Pvn H,i I'UitlS

MONTACUTA. 209
protruded considerably beyond the shell and fringed with brilliant
silvery, close-set, symmetrical, blunt, short but distinct
indented points, which extend on both sides to near the beaks :
tube scarcely perceptible, lying within the posterior margin of
the shell : foot very large, long, broad, subtriangular, hyaline,
sprinkled with flake-white spots ;
it is slightly bent or genicu-
late, and has a conspicuous byssal groove.
Shell triangularly oval, or of a somewhat rhomboidal
figure, compressed, rather solid and opaque, of a more or less
dull aspect : sculpture, irregular lines of growth, and (occa-
sionally) white longitudinal
lines or streaks as in the last
species : colour milk-white : epidermis not very thin, prismatic,
and marked with numerous minute concentric striae
which impart to it a silky appearance under the microscope :
margins abruptly truncate at the smaller or posterior end,
nearly straight or slightly curved in front, expanding and
rounded at the anterior end, and very gently sloping behind
from the beak to that side : beaks small and blunt, prominent,
somewhat incurved, with an indentation below ; they
are placed very much nearer the posterior side, which is not
one-third the size of the other : hinge-line nearly rectan-
gular, occupying
about one-third of the circumference : car-
tilage yellowish, smooth, thick and globular, contained in a
triangular pit lying directly under the beak in the left valve,
and forming a cup-shaped process or ossicle at its base : hinge-
plate narrow but strong, deeply
excavated in the middle for
the reception of the cartilage, which sometimes encroaches on
the beak to such an extent as to make the latter appear broken
or eroded : teeth, in the right valve rather short, leaf- like, and
in the left valve longer, and parallel with
diverging inwards ;
the hinge-line ; those on the anterior side are the largest in
both valves : inside nacreous and glossy, with a :
plain margin
scars obscure. L. 0-125, B. 0*14.
Habitat : Muddy
gravel and in the crevices of old
bivalve shells, from 10 to 70 fathoms, everywhere from
Unst to Guernsey. As an upper tertiary fossil it is
James Smith from
recorded by Grainger from Belfast, by
Bridlington, and by Searles Wood from the Red and
Coralline Crag. Its foreign range extends from Norway
to Sicily. Malm has dredged it on the Swedish coast in
10-25 fathoms, M fAndrew at Vigo in % and. off Sicily