360
THE ESSEX NATURALIST
species of the Mersea Island molluscs which come into this cate-
gory have been included in the list.
The thirty-one species, all either prosobranch or pulmonate
gastropods, were recorded from collections made during the
summers of 1961 and 1962. They are listed below with some
ecological notes, according to the system and nomenclature
employed in the Census of British Non-Marine Mollusca (1951).
The identifications have been made mostly with reference to Ellis
(1926) and Macan (1960).
Species list
Class Gastropoda
Sub-class Prosobranchia
Family Hydrobiidae
Hydrobia ventrosa (Montagu). Common in the saltings along the
edges of Pyefleet and at East Mersea. Shells commonly
taken in the litter at high water mark.
Peringia ulvae (Pennant). Very common on the tidal flats, in
saltings and brackish water ditches. Shells are often deeply
eroded by nematodes.
Potamopyrgus jenkinsi (Smith). Very common in fleets and
ditches on the drained marshes and also occasionally in
fresh-water drains running onto the saltings. The popula-
tion in the fleet at West Mersea contains a high proportion
of forms with keeled shells. The keeled and normal form
are shown in Fig. 2.
Figure 2.—Potamopyrgus jenkinsi keeled and normal shell forms.
Family Assimineidae
Assiminea grayana Fleming. Only found in the saltings and can
be very common beneath the litter at high water mark.
Sub-class Pulmonata
Order Basommatophora
Family Ellobiidae
Carychium minimum Muller. Frequently found in ditches and
hedgerows, mostly close to the drained marshes.