c or
endogastric. The direction of the coil of the shell cannot be determined
by the position of the siphuncle, which traverses the septa centrally,
ventrally or dorsally. Contracted shell apertures occur also in
Ammonitoidea, the condition reaching an extreme in _Morphoceras_, where
the original aperture is subdivided by the ingrowth of the sides, so
that only five small separate apertures remain. Of these the central
probably corresponded to the mouth, two lateral to the eyes, and the
remaining two to the pedal appendages.

_Head, Foot, Mantle-skirt and Sub-pallial Chamber._--In the pearly
nautilus the ovoid visceral hump is completely encircled by the free
flap of integument known as mantle-skirt (figs. 2, 3, d, e). In the
antero-dorsal region this flap is enlarged so as to be reflected a
little over the coil of the shell which rests on it. In the
postero-ventral region the flap is deepest, forming an extensive
sub-pallial chamber, at the entrance of which e is placed in fig. 3. A
view of the interior of the sub-pallial chamber, as seen when the
mantle-skirt is retroverted and the observer faces in the direction
indicated by the reference line passing from e in fig. 3, is given in
fig. 4. With this should be compared the similar view of the
sub-pallial chamber of the Dibranchiate _Sepia_. It should be noted as
a difference between _Nautilus_ and the Dibranchiates that in the
former the nidamental gland (in the female) lies on that surface of
the pallial chamber formed by the dependent mantle-flap (fig. 4, g.n.;
fig. 1, V), whilst in the latter it lies on the surface formed by the
body-wall; in fact in the former the base of the fold forming the
mantle-skirt comprises in its area a part of what is unreflected
visceral hump in the latter.

[Illustration: FIG. 2.--Spirit specimen of female Pearly Nautilus,
removed from its shell, and seen from the antero-dorsal aspect (drawn
from nature by A.G. Bourne).

m, The dorsal "hood" formed by the enlargement of the outer or
annular lobe of the fore-foot, and corresponding to the sheaths of
two tentacles (g, g in fig. 6).

n, Tentacular sheaths of lateral portion of the annular lobe.

u, The left eye.

b, The nuchal plate, continuous at its right and left posterior
angles with the root of the mid-foot, and corresponding to the
nuchal cartilage of Sepia.

c, Visceral hump.

d, The free