, 

 and Pilot Knob. 



The wide occurrence of Peromyscus eremicus through the region is 

 thus indicated. Yet our trapping showed distinct associational pref- 

 erences. The overflow bottom is evidently rarely invaded, there being 

 but slight overlapping of the habitat of Peromyscus maniculatus 

 sonoriensis; and the rocky hills and mesas are eschewed, so that in 

 this direction the domain of Peromyscus crinitus stephensi is seldom 

 encroached upon. P. eremicus was found in greatest numbers on 

 arenaceous alluvium not included in the overflow area of the river 

 bottom. It was present on the second bottom outside the mesquite 

 belt, and along desert washes leading down from the interior; also 

 on the desert mesa where wind-blown accumulations of fine sand 

 afforded the same sort of ground, that is, ground of a fine but loose 

 consistency, easy to burrow into. 



The breeding season of the desert white-footed mouse, as indicated 

 by the following dates, covers at least three months. Nearly or quite 

 full-grown, blue-pelaged young were taken as early as March 2, and 

 from then on at increasingly frequent intervals until the last of April. 

 Females were taken March 5 with three embryos, March 8 with three 

 embryos, April 13 with four embryos and April 19 with four embryos. 



Sigmodon hispidus eremicus Mearns 

 Western Desert Cotton Rat 



This rodent proved to be strictly riparian in its associational prefer- 

 ences, and. furthermore, was found only along the lower course of 

 the river, below Ehrenberg. It belongs to a southern zone, tropical 

 or semi-tropical, and is one of the lowest zonal elements entering 

 the region ; moreover, it lias not pressed its way far up the river. 



I should judge its habits and food requirements to be closely 

 similar to those of Microtus, of cooler regions. But as far as known, 

 the ranges of Sigmodon and Microtus do not quite meet anywhere. 

 Certainly there are no Microtus in the Colorado River bottom as low 

 down as Needles. Sigmodon thus has a clear right of way, from a 

 competitive standpoint. 



The three localities of capture are all on the California side; yet 

 there is every reason to suppose that cotton rats occur on the Arizona 

 side as well, being surely as readily able to swim as harvest mice. 



