, our knowledge of all the rest is based
upon the study of their fossil shells. A vast number of species of
shell similar in structure to that of _Nautilus_ are known, chiefly
from Primary and Secondary formations. These are divided into two
sub-orders by differences in the form and structure of the initial
chamber. In the Nautiloidea this chamber has the form of an obtuse
cone, on the apex of which is a slit-like mark or cicatrix, elongated
dorso-ventrally and placed opposite to the blind end of the siphuncle,
which indents the front wall of the initial chamber but does not enter
its cavity. In the Ammonoidea, on the other hand, the initial chamber
is inflated, and is spheroidal, oval or pyriform in shape, with no
cicatrix, and separated from the first air-chamber by a constriction.
The siphuncle also commences with a dilatation which deeply indents
the front wall of the initial chamber, called the protoconch, but does
not penetrate into its cavity. Munier-Chalmas has shown that the
cavity of the protoconch is traversed by a tubular organ, the
"prosiphon," which does not communicate with the true siphuncle, the
place of which it is supposed to take in the early life of the animal.
It is generally held, as suggested by Alpheus Hyatt, that the initial
chamber of the Nautiloidea corresponds not to the protoconch of the
Ammonoids, but to the second chamber of the latter, and that there
existed in the young Nautiloids a true initial chamber, a protoconch
which was either uncalcified or deciduous. The shell of the living
nautilus does not decide this question, as its early stages are
unknown, and there is a little vacuity in the centre of the spirally
coiled shell which may have been originally occupied by the true
protoconch.

The septa in the Nautiloidea are generally concave towards the
aperture of the shell, their curvature therefore directed backwards
(fig. 1); in the Ammonoidea, on the other hand, the convexity is
usually towards the aperture, the curvature therefore directed
forwards. The lines along which the edges of the septa are united to
the shell are known as "sutures," and these in the Nautiloidea are
simply curved or slightly lobed, whereas in the Ammonoidea they are
folded in various degrees of complexity; the projections of the suture
towards the mouth of the shell are called saddles, those in the
opposite direction lobes. The siphuncle