Pvn H,i I'UitlS

416 MACTRIDJS.
below them a bright orange line often encircles the siphonal
sheath; a conical valve is now and then seen to protrude
from the opening of the excretal tube :
gills varying in colour
from light orange to reddish-brown ; upper pair larger than
the other, obliquely and very finely pectinated: palps long,
narrow, pointed, brown of different shades, more distinctly
striated than the gills, particularly on the inner surfaces :
large, fleshy, and white.
foot
Shell representing in form an isosceles triangle, convex,
usually solid and thick, opaque, somewhat but not very glossy,
owing to the continual abrasion of the surface by the sand in
which it is buried :
sculpture, numerous slight and irregular concentric
striae, which often become laminar and almost rib-like
in front and at the sides, and always at the dorsal margin ;
lines of growth strongly marked ;
on the anterior and ventral
sides in some specimens may also be observed a few slight
raised lines which radiate from the beak : colour
longitudinal
pale yellowish-white, sometimes stained with ochre from a
sediment deposited in shallow or tidal waters: epidermis
yellowish-brown, silky in the young and coarse in the adult,
being in the latter state more persistent at the sides and
towards the edges : margins curved in front, obtusely angular
at each end, but more pointed at the posterior side, with a
rounded slope from the beak to either extremity ; the sides
(especially the posterior) are more or less angulated
very small and rather blunt, incurved, turning a little towards
the anterior side ;
: beaks
umbones more or less tumid and projecting :
ligament short and slight, yellowish-brown: cartilage trian-
gular, compact and strong, golden-yellow, placed obliquely in
a direction opposite to that of the beaks and immediately
under the ligament :
hinge-line obtusely angular : hinge-plate
extremely broad, thick, and flexuous, the cartilage-pit projecting
in the middle : teeth, in the right valve a short laminar
and erect cardinal on the anterior side, nearly parallel with the
hinge-line, and a much slighter and scarcely raised cardinal
forming one of the sides of the cartilage-pit ; laterals in this
valve two on each side, the inner surfaces of which are strongly
pectinated or grooved in a perpendicular direction ; the left
valve has two short cardinals, also on the anterior side of the
cartilage-pit, united at the base or point of the fork, and of
equal height and length laterals in that valve one on each
;
side, deeply grooved like the laterals in the right valve, but on
both sides and not merely on their inner surfaces : inside more