They feed on the
decaying leaves of the iris and other water plants, and from the number of
divisions on the shell are believed to live for sometimes twenty years. Of
the many varieties, one, the largest, the horn-coloured _planorbis_,
emits a purple dye. Two centuries ago Lister made several experiments in
the hope that he might succeed in fixing this dye, as the Tyrians did that
of the murex, but in vain. There are eleven varieties of this creature
alone. It is easier to find the shells than to discover the living
creature in the river. For many the deep, full river is not a suitable
home; they only come there as the water does, from the tributary streams.
Far up in some rill in the chalk, from the bed of which the water bubbles
up and keeps the stones and gravel bright, whole beds of little
pea-cockles may be found, lying in masses side by side, like seeds sown in
the water-garden of a nymph.
[1] I have a series of _neretina_ shells from the Philippines, much
larger in size and brown in colour, in which many of the same kinds of
ornament occur.
[2] A fresh-water mussel shell from North America in my possession is
coloured green, and so marked and crimped as to resemble exactly a patch
of water-weed, such as grows on stones and piles.
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