stuffed with these mussel shells. They lie all ways up, endways, sideways,
on their faces, on their backs. The pearl lining shines through the sand,
and the mussels gleam like silver spoons under the water. They crack and
crunch beneath your feet as you step across to search the mass for the
smaller and rarer shells. Many of those in the water contain living
mussels, yellow-looking fat molluscs, greatly beloved of otters, who eat
them as sauce with the chub or bream they catch, and leave the broken
shells of the one by the half-picked bones of the other. There was a
popular song which had for chorus the question, "Did you ever see an
oyster walk upstairs?" These mussels _walk_, and are said to be
"tolerably active" by a great authority on their habits. They have one
foot, on which they travel in search of feeding ground, and leave a
visible track across the mud. There are three or four kinds, two of which
sometimes hold small pearls, while a third is the pearl-bearer proper.
_Unio pictorum_ is the scientific name of one, because the shells
were once the cups in which the old Dutch painters kept their colours, and
are still used to hold ground gold and silver for illuminating. The
pearl-bearing mussel is longer than the other kinds, flatter and darker,
and the lining of mother-of-pearl is equal to half the total thickness of
the shell.[2]

[Illustration: SHELLS OF THE THAMES. _From a photograph by E. Seeley_]

Though not so striking from their size and pearly lustre, there are many
shells on the Thames sandbanks not less interesting and in large numbers.
Among these are multitudes of tiny fresh-water cockle shells of all sizes,
from that of a grain of mustard seed to the size of a walnut, flat, curled
shells like small ammonites, fresh-water snail shells of all sizes, river
limpets, _neretinae_, and other and rounder bivalve shells allied to
the cockles. The so-called "snails" are really quite different from each