1914] Grinnell: Mammals and Birds of the Colorado ValUy 227 



On both sides of the river in the vicinity of Pilot Knob there was 

 considerable beaver sign, consisting of well-beaten trails up the banks, 

 foot and tail prints, and cut willow saplings. At all points where 

 prospects seemed at all favorable, traps were set, but our nearest 

 approach to success was the securing of a front foot (no. 10731) in 

 a trap on the Arizona shore opposite Pilot Knob. The shore sloped 

 so gently at this place that the trap could not be set in water deep 

 enough to insure drowning of the victim and yet occupy a position 

 anywhere near the foot of the used slide. The animal had evidently 

 pulled loo.se. 



Further troubles in our efforts to trap beavers resulted from the 

 continual rising or falling of the river and from the heavy deposit 

 of silt. The latter so rapidly filled in the steel traps (Xewhouse nos. 

 3 and 4), even when set upon broad strips of bark and staked out two 

 feet or so from the actual bank, that beaver or any other animals could 

 walk upon them without setting them off. 



At the last trapping place, the lowering of the water repeatedly 

 exposed both the traps and stakes, but tracks showed that during the 

 night a beaver had gone pa.st. nevertheless, indicating no particular 

 shjTiess or suspicion. 



From the reports given us by different people along the river, it is 

 probable that beavers were abundant in suitable parts of its course up 

 to a few years ago. As many as 250 skins are said to have been taken 

 in a single year by one trapper. With observance of the protective 

 laws now in force in both Arizona and California there is a chance 

 for beavers to become sufficiently plentiful again to warrant a brief 

 open season each year when the fur is prime. I see no reason why 

 the Colorado River should not produce a regular output of skins, 

 provided the nimiber taken annually be adjusted to the rate of increase. 



Peromyscus maniculatus sonoriensis ' Le Conte) 

 Sonora White-footed Mouse 

 An abundant inhabitant of the bottomlands everA*where, this in 

 spite of the annual overflow which might be expected to drown out 

 a mammal of this non-aquatic genus. 



The 65 specimens secured (nos. 10100-10164) represent the follow- 

 ing localities : California side : five miles below Needles, oppasite The 

 Needles, Chemehuevis Valley, at Riverside Jlountain, above Ehren- 

 berg, near Palo Verde, eight miles below Pieacho, four miles south of 



