Pvn H,i I'UitlS

168 ARCIDiE.
fry have a square shape, and are only sculptured by con-
centric strise. Their inside margin is quite plain, and
has no appearance of the crenulations which are deve-
loped in a subsequent stage of growth. The impressions
left by the ligament on the triangular space between the
beaks in full-grown shells are very distinct. I do not
find that the remarkable structure of the ligament,
which is evident from these impressions, has ever been
noticed. The hinge-process is liable to become abnormal
or monstrous. A curious instance of it was exhibited
in a specimen found by Mr. Bariee in the Shetlands.
The hinge-plate had none of the ordinary cardinal teeth ;
but, by way of substitute, each valve was provided with
a rather strong laminar and horizontal tooth on each
side, which locked into a corresponding groove in the
opposite valve. The teeth occasionally decay and become
carious in living specimens. Whether the animal
suffers from tooth-ache would be a novel subject for dis-
cussion by the Odontological Society. Aged individuals
are often almost toothless, in consequence of the liga-
ment pushing so far forward on the hinge-plate, as to
obliterate all the central teeth : it reminds one of the
hardened gums of an old man who has lost the greater
part of this extremely useful apparatus. The shells are
often seen in grotto-work; and Mr. M'Andrew says
that at Algarve on the coast of Spain they are used,
instead of lead, by the fishermen for sinking their lines.
The anterior side of the shell, while the animal is alive,
is frequently fringed with the tubes of a Hydroid polype
(one of the Tubulariidce) , which seems to take advantage
of the strong gyratory current produced by the mollusk
for its own food-seeking purpose. This may be an
analogous case to the supposed parasitic nature of Mon-
tacuta substriata, which is always found attached to the