18
THE NON-MARINE MOLLUSCA OF THE RIVER LEA
Hygromia hispida (Linn) was by far the commonest
Helicoid present, but the examples are by no means large,
the finest being but 9 mm. in diameter, the average height being
5.5 mm.
Helicigona lapicida (Linn) was not common, and all the
examples found were obtained from one patch of shell-marl near
the south side of the Lockwood Reservoir. It is a new discovery
for this neighbourhood, though it has been known from the
alluvium of the Lea at Canning Town.
Helicigona arbustorum (Linn) was an extremely common
form, and varied much in shape and size, the largest example
being 18 mm. in height, and 20 mm. in diameter, whilst the
smallest was 13 mm. in height and 16 mm. in diameter.
Helix aspersa, Mull, was answered for by two examples,
both very small, being 25 mm. and 24 mm. in height and 26
mm. and 27 mm. in diameter.
Helix nemoralis, Linn, was very common, the shells being
often well preserved, and still retaining their coloration.
The band formulae noted were :—
It will be noticed that the banded examples are greatly in
excess of the unicolorous examples, and that there is a marked
tendency for the bands to coalesce, a condition probably arising
from some condition in the environment.
Helix hortensis, Milii, was a new record for these beds.
The band formulae observed were :—
Balea perversa (Linn). One single example constituted
an interesting addition to the fauna of the alluvium of the Lea.
The species is extremely rare in the fossil state, being only known