88
XII. On the Land and Fresh-water Mollusca of the
District around Colchester; a contribution towards
a list of the Fauna of Essex.
By Henry Laver, M.R.C.S., F.L.S.
[Read September 24th, 1881.]
The distribution of the Mollusca of a district depends so
much ou its geological features that no excuse is needed in
prefacing an account of the Conchology of Colchester by
giving its geological conditions. The town itself, south of
the River Colne, is situated on sands and gravels, and the
same soil is continued for five miles from that river in the
southerly district, and also for four miles to the west of the
town, as well as for two miles on the east. Beyond this
distance, in the latter direction, there are clays, sands, and
gravels of the Tertiary formations. Southwards the sands
and gravels are succeeded by London Clay, and westwards by
Boulder Clay and Brick Earths, as at Copford, where are the
well-known brick pits and the remarkable deposit of Shell
Marl. North of the town, after crossing the river valley,
London Clay appears again, and continues until the Stour is
approached, when sand is the prevalent formation.
From this short sketch it will be seen that geologically the
district is one from which many molluscan forms will be
wanting. There is an entire absence of bog, and on the
whole the district may be considered dry; but it is fairly
timbered, and contains a few woods, principally composed of
oak. The greater part is under the plough, and there are no
commons worth mentioning. With all these unfavourable
conditions, I think the following list will prove that we are
richer in species than might have been expected.1
1 [The number of species enumerated by Mr. Laver within his district
is 75, compared with 121 species given by Jeffreys in 'British Conchology'
as the total number of Land and Freeh-water Molluscs at present in-
habiting Great Britain.—Ed.]