358 PLEISTOCENE GEOLOGY OF THE THAMES VALLEY.
Unio littoralis, Lam. Not found by us. Examples in
the Natural History Museum.
Unio pictorum, Linn. Not found by us. Examples are
preserved in the Prestwich collection in the Natural History
Museum.
Unio tumidus, Retz. Abundant, but most of the speci-
mens were small, and owing to their condition it was only with
extreme difficulty that perfect examples were obtained.
Anodonta cygnea, Linn. Numerous broken examples in
the clay.
Corbicula fluminalis, Müll. Very common, but though
examples were obtained ranging from the fry to the adult none
of the specimens can compare in point of size with Crayford
examples. A pair of valves obtained from the base of the shell
bed still retained traces of their coloration.
Sphaerium corneum, Linn. Common. Most of the
examples were intermediate between typical corneum and var.
maenanum.
Pisidium amnicum. Abundant. Many of the examples
were very large and much inflated, a variation which is appar-
ently confined to Grays. Numerous specimens of the variety
danubialis were found. This variety is well marked, resembling
in shape P. fontinale, but it is of course very much larger. This
variety was first found at Crayford, and on examples being sub-
mitted to Dr. O. Böettger, he identified them as var. danubialis.
We have not seen recent British examples, though it is living on
the Continent.
Pisidium astartoides, Sandb. Very common. This is a
well marked species and though strongly sulcate specimens of
P. amnicum may somewhat approach it, yet its shape and the
sulcate umbo at once distinguish it.
Pisidium pusillum, Gmel. Recorded by S. V. Wood.
Pisidium fontinale, Drap. Very abundant.
There is now a total of sixty-one species known or recorded
from Grays ; forty-nine being from existing examples and
twelve on authority, a longer list, with the exception of Ilford,
than from any other Pleistocene deposit in the Thames Valley.