14
ON THE MOLLUSCA OF THE SHELL-MARL
These lists are obviously the result of a somewhat cursory exami-
nation. There is a discrepancy in collating them with mine which
should have mention. "Bythinia" occurs in their list, no specimen
of which genus I have ever found. On the other hand I have every-
where found Cyclostoma elegans to be a common shell, but its name
does not appear in the Memoir lists. It may be that a wider study
will throw further light upon this difficulty. In any case, it does not
materially affect our conclusions.
In the first place, the distribution of the Mollusca in Essex which
obtained when these deposits were formed differs from that of the
present day, as a comparison of a few species will show :—
The first two species in the above list are now confined (according
to Rimmer) to the North of England, Scotland, Ireland and
Wales ; the third, H. carthusiana, to the Downs of Kent and Sussex.
Clausilia rolphii, according to the same authority, has only been
noticed in a few places in England. Zonites purus and Cochlicopa
tridens are perhaps now not anywhere common in England. But the
most striking feature in all the lists is the comparative absence of
freshwater forms. This will appear the more remarkable when we
consider that those alone now, owing to the complete drainage of
the country, are the shells that stand a chance of preservation; and
that in point of numbers and universality they probably far exceed
the land-shells. Moreover, the conditions under which the marl was
laid down would appear to have been favourable for their existence,