Pvn H,i I'UitlS

98 AVICULIDiE.
(contained in the '
Zoological Miscellany' for 1814) lie
mentioned that " one indigenous species " had been dis-
covered at Plymouth by Mr. Prideaux. In his '
Synopsis
of the British Mollusca' he gave the last-mentioned
species
the name of Britannica.
Pema alata (Crenatula Travisii of Turton) is a native
of tropical seas, but was accidentally imported into this
which came into Scar-
country on the bottom of a ship
mollusca have been introduced
borough. Many foreign
in the same way. They continue to live for some time
after entering our colder seas, but they never become
acclimatized.
Genus II. PINNA *, Lister.
Frontispiece
and PL III. f. 1.
Body oblong and attenuated : palps small : byssus silky and
copious.
Shell forming an elongated triangle, equivalve, widely
gaping in front, and slightly on the anterior side for the passage
of the byssus : margins entire : beaks terminal and pointed :
hinge toothless.
We now approach the Mussels, to which the shells
composing the present genus bear a considerable resem-
blance. The principal distinction is that the former
have the valves entirely closed, while in the latter they
as well as that the beaks
gape widely at the larger end,
in Mytilus and its allies are not placed at one end of the
shell as in Pinna. According
to Da Costa the shell of
Pinna is called in France "jambon" and "jambonneau"
; and it looks exceedingly like a small ham. In
another point of view it is a wingless Avicula. Pliny's
account of the little pea-crab, which is so often found in
this mollusk (as well as in Mytilus modiolus and Cyprina
* From the irivva of Aristotle, who first mentioned tins mollusk.