VARIATION IN THICKNESS. 
77 
and also of diminished size ; the Cheadle specimens of the same 
species, though smaller and thinner, are less transparent than the 
Burnley shells, and are, according to the careful and connected 
observations obligingly undertaken at my request by Mr. Masefield, 
subjected to a temperature varying between 60° and 98° Fahr. ; 
when the water rises towards the latter temperature these mollusks 
crawl out from it and remain attached to the sides of the condensing 
reservoir at some point above the water level until evening, when the 
temperature owing to the stoppage of the engines, begins to fall again, 
and on its decreasing to about 98° the mollusks have been observed 
to re-enter the water, that temperature appearing to be the maximum 
heat they are able or willing to endure. 
Fluviatile species inhabiting great depths, much beyond those in 
which they are normally found, also show marked attenuation of shell 
substance, probably on account of the lower temperature to which 
they are exposed hindering the free exercise of the shell-secreting 
function. The Limnan stagiicclis dredged by Mr. Bryant Walker in 
High Island Harbour, Lake Michigan, at a depth of 34 feet, is 
remarkable for the delicacy of its shell, and this is also a very 
striking character of the various abyssal forms of Limncva from the 
depths of the Lake of Geneva. 
In Helix aspersa v. tenuior found in Guernsey, where calcareous 
strata are absent and the shell more largely composed of animal 
matter, we have a very good example of the effect of a deficiency of 
shell-forming material, the weight of a fairly characteristic specimen 
of this variety, kindly given me by Rev. Dr. McMurtrie, being only 
four grains, whereas the average weight of typical shells is about 
32 grains. Helix nemondis shows similar, though less striking, 
results, as specimens found by Mr. J. Ray Hardy upon the Volcanic 
Slate, near the summit of the Macgillicuddy Reeks, co. Kerry, were 
so excessively thin that many of the most fragile collapsed under 
the necessary compression used in gathering or cleaning them, the 
most delicate shell preserved intact weighing 3‘3 grains, or a little 
more than one-quarter of the weight of ordinary individuals. 
This deficiency of calcareous matter has also, according to Clessin, 
often a marked effect upon the form of certain species, the .shells of 
Clausilia being said to diminish in length and Helix lapicidu to 
become rounded at the periphery. 
