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.
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