The Naturalist on the Thames | Page 6

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 and fish, unfrightened by the boat traffic, are tamer and more visible.
[Illustration: A FLOWERY BANK NEAR COOKHAM. _From a photograph by E. Seeley_.]
The things in the waters and growing out of the waters are very, very old. The mountains have been burnt with fire; lava grown solid has turned to earth again and grows vines; chalk was once sea-shells; but the clouds and the rivers have altered not their substance. Also, so far as this planet goes, many of the water plants are world-encircling, growing just as they do here in the rivers of Siberia, in China, in Canada, and almost up to the Arctic Circle. The creatures which lived on these prehistoric plants live