in the _Nautilus_ pierces the
centres of the septa, and in fossil Nautiloids it is usually central
or sub-central. In a few cases it is marginal, and in that case may be
external, i.e. ventral, or internal, i.e. dorsal. In Ammonoids the
siphuncle is always marginal, and usually external. Its walls in the
living _Nautilus_ are strengthened by the deposit of calcareous
granules, and in some fossil forms the wall is completely calcified.
But this proper calcified wall is quite distinct from calcareous tubes
surrounding the siphuncle, which are developed from the septa. In the
pearly nautilus each septum is prolonged backwards at the point where
it is pierced by the siphuncle, forming a shelly tube somewhat like
the neck of a bottle. In many fossil forms these septal necks are
continued from the septum from which they arise to the next, so that
the siphuncle is enclosed in a complete secondary calcareous tube. In
the majority of Nautiloids the septal necks are directed backwards,
and they are said to be retrosiphonate. In the majority of the
Ammonoids the septal necks are continued forwards from the septa to
which they belong, and such forms are termed prosiphonate.

The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic
periods. The Nautiloidea are the most ancient, appearing first in the
Upper Cambrian, the genera being most numerous in the Palaeozoic
period, and comparatively few surviving into the Secondary. On the
other hand, the Ammonoidea are scarce in Palaeozoic formations, being
represented in deposits earlier than the Carboniferous only by
comparatively simple types, such as _Clymenia_ and _Goniatites_. In
the Secondary period Ammonoids were very abundant, both in genera and
species and in individuals, and with few local exceptions none are
known to have survived even to the commencement of the Tertiary. In
the widest sense the genus _Nautilus_ has existed since the Ordovician
(Silurian) period, but the oldest types are not properly to be placed
in the same genus as the existing form. Even with this qualification
the genus is very ancient, shells very similar to those of the living
_Nautilus_ being found in the Upper Cretaceous.

It has been maintained by some zoologists that the Ammonoidea were
Dibranchiate, though it would not follow from this that the shell was,
therefore, internal. They are, however, generally classed with the