Pvn H,i I'UitlS

MONTACUTA. 211
rior side produced considerably beyond the shell, and forming
a kind of frill, which becomes gradually smaller and more
even as it passes along the front of the shell towards the posterior
side ; it is fringed with very delicate, rather short, and
blunt filaments, which extend completely round the edges of
the valves, with the exception of a small space at the umbones :
tube consisting of a small inconspicuous excretory orifice : foot
very large and muscular, slightly bent in the middle, tapering
to a blunt point in front and abruptly truncate behind ; its
base or sole is somewhat wavy and grooved through its entire
length.
Shell oblong, convex, thin, rather opaque, glossy : sculp-
ture, irregular lines of growth and occasionally
a few exceed-
ingly slight longitudinal scratches : colour greyish-white :
epidermis as in the last species, but it is usually obscured by
a thick ferruginous crust : margins somewhat truncate but
rounded at the smaller or posterior end, thence obliquely
slanting and slightly curved in front, expanding and rounded
at the anterior end, and very gently sloping behind from
the beak to that side, with a short ledge on the other side
which forms an obtuse angle at the : posterior extremity beaks
small, blunt, and rather tumid, not very prominent, and nearly
straight; they are placed at about one-third the distance
from the posterior end : hinge-line almost straight, occupying
about one-third of the circumference : cartilage large and
solid, light yellowish- brown or horn colour, obliquely twisted
and clasping the hinge-plate on each side, lying close to the
beaks at the posterior side ; the pit or groove containing it
slants abruptly and obliquely down towards the posterior side,
and has its walls and base much thickened : hinge-plate narrow
but thick : teeth, in the right valve one on the anterior
side, which runs from the cartilage at nearly a right angle to
the hinge-line and so far resembles a cardinal tooth, but it
then takes a sharp twist in the direction of the hinge -line,
where it becomes laminar and is gradually attenuated to a
sharp point ; the other tooth in the same valve on the posterior
side is shorter, triangular and pointed, placed on a lower level
and parallel with the posterior slope ;
in the left valve the
tooth on the anterior side is pointed near its commencement
and forms a rather long laminar ridge in a parallel line with
the hinge, the other tooth being of the same shape as the corre-
sponding one in the right valve and taking a similar direction :
inside partly nacreous but mostly of a dull hue, with a plain