bal 
1910.] NATURAL SCIENCES OF PHILADELPHIA. 47 
with little vegetation northward and a more shaded slope on the 
south side, having more vegetation, but deeply interrupted by side 
canyons and ravines. These conditions may be illustrated by the 
map of White Tail Canyon (p. 75). It will be noticed thereon that 
on the southern side of the canyon the collecting stations are nearly 
all on northwestern slopes, none on northeastern. The latter are 
barren, and except near the top of the ridge form impassable barriers 
to mollusks. In the more arid portion of the range, snails are rarely 
found, often wholly wanting, on the slopes with mainly southern 
exposure. These conditions of exposure determine the limits of snail 
colonies, absolutely inhibiting migrations of much extent, though the 
anastomosing heads of adjacent canyons sometimes supply favorable 
slopes. 
The isolation of snail colonies is further favored by the habits of 
most of the Helices, which live deep in rocky talus or slides. They 
probably crawl about in the open only on rare occasions, and are 
wholly incapable of crossing slopes where shelter is lacking. We 
have never found living Helices on the surface in the Chiricahua 
Mountains, and with the exception of Oreohelix chiricahuana, living 
individuals were always found well buried in the rocks. Of some 
species, not even dead shells have been found on the surface. Some 
of these races apparently live and die under the surface. 
The progressive growth of the canyons by the deepening of lateral 
ravines and formation of new ones constantly accentuates the isolation 
of colonies by forming new slopes, of which one in each case is likely 
to be arid and therefore a barrier to the spread of snails. Moreover, 
the removal by erosion of stratified rocks, especially limestone, exposes 
ridges of granitic or eruptive rocks, in which snails are generally 
scarce, and some genera never present. Since the period of isolation 
of the several ranges, there has therefore been progressive isolation 
of colonies within each range. 
Il. INFLUENCE OF ENVIRONMENT ON THE SHELL, 
Relation of Climate to Shell-texture.—Notwithstanding the aridity 
of the climate, the Chiricanuan snails show none of the characteristics 
which some recent authors have considered to be the direct reactions 
to the desert environment. With the exception of Oreohelix chirica- 
huana, none of the species are conspicuously earthy, and none differ 
markedly in sculpture from snails of more humid districts. 
Rude, irregular sculpture and opaque chalky substance charac- 
terize land snails which live exposed to the sun. In such places their 
