364 PROCEEDINGS OF THE ACADEMY OF [June, 
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 voleanic, and the stony tracts are therefore 
unsuitable for Holospira. 
The progressive impover:shment of the fauna leaves, in the Santa 
Cruz Valley and westward, a few Lower Sonoran Pupillide, Zonitide, 
Thysanophora (hornit), 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. 
