With this he gave him four shells, two young, and two grown up ones,
which are called the Pteroceras Scorpio; and three others besides, one
young and two grown up ones, which go by the name of Cypraea Exanthema.
He told Charley to put all these shells together in his Museum, because,
in certain particulars, they are alike, and all have, besides their own
special names, the same generic name of Gasteropoda. They are so called,
because they have something like a foot proceeding from the body which
they use for moving about. Some of them have a distinct head, furnished
with feelers, and eyes, and some means of smelling and hearing. Commonly
the shell has but one valve, but sometimes more. Their shell is
secreted or made out of their skin, which is called a mantle. I ought to
tell you also, that all these shell-fish have another name, still more
general, which is Mollusca, or Molluscs.
[Illustration: ADULT SHELLS. YOUNG SHELL.