Pvn H,i I'UitlS

442 MACTRlDiE.
4. S. te'nuis*, Montagu.
Mactra tenuis, Mont. Test. Brit. Suppl. p. 572, t. 17. f. 7. Syndosmya
tenuis, F. & H. i. p. 323, pi. xvii. f. 11.
Bodi white : mantle having its edges delicately fringed :
tubes cylindrical and narrow ; the alimentary one capable of
being extended twice the transverse admeasurement of the
shell, the other usually recurved and shorter by one-third ;
orifices apparently plain: foot sernitransparent, extremely
flexible.
Shell rather triangular than oval, moderately convex,
nearly equilateral, thin, opaque, generally not glossy although
iridescent :
sculpture, fine and close-set concentric strias, the
interstices of which are traversed by twice as many microsco-
lines of growth irregular but conspi-
pical longitudinal strise ;
cuous : colour : ashy-white epidermis laminar, light yellowishbrown
: margins gently curved in front, rounded on the anterior
side, truncate and nearly straight on the posterior side, which
dorsal margins straight,
is bluntly angulated and gapes a little ;
and forming an angle of about 60 degrees with the beak as
an apex, slightly excavated or depressed on both sides : beaks
small and calyciform, inclining a little to the posterior side ;
umbones projecting: ligament minute and narrow, dark horncolour:
cartilage curved, yellowish-brown: hinge-line more rect-
angular than obtuse- angled : hinge-plate of moderate strength
and thickness, occupying in the right valve about one-fourth,
but in the left not one-tenth of the circumference : teeth, in
the right valve two small erect and parallel cardinals, set (as
usual) transversely to the hinge, besides one short double
laminar lateral on the anterior side and a similar but single
these laterals are triangular and
lateral on the posterior side ;
rather abruptly truncate at their further extremities ; the
left valve has only one cardinal, similar to those in the right
valve, and no lateral tooth (unless an obscure sunken fold or
socket on each side of the hinge can be so called) : inside
somewhat nacreous, but not polished, indistinctly striated
lengthwise, microscopically and closely freckled ; margin bevelled
and sharp : pallial scar well defined ; sinus large and
triangular, with its blunt apex below the beak, and its base
parallel with the front margin : muscular scars rather deep,
anterior oval, posterior trapezoidal. L. 0*3. B. 0-4.
* Thin.