THE POST-PLIOCENE NON-MARINE MOLLUSCA OF ESSEX. 100
There are only five living species which, although found in
the Pleistocene are unknown in the Holocene. We have already
noticed Helicella virgata, the unique example of which was found
at Ilford. Buliminus obscurus is only known from Copford and
from a post-Roman hill-wash at Otford, Kent. The absence of
Paludestrina stagnalis [=P. ulvae] is due to the fact that it is a
brackish-water form ; whilst Planorbis lineatus is known from
several Holocene beds in the London district, and its non-
occurrence is merely an accident and Buliminus montanus is
found in the Holocene tufa at Blashenwell, Dorset.
Of the seven forms only known from the Holocene no less
than four have been recorded from the older series in other
parts of England Vivipara contecta at Crayford, Pomatias
elegans and Caecilianella acicula from Ightham, whilst though
we have seen no Pleistocene examples of Planorbis corneus
from Essex, yet since it is a common form of that period
and has been recorded from three localities in all probability it
existed in the county during the period in question. The
remaining three species are presumably late arrivals in Essex,
viz.:—Helix aspersa, Vivipara vivipara, and Neritina fluviatilis.
In considering those species, twenty-seven in number, which
are quite unknown in a fossil state, it is perhaps better to
exclude the whole of the slugs as well as the Brackish-water
forms, thus leaving fourteen species. Of these four have been
recorded from other Post-Pliocene beds : Vitrea alliaria from the
Pleistocene of Ightham ; Balea perversa occurs in the Pleistocene
of P>arnwell, and in a Holocene deposit at Crossness: Sphaerium
rivicola occurs in beds of both ages in the London District
and Pisidium nitidum is found abundantly in old lake beds
in Scotland. Pyramidula rupestris has been recorded from
West Wittering, but as we have not seen the specimens we
cannot say whether correctly or not. A single example of
Vitrina pellucida is known from the Cromer Forest-bed, but
no specimen of later date is known. The remaining eight
species as yet unknown in a fossil state ; they are :—
Vitrea glabra Limnaea glabra
Helicella cantiana Sphaerium ovale
Hygromia fusca ,, lacustre
Helix pomatia Pisidium milium.
Of course it is quite possible that some of these forms may
yet be found preserved in some undiscovered deposit in this
country, still as far as our knowledge goes at present they must
be deemed modern additions to the fauna.