The Naturalist on the Thames | Page 6

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 other, some, the paludinas, being large, thick-striped shells,
while the limnaeas are thin, more delicately made, some with fine,
pointed spiral tops, and others in which the top seems to have been
absorbed in the lower stories. There are eight varieties of these

limnaeas alone, and six more elegant shells of much the same
appearance, but of a different race.
The minute elegance of many of these shells is very striking. Tiny
physas and succineas, no larger than shot, live among big paludinas as
large as a garden snail, while all sizes of the larger varieties are found,
from microscopic atoms to the perfect adult. Being water shells, and
not such common objects as land shells, these have no popular names.
The river limpets are called ancylus fluviatilis. Some are no larger than
a yew berry, and are shaped like a Phrygian cap; but they "stick" with
proper limpet-like tenacity. On the stems of water-lilies, on piles, on
weeds and roots in any shallow streams, but always on the under side
of the leaves, are the limpets of the Thames. The small ammonite-like
shells are called planorbis, and like most of the others, belong also to
the upper tertiary fossils. 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.

THE ANTIQUITY OF RIVER PLANTS
In the still gossamer weather of late October, when the webs lie sheeted
on the flat green meadows and spools of the air-spiders' silk float over
the waters, the birds and fish and insects and flowers of the best of

England's rivers show themselves for the last time in that golden
autumn sun, and make their bow to the audience before retiring for the
year. All the living things become for a few brief hours happy and
careless, drinking to the full the last drops of the mere joy of life before
the advent of winter and rough weather. The bank flowers still show
blossom among the seed-heads, and though the thick round rushes have
turned to russet, the forget-me-not is still in flower; and though the
water-lilies have all gone to the bottom again, and the swallows no
longer skim over the surface, the river seems as rich in life as ever; and
the birds