Pvn H,i I'UitlS

174 ARCID&.
he referred to the latter species as " A. raridentata, var.
major" Having carefully inspected and compared a
great nmnber of specimens, recent and fossil, reputed
to belong to both species, and having had the advantage
of examining on the spot the grounds upon which Sars
formed his opinion, I am not satisfied that these species
ought to be united. At first sight, indeed, it might
seem as if A. glacialis were only a large form of A.
pectunculoides, and that the difference of size was explained
by the former having a more northern habitat.
But since A. pectunculoides has been found in Green-
land, and A. glacialis is not uncommon in Iceland, while
each constantly exhibits its own distinctive features, and
attains nearly an equal size in the Arctic seas, we must
inquire whether there is any intermediate link or variety
connecting the two forms. I believe this question must
be answered in the negative, so far as our present know-
ledge extends. The shell of A. pectunculoides is roundish-oval
or trapezoidal ; that of A. glacialis is obliquely
oblong. The former is more gibbous or convex than
the latter ; the length, or distance from the umbo to the
front margin, is proportionally greater ; the beaks are
more prominent and placed nearer the centre of the
hinge-line ; the posterior margin is rounded, instead of
wedge-shaped (as in A. glacialis) ; the sculpture is much
finer, and the strise more numerous, even in specimens
of a corresponding age and size ; and the teeth are fewer
and set less diagonally than in the other species. The
only description Which has been published
of the ani-
mal of A. glacialis is contained in Dr. Gray's Supplement
to the Appendix to Parry's First Voyage, and is as
follows :—" Animal :
compressed, subquadrate,
mantle lobes separate ; foot flat,
front two cut with one or two
fibres from the lower edge ; trachea none." For want