WITH A LIST OF THE MOLLUSCA OCCURRING THEREIN.
7
rule, they are pretty well distributed throughout the strata in which
they occur, though two — Cochlicopa tridens and Cyclostoma elegans—
are found but rarely in the black earth, except at one spot (fig. 1,
"E"). The Cyclostoma specimens had still some of the opercula in
place, as had also a few found in the chalky-marl, so that it is not
probable the shells had travelled far, although at the present time, I
believe, this species is not found living within thirty-five miles of
Chignal. It seldom lives off chalk or limestone.
The table of shells (page 8) found in the beds may be profitably
compared with the list of species found in the Clacton and Copford
deposits, as given by Mr. W. H. Dalton in the Geological Survey
Memoir, "Geology of the neighbourhood of Colchester" (p. 19). I
have followed Dr. Jeffreys' arrangement and nomenclature, but have
inserted the names used by Mr. Dalton when they differ.5
Altogether I have met with fifty-two species, of which all but one
—Physa fontinalis—occur in the black earth ; thirty-two are found
in the chalky-marl, and forty four at least now inhabit the neighbour-
hood. Of the total fifty-two species, all but six occur at Copford, and
all but fourteen at Clacton, while three species occurring at Chignal
are found neither at Copford nor Clacton.
The number of fresh-water species (10) is small, as well as the
number of individuals. Most of them are such as frequent ponds or
ditches, though at least four live in the brook at the present time.
These fresh-water shells are exceedingly scarce in the chalky marl.
Some of the specimens of Valvata piscinalis are rather higher in the
crown than the present type. Unio pictorum and Anodonta cygnea
now inhabit the brook, but do not appear in the deposit, being pro-
bably too heavy to be drifted by flood-water; but this objection would
not apply to Sphaerium lacustre, Neritina fluviatilis, Bythinia leachii
(ventricosa), Physa hypnorum, Ancylus fluviatilis, Ancylus lacustris,
several species of Lymnaea and Planorbis, all of which now inhabit
the neighbourhood.
Altogether forty-one species of land-shells appear, and the number
of individuals is generally pretty numerous. Eleven of these do not,
I believe, now inhabit the neighbourhood, although Helix rufescens is
common ten miles to the N.W., and Cyclostoma elegans thirty-five
miles N.W., but neither exists nearer to my knowledge. Some of
5 The method adopted for separating the shells from: the soil is very simple, but at the same
time very effective. I merely take the fine dry mould and throw it into water, when the smaller
species float in numbers, and may be skimmed off, while the larger ones may afterwards be
picked out by hand.