Pvn H,i I'UitlS

176 ARCID.E.
compressed in the middle and indistinctly furrowed by a longitudinal
groove, which runs from the beak to the front margin,
convex, rather solid, scarcely glossy :
sculpture, numerous fine
and rounded ribs, radiating from the umbo, and equally numerous
but laminar transverse striae which are raised or imbricated
over the longitudinal ribs— a reticulated appearance resulting
from their crossing each other : colour milk-white : epidermis
thin and silky : margins rounded on all sides except behind,
with a slight inflection in front, sloping abruptly on the
anterior side, and wedge-shaped on the posterior side ;
dorsal
angle well developed : leaks small, blunt, and glossy, placed
near the anterior margin and close to the hinge-line : ligament
yellowish, slight, and narrow : hinge-line nearly straight,
curved :
occupying three-fourths of the shell's breadth, gently
teeth arranged in two rows, that on the anterior side consist-
ing of 4 or 5, and the other of 10 or 12, set nearly at a right
angle to the hinge-line, but slightly diverging as they approach
each end ; the intermediate space is smooth : inside
porcellanous, marked with a few longitudinal striae, which
only extend to the pallial scar ; margin thickened and broad,
with the edges faintly crenulated : muscular sca?*s very large
and conspicuous. L. 0*15. B. 0*2.
Habitat: 80-85 fathoms, off Unst, in shell-sand.
Two valves only (right and left) have been found, one
by myself and the other by Mr. Waller ;
both are fresh
and perfect. It is a Scandinavian species, and has been
taken by Danielssen in 60-80 fathoms at Vadso, by
Sars in West Finmark, and by Malm in 100 fathoms
on the Bohuslan coast. It was discovered by Philippi
in newer tertiary strata at Lamato in Calabria; and
Mr. M 'Andrew has shown me two recent valves which
he received from the late Professor Forbes, probably
iEgean.
This interesting addition to the British mollusca is
intermediate between A. pectunculoides and A. lactea.
From the former it differs in being shorter in proportion
to its breadth (the shape also being more oblique), and in
having the beaks placed much nearer to the anterior side,