94 THE POST-PLIOCENE NON-MARINE MOLLUSCA OF ESSEX.
CHIGNAL ST. JAMES.
This deposit is situate on the right bank of the Cann, a
tributary of the Chelmer, 31/2 miles north-west of Chelmsford and
51/2 from the brook's source. The first published account
of the Mollusca from this locality was in 1879 by Mr. J. E.
Clark (20), who listed sixty-one species on the authority of
Mr. Miller Christy, but of this number eleven are noted as
only occurring in the surface soil. A section and map was
given, and Mr. Clark pointed out that some of the species were
locally extinct, whilst on the other hand some species now
abundant in the locality were absent. The next notice was in
the Essex Naturalist in 1889 by Mr. Miller Christy (25),
abundant details of the deposit being given, with a list of the
Mollusca, and notes on the occurrence and absence of certain
species. Mr. Christy's specimens are now in the Essex Field
Club's Museum, and we have to thank Mr. W. M. Webb for
placing at our disposal his notes on these examples and for
enabling us to examine critical specimens. There are forty-
eight species represented, five of which are new, viz., Vitrea
excavata, Vitrea nitida, Vertigo moulinsiana, Physa hypnorum, and
Pisidium pusillum.
Eight names have been rejected, viz. :—Agriolimax agrestis,
Helix [=Pyramidula] rupestris, Helix [=Helicella] caperata,
Balea perversa, Succinea elegans, Physa fontinalis, Sphaerium
corneum, and Pisidium nitidum.. These are unrepresented by
specimens, and many of them are due to oversights in identification.
The surface soil in one place also contains a large number of
shells which of course are of much later date than those in the
deposit proper.
It is noteworthy that no less than ten species have been
found in the newer bed which do not occur in the older one,
viz. :—
Limnaea pereger Planorbis contortus
,, auricularia ,, carinatus
,, palustris ,, marginatus
Bythinia leachii ,, corneus
Pisidium amnicum ,, vortex.
Of course it is quite possible that these species existed in
the neighbourhood at the time of the deposition of the older bed
and that their absence is the result of accident ; yet, considering
that they are all freshwater forms, it is by no means improbable