Pvn H,i I'UitlS

318
CYPRINID^.
B. Smootli or slightly striated.
3. A. triangularis *, Montagu.
Mactra triangularis, Mont. Test. Brit. p. 99, t. 3. f. 5. A. triangularis,
F. &H. i. p. 467, pl.xxx. f.4.
Shell forming almost an isosceles triangle, and more or less
equilateral, with a somewhat oblique outline, compressed and
sloping gradually from the umbonal part to the margins, like
a sharp wedge, thick for its size, rather glossy : sculpture,
irregular lines of growth, or a few very slight
and indistinct
concentric ridges : colour white beneath the epidermis, which
is of various hues from pale yellow or orange to purplish -
brown or chocolate, rarely milk-white, often marked with
streaks of a darker tint, which radiate from the direction of
the beaks towards the front, where and on the posterior side
they are chiefly conspicuous ; the epidermis appears under a
high power to be microscopically punctured all over, but not
in rows : margins rounded in front, with an oblique curve to
the posterior angle, more or less straight on the anterior side,
and inclined to straight or but slightly curved on the posterior
side : beaks extremely prominent but blunt, almost central,
recurved a little towards the anterior side : lunule heart-
shaped and deep : corselet slight and short : ligament very short
and protuberant, of a yellow or brown tinge, according to the
colour of the epidermis, partly sheathed in a cardinal groove :
hinge-line acute-angled :
hinge-plate extremely
thick and
one-third of the circumference :
broad, occupying scarcely
teeth as in the other species, but the chief cardinals are much
stronger in proportion, and the third or smallest in each valve
is barely perceptible, the laterals being fine and ridge-like :
inside polished and nacreous, sometimes faintly striated length-
wise ; margin either thickened and closely denticulated, or
else bevelled and quite smooth : pallial and muscular scars
distinct, especially the latter. L. 0*125. B. 0-125.
Habitat :
Local, but gregarious, on all our coasts
from the northern extremity of Shetland to the Channel
Isles, in sand, at depths of from 3 to 60 fathoms ;
remarkably
it is
abundant at Lewis in the outer Hebrides
and at Guernsey. It occurs, but not commonly, in all
our upper tertiaries and associated with high-northern
* Triangular.