LAND AND FRESH WATER MOLLUSCA. 171
If we examine carefully a handful of different univalves, we shall
find that as regards the external appearance of the shell, they are
all more or less modifications of one type. Take for example the
shell of the common garden snail, belonging to the genus Helix, and
imagine it to be in a soft and plastic condition, capable of being
modelled. By a little manipulation, elongation and compression, we
can so modify the shape as to cause it to resemble in turn examples
of many different genera—Paludina, Limnaea, Bulimus, Pupa, Clau-
silia, Planorbis, etc., and as a sort of memoria technica, this may
serve at starting to help us to distinguish these genera and remember
their names.
By degrees we shall discover that each has its characteristic
habitat in which, in many cases at least, a peculiarity of shape
or colour tends to favour its concealment. The shells of the
genus Clausilia, for example, so closely resemble little rough
excrescences on the bark of trees to which they are found adhering,
that it requires a practised eye to detect them. Some of the
species of Pupa to be found amongst moss and general debris at
the roots of trees, are so like a chrysalis, that at first sight they might
be mistaken for one; while the flattened coil shells belonging to the
genus Planorbis, have such a similarity to worm-casts that have sub-
sided, as on that account almost to escape notice. Land shells are
most abundant on limestone soils, which are most congenial to their
existence, for the shell is composed almost entirely of carbonate of
lime. The plants upon which these molluscs feed, are the sources
whence the mineral matters are derived, and plants growing on
calcareous soils contain proportionately a larger amount of salts than
those affecting clayey or sandy soils, and are therefore in greater
request. So also many species of horse-tail (Equisetum) and grasses,
which contain a large percentage of earthy salts, are, on this account,
much frequented by land snails. Most of the terrestrial mollusca
love concealment, and to a certain extent moisture. In dry weather
they are to be looked for in damp situations under stones, and logs,
fallen branches of trees, and at the roots of moss. After rain these
molluscs may be found crawling over the moist surface of tree trunks
and palings, or like the rock snail Helix lapicida, on rocks, ruins, or
ivy covered walls.4
4 Prof. R. Tate makes some suggestive observations ("Collecting Natural History Objects")
as to the localities and habits of certain species :—" Our steps are directed inland, and as we
proceed the hedgerows, mossy banks, and margins of watercourses were diligently searched,
finding a Helix here, a Pupa or a Succinea there. Gaining the woods, we turn over the damp