7G 
VARIATION IN THICKNESS. 
while those of Helix hhpida are usually more or less aiigulated or heiit, 
with the tips directed forwards or towards the aperture of the shell. 
These periostracal hairs are not coiihued to the Gastropods, hut are 
equally possessed by the different species of Phldiiim, Spliarium, 
etc. ; those upon the .shell of Sp/urriitm corneum are of microscopic 
size, somewhat short, comparatively thick, bent at the tips and most 
numerous in the umbonal region. 
Substance, or thickness, is frequently the outcome of an ample ora 
limited supply of calcareous matter, though the variation in this 
character may also arise from an apparent physiological inability to 
utilize the material presented, which action may be intensified or 
diminished by chemical, meteorological, or other conditions. This 
weight or thickness varies not only according to the species, some 
having thin and almost purely chitinous shells while others are heavy 
and dense ami almost entirely composed of calcareous matter, but 
also according to age, young specimens being always more delicate 
than adults living under the same conditions and containing propor- 
tionately less calcic carbonate than the more aged shells, in which the 
nacreous or inner layer may become abnormally thickened if the life 
of the individual be from any cause greatly prolonged. 
It has also been satisfactorily and conclusively shown that a tem- 
perature above or below the optimum, which optimum varies for the 
different specie.s, results in the minimising or even the ce.s.sation of 
the shell-secreting function, and the growth, if any, is characterized by 
greater delicacy as the temperature recedes from the optimum. Thus 
the varieties ghichdh mxA thermcdis oi L'lminm jieregra which inhabits 
glacial and thermal springs respectively, are ecpially distinguished for 
the extreme delicacy and dwarf size of their shells. Specimens of 
Llmnau perefjra found living in the warm 
water of engine cisterns at Cheadle, Burn- 
ley, and other places, are characterized 
by the same extreme tenuity and 
diminished size. This form is almost 
ei|ually well-known by the two distinct 
names, dlaphami and thenmdh, the first 
descriptive of the character of the shell 
and the second referring to its habitat. 
The Burnley shells are, as I am informed by Mr, h'. G. Long, sub- 
jected to a temperature of 84° Fahr., and are especially transparent 
Fig. 170 . — Linnuea peregra var. 
ilu'7‘///aiis Foub^e X lA, 
Mill cistern, (iannow, llurnley, 
Collected by Mr. R. Wiggle.sworth, 
Sliowing the effect upon tlte .shell 
of too great a degree of warmth in 
the iiihal>iled water. 
