ARKS. 200 
depths of water from tide-marks outwards. Some 
of them affix themselves firmly to rocks, by means 
of a stout byssus. 
GENUS ARCA. 
From the square boat-like form of the valves in 
this genus, resembling that commonly attributed 
in engravings to the Ark of Noah, these shells have 
derived their name. The valves are generally 
equal, much broader than long, and more developed 
on one side than on the other; four-sided in out- 
line, and usually solid in texture. Their surface 
is covered with radiating close-set grooves, con- 
cealed, however, by a rough, loose, horny epidermis. 
The hinge is nearly or quite straight ; consisting 
of numerous minute teeth, which are parallel in 
the centre, but diverge at the sides. 
The animal is oblong; the mantle, which, as 
stated above, is open, is either fringed or simple 
at its edges. The foot is large, oblong, bent, 
grooved throughout, and capable of expanding 
into a disk, with plain, or slightly plaited edges ; 
it is furnished at its base with a gland from which 
is spun a strong compact byssus. The mouth is 
surrounded by lips, which are formed by the ex- 
tremities of the gills. 
These Mollusks inhabit all depths of water, but 
are generally found near the shore; perhaps, how- 
ever, this is because their peculiar habits render 
them less accessible in other situations. They 
chiefly live in holes and fissures of rocks, moored 
by their powerful byssus. M. Rang remarks that 
they sometimes adhere by their disk-like foot: 
and Mr. Swainson states, that when young, they 
