VARIATION IN SIZE. 
S3 
expression of the various climatic and other modifying circumstances 
to which the mollusks have been subjected. A cold unfavourable 
season would result in a smaller and more delicate shell, with pro- 
bably more or less distinct transverse indications of the growth-checks 
sustained, while another season of a milder and more favourable 
character would result in larger and more uniformly grown specimens. 
In Malham Tarn, a large body of water upon an elevated plateau, 
1,250 feet above the sea, in the West York- 
shire Highlands, Mr. W. Denison Roebuck 
and Mr. .J. Darker Butterell, in Sept., 1883, 
collected numerous specimens of Limncca 
stagnali!^, which I found to clearly reflect 
the low temperature and the thermometric 
vicissitudes of the lake by a shell of great 
comparative delicacy and small size, with the 
many growth-checks strongly emphasized 
by the whitish transverse thickenings 
crossing the whorls and showing that part 
of the shell-forming energy has evidently been here diverted to more 
vital purposes. Other specimens obtained 
during August, 1890, from the same place, 
did not exhibit any striking diversity from 
ordinary examples, and this was probably 
owing to the active growth-periods being 
during comparatively mild and favourable 
weather. 
An examination of the meteorological 
statistics preserved by the Philosophical 
Society at Leeds confirm and emphasize the 
probable correctness of my conclusions. 
The early spring of 1883 was characterized 
by severe cold, frerpient snow storms, and 
considerable fluctuations of temperature, 
while that of 1890 was distinctly more 
equable, and showed fewer and .slighter variations of temperature. 
For every other species there are probably also certain conditions 
of temperature, etc., exceptionally flxvourable to their growth and 
development, which may be termed the optimum, subject to which 
conditions progress and growth are most rapid and decisive, being 
Showing the even regular 
growth and larger size, as at- 
tained under more favourable 
circumstances. 
Fig, 188. — Lij7in(pa stagnalis 
var. fragilis-variegata Roe!)., 
Malham Tarn, Vorks., Sept , 
1883, 
Collected by Mr.W. D. Roebuck, 
Showing its maturity by the 
full number of whorls and the 
inclement nature of the growth 
periods by a small thin shell, 
with many strong indications of 
growth checks. 
