'TAYLOR : ON THE VARIATION OF MOLLUSCA. 297 



Ancylus even when occupying the same water, is said to be 

 remarkable for unusual thinness and delicacy. 



The dry and arid regions in different parts of the world 

 are tenanted mainly by mollusks with thick and uniformly dull- 

 white shells, of which the Palsearctic species are grouped under 

 the name of Leucochroa, and are principally found on the sterile 

 lands to the South of the Mediterranean. In what is called the 

 arid regions of Central North" America, comprising the territories 

 of Idaho, Utah, Montana, &c., the same character of shell 

 is reproduced in Helix idahoensis, cooperi, &c. The Rev. 

 Canon Tristram in alluding to the mollusks of the Sahara 

 remarks that the snail-shells found there, were much thicker 

 than those of the same species from the more temperate parts 

 of Europe, and he is disposed to regard this modification as an 

 additional means of preventing evaporation in so dry a climate. 

 The elimination of the banding would appear to be desir- 

 able on account of the darker coloring, greater tenuity and 

 possibly less calcified condition of those parts, which would 

 therefore offer less obstacles to the dessication of the mollusk 

 than the thick white shell which would absorb the least possible 

 amount of heat. The uniformly dull-white specimens of Helix 

 vii'gata, ericetonim and other shells of a similar character in our 

 own country, agree also with the desert forms in being of 

 thicker texture than usual. I am disposed to think they are 

 analogous to them, and tend to be evolved by conditions similar 

 though less in degree to those to which I have just referred. 



In confirmation of this view Strobel has noticed that in 

 Moravia the thick uniformly white variety of Helix virgata is 

 exclusively found on the open cultivated lands, and attributes 

 these peculiarities to dryness and warmth. He noticed the 

 banded variety to be more restricted to the wooded grounds. 



Those mollusks which usually live in shade and retirement, 

 only coming forth at twilight and evening, or by the stimulus of 

 moist, damp or dull days, are provided with shells composed 

 largely of animal matter, of uniform and obscure colors, and 



