SUBSIDENCE OF THE THAMES ESTUARY. 167
Paladelhia radigueli (Bourg), Bithynia tentaculata (Linn.),
Corbicula fluminalis (Mull.).
Marine shells from estuarine clays near causeway.
Oyster, Cockle, Mussel, Periwinkle.
Shells of edible marine molluscs from the Shell-mound.
Oyster, Cockle, Mussel, Periwinkle.
Shells of edible molluscs in pockets of causeway.
Oyster, Cockle, Mussel, Periwinkle, Whelk (1), Paphia
decussata (Linn.) (1). Paphia decussata appears on
prehistoric sites in Sussex. The Oyster was apparently
indigenous on the East Coast. In Southchurch it was
eaten before the Roman period ; it was present in the
Shell-mound which seems to date from the junction of the
Bronze and Early Iron Ages ; it was mingled with
Hallstatt-La Tene pottery in the pockets of the causeway
and was also present with the Romanized pottery and
shells of Helix aspersa on the islet road. At Gullane
Bay [45] and North Berwick [46] it occurred with pottery
of the Beaker period. During the Roman period it became
an article of diet in the southern counties [47].
In addition to these marine forms the shells of Helix aspersa
occur on the surface of the islet road ; the significance
of its presence has been mentioned previously.
The Shells of the Causeway.
A. Freshwater types.
a. Characteristic of permanent water in shallow sedgy
meres ; from the submerged portion of the corduroy
road ; 2 species, abundant.
Limnaea pereger (Mull.), Planorbis leucostoma (Mull.).
b. Characteristic of swamp with small pools liable to
desiccation ; from the islet road ; 8 species, scanty.
Limnaea truncatula (Mull.) Pisidium cinereum (Alder)
Planorbis spirorbis (Linn.) ,, nitidum (Jenyns)
Aplexa hypnorum (Linn.) ,, personatum (Malm)
Succinea elegans (Risso) ,, obtiisalastrum
(B. B. Wood).
B. Marine type.
Peringia ulvae (Penn.) One specimen was found on
the islet road ; it may have been accidentally intro-
duced with the shells of edible marine molluscs
or by a bird.
M