88 THE LOCAL EXTINCTION OF MOLLUSCS.
the attention of Essex naturalists for many years ; I allude to
the almost complete extinction of Cyclostoma elegans, compared
with its great abundance in Roman and comparatively-recent
times. This shell was thought to be now quite extinct in Essex,
but in the year 1890 I found a small colony of them alive in
Felstead, and they are still in existence (see Essex Naturalist,
vol. IV., p. 92). Cyclostoma is always, I believe, described as
associated with chalk or limestone. In Essex this chalk was
supplied by the Boulder Clay, or by the tufa which is formed by
the re-deposit of chalk at places where springs issue. But the
Boulder Clay is a wasting product, and is only occasionally
found on the valley slopes, neither is it continuous over very
large spaces on the higher ground,3 and where it is, drainage and
other artificial agencies have deprived the snail of its shelter and
broken up its colonies (always an important step towards local
extinction), so that Cyclostoma elegans has now almost completely
disappeared.
Another shell that is generally described as affecting a lime-
stone habitat is Helix lapicida. This species is, I feel sure,
verging also on total extinction, although still living in some
parts of Essex. There are no data, so far as I am aware, to
prove that it ever approached Cyclostoma in numbers, but there is
sufficient evidence to show that its wide-spread distribution was
due to conditions other than those now in operation. The stage
of separation and isolation is now very far advanced, and Helix
lapicida is always quoted as being local and in very small
numbers. Where such a condition of distribution obtains over
a very wide area, it is pretty clear that the species was once
common in that area. I suggest that the wasting away of cal-
careous matter from the surface soil is a probable cause for the
disappearance of this species.4 Perhaps the greatest factor of all
here at work in bringing about local extinction is the drainage of
the land ; this affects the Molluscs indirectly in many ways. In
former times, when even the uplands were sometimes sour
morass, growing mosses in abundance, and the county was well
wooded, a certain amount of moisture was stored up in the day-
time, to be given out as aqueous vapour and received back as
3 In reply to objections by Mr. T. V. Holmes and Mr. Whitaker, that "the Boulder Clay
is continuous over the greater part of N. Essex and forms the surface of most of the high
ground," Mr. French remarked that in "my neighbourhood (Felstead) Boulder Clay is certainly
not continuous. A reference to the geological map is misleading, because the map only takes
into account the subsoil. Wherever tillage is in progress, the surface soil, which is that in
which Molluscs work, is deprived of its calcareous ingredient."—Ed.
4 The greatly-increased preservation of birds may account for the disappearance of some
species of snails.—T. V. H.