Pvn H,i I'UitlS

100 AVICULID.E.
Shell tapering to a point at one end and expanding at the
other to an obtusely rounded or slightly truncate edge, convex,
with a gradual slope towards the sides, rather thin for its size,
more or less : glossy sculpture, 8-12 long delicate ribs on the
dorsal portion, radiating from the beaks, and sometimes numerous
shorter ribs on the anterior portion of the shell, extend-
ing to that margin ; these ribs often rise at irregular intervals
into vaulted prickles in consequence of an exfoliation of the
outer layers ; the whole surface is also covered with the mi-
croscopical punctures peculiar to this family : colour yellowish-brown
: margins very thin in front and on the anterior
side, but thickened on the dorsal side, which is straight or
occasionally a little incurved, and sloping gradually on the
other side (with the exception of a slight indentation for the
byssus) to a blunt angle, where it meets the front margin :
beaks strong and exceedingly prominent, much worn and
broken in adult specimens, and furnished inside with a series
of concentric plates, in consequence of the successive shift-
ings and advance of the anterior adductor muscle : hingeline
extremely produced, occupying two-thirds of the entire
length : ligament horny, narrow, very strong and elastic :
so as to contain it in a
hinge-plate folded over the ligament,
kind of sheath or groove, closed or firmly soldered for a short
distance beyond the posterior termination of the ligament :
inside highly nacreous and iridescent, of a darker colour on
the upper half than below: muscular scars very distinct.
L. 10-5. B. 6.
Habitat :
Sparingly and locally
distributed on all the
British coasts, but gregarious., from low-water mark to
80 fathoms, in muddy or sandy gravel. Fragments have
been found in the Coralline Crag. Its foreign range
extends south to both sides of the Mediterranean, and
also to the Canary Isles. It has not been noticed as
occurring anywhere
north of Shetland.
The " fan-mussel " is by far our largest shell, mea-
suring sometimes fifteen inches in length and eight in
breadth. It varies greatly in proportions and sculpture.
Specimens in some situations have a tendency to expand,
and become distorted in consequence of their confined
position. The fry have spoon-shaped and some-