234 Univeisify of California riibUcations in Zoology [Vol. 12 



the riparian strips, and no evidence was forthcoming of its occurrence 

 out on the desert mesa, in the hills, or even up the desert washes. As 

 compared with rodents of the desert proper this species might for 

 the sake of emphasis be termed semi-aquatic, for it was trapped within 

 a few feet of the main river, and its tracks were noted on fresh mud 

 at the water's edge. Individuals were caught on willow-grown islands, 

 w-hich were subject to submergence with rising water, so that the 

 animals would be forced to live in trees or take to water. No signs 

 of nests were observed in trees anywhere, and it seems reasonable to 

 suppose that these wood rats voluntarily swim narrow channels, 

 especially when pressed by hunger. 



Although foraging widely over the bottomlands, the permanent 

 abodes of these rats appeared to be chiefly located at the lateral rims 

 of the riparian belts, .just at the upper reach of high w^ater. This, too, 

 marked the belt of mesquites, so that the mesquite association can 

 be confidently assigned as the distinctive habitat of Xcotoma a. venitsta. 



At no point did we find the "enormous nests" of this species 

 described by Mearns (1907. p. Hi) as found by him below Yuma. 

 His name, Xcotoma cumulator, for this species was selected on this 

 account. A few small stick houses were found by us in the mesquite 

 strip above Mellen, and nests of similar scant proportions, at other 

 places down the river. In all cases the rats appeared to have ready 

 access to subterranean burrows. At the mouth of the Gila River, near 

 Yuma, many burrows were found in a tract of guatemote witliout 

 trace of stick nests, and a number of the animals were caught. It is 

 quite possible that recurring unusually high flooding discourages the 

 rats in the portion of their range explored by us from amassing much 

 material, since it is subject to being floated off. 



We found small young as early as February 24 and 25 at Mellen. 

 and from that date on to May 5. near Yuma. It is probable that the 

 breeding season is just about closed by the time the annual overflow 

 begins, so that yoiing as well as adults are able to seek safety for the 

 brief period of exile by assuming arboreal habits or swimming to 

 higher ground. 



The series of 68 specimens of Xeofoma albiyula vcnusfa obtained 

 (nos. 10463-10530) represent localities of capture as follows: Cali- 

 fornia side: near Riverside Mountain. 2; Palo Verde, 2; twenty miles 

 above Pieacho, 8; eight miles below Picacho, 2; five miles northeast 

 of Yuma. 8: Pilot Knob. 3. Arizona side: Jlellen, 12; Ehrenberg, 3; 

 twenty-five miles below Ehrenberg. 1: ten miles below Cibola, 7; five 



