Pvn H,i I'UitlS

PECTUNCULITS. 167
Habitat : Sandy and shelly gravel and nullipore on
every part
of our coast from the Shetland to the Channel
Isles, in 7-90 fathoms. It is gregarious, as well as gene-
rally diffused ; and it occurs in all our upper tertiaries.
Sars has recorded it from the Loffoden Isles, and Lilljeborg
from Christiansund ; and it ranges southward on the
east to the iEgean, and on the west to Madeira and the
Canaries. Brocchi and Philippi have enumerated it as
fossil in the Subapennine and Sicilian tertiaries.
This is the " Dog-cockle " of Da Costa. The animal
is sluggish and shy. I never saw it crawling. Mr.
Clark observes that " the animal does not execute a
direct progressive locomotion, but only turns the shell
round on its disk or from side to side." The great
thickness of its shell does not effectually protect this
succulent mollusk from all its enemies. I was in-
formed by Dr. Lukis that the dredge often brought up
large empty shells, with the valves united, but having
the work of
pieces broken off. This has been probably
the cat-fish, whose enormously strong jaws and teeth
nothing but a solid stone can resist. Mr. Cleghorn
attributed the imperfect state of all boulder- clay shells
to this cause; but with respect to Cyprina Islandica,
which is the most common shell in such deposits, I will
in the proper place suggest another explanation. Spe-
cimens of P. glycymeris vary considerably in the degree
of convexity, as well as in the obliquity of their outline,
and in colouring. The typical or usual form is more
produced at the posterior side, especially
in the adult
state ; the variety pilosa is more orbicular ; in the va-
riety decussata the longitudinal strise are deeper, but
less numerous ; and small, round coloured spots distin-
guish the pretty variety nummaria. The largest specimens
I possess are about three inches in diameter. The