219.--_Cerithium hexagonum_. Eocene.]
With regard to the _Cephalopods_, the chief point to be noticed
is, that all the beautiful and complex forms which peculiarly
characterised the Cretaceous period have here disappeared. We no
longer meet with a single example of the Turrilite, the Baculite,
the Hamite, the Scaphite, or the Ammonite. The only exception
to this statement is the occurrence of one species of Ammonite
in the so-called "Lignitic Formation" of North America; but the
beds containing this may possibly be rather referable to the
Cretaceous--and this exception does not affect the fact that
the _Ammonitidoe_, as a family, had become extinct before the
Eocene strata were deposited. The ancient genus _Nautilus_ still
survives, the sole representative of the once mighty order of the
Tetrabranchiate Cephalopods. In the order of the _Dibranchiates_,
we have a like phenomenon to observe in the total extinction
of the great family of the "Belemnites." No form referable to
this group has hitherto been found in any Tertiary stratum; but
the internal skeletons of Cuttle-fishes (such as _Belosepia_)
are not unknown.
[Illustration: Fig. 220.--_Limnoea pyramidalis_. Eocene.]
[Illustration: Fig. 221.--_Physa columnaris_. Eocene.]
[Illustration: Fig. 222.--_Cyclostoma Arnoudii_. Eocene.]
Remains of _Fishes_ are very abundant in strata of Eocene age,
especially in certain localities.
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