78 University of California Publications in Zoology [Vol.12 



Passerculus sandwichensis nevadensis: Amphispiza bilineata deserticola: 

 excl. ; winter min.; summer 



Passerculus sandwichensis alaudinus: Amphispiza nevadensis nevadensis: 

 excl.; winter max.; winter 



Zonotrichia leueophrys gambeli: Lanius ludovicianus exeubitorides: 

 min.; winter min.; resident (?) 



Spizella breweri: max.; winter Thryomanes bewiclci eremophilus: 



min.; winter 



Citellus tereticaudus tereticaudus: Lepus californicus deserticola: min. 



max. Sylvilagus auduboni arizonae: min. 



Peromyscus eremicus eremicus: max: Taxidea taxus berlandieri: excl. (?) 



Thomomys albatus: excl. Myotis californicus pallidus: Max. 

 Dipodomys deserti deserti: max. (?) 



Dipodomys merriami merriami: min. Pipistrellus hesperus hesperus: min. 

 Perognathus penicillatus penicillatus: 



Remarks upon the Saltbush Association. — Ranchers in the Colo- 

 rado River region distinguish two portions in each of the valleys. 

 the "first bottom" and "second bottom." These are of course dupli- 

 cated in reverse position on opposite sides of the river. The first 

 bottom is the overflow area, and comprises all the associations treated 

 up to this point in the present chapter, from the river to the mesquite 

 association, inclusive. These associations together constitute the 

 riparian belt proper. The second bottom is in the nature of a terrace 

 or bench, and is situated above the reach of high water, extending 

 from the mesquite association desertwards to the base of the mesa bluff. 



This second bottom is usually quite level and varies from a mere 

 strip, of few yards in width, to a tract as much as a mile wide. It 

 is for the most part clothed sparsely with xerophytic or halophytie 

 vegetation, a predominating species in which is the saltbush (Atriplex 

 polycarpa) ; hence the name adopted for the association represented. 



The soil is almost always of fine sand, often wind-blown. The pre- 

 vailing westerly winds have caused a growth of sand-dunes on the 

 southeast edges of the second bottom at several points, notably on 

 the Arizona side above Mellen. The driving sand is often arrested 

 about a bash and as the resulting dune grows, so does the bush com- 

 mensurately. A scrubby form of mesquite may thus constitute the 

 core of a sand-dune. Several small plants are peculiar to these wind- 

 formed hillocks, one of which is the sand verbena {Abronia villosa). 



