PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 357
Planorbis vortex, Linn. One example of this hitherto
unrecorded species. It is a rare form in any Pleistocene
deposit.
Planorbis spirorbis, Linn. Several examples.
Planorbis contortus. Linn. An imperfect example.
Recorded by S. V. Wood, but no examples known.
Planorbis fontanus, Lightfoot. Two examples, one of
which has been lost. This is a new record for Grays. It is
known from the Pleistocene of Westminster, Clacton, Copford
and Barnwell.
Planorbis lineatus, Walk. Recorded by S. V. Wood ;
perhaps an error for P. fontanus,
Paludestrina ventrosa, Mont. An abundant form.
This is the more remarkable as at the present day it is a
brackish water form, and there is no other Palaeontological
evidence of estuarine conditions at Grays.
Paludestrina marginata, Mich. Several examples of
this species which is now quite extinct in this country though
still living on the Continent, in S.W. France, N.E. France and
Belgium. Though known from several deposits in this country
of Pleistocene age it is not known from any newer beds.
Bythinia tentaculata, Linn. Abundant.
Bythinia leachii, Shepp. Not found by us, though
examples are in the Natural History Museum from Grays.
Valvata piscinalis, Müll. The most abundant species.
Numerous examples of the form known as var. antiqua, Sow,
were found as well as every intermediate state between it and
var. depressa, Pfr. Though the former variety is on the Conti-
nent considered a good species under the name of V. contorta,
Menke, in our opinion it can only be considered an extreme
variety of the polymorphic species V. piscinalis. We have
never seen recent British examples of var. antiqua though
according to Jordan1 it still lives in Southern Scandinavia,
Jutland, Holland, Germany and N. France.
Valvata cristata, Müll. Seven examples.
Die Binnemollusken der Nördhch gemassigten Lander von Europa und asien und der
arktischen Lunder.