1914 I Grintu 11: Mammals and Birds of the Colorado Valley 229 



sonoriensis can stand the fatalities due to the river's irregular habits 

 and still maintain a large population, without any modification in 

 birth rate. 



Mice of the Peromyscus maniculatus group are known to be much 

 more prolific than Perognathus or Dipodomys, not so much because of 

 the larger litters but because there are several litters each year in the 

 first-named rodent, and only one as a rule in the last two. On the 

 Colorado the results of our collecting give but little information as 

 to the breeding of P. m. sonoriensis. A female taken March 25 con- 

 tained five embryos; one taken May 5 contained three embryos. 



Peromyscus crinitus stephensi Mearns 

 Stephens Canon Mouse 



As judged from the results of our three months' trapping, this 

 was the least numerous of the smaller rodents of the region. Only 

 thirteen specimens were procured, apportioned by locality as follows : 

 Opposite The Needles, five (nos. 9978-9982) ; above Ehrenberg, one 

 (no. 9983) ; twenty miles above Picacho, two (nos. 9984, 9985) ; Pot- 

 holes, two (nos. 9986, 9987) ; Pilot Knob, three (nos. 9988-9990). 



All of these localities are on the California side of the river. It 

 would thus appear that in its lower course the river forms an absolute 

 barrier to the eastward spread of this mouse. Yet it has been taken 

 on the Arizona side of the Colorado, whence, considerably above Fort 

 Mohave, Osgood (1909, p. 233) has recorded three specimens. 



The associational preferences of this mouse are most pronounced. 

 All of the specimens were captured near or among rocks in ravines or 

 on steep hillsides. It is thus a rupicoline species with much the same 

 associational exclusiveness as Perognathus spinatus. As in the case 

 of the latter, each mountain mass probably possesses its more or less 

 completely isolated colony of these mice, separated from its neighboring- 

 colony by the uninhabited interval of level mesa or valley. 



Peromyscus eremicus eremicus (Baird) 

 Desert White-footed Mouse 

 The series of 109 specimens of this mouse (nos. 9991-10099) are 

 representative of the following localities: Arizona side: Mellen, foot of 

 The Needles, above Bill Williams River. Ehrenberg, twenty-five miles 

 below Ehrenberg, ten miles below Cibola, five miles north of Laguna, 

 and Yuma; California side: opposite The Needles. Chemehuevis Valley, 



