Pvn H,i I'UitlS

CARDIUM. 2G5
which lately hindered our passage and made it so difficult
aud unpleasant. The present group is free from all
those anomalies that rendered the position and arrangement
of many genera in the last two families extremely
critical and indefinite, taxing the discriminative powers
of the systematist to an extent which makes concho-
logical nature almost faint under the task. The Cockles
are easily characterized and cannot be well confounded
with any other family.
Although the mantle-scar is usually entire, Mr. Clark
has observed that " in some species an imperfect pallial
siphonal sinus may
be seen." I have in vain searched
for such a mark. According to Dr. Carpenter the
microscopical structure of these shells is very much
the same as in the Area family. He says that in various
species of Cardium, which he had examined, he found
"a considerable amount of tubular structure in the
external layer, while in Isocardia there is scarcely a
vestige of it— thus bearing out the general statement
that the presence of the tubular structure has a relation
to the costations, foliations, or other sculpturing of the
external surface."
Cockles inhabit sandy and soft ground ;
and they are
widelv distributed over all seas, from low-water mark to
considerable depths.
We have only the typical genus : the others (Car-
dissa, Papyridia, and Aphrodita) are tropical
forms.
Genus CAR'DIUM *, Linne. PI. V. f. 9.
and arctic
Body globose : mantle, thin at the edges and tubercled be-
hind, very slightly adhering to the shell: tubes unequal in
size, but nearly of the same length, covered with cirri or fila-
* Heart-shaped.
X