20 THE NON-MARINE MOLLUSCA OF THE RIVER LEA
Copenhagen, who kindly identified the species for us, informs us
that it is now living in Siberia, Finland, and Northern Scandi-
navia, while it occurred in Denmark solely in deposits of the
Oak-period (=Bronze-age). Up to the present we have noted
it from the dried "Thames mud" of the Embankment on the
site of the New Scotland Yard (in which it is probably a derived
fossil), from the Holocene gravels at the same spot and at the
Houses of Parliament; from Betteridge Road, Fulham; Kew,
Staines, and Clifton Hampden, Oxfordshire. It will be noted
that these are all connected with the Thames river system.
Why a form, which was abundant within comparatively modern
times, should have become totally extinct, is indeed an interest-
ing problem, for which we can offer no solution. (Fig. 2.)
Planorbis fontanus, Lightft., forms another new record
for these beds.
FIG. 2. PLANORBIS STROEMII, WEST.
FIR. 3. PISIDIUM SUPINUM, A. SCHM.
Both magnified four diameters.
No less than 9 species of Pisidium were obtained, of which
the most noteworthy is P. supinum, A. Schmidt, a form which
has hitherto been undetected in these islands. For its identifi-
cation we are again indebted to Mr. A. C. Johansen. The
examples are large, but are by no means so fine as those which
occur in the Pleistocene of Grays. We are of opinion that this
form will eventually be found to have been widely distributed in
England during Pleistocene and Holocene times, though as yet
it has not been detected living. (Fig. 3.)
CONCLUSIONS.
We have now a list of 73 species of Mollusca from the
Alluvium of the River Lea, 70 coming from the older