364 PROCEEDINGS OF THE ACADEMY OF [Juiie, 



elimination of the Transition Zone forms. Holospira, too, becomes 

 rare. In the Santa Cruz River Valley it is known by one species — 

 at the present time, one specimen; and none are known from further 

 west. The spread of this genus is not controlled by humidity. It 

 lives in the driest and hottest situations, often at low elevations, 

 but it is confined to limestone tracts, and limited by volcanic or 

 metamorphic rock. The mountains westward, in the region under 

 consideration, are mainly volcanic, and the stony tracts are therefore 

 unsuitable for Holospira. 



The progressive impoverishment of the fauna leaves, in the Santa 

 Cruz Valley and westward, a few Lower Sonoran Pupillidce, Zonitid{e, 

 Thysanophora (hornii), and the true desert snail, Sonorella. Sonorella 

 will live in the most arid places, where the rainfall does not exceed 

 5 or 6 inches, so long as there is abundant rock shelter and a certain 

 amount of shade, such as the shadow of a cliff or a small bush. 

 Northern slopes are preferred. In exploring a new mountain or hill 

 in the really arid country one aims for the northern or northwestern 

 slope under the highest crags. If coarse talus or rock "slides" are 

 found, persistent quarrying should produce Sonorella. In less arid 

 mountains, such as the Santa Ritas, the most productive collecting 

 stations are in the deep, verdant canyons. 



The exact location of collecting stations, and especially of type 

 localities, which we attempt in these papers, may seem meticulous 

 to many zoologists. In humid areas, or in dealing with less sedentary 

 animals, such exactness would hardly be worth while; if a type 

 locality is fixed within a few miles, it is near enough. But here we 

 deal with a region of intense local differentiation and with creatures 

 which are often confined within narrow bounds by physical conditions. 

 The hunt is difficult and laborious. The colonies are often so small, 

 the country so vast, that, without careful directions, one might make 

 a season's campaign in the more complex ranges without relocating 

 some former find which it might be important to investigate further. 

 It is, moreover, important to show exactly what ground has been 

 covered, in order that further exploration can be made to the best 

 advantage, that the unexplored parts of the ranges may be gone 

 over. In future it will be of interest to be able to trace the changes 

 and fate of the smaller and more isolated colonies, such as that of 

 Sonorella eremita, which covers an area of only a few square rods,, 

 many miles from any other snail colony. We suggest that future 

 collectors continue our serial station numbers in each range, instead 

 of beginning again at No. 1. 



