50 



cable to the valley north of Yuma, is due to such factors as 

 the great sinks of the Salton and Pattie Basins, with their 

 fluctuating saline lakes, extensive dune areas, and the high 

 alkalinity of their soils; the existence of hot springs, mud 

 volcanoes, and solfataras; the presence of an isolated mountain 

 range practically surrounded by the delta and its neighboring 

 basins; the unbroken, plant-covered slopes on the western 

 side of Pattie Basin; etc. 



Since I had no opportunity to trap small mammals during 

 our hurried sojourn in Lower California, it would be rash for 

 me to try to demarcate exact associations in the Pattie Basin 

 and the borders of the fluvial land. Yet in comparing 

 Rhoads's annotations, on twenty-one species of terrestrial 

 mammals from the delta region, with Grinnell's more ample 

 account of species inhabiting the river valley north of the 

 international boundary, one is struck by certain reflections 

 of the changed and unusual physiographic relations. For 

 instance, two white-footed mice, Peromyscus maniculatus 

 sanoriensis and P. eremicus eremicus, occupy well-defined and 

 approximately exclusive strips of country along the Colorado 

 between Needles and Yuma. P. m. sonoriensis is the bottom- 

 land form, finding its optimum life-conditions in the Willow- 

 Cottonwood Association, and occurring much less abundantly 

 in the Arrowweed, Quail-brush, and Mesquite Associations. 

 Grinnell trapped 65 specimens, and failed to find the species 

 at any point beyond the mesquite belt. P. e. eremicus, on 

 the other hand, is the upland-desert form, belonging altogether 

 to associations outside the mesquite belt. The capture of 

 109 specimens showed that it finds its optimum in the Saltbush 

 Association, and extends its habitat sparingly above. 



Now in Lower California, Rhoads found P. e. eremicus 

 plentiful in the Cocopah Mountains, where he took 19 speci- 

 mens. He characterizes this mouse as a ''rock-loving moun- 

 tain species." P. m. sonoriensis was ''excessively abundant 

 in the bottoms," as might have been expected, but "four 

 specimens were also trapped in the Cocopahs with the pre- 

 ceding species." Here then is at least slight evidence of an 

 intermingling which doubtless obtains, in various botanic and 



