Pvn H,i I'UitlS

120 MYTTLID^:.
invests itself in a case of gravelly and shelly fragments.
The fry are of a rhomboidal shape. The size of adult
specimens does not appear to exceed three-quarters of
an inch, those from the south being much smaller than
northern ones. The shell resembles that of Modiolaria
in the denticulation of the dorsal margin. It differs
from the young of M. modiolus in having this peculiar
character, as well as in being much thicker, more con-
vex, of a darker colour, and angulated behind.
Allied to M.phaseolinus in respect of the denticulated
hinge and dorsal margin is the M. crenatus of Lamarck,
which was imported in 1816 into. Portsmouth harbour
on the bottom of H. M. S. Wellesley from Bombay;
but it has not become naturalized. M. bidens of Linne
and M. Africanus of Chemnitz were subsequently intro-
duced in the same manner, and with a like result, into
Scarborough and Plymouth. The former is a West-
Indian, and the latter a West-African species.
Modiola
agglutinans of Cantraine (M. vestita, Philippi) is said to
have been once found in Ardmore Bay, near Youghal,
and was described by Captain Brown under the name
of Modiola Ballii. It is a Mediterranean species. The
was found
Mytilus (Lithodomus) aristatus of Dillwyn
many years ago in a piece of ballast limestone imported
into this country from the West Indies, and was for-
merly included among
British shells.
Genus II. MODIOLA'RIA * Beck. PI. III. f. 3.
Body suboval : mantle folded in front into a wide incurrent
tube, and behind into a conical excurrent tube :
foot strapshaped.
Shell rhomboidal, sculptured by two rows (one on each
the middle
side) of striae which radiate from the beaks, leaving
* Allied to the genus Modiola of Lamarck.