ALLUVIUM AT WALTHAMSTOW, ESSEX. 19
from the Pleistocene of Barnwell and the Holocene of Exedown
(near Wrotham), Crossness and the Hampshire tufaceous
deposits.
Clausilia bidentata, Strom. This widely distributed form
has hitherto been undetected in these beds. It was represented
by six examples.
Succinea elegans, Risso, proved a more abundant form than
S. putris, Linn. One example of ths former, the largest we have
seen, measured no less than 21 mm. in length. The largest
individual of S. putris was 18.5 mm. in length, the average size
being 12.5 mm., whilst the average of S. elegans is rather less.
Carychium minimum, Mull., was not common, but it is
a new record.
Limnaea auricularia, Linn, on the other hand was very
abundant, many of the examples attaining a large size, but the
finest of all were obtained from the modern deposits.
Limnaea pereger, Mull., was equally common with the
last-named in the older beds, but was decidedly scarce in the
more recent deposits.
Limnaea stagnalis, Linn., attained a large size, and was
not uncommon. The largest individual measured 52 mm. in
height, with a maximum breadth of 22 mm.
Limnaea truncatula, Mull., was another common form,
and was decidedly above the average in size.
With the exception of Planorbis glaber, Jeff., and P. lineatus,
Walker, all the British species of Planorbis were present. It
was the material from these beds that first enabled us to
differentiate the form P. stroemii, West, which had not hitherto
been noted in these islands, either living or fossil.2
The species is near to P. albus Mull, but is distinguished
from it by its larger size, the constant presence of a keel, and
the absence of spiral striae: it also resembles an almost keelless
variety of P. carinatus.
At Walthamstow P. stroemii was a far more abundant form
than P. albus in the older beds, but in the modern deposits the
former is very scarce, whilst the latter is abundant. In our
opinion the examples in the recent beds have been derived from
older deposits. Mr. A. C. Johansen, of the Zoological Museum,
2 Proc Malacological Soc. Lond. Vol. IV. (1901), p. 236.