umber
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. 205), distinguished by the possession of
a straight fissure in the upper end of the guard. This also
disappears at the close of the Cretaceous period; and no member
of the great Mesozoic family of the _Belemnitidoe_ has hitherto
been discovered in any Tertiary deposit, or is known to exist
at the present day.

[Illustration: Fig. 203.--_Turrilites catenatus_. The lower figure
represents the entire shell; the upper figure represents the
base of the shell seen from below. Gault.]

[Illustration: Fig. 204.--a, _Ptychoceras Emericianum_,
reduced--Lower Greensand; b, _Baculites anceps_, reduced--Chalk;
c, Portion of the same, showing the folded edges of the septa;
d, _Crioceras cristatum_, reduced--Gault; e, _Scaphites oequalis_,
natural size--Chalk; f, _Hamites rotundus_, restored--Gault.]

Passing on next to the _Vertebrate Animals_ of the Cretaceous
period, we find the _Fishes_ represented as before by the Ganoids
and the Placoids, to which, however, we can now add the first
known examples of the great group of the _Bony Fishes_ or
_Teleosteans_, comprising the great majority of existing forms.
The _Ganoid_ fishes of the Cretaceous (_Lepidotus, Pycnodus_,
&c.) present no features of special interest. Little, also, need
be said about the _Placoid_ fishes of this period. As in the
Jurassic deposits, the remains of these consist partly of the
teeth of genuine Sharks (_Lamna, Odontaspis_, &c.) and partly
of the teeth and defensive spines of Cestracionts, such as the
living Port-Jackson Shark. The pointed and sha