Pvn H,i I'UitlS

ARCA. 1 79
time to suppose that the animal excavated rocks ; and
he gave this shell the specific name of perforans, believ-
ing
that it was not the A. lactea of Linne. The shell
varies considerably in the proportion of its different
parts, as well as in the comparative tenuity of sculpture.
It never grows to much greater dimensions than I have
stated in the description. My largest specimen, which
was evidently a veteran, and must have outlived most
of its generation, is not much more than three-quarters
of an inch in length.
I should have been inclined to consider the present
species the A. modiolus of Linne, if it were not for the
expression that it is exactly the shape of Mytilus mo-
diolus and the size of a large bean. The rest of his de-
scription agrees with it in every particular. He even
placed A. modiolus in one section as having a plain
margin, and A. lactea in another as having a notched
margin. He says both inhabit the Mediterranean, and
that A. lactea is " diaphana" which is certainly not
the case in our shell. In all probability his A. lactea is
the A. imbricata of Poli, a thinner shell and having
the inside margin strongly notched. Brocchi applied
Muller's name of nodulosa to the present species, be-
cause A, lactea was described by Linne as possessing
and for the same reason
the last-mentioned character ;
Poli, Olivi, Chierighini, and Costa adopted the name of
A. modiolus. But I will not venture to expunge the
generally received name of lactea, and to substitute for
it another which is referred by many conchologists to a
common West-Indian shell— especially as so much ob-
scurity still involves several of the Linnean species,
notwithstanding
the laborious research devoted to the
subject by Mr. Hanley. What we call A. lactea may
be the A. barbata of Muller's '
Prodromus/ although