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. 



II. 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 



