48 PROCEEDINGS OF THE ACADEMY OF [Feb., 



active life and periods of growth are certainly limited to dewy nights 

 and times of ram, 5 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. Pulmo'nate 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, 6 but 

 as a protective adaptation to excessive sunlight, which would readily 

 penetrate thin shells, or those composed largely of conchiolin. 7 



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, 8 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, 9 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 



5 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 fullv produced. 



6 As claimed by Dr. W. H. Dall, Proc. A. N. S. Phila.. 1896, pp. 411, etc. 



7 A long list of conspicuously opaque, chalky snails which aestivate in the 

 full glare of the sun could be given. We may mention Leucochroa, Cerion incanum 

 maritimum, etc., Bulimulus alternatus, Oreohelix strigosa huachucana. Allied 

 species which live in more shaded places tend to lose their opacity; cf. Oreohelix 

 barbata and 0. clappi. 



8 At least within limits of two or three thousand feet. 



9 We have already given data in support of this fact in Proc. A. N. S., Phila., 

 1906, pp. 552-555, and in other places. 



