Cones (_Conus_),
Volutes (_Voluta_), Cowries (_Cyproea_, fig. 218), Olives and
Rice-shells (_Oliva_), Mitre-shells (_Mitra_), Trumpet-shells
(_Triton_), Auger-shells (_Terebra_), and Fig-shells (_Pyrula_).
Along with these are many forms of _Pleurotoma, Rostellaria_,
Spindle-shells (_Fusus_), Dog-whelks (_Nassa_), _Murices_, and
many round-mouthed ("holostomatous") species, belonging to such
genera as _Turritella, Nerita, Natica, Scalaria_, &c. The genus
_Cerithium_ (fig. 219), most of the living forms of which are found
in warm regions, inhabiting fresh or brackish waters, undergoes a
vast development in the Eocene period, where it is represented
by an immense number of specific forms, some of which attain
very large dimensions. In the Eocene strata of the Paris basin
alone, nearly one hundred and fifty species of this genus have been
detected. The more strictly fresh-water deposits of the Eocene
period have also yielded numerous remains of Univalves such as
are now proper to rivers and lakes, together with the shells of
true Land-snails. Amongst these may be mentioned numerous species
of _Limnoea_ (fig. 220), _Physa_ (fig. 221), _Melania, Paludina,
Planorbis, Helix, Bulimus_, and _Cyclostoma_ (fig. 222).

[Illustration: Fig. 215.--_Turbinolia sulcata_, viewed from one
side, and from above. Eocene.]

[Illustration: Fig. 216.--_Cardita planicosta_. Middle Eocene.]

[Illustration: Fig. 217.--_Typhis tubifer_, a "siphonostomatous"
Univalve. Eocene.]

[Illustration: Fig. 218.--Cyproea elegans. Eocene.]

[Illustration: Fig. 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 th