Pvn H,i I'UitlS

AMPHIDESMA. 413
Amphidesma casta neum *, Montagu.
Donax castanea, Mont. Test. Brit. App. p. 573, t. 17. f. 2. Ervilia castanea,
R & H. i. p. 341, pi. xxxi. f. 5, 6.
Shell obliquely and triangularly oval, or inclining
to ob-
long, convex, thick, opaque in the adult and semitransparent
in the young, glossy : sculpture, slight and irregular concentric
striae, which now and then become laminar on the anterior
side : and in fresh specimens may also be observed numerous
and minute longitudinal striae, especially on the posterior side,
which radiate from the beaks and extend to the margins :
colour reddish-chestnut, of a deeper tint on the posterior side,
with a short chocolate streak or ray in each valve, curving
from the front or ventral margin to about two-thirds of the
distance from the beak ; this streak is narrow at the top and
rapidly enlarges to its base as in Donax politus ; occasionally
the colour is milk-white : :
epidermis slight, yellowish margins
nearly straight or but little curved in front, rounded and semioval
at each end, sloping behind from the beaks at an obtuse
angle, where the dorsal margins are straight, and half as long
again on the posterior as on the other side : beaks veiy small,
blunt and calyciform, incurved, turning a little towards the
posterior side umbones rather tumid but not :
;
projecting ligament
extremely small and caducous : cartilage thick, contained
in a triangular pit which shelves inwards and is enclosed by
strong walls or sides : hinge-line gently curved : hinge-plate
very strong, solid, and broad : teeth, in the right valve a large
erect, triangular and oblique cardinal on the anterior side, and
a smaller diverging one on the other side of the cartilage-pit,
besides a long but indistinct laminar lateral on each side in
;
the left valve are similar cardinals but of more equal size, and
the laterals are usually obliterated or obscure : inside polished
and glossy; margin minutely and closely crenulated: pallia!
scar well defined, with a rather short oval sinus which does
not reach half way across : muscular scars oval, the anterior
being the deeper and more conspicuous. L. 0-3. B. 0-5.
Habitat : Coasts of Cornwall and Scilly Isles, in
20-54 fathoms, but more frequently thrown up on sandy
beaches ; Herm (Macculloch) ; Cork (Humphreys and
J. G. J.) ; dredged off Arran Isles, co. Galway, in 10-12
* Chestnut- colour.