Pvn H,i I'UitlS

MODIOLARIA. 121
portion smooth : berths incurved : hinge mostly toothless, but
sometimes crenulated : hinge-plate finely notched.
This genus differs from Mytilus in the mantle being
folded into a distinct tube for the supply of food and
aerated water, as well as in the remarkable sculpture of
the shell. The hinge-plate is crenulated as in Mytilus
phaseolinus . The
foot is very flexible and extensile ; and
when fully stretched out, it is two or three times as long
as the shell, and becomes almost thread-shaped. The
animal can crawl rather fast along a level surface by
extending and fixing the point of its foot in front, using
it as a fulcrum, and then dragging itself forwards ;
and
it occasionally floats on its back like Kellia suborbicu-
laris and other small bivalves. One species (M. mar-
morata)
burrows into the tunic or outer coat of Asci-
dians, and others form nests or wrapping-cases with the
aid of their byssus. It would seem, however,
that the
European habit of infesting Tunicata is not possessed by
any Transatlantic species.
Although Beck has the credit of founding the genus
Modiolaria, and it has been adopted by Loven, Mid-
dendorff, and other writers on the northern mollusca,
it has never been described or characterized by any of
them. The same remark applies to the genera La-
nistes of Humphreys and Lanistina of Gray,
which are synonyms of the present genus.
both of
Modiolaria
may be distinguished from Brown's genus Crenella by
the shell of the latter being covered all over with stride
or ribs, and by its having a strong and crenulated
tooth in the hinge of each valve. The animal also
differs in some important particulars, which will be pre-
sently noticed. Whether all these differences are generic
or subgeneric, may be a moot question ; but as the
lines of demarcation between any one group of animals
G