was
becoming more and more internal. The shell of _Spirula_ (fig. 18) is
coiled somewhat like that of _Nautilus_, but the coils are not in
contact, the direction of the coil is endogastric or ventral instead of
exogastric, and the shell is very much smaller than the body. Like that
of _Nautilus_ it is divided by septa and traversed by a siphuncle. The
relation of the animal to the terminal chamber is as in _Nautilus,_ but
the body extends far beyond the aperture, and folds of the mantle grow
up over the shell and cover it everywhere except part of the dorsal and
ventral surfaces.

[Illustration: After Chun, from Lankester's _Treatise an Zoology_

FIG. 18.--_Spirula._

A, Dorsal aspect.

B, Ventral aspect.

a, Arms.

e, Eyes.

fi, Fins.

fu, Funnel.

pa, Mantle.

po, Posterior fossa.

sh, Shell.

te, Tentacular arms.

td, Terminal pallial disk]

[Illustration: FIG. 19.--Digram of shell Belemnite (after Phillips). r.
Horny pen or "proostracum": A, conical cavity or "alveolus," in which
the chambered "phragmacone" (p) is contained: g, "guard," or "rostrum."]

The next modification in the enclosed shell is the addition to it of
secondary deposits of calcareous matter, by the inner surface of the
shell-sac. Successive layers are deposited on the posterior part of the
original shell, whether coiled or straight, and these layers form a
conical mass, which may attain great thickness. A somewhat coiled shell
with such a deposit is seen in _Spirulirostra_ (fig. 17, C) of the
Miocene. In the next stage of modification secondary secretion forms a
long and broad projection of the dorsal lip of the aperture; this is
well developed in the belemnites (fig. 19). Thus in these modified
shells three parts are to be distinguished: the original septate shell,
which has been called the phragmacone; the posterior conical deposit,
called the rostrum or guard; and the anterior somewhat flat projection,
called the proostracum. In the living Dibranchiata other than _Spirula_
the phragmacone and rostrum have become very rudimentary. The shell of
_Sepia_ (fig. 20) consists almost entirely of the proostracum, the
little ventral hollow posteriorly representing the phragmacone, and the
posterior pointed projection, the rostrum. In the _Oigopsida_ the shell
is represented by a proostracum which is no longer calcified by forms a
chitinous plume or gladuius, and a similar rudiment occurs