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.
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