Pvn H,i I'UitlS

156 ARCIDiE.
filaments : tubes united for more than half their length,
considerably produced, slender and smooth ; the incurrent
(or branchial) tube is shorter than the excurrent (or anal)
tube, and the latter has a square orifice, with finely pointed
angles : foot oblong, compressed, white, deeply grooved, and
capable of being expanded into a creeping-disk with notched
margins.
Shell inequilateral, sloping from the beak with an oblique
curve to the posterior extremity, which somewhat resembles
a duck's bill, compressed, rather solid and opaque, scarcely
glossy : sculpture,
about 30 transverse laminar ribs which
vary very much in strength and compactness, besides extremely
fine glittering and prismatic lines, which radiate from the
beaks, but do not usually extend to the front margin unless
in very young shells ; and there are also two ridges, which
diverge from the beak in each valve, and embrace the truncated
point at the posterior angle : colour pearl-white under
the epidermis, which is yellowish-brown and generally of a
dull hue : margins rounded at the anterior side, more or less
curved in front, and elongated at the posterior side to a blunt,
upturned, and truncate point, where the valves slightly gape :
beaks small, rounded, nearly straight, destitute of transverse
ribs, and : glossy lunule lance-shaped, ribless, depressed, with
slightly prominent edges or lips, and defined by
the inner
ridge of the posterior slope : ligament slight : cartilage oval
and small, contained in an : oblique pit hinge-line curved on
the anterior side of the dorsal area, and inflected on the other
side, occupying about two-fifths of the circumference : hingeplate
grooved, and increasing in width in proportion to the
growth of the shell: teeth vault-like or concave, slightly
curved outwards, higher in the centre of each row, and dimi-
there are about 16 on the
nishing in size towards either end ;
anterior side, and about 20 on the posterior side : inside porcellanous
and glossy, somewhat thickened near the margin,
which is smooth, with a distinct ridge running down the middle
of the beak -like extension of the posterior side : pallial and
muscular scars well marked. L. 0'3. B. 0-55.
Var. brevirostns. Shell smaller, longer in proportion
to its
breadth, with the posterior extension much shorter, more convex
or tumid : ribs finer and more crowded.
Habitat :
Everywhere
on our northern coasts, in
muddy gravel and sand, from 20 to 90 fathoms. Captain