Pvn H,i I'UitlS

106 MYTILIM.
M. hesperianus of Lamarck. It attains a considerable
size, and I have a specimen which measures nearly five
inches in length. I may here observe that Mr. Barlee's
British Mollusca '
shell, recorded in the f
as belonging
to this species and eight inches and a half long, is M.
modiolus. Var. 2. On all rocky coasts, filling crevices
and crowded together, so as to prevent the free deve-
lopment of each individual. Var. 3. Solitary
and there-
fore larger and more expanding. Var. 4. On floating
buoys and sunken wrecks, often at a considerable distance
from land. Young shells of the ordinary kind
often resemble the last variety in substance and markings.
The common mussel is found in all our upper tertiaries ;
but only the sublittoral variety [M. hesperianus) occurs
in the Coralline Crag. It is distributed throughout the
northern hemisphere from the polar circle to the iEgean
Sea and the coast of Morocco, and it seems to thrive
equally on both sides of the Atlantic. The M. trossulus
of Gould, from Vancouver's Island, probably differs in
no other respect than being called a " representative "
species.
Although M. edulis here inhabits the coast-line only,
Dr. Walker is said to have dredged it in Baffin's Bay at
a depth of 140 fathoms. I once obtained a fresh single
valve in between 70 and 80 fathoms about forty miles
off the Shetlands ;
but it had perhaps been voided by a
coal-fish (Gadus carbonarius) , which frequents the shore
in the spawning- season. The mussel is occasionally found
with the common periwinkle [Littorina litored), living
on the shore in a stream of perfectly fresh water during the
recess of the tide. The common cockle (Cardium edule)
and My a arenaria have the same habit; and I have
even seen the two latter species associated with fresh-
water mollusca. All of these can exist for many days