Pvn H,i I'UitlS

LTJTRARIA. 429
Shell elliptical, compressed, moderately solid, and glossy :
sculpture, irregular and slight concentric striae alternating with
stronger lines of growth : colour yellowish-white under the
ejptdermis, which is olivaceous, glossy and like oilskin, wrinkled
or puckered, delicately fibrous at the sides and in front, and
minutely striated on the umbonal part in a longitudinally
radiating direction : margins nearly straight or slightly indented
and flexuous in front, and sloping obliquely to each
side, the anterior being rounded or semioval and the posterior
bluntly angulated ; dorsal margins scarcely (if at all) curved,
inclining forwards, and diverging from an obtuse angle : beaks
small and sharp, incurved; umbones not much projecting:
ligament small and short, greenish-brown : cartilage triangular
and thick, narrower in the middle and spreading out at each
end in the hinge-cavity, : yellowish-horncolour hinge-line
nearly straight :
middle to form a support for the : cartilage teeth, in the right
valve two diverging cardinals on the anterior side, and a short
hinge-plate long and strong, projecting
in the
slight and laminar lateral on the posterior side of the cartilage;
in the left valve an erect and large double cardinal immediately
below the beak, flanked by a smaller but pointed one
(like a leaf) on the posterior side, and by a sharp triangular
lateral on the anterior side, besides a slight laminar lateral on
the posterior side, corresponding with that of the opposite
valve : inside porcelain-white, polished and iridescent in the
scar-marks, and microscopically wrinkled or frost-like in every
other : part pallial scar double, with an oblong sinus extend-
about two-thirds of the distance across : muscular sears
ing
deep, placed high up near the dorsal margins, anterior pearshaped,
posterior triangularly oval. L. 3. B. 5*5.
Var. alterutra. Shell smaller, thicker, and proportionally
front and back margins nearly parallel, anterior side
broader ;
obliquely truncate.
Habitat : Every part of our seas, from the lowest
verge of spring-tides to 15 fathoms, in soft or slushy
sand. The reason of this species being chiefly found in
estuaries and at the mouths of rivers appears to be
washed down
owing to an accumulation of mud being
from the land and deposited outside the line of mean
tide-marks, which suits the Lutraria as a burrowing-bed.
The variety has been found by Professor King on the