1914] Grinnell: Mammals and Birds of the Colorado Valh-y 237 



would, ultimately, barring accidents, tend to reach the same sort of 

 environs they were used to, namely, the rocky hill slopes. 



While such a series of propitious events is in but a remote degree 

 possible, it appears to me the most logical way of explaining the 

 extension of the range of desertorum across the Colorado to the Arizona 

 side. As above intimated, this occurrence, if a fact, does not militate 

 against Goldman's hypothesis that lepida and desertorum may have 

 been held apart by the Colorado River, and are still so held apart 

 in most of its course. 



Ondatra zibethica pallida (Mearns) 

 Pallid Muskrat 



Evidence, hearsay and direct, indicates the presence of mu.skrats all 

 along the Colorado River, from above Needles to below the Mexican 

 line. They occur both in the main stream and in the varioiLS diverging 

 sloughs of the big valleys. Our nine specimens (nos. 10652-10660) 

 were all secured on the California .side of the main stream, three near 

 Palo Verde and six near Pilot Knob. 



No signs of houses were seen anywhere, the muskrats appearing to 

 resort entirely to holes in banks where the current was sluggish. Near 

 Palo Verde. April 1, a system of burrows was dug out by Stephens 

 and Jones. This system of holes was in the bank of a tule-bordered 

 slough about a half mile above its confluence with the main river. 

 The entrances all opened considerably beneath the surface of the 

 water at its level at the time. From these the burrows sloped upwards 

 as they extended back from the slough, until those parts farthest 

 from the water were two to three feet above its level at that stage. 



Some green tule stems were found in the passage-ways. One of 

 the blind leads was filled in with packed tule stems, mostly dead ones, 

 but moist and crushed. The regular nests consisted of drj^ tule .stems, 

 some of them shredded, and laid two to three inches deep. Some of 

 the short blind leads showed fresh claw scratches in the earth at their 

 ends. The passage-ways were ordinarily five inches in diameter, in 

 places more, and usually kept near the surface of the ground, follow- 

 ing its irregularities pretty closely. In places there was not more 

 than three inches of earth aliove the liurrow, and there was seldom as 

 much as a foot. The .system was in fact discovered by one of the 

 party's stepping upon a thin place and breaking through. 



The impression gained from a survey of the uncovered burrows 

 was that the excavator had at the beginning burrowed up from the 



