Pvn H,i I'UitlS

ARCA. 177
and from the latter in its smaller size and coarser sculpture,
and in having only half the number of teeth. The
furrow or indentation in the middle is also
longitudinal
a peculiar character of the present species. Norwegian
specimens are twice the size of ours, and these last are
larger than Forbes' s specimens.
It is the A. Korenii of Danielssen (' Beretning om en
Zoologisk Reise/ 1859)
and A. lactea of Malm. The
A. obliqua of Reeve is a West-African shell, and a very
different species.
3. A. lac'tea *, Linne.
A. lactea, Linn. Syst, Nat. p. 1141 F. & H. ; ii. p. 238, pi.
xlvi. f. 1-3.
Body white : mantle having its border finely notched, and
pale-red towards the middle of the dorsal area ; the under
surface of the ventral line is marked with irregular flake -
brown blotches on a pale-yellow ground, and the upper surface
is marked for some little depth with a sand-like rusty-brown
belt and a darker interrupted line nearer the margin :
gills
symmetrical and equal in size, very thin and gradually taper-
ing, pale yellow : foot extensile, fleshy, and pure white : bi/ssus
short, horny, composed of several leaf-like threads.
Shell varying in shape from rhomboidal to triangular,
tumid, solid, of a dull hue :
sculpture, numerous and fine longitudinal
ribs, which become fewer and stronger at the sides,
besides slighter and rather more numerous transverse ribs,
which cross the other, making the front surface appear reticu-
lated, and forming rows of minute tubercles at the point of intersection
; sometimes a few smaller and intermediate longitudinal
ribs are perceptible in front: colour yellowish-white: epidermis
brown, laminar and downy, thicker and forming towards the
front and sides longer threads, which occasionally resemble
short bristles : margins rounded at the anterior side, straight
or nearly so in front, obtusely wedge-shaped and somewhat
truncate on the posterior side, with a blunt keel or ridge separating
that side from the rest of the shell, and which is very
distinct and sharp on the umbonal area: byssal sinus some-
* Milk-white.
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