Pvn H,i I'UitlS

138 ARCID.E.
addition to, or subtitution of, the ligament : hinge-plate furnished
with a row of close-set teeth in each valve, which
mutually interlock : muscular scars lateral, deep or distinctly
marked.
The old and characteristic genus Area of Linne has
multiplied so fast since his time,
very numerous family.
that it now forms a
The quota which has been con-
tributed to it by palaeontology, or the record of extinct
races and generations, is nearly equal to that of recent
genera ; and in this sense the march of science may be
said to advance with almost the same rapidity in a re-
trograde as in a forward direction. Both zoologists
and palaeontologists are strenuous in the race ; but in-
stead of being hostile rivals, the only object of their
emulation is to assist each other and thus promote the
common cause.
The character which makes this group (whether we re-
so distinct
gard it as a genus, a family, or a set of families)
and self-contained is the peculiar structure of the hinge.
Instead of having, like other bivalves, only one, two, three,
or, at the most, four tubercular teeth under the beak, and
occasionally a single or double laminar fold on each side,
all the Arcidce are furnished with a symmetrical row
of these processes, occupying the whole of the hinge in
each valve, and interlocking like the real teeth of many
kinds of fish. This apparatus, aided by stout retractor
muscles and an elastic ligament, and in some genera by
a strong internal cartilage, enables the mollusk to keep
its house closed against most of its predatory enemies ;
and it is only when the shell is drilled by some canalifer-
ous Gasteropod, or else swallowed whole by a voracious
fish or by a member of the Bulla family, that its days
are numbered. Those species of Area which habitually
shelter themselves in the crevices of rocks, and are