506 University of California Publications in Zoology [Vol.12 



leafed tree yucca (Yucca mohavensis) , a smaller species than the 

 twisted form so common on the western stretches of the Mohave Desert. 



Long belts of drifting sand traverse the California deserts at vari- 

 ous places. These obviously owe their geographic position to the pre- 

 vailing direction of wind, in connection with the place of occurrence 

 of original stream deposits. Running east and west two miles south of 

 Blythe Junction is a wind-drifted strip of fine sand blown into small 

 dunes that are being carried eastward across the desert. In places 

 this eolian material has been driven up over mountains and forms 

 white cascades down their eastward slopes. The flora of the dunes 

 south of the Turtle Mountains is dominated by a large species of 

 Ephedra, or desert tea, and bunches of the wiry "galleta grass." 



The plant and animal associations (see Ruthven, 1907, and Grin- 

 ned, 1914) in the region under discussion can be definitely correlated 

 with the terrain, and the two together may conveniently be classed as 

 environments, of which the following eight, at least, may be recognized 

 in this region (see pi. 19). 



( 1 ) The rocky hillside environment, characterized by the low, pale, 

 rounded shrub Encelia, and a small species of Ephedra. 



(2) The canon-bed environment, with the green catclaw (Acacia) 

 as its most typical plant. 



(3) The rocky mesa environment, possessing few species of either 

 plants or vertebrate animals. 



(4) The wash-bed environment, which is a direct continuation out- 

 side the mountains of the canon bed and is in general slightly below 

 the level of the rocky mesa. The characteristic association of legumin- 

 ous desert trees lives in this environment. 



(5) The low plain environment, distinguished by the predom- 

 inance of the creosote bush (see pi. 20, fig. 3). 



(6) The high plain environment, with the tree yucca, in addition 

 to the low plain flora (see pi. 20, fig. 2). 



(7) The drif ting-sand environment, supporting a large Ephedra 

 and patches of galleta grass (pi. 21, fig. 4). 



(8) The canon spring environment, represented in this region only 

 by the three or four permanent springs of the Turtle Mountains, and 

 characterized by the hydrophilous arrowweed association. 



The water-supply, except after the rare showers, is extremely 

 scanty. Behind dykes in three or four of the larger canons feeble but 

 lasting springs issue from the wash gravel. At some times of the 

 year water can be found under the sand, or exposed in "tanks," pot- 



