204 ON THE LAND AND FRESH-WATER MOLLUSCA
The present list consists of seventy species, all of which have
been collected by myself, chiefly in Wanstead, Barking Side, Leyton-
stone, Leyton, Walthamstow, East Ham, and the valley of the Roding
from Woodford Bridge down to Barking Creek.1
In the general "Census" compiled by Mr. John W. Taylor,
F.L.S., and W. Denison Roebuck, F.L.S., which is printed as
chapter v. in a little book entitled "Land and Fresh Water Shells," 2
they give the number of species as forty-seven in North Essex, and
thirty-two in South Essex ; but it must be remembered that they
only include in their list such species as have been verified by
specimens sent to the referees of the Conchological Society. Such
records as those of Dr. Laver and Mr. French are therefore not
included, and thus the "Census" is not a complete record.
The soil of the Becontree Hundred is mainly composed of London
Clay and gravels ; whilst the valley of the River Roding is covered
with alluvium. This river, which is very tortuous, is subject to
heavy floods after rain, when large tracts of the lower land, especially
between Woodford Bridge and Wanstead Park, are covered with the
overflow; and the roads at the same time are covered with water
from six inches to twenty inches in depth—the former depth near
Woodford Bridge and the latter in the lane at Barking Side, near
Fern Hall Farm and occasionally in the road at Red Bridge. At
the latter place I have seen on one occasion during the last ten
years, after an exceptionally heavy storm, over two feet of water in
the road.
Many species, such as Paludina vivipara and the Unios, were
formerly extremely common in the River Roding, but of late years
they have been kept down, probably by the ever-increasing amount
of sewage. This, however, is being remedied by the establishment
of filtering beds, and in consequence the water in the Roding is
now much clearer.
1 At the reading of the paper I had the pleasure of exhibiting a selection of all the shells
and of some of the slugs. The latter are, however, difficult of preservation in anything like a
natural condition.
2 By J. W. Williams, "The Young Collector Series," Swan & Co., 1889.