Pvn H,i I'UitlS

144 ARCID^.
numerous fine striae which radiate from the beak, and a few
irregular lines of growth which form dark zones but do not
intersect the longitudinal striae ; these striae are wanting
on the lunule and dorsal area : colour greyish- white under
the epidermis, which is yellowish -green and very closely and
microscopically wrinkled in a transverse direction : margins
angular and somewhat truncate on the anterior side, curved
in front, and obtusely wedge-shaped and rounded on the
posterior side ; beaks prominent bat blunt, slightly
recurved :
lunule marked obliquely by the lines of growth, and strongly
indented or grooved across below the beaks it is ; defined by
a slight furrow, and projects a little outwards, so as to form a
blunt and indistinct keel : ligament slight : cartilage oval, con-
tained in a short projecting cavity underneath the beaks :
hinge-line somewhat curved : Jiinge-plate as in the last species :
teeth sharply pointed and slightly recurved, about 15 on the
anterior, and 25 on the posterior side : inside nacreous and
highly iridescent, striated and notched as in N. sulcata, but in
the present species the crenulations extend to the posterior
angle
: muscular scars oval and distinct. L. 0*475. B. 0*475.
Yar. 1. radiata. Shell larger, flatter, more decidedly triangular
and produced at the posterior side, and marked with
numerous purplish-brown streaks, which radiate from the
beaks outwards. L. 0*6. B. 0*6. N. radiata, F. & H. ii.
p. 220, pi. xlvii. f. 4, 5, and xlviii. f. 7.
Yar. 2. lumidida. Shell smaller, more triangular and con-
vex. N. tumidula, Malm, Proc. Scand. Soc. (1862), p. 621.
Habitat :
Common
in sand and gravel on all our
coasts from Shetland to the Channel Isles, at various
depths ranging from 3 to 85 fathoms. Capt. Beechey
in 145 fathoms.
dredged it off the Mull of Galloway
Var. 1. More local, but widely distributed in the British
seas, as well as from the Swedish coast (Malm) to
Algeria and Sicily (M f
Andrew). Var. 2. Off Unst, in
85 fathoms ; very rare. This variety is also Scandi-
navian, and was obtained by Dr. Torell in 80 fathoms.
The typical form and the first variety occur in all the
upper tertiaries, both in Great Britain and the south of
Europe. Beyond our shores the recent species inhabits