Pvn H,i I'UitlS

248 LUCINID^.
gular lines of growth and microscopical frost-like markings :
colour white : epidermis filmy and inconspicuous : margins ob-
liquely truncate on the anterior side, and forming nearly a
right angle with the dorsal margin, which is almost straight or
slightly incurved, rounded in front, and flexuous on the posterior
side, which is deeply indented by a wide furrow or
groove running from the beaks at an angle of about 30 degrees
and gradually enlarging outwards, as well as by another smaller
and parallel furrow close to the ligamental area : beaks small
and pointed, but not prominent, much recurved, almost con-
tiguous, a little nearer to the anterior side : lumde rather deep
and heartshaped : corselet prominent, well defined by the upper
furrow, and forming two sharp and pouting lips : ligament
rather long, yellowish-brown or light horncolour, not projecting
beyond the lips of the corselet, but distinctly visible
outside ; ligamental groove narrow and shallow : hinge-line
obtusely angular, occupying
not one-fourth of the circumfe-
rence : hinge-plate rather broad and thick, nearly flat, almost
below the
all of it lying on the posterior side ; immediately
beaks it is indented in the right valve and slightly reflected in
the left, which gives that valve the appearance of having an
indistinct or obscure cardinal tooth : inside highly glossy and
iridescent, sometimes slightly striated lengthwise ; margin thin
and plain: pallial scar broken in a zigzag line, in conse-
quence probably of a corresponding structure in the edge of
the mantle: muscular scars large. L. 0375. B. 0*35.
Var. polygons,. Rather longer in proportion to the breadth,
marked by three or four longitudinal ridges, giving the shell
an angular form, the grooves on the posterior side being much
deeper
than usual.
Monstr. Having a longitudinal groove down the middle,
which is longer and deeper in the left valve than in the right.
Habitat :
Gregarious
in soft mud and sand on all
the British coasts, from Shetland to the Channel Isles,
in 3-87 fathoms. The variety is from the deepest water
on the " outer haf/' about forty miles east of the
Whalsey Skerries, and the monstrosity from Falmouth
and Tenby. This species is rather plentiful in the
" alluvial" deposits at Belfast (Hyndman and Grainger);
Clyde beds (Smith); Coralline Crag, Sutton (S. Wood).