170
LAND AND FRESH WATER MOLLUSCA.
operations of this Society. If so, I need only express the hope that
they will continue their investigations, and by the friendly co-operation
which happily characterizes these pleasant field meetings, aid in the
preparation of a catalogue of the Essex Mollusca which may be not
only of special interest to this Society, but useful to conchologists in
other parts of the country.
Those who have already taken up the subject will doubtless have
become sufficiently interested in it to require no further stimulus from
any observation that I can now offer ; but on the other hand there
may be some to whom the subject is altogether new, and who con-
sequently may be glad to receive a few hints to enable them to make
a start as collectors and students of a most interesting class of animals.
One great difficulty which attends the collection of marine shells will
not arise here. Amongst the Marine Mollusca, the number of species is
so enormous, and the difficulty of obtaining them (except by purchase)
is so great as to deter many people from attempting to collect them.
Now the number of British Land and Freshwater Mollusca is com-
paratively limited. Roughly speaking there are only about 120
species, excluding such as have been evidently introduced, or
described as British on insufficient authority, but including the slugs,
which though generally regarded as shell-less, have the shell either
rudimentary and of an indefinite form, or shield shape placed beneath
the mantle. Of these 120, about 75 are terrestrial, and 45 aquatic. It
is of course, to be understood that in speaking of them as British, it is
not to be implied that they are not found out of the British Islands.
The majority are commonly distributed over the north of Europe, while
many are found not only in Southern Europe, North Africa and Asia
Minor, but even in Siberia and the northern parts of Asia and America.
To get a general idea of some of the commoner forms of shells the
subject is at once simplified by dividing the whole number of species
into two classes, the Bivalves (Conchifera) of which the common
mussel furnishes an illustration, and the Univalves (Gastropoda) of
which the garden snail is a familiar example. Amongst the aquatic
shells there are both bivalves and univalves, but with the terrestrial
species only univalves occur. The reason for this is to be found in
the difference of structure which exists in the animal itself, par-
ticularly in the organs of respiration, and which enables one species
to breathe and live where another would die.3
3 Mr. Harting here gave some details of the classification of the British species, but this would
best form the subject of a separate paper.—Ed.