48 PROCEEDINGS OF THE ACADEMY OF |Feb., 
active life and periods of growth are certainly limited to dewy nights 
and times of rain,® whether they live in a reasonably humid region, as 
Cerion on the coast of Cuba, or in an arid region, as some Oreohelices 
of the Rocky Mountains. Pulmonate land snails which pass their 
lives hidden from the sun or shaded by dense foliage, do not have 
notably chalky shells, even in Arizona where a great part of the year 
is dry. We regard the opacity and cretaceous texture of exposed 
snails as in no way due to the direct action of climatic factors,® but 
as a protective adaptation to excessive sunlight, which would readily 
penetrate thin shells, or those composed largely of conchiolin.’ 
Oreohelix chiricahuana and in some cases Holospira are the only 
Chiricahuan snails which live on or close to the surface, uncovered or 
only partially protected; these are also the only snails which have 
the shell notably chalky and opaque. The snails of the dryest 
(Dos Cabezas) part of the range are not “desert snails” in appearance. 
They live where little light or none can penetrate. 
Relation of Exposure to Size.—The size of individual snails (excepting 
the heliophilous group discussed above) is almost wholly a function 
of the exposure. Snails living on northern or northwestern exposures 
are invariably larger than those from southern or eastern exposures, 
regardless of elevation,’ unless other conditions are conspicuously 
unfavorable. This is apparently due to the more abundant crypto- 
gamic or other plant food, the growth of which is favored by the more 
humid slopes,® as well as the greater duration of humid growing periods. 
As would naturally be expected, the difference in size of the snails 
is more marked in rather dry or semi-arid regions than in those abun- 
dantly watered and shaded. Our measurements of Sonorella support 
the observations already published on other species. 
In some cases the divergence has proceeded so far that diversity 
of size becomes an important specific character, as in the case of 
Sonorella leucura and micra, living on opposite sides of White Tail 
8 Additions to the shell are made only when the animal is active, during 
humid periods, and thus exposed neither to arid winds or alkaline dust. Growth 
of the shell does not proceed when the snail is retracted and at rest, but only 
when it is in full activity, with the mantle fully produced. 
6 As claimed by Dr. W. H. Dall, Proc. A. N.S. Phila., 1896, pp. 411, ete. 
7A long list of conspicuously opaque, chalky snails which estivate in the 
full glare of the sun could be given. We may mention Leucochroa, Cerion incanum 
maritimum, ete., Bulimulus alternatus, Oreohelix strigosa huachucana. Allied 
species which live in more shaded places tend to lose their opacity ; cf. Oreohelix 
barbata and O. clappi. 
8 At least within limits of two or three thousand feet. 
® We have already given data in support of this facet in Proc. A. N. S. Phila., 
1906, pp. 552-555, and in other places. 
