1914] Grinnell: Mammals and Birds nf the Colorado Valley 225 



In measurements of the two groups, the Arizona-side examples of 

 teretieaudus are shorter-tailed, as shown in the following comparison. 

 Average of eight adults from the Arizona side as compared wnth aver- 

 age of seven adults from the California side (in parenthesis) is: total 

 length, 234.1 (240.6) ; tail vertebrae, 76 (88.3) ; hind foot, 35.1 (35.5) ; 

 ratio, tail to body, 48 per cent (58 per cent). These mea.surements 

 of the California side examples are close to those of many others of 

 the Colorado Desert west of the river. The shortnes.s of tail is fairly 

 well borne out in the mea-surements of Citellus teretieaudus from east 

 of the river along the Mexican line as given by Mearns (1907, p. 338). 

 The present series, however, is inadequate to settle the point satis- 

 factorily. 



It will be observed that in relative darkness of the Arizona side 

 examples, we have here a parallel to the ease in Ammospermophilus, 

 while in ratio of tail to body the reverse relationships are presented. 

 The degree of difference exhibited in Citellus. even if actual, is very 

 slight as compared with the differences between Ammospermopln'l us 

 h. harrisi and A. I. leucurus. 



Castor canadensis frondator Mearns 

 Sonora Beaver 



Signs of beavers were seen at many places along the river through 

 the big valleys, from near Riverside Mountain to Pilot Knob. We 

 heard of their occurrence also above Needles. At the pre.sent time, 

 however, beavers are scarce along the Colorado River, this being said 

 to be due to the unrelenting pursuit of them by professional trappers. 

 Various evidences of the late presence of trappers were seen by us 

 below Ehrenberg. 



Verv^ little of the beaver sign seen during our cruise of the river was 

 fresh. The first noted was on the California side near the base of 

 Riverside Mountain, March 16. Many .young cottonwoods had been 

 felled at the edge or within twelve feet of the main river channel, 

 where its bank sloped steeply into deep water. The trees had been 

 cut off about one foot above the ground and all felled down-stream, 

 or, rather, diagonally, down-stream and towards the river. The largest 

 Cottonwood was fourteen inches in diameter where cut, the next two 

 seven inches in diameter. Other cuttings were noted down to willow 

 shoots one-half inch in diameter. 



