Pvn H,i I'UitlS

170 arcidjE.
as in the last genus ; but the bundles are not in all cases
placed diagonally, being in some species across the space between
the beaks : hinge-line straight : teeth laminar, and set at
or a little more or less than a right angle to the hinge-line, con-
tinuous in most species, but divided into two distinct rows in
others : pallial scar entire : muscular scars oblong, symmetrical
and strongly marked.
Area rivals Pectunculus in its high descent, as well
as in its fertility; and it far excels it in the number
of species. It has, besides, a wider distribution, one
species (A. pectunculoides) being found both on the coast
of Greenland and in the iEgean. Its bathymetrical
range is considerable, extending
from low-water mark
to between 300 and 400 fathoms. Unlike Pectunculus,
it spins a byssus, and is by this means attached to sub-
marine substances, from which it can disengage itself
at pleasure ; or it takes permanent shelter and makes its
abode in the cranny of a rock, the shell occasionally
becoming distorted by
tation.
the narrow limits of its habi-
The generic name is encumbered by a load of no less
than thirty synonyms, which have been from time to
time imposed by the fanciful ambition of systematists,
or for the more laudable purpose of distinguishing particular
groups of species. It will probably always remain
a question whether subgenera are advisable, the settle-
ment of it depending in a great measure on the defini-
tion of a genus. The characters of all genera cannot be
equivalent ; or perhaps we have not yet found the right
key to Nature's lock. All are agreed as to the existence
and they must be discriminated by certain
of varieties ;
names, in the same way as species, genera, and higher
groups. One of Linnets botanical axioms may be cited
in support of this method of identification :—'' Nomina
si nescis, perit et cognitio rerum." Whether subgenera