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