The species of _Platyostoma_
(fig. 72, h) also belong to the same family; and the entire
group is continued throughout the Devonian into the Carboniferous.
Amongst other well-known Upper Silurian Gasteropods are species
of the genera _Holopea_ (fig. 72, g), _Holopella_ (fig. 72.
e), _Platyschisma_ (fig. 72, d), _Cyclonema, Pleurotomaria,
Murchisonia, Trochonema_, &c. The oceanic Univalves (_Heteropods_)
are represented mainly by species of _Bellerophon_; and the Winged
Snails, or _Pteropods_, can still boast of the gigantic _Thecoe_
and _Conularioe_, which characterise yet older deposits. The
commonest genus of _Pteropoda_, however, is _Tentaculites_ (fig.
73), which clearly belongs here, though it has commonly been
regarded as the tube of an Annelide. The shell in this group
is a conical tube, usually adorned with prominent transverse
rings, and often with finer transverse or longitudinal striae as
well; and many beds of the Upper Silurian exhibit myriads of
such tubes scattered promiscuously over their surfaces.
The last and highest group of the _Mollusca_--that of the
_Cephalopoda_--is still represented only by _Tetrabranchiate_
forms; but the abundance and variety of these is almost beyond
belief. Many hundreds of different species are known, chiefly
belonging to the straight _Orthoceratites_, but the slightly-curved
_Cyrtoceras_ is only little less common.
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