Pvn H,i I'UitlS

lies in having two cardinal teeth,
OSTREA. 37
as in Placuna and Pla-
centa, and is not applicable to the present species.
Family II. OSTRE'ID.E, Broderip.
Body round : mantle having rather thick edges
in front :
cirri short :
gills simple. There is no foot, or muscle for external
attachment. The animal is fixed in the earlier stage of
its growth, and sometimes in its adult more convex valve of its shell.
state, by
Shell circular, longitudinally oval or oblong,
the lower or
or of an irre-
gular shape, and inclined to be wedge-like, inequivalve : hinge
toothless, but having its margins sometimes notched: cartilage
internal, short and curved, placed horizontally on the hinge-
line.
Some genera are exotic, and others are extinct or
known only as fossil. We have but the typical genus.
The Oyster family differs from that of Anomia in the
gills being simple, in having no foot or plug
of attach-
ment, and in the shells being either free or adhering to
other substances by the lower valve, which is invariably
larger and deeper
than the other.
Genus OS'TREA *, Linne. PI. I. f. 5.
Body compressed.
Shell composed of numerous imbricated or tile-like plates,
which overlap one another in succession : beaks disunited :
cartilage strengthened by a ligament
on each side of it.
The so-called species of Ostrea are exceedingly nume-
rous, and many of them are only distinguishable by very
slight characters. Almost every sea appears to have
several species or varieties. Their general form is very
inconstant and often irregular. It is more than probable
that when a sufficiently extensive series from each
* Oyster.