Pvn H,i I'UitlS

256 CARBIT1D.E.
Family X. CARDITDXE, Gray.
Body oblong or roundish -oval : mantle thin, open in front,
and folded behind into a short excretal tube : foot tongue-
shaped and extensile, byssiferous.
Shell shaped like the animal, equivalve, inequilateral :
ligament external: hinge strong, furnished with one or two
cardinal teeth, which are set more or less obliquely, and with
scar in
a laminar posterior lateral tooth in each valve : pallial
nearly every case entire : muscular scars large, round, and
deep.
I give this family provisionally as British, not know-
ing where else to place the abnormal genus Cyamiwn,
as well as a species of Cypricardia, which at present I
am not sure is indigenous. The present members of this
family are dispersed over the ocean, but chiefly confined
to the southern zone : their ancestors flourished in the
northern hemisphere, and multiplied exceedingly during
the tertiary epoch.
With respect to the shape of the pallial scar, being
the impression produced by the adhesion of the mantle
to the inside of the shell near the front margin, I think
an undue importance has been attached to this cha-
racter. When it is sinuous or indented, it certainly
shows that the animal has two distinct tubes springing
but when
from a common sheath, at the posterior side ;
it is simple, or even flexuous, we cannot learn from it
whether there are two such tubes, or only one, nor
what their length may be, nor even whether there is
any
tube at all. Cardium and Venus have each a double
tube on the posterior side ; but the pallial scar in the
former is simple or entire, and in the latter sinuous.