THE MARINE FAUNA OF THE BLACKWATER ESTUARY 37
Creek and on Mersea Flat. Ninety-two living specimens were
taken from 0.2m2 of a mussel bed on the lower shore off Mill
Creek on 3.2.1964. Many variations of colour pattern occur but
these do not seem to have any ecological significance.
Class GASTROPODA
Sub-class PROSOBRANCHIA
Order Archaeogastropoda
Patellidae
Patella vulgata Linne Common Limpet.
Not common in the Blackwater. Some specimens reported
from the shore of Mersea Island and three specimens collected
from the Bradwell shore (14.6.1961 and 11.1.1962). The rarity of
the species is probably due to the lack of suitable habitat, as it is
quite common on rocks at Dovercourt in the north of Essex.
Trochidae
Calliostoma zizyphinum conuloide (Lamarck) Painted Top
Some shells have been found washed ashore on St. Peter's
Flats. Freshly vacated shells were dredged from the Wallet (1962)
and also collected on the shore at Walton-on-the-Naze (6.1962).
Gibbula magus tuberculata (da Costa)
Recorded from the Blackwater as Trochus magus during the
Essex Field Club excursion of 1881 (Cole, 1881). There are no
subsequent records.
Gibbula cineraria (Linne) Grey Top
Fairly common on clean shelly or stony bottoms sub-littorally
and on the lower shore. Distribution in the Blackwater is from
Osea Island seawards. Small shells are distinctly conical but they
become more inflated with age.
Gibbula umbilicalis (da Costa) Purple Top
Recorded from the Blackwater as Trochus umbilicalis during
the Essex Field Club excursion of 1881.
Order Mesogastropoda
Lacunidae
Lacuna crassior (Montagu)
Living specimens occasionally obtained in dredge samples from
clean shelly bottoms. Shells are generally common on most shores.
Lacuna parva (da Costa)
Living specimens occasionally recorded and dead shells are
common on the shores. This is mostly a western and south-western
species but does extend into the North Sea.
Littorina littorea (Linne) Common Periwinkle
Very common on the middle and lower shore and also on salt-
ings in most parts of the Blackwater. Some specimens have also
been taken sub-littorally along the Bradwell shore and in Thirslet