ALLUVIUM AT WALTHAMSTOW, ESSEX. 21
beds, and 3 from the modern deposits. These last are :—
Helicella cantiana (Mont.)
Sphaerium rivicola, Leach.
— lacustre (Mull.)
The first-named must be considered a recent introduction,
because though widely distributed over Essex at the present
time, it has not hitherto been found fossil in any deposit in the
country, however modern, and the available evidence seems to
show that it was introduced at a later period. Sphaerium rivicola,
on the other hand, is known from both the Pleistocene and
Holocene. Its absence from the Shell-marl is probably acci-
dental, and the same may be said of Sphoerium lacustre. The
four species which were not identified by us in these recent
excavations are :—
Acanthinula lamellata (Jeff.) Vivipara contecta (Millet.)
Helicella caperata (Mont.) Unio pictorum (Linn.)
The last-named was to be seen in the modern beds, but not
in the older deposits.
We have already noted two species, Planorbis stroemii, West,
and Pisidium supinum, A. Schm, as being quite extinct in this
county, though common in these deposits, but the remaining
64 species are all living in the area drained by the River Lea.
One thing is noteworthy, and that is the almost total absence of
abnormal or monstrous forms, with the exception of Planorbis
stroemii. This form is very often slightly distorted. There were
a few "mended" examples of Helix nemoralis and Helicigona
arbustorum; two slightly scalariform specimens of Bithynia
tentaculata and one of Limnea palustris ; two distorted examples
of Planorbis spirorbis and three of P. vortex. These are all
observed, although we have had many thousands of shells
through our hands.
A great advance has been made in our knowledge of these
beds in the last few years. In 1890 only 40 species were known
as occurring in them. In 1897 we were able to list 52, and now
the record stands at 70, or if we include the forms from the
modern deposits, at 73. Fresh excavations will no doubt add
to the list, since there are many forms living in the Lea Valley
which are unknown from these beds. We would urge all mem-
bers of the Essex Field Club to lose no opportunity of collecting
from any future exposure in the Lea alluvium.