108 THE POST-PLIOCENE NON-MARINE MOLLUSCA OF ESSEX.
Planorbis glaber is a form which is certainly less common
than formerly. In spite of a former protest, many persons
persist in misidentifying this species with P. parvus, Say., so that
it may be as well to take this opportunity of repeating Dr. Dall's
verdict, which is : "Planorbis glaber, Jeffreys, in my opinion, is
distinct from P. parvus. The latter is flatter and less deeply
umbilicated, and shows much more of the whorls within the
outer one. It is also distinctly less polished in the average
specimen."2 As P. glaber is known from Pliocene deposits in
this country, it is probable that these forms are descended from
a common ancestor.
Helicella caperata is the only species that has apparently
increased in size since the Pleistocene, all the other forms having
certainly diminished, as the figures we have given well shew.
There can be no doubt that the Pleistocene molluscan
fauna was in every way a finer one than that now existing, in
spite of the fact that perhaps a dozen species have been intro-
duced through human agency ; though this does not appear to
be so numerically in our table owing to the imperfection of the
record. Nevertheless the figures for the older period now given
show a decided increase on those to be found in "The Pleistocene
(non-marine) mollusca of the London District" (31), and future
work is certain to augment them.
CONCLUSIONS.
On comparing the Table accompanying this paper with the
one published in ''The Pleistocene Mollusca of the London
District" (31) a great advance in our knowledge of the
geological history of our non-marine shells will be noted.
Species then only known from the Holocene have had their
range extended to the Pleistocene ; gaps in the records have
been filled up, and many doubtful records have been confirmed.
Of the seven species which we have noted as having become
extinct in Essex in Holocene times it is extremely probable that
three of them may yet be found living in the county, viz., Acicula
lineata, Vertigo angustior, and Clausilia rolphii. They are all local
forms and are to be found living in the adjoining counties, so
that it is not too much to hope that they may yet be added to
the list of Essex Mollusca.
2 Co-types of these two species sent over by Dr. Dall for the purpose are preserved in the
Natural History Museum.