THE POST-PLIOCENE NON-MARINE MOLLUSCA OF ESSEX. 105
CHELMSFORD.
In 1895 Mr. W. M. Webb recorded eight species of shells
from the Pleistocene brick-earth at Chelmsford (33). They are
all common forms of that period, and chiefly interesting because
it is an entirely new locality for Molluscan remains. It has,
however, been considered advisable to tabulate them, because
there is a possibility that they may be of a slightly different age
from the other Essex Pleistocene shells.
[NOTE. Since the above was in type we have had the
opportunity of examining the late Professor Prestwich's
collection (now preserved in the Natural History Museum), and
we were pleased to find in it examples of Unio pictorum from
Ilford and Grays. This is a new record, or rather authentication,
for the Pleistocene of the Thames Valley and the view that it
was a late introduction into that part of England must now be
discarded.]
AGE OF THE DEPOSITS.
As to the relative age of the Beds, it is impossible to speak
with certainty. Roughly speaking, the Holocene deposits can
be divided into two groups : Canning Town, Lea Valley, and
Tilbury belonging to one ; Chignal St. James, Felstead, Roxwell,
Shalford, and Copford forming another : while Witham has
affinities with both. In all probability, the differences between
these groups arises not from a later or earlier age of deposition,
but from a difference in origin ; the former belonging to the
larger, while the latter was deposited by the smaller, streams.
The extraordinary abundance of Vertigo at Copford and Chignal
is unparalleled, but in the Journal of Conchology, July, 1893, p. 203,
is an account of the rejectamenta of a small river at Portsalon,
Co. Donegal, by Mr. R. Standen, in which a similar state of
affairs was found to occur, and there can be little doubt that
these deposits were formed in a like manner.
With regard to the Pleistocene deposits, Grays and Ilford
are probably of the same age, while Clacton and Harwich may
also be classed together ; the relation, however, of the two latter
to the former is still a matter of pure conjecture.
NOTES ON SOME OF THE SPECIES.
Individual variation, though prevalent at the present day, as
a glance at any "variety" list will shew, seems to have been
even more marked in former times, and it is a matter of con-