Pvn H,i I'UitlS

172 arcid^:.
aspect : colour yellowish-white, faintly tinged with brown :
epidermis laminar, thicker towards the margins, and when
fresh forming a line of short hairs on each of the longitudinal
strise : margins rounded on all sides except
the dorsal or
hinge-line ; anterior margin only half the depth of the posterior
one ; ventral margin slightly indented by the byssal
chink : beaks not widely separated, small but prominent and a
little recurved : ligament reddish-brown, slight and never perfect,
composed of numerous line threads, which cross the de-
pressed and narrow area at the back, and leave their impress
in the shape of minute strise :
liinge-line quite straight, and
forming almost a right angle at each end, occupying nearly the
whole breadth of the shell :
hinge-plate narrow in the middle
and widening towards each end, so as to afford a broad angular
space for the reception of the teeth on either side : teeth
3 or 4 on the anterior side, and 4 or 5 on the posterior side,
indistinctly and irregularly notched on their outer edges, set
more or less obliquely and sometimes nearly parallel with the
hinge-line ; besides these teeth, and on that part of the hinge-
plate which lies between the two rows, is a series of minute
crenulations (like the ordinary teeth in A. lactea and allied
species), which cross the hinge -plate and lie nearly at a right
and somewhat
angle with the side teeth : inside porcellanous
nacreous, obscurely marked by remote longitudinal striae ; mar-
gin often indistinctly notched, especially
at the sides :
pallial
sear slightly nexuous : muscular scars very large and conspicuous.
L. 0-175. B. 0-2.
Habitat : The Hebrides and Shetland Isles, from 35
to 90 fathoms, in muddy and sandy gravel. Mr. M fAn-
drew has dredged it off Cape Clear in 60 fathoms,
and Capt. Hoskyn off the west coast of Ireland in 100
fathoms. It is an abundant fossil in the Coralline Crag
at Sutton. According to Scacchi and Philippi
it like-
wise occurs in the upper tertiaries of the south of Italy ;
and Nyst has recorded it from a similar formation in
Belgium. In the Arctic seas it attains a remarkably
large size. Dr. Wallich took, at a depth of 108 fathoms,
on the east coast of Greenland, a specimen whose di-
mensions nearly equal those of A. glacialis. Speci-