Pvn H,i I'UitlS

NUCULA. 141
cartilage ; while Limopsis has the form of Pectunculus,
with a cartilage and a cavity for its reception, as in the
so-called Nuculida, although placed differently,
viz. out-
side the hinge, instead of in the middle of it. Stimpson
advocates the separation of Nucula from the Arcidce be-
cause of the want of a byssus ; but Pectunculus has none.
This appendage is only necessary when the habitation is
rocky or " hard/' In sandy and muddy, or " soft "
ground the foot is used for burrowing, instead of spin-
ning a byssus; and it is consequently larger in Nucula,
Leda, Limopsis, and Pectunculus, which live in such
situations, than in Area, which attaches itself to rocks
and old shells. Recluz suggested the removal of Leda
from the Arcidae and Nuculidce, and considered that all
of them belonged to different tribes ;
but our progress in
classification has not yet advanced sufficiently to allow
of such extreme subdivision. Gray placed Nucididce
between the families represented by Solen and Mya, and
in another order than that which contains Area, Pec-
tunculus and Limopsis. Leach's arrangement is rather
more eccentric or opposed to general views. His fami-
lies of Nuculadce and Pectunculidae lie between Mactra
and Venus, and his Arcadce [longo intervallo) between
Pinna and Avicula. The late Mr. G. B. Sower by was
the first to point out the separation of Leda and Nucula
from Area.
A. Inner margin notched.
1. Nucula sulcata*, Bronn.
N. sulcata, Bronn. Italiens Tertiargebilde, p. 109, no. 633. N. decussata,
F. & H. ii.
p. 221, pi.
xlvii. f. 1-3.
Shell obtusely triangular, rather convex, solid, of a dull
hue: sculpture, numerous transverse striae or ridges, which
* Furrowed.