THE CEPHALOPODA 293 
the form of the aperture and the position of the ‘ hyponomous ” 
sinus, which corresponds to the funnel (Fig. 261, zs). In some 
Nautiloidea, ¢.7. the dextral or sinistral 77ochoceras, and in sundry 
Ammonoidea, ¢.g. the sinistral Zwrrilites and Cochloceras and the 
dextral Bostrychoceras, the coil may be produced into a _helicoidal 
or turriculated spire. And in other cases again the last whorls 
of shell, whether it be discoidal or helicoidal, may be partly un- 
coiled, as may be seen, for example, in Litwites, which is largely 
uncoiled, or in Ophidioceras, in which only a small extent of the 
shell is uncoiled. Finally, the shell may become secondarily 
rectilinear in the adult, as in Baculites, among the Ammonoidea. 
In Spirula the shell is coiled in one plane, but it is endogastric, 
that is to say, coiled in the opposite direction to that of Nautilus 
(Figs. 268 and 270), and it is largely internal. In certain fossil 
Dibranchia the multilocular shell, whether it be straight or partially 
coiled, has become internal (Belemnitidae, Spirulirostra) and forms 
the phragmocone (Fig. 262, C). In such cases it is surrounded by a 
caleareous secretion of the reflected portion of the mantle, which 
is not homologous with the shell of other Molluscs, and forms the 
pointed rostrum or guard at the end opposite to the head (Fig. 262) 
and the cephalic plate or pro-ostracum at the anterior or dorsal end. 
Thus there is, in the shells of these Cephalopoda, an element which 
is not represented in the shells of other Mollusca. 
In the living Dibranchia, with the exception of Spirula, the 
phragmocone and the rostrum of this internal shell have become 
very rudimentary. In Sepia, for example, the shell is composed of 
parallel layers united together by short pillars of calcareous sub- 
stance, and has a stratified and alveolar structure: at its posterior 
end a little hollow marks the position of the phragmocone, and a 
short pointed external projection represents the rostrum, the bulk 
of the shell being formed by the anterior pro-ostracum, on which the 
retractor muscles of the cephalopedal mass are inserted. In the 
Oigopsida the guard is no longer calcified, and the shell has the form 
of a chitinous plume or gladius, but in Ommuatostrephes there is a 
small posterior conical cavity representing the remains of the 
phragmocone. 
In the Loliginidae and Sepiolidae the shell is similarly repre- 
sented by a chitinous gladius (Fig. 263), but in these families it is 
so much reduced that it only occupies the anterior portion of the 
body. In Jdiosepius this shell is nearly obsolete, and it is absent 
altogether in certain Sepiolidae and some allied forms such as 
Stoloteuthis, Inioteuthis, Sepioloidea, and Sepiadarium. Finally, in the 
Octopoda there is no longer a true shell, but only some simple 
chitinous rudiments, on which the retractor muscles of the head 
and funnel are inserted ; these may be paired, as in the case of the 
lateral stylets of Octopus ; or unpaired, as in the case of Cirrhoteuthis. 
