The _Baculite_ is the simplest of
all the forms of the _Ammonitidoe_; and all the other forms,
however complex, may be regarded as being simply produced by the
bending or folding of such a conical septate shell in different
ways. The _Baculite_, therefore, corresponds, in the series of
the _Ammonitidoe_, to the _Orthoceras_ in the series of the
_Nautilidoe_. All the above-mentioned genera are characteristically,
or exclusively, Cretaceous, and they are accompanied by a number
of other allied forms, which cannot be noticed here. Not a single
one of these genera, further, has hitherto been detected in any
strata higher than the Cretaceous. We may therefore consider that
these wonderful, varied, and elaborate forms of _Ammonitidoe_
constitute one of the most conspicuous features in the life of
the Chalk period.
[Illustration: Fig. 201.--Different views of _Nautilus Danicus_.
Faxoee Limestone (Upper Cretaceous), Denmark.]
[Illustration: Fig. 202.--_Ancyloceras Matheronianus_. Gault.]
The _Dibranchiate Cephalopods_ are represented partly by the
beak-like jaws of unknown species of Cuttle-fishes and partly
by the internal skeletons of Belemnites. Amongst the latter,
the genus _Belemnites_ itself holds its place in the lower part
of the Cretaceous series; but it disappears in the upper portion
of the series, and its place is taken by the nearly-allied genus
_Belemnitella_ (fig.
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