90 THE LOCAL EXTINCTION OF MOLLUSCS.
SPECIES.
PAST DISTRIBUTION.6
PRESENT DISTRIBUTION.
Turning our attention to the Freshwater Molluscs, other
agencies affecting extinction may be assumed to come into play.
Reduction of quantity of suitable food may be one factor, but of
this I have no good evidence to offer. As regards lowering of
temperature, something may be said. One species, at least,
found readily in the ancient river alluvium, is now rare or locally
extinct, although it still flourishes as the water becomes deeper
towards the estuary. This is Neritina fluviatilis. The conditions
under which the alluvium was laid down were those of stiller
and deeper, and therefore, perhaps, warmer water ; i.e., not so
readily affected by cold springs, which now appear to act as a
deterrent of both animal and vegetable life, and of which I
could instance many examples.7 Other species have been known
for many years in the lower reaches of the river, but they have
been unable to make any progress up stream. Two or three
species deserve particular attention among those that are dying
out. These delight in ascending ditches that have an immediate
communication with the river. Such are Succinea putris and
S. elegans, Limnaea palustris, L. truncatula, Planorbis carinatus,
P. contorta, &c. The Planorbis will flourish, even though their
river-communication be cut off, but my experience is that
6 In the shell-marls and peat which, as shown in the paper on the Felstead Shell-Marl
above referred to, were accumulating down to Roman times. They in this district all overlie
gravels.—J. F.
7 [We have no evidence of any reduction of temperature in recent times ; but drainage and
the development of agriculture would certain!)' affect plant lite and the increase or decrease of
particular species.—T. V. H. As to evidence of reduction of temperature, Mr. Christy's
remarks, in his recent paper on "Essex as a Wine-producing County" (ante pp. 44--46) may be
usefully borne in mind.—Ed.]