6ths of
an inch below and behind the other. They are in close juxta-position,
being separated only by a thin triangular fold of membrane, which
constitutes the inner lip of the one and the outer lip of the other.

The inner aperture is the larger, measuring 3/16ths of an inch in long
diameter, and having the form of a triangle with its base directed
posteriorly. The outer aperture is not more than 1/8th of an inch long.
The two apertures lie just above the edge of the fold of membrane which
runs from the inner root of the larger or outer branchia, across the
branchial cavity and beneath the rectum, to the other side.

These apertures lead into five sacs, which collectively constitute what
has been described as the pericardium. The sacs into which the superior
apertures open, by a short wide canal with folded walls, are situated on
each side of and above the rectum. Their inner boundaries are separated
by a space of not less than 5/8ths of an inch in width, in which lie
the vena cava and the oviduct. Each cavity has a rounded circumference,
and a transverse diameter of about half an inch. In a direction at right
angles to this diameter the dimensions vary with its state of
distension; but a quarter of an inch would be a fair average.

The anterior or outer wall of the cavity is formed by the mantle; the
posterior, inner, or visceral wall by a delicate membrane. The former
separates it from the branchial cavity; the latter from the fifth sac,
to be described by-and-by. I could find no natural aperture in the thin
inner wall, so that I conceive no communication can take place between
either of these sacs and the fifth sac.

Two irregular, flattened, brownish, soft plates depend from the
posterior wall of the sac into its cavity; their attached edges are
fixed along a line which is directed from behind obliquely forwards and
upwards.

The outer and smaller of the inferior apertures on each side leads into
a sac of similar dimensions and constitution to the preceding, but
having a less rounded outline in consequence of its being flattened in
one direction against its fellow of the opposite side, from which it is
separated only by a delicate membranous wall, whilst on another side it
is applied against the inferior wall of the superior sac, and is in like
manner separated from it only by a thin and membranous partition.

Like the upper sacs, each of these has two dark-brown, lamellar,
glandular masses depending from its