The type most abundantly represented,
especially in America, is _Platyceras_ (fig. 99), comprising thin,
wide-mouthed shells, probably most nearly allied to the existing
"Bonnet-limpets," and sometimes attaining very considerable
dimensions. We may also note the continuance of the genus
_Euomphalus_, with its discoidal spiral shell. Amongst the
_Heteropods_, the survival of _Bellerophon_ is to be recorded;
and in the "Winged-snails," or _Pteropods_, we find new forms
of the old genera _Tentaculites_ and _Conularia_ (fig. 100).
The latter, with its fragile, conical, and often beautifully
ornamented shell, is especially noticeable.
[Illustration: Fig. 100.--_Conularia ornata, of the natural size.
Devonian, Europe.]
[Illustration: Fig. 101.--_Clymenia Sedgwickii_. Devonian, Europe.]
The remains of _Cephalopoda_ are far from uncommon in the Devonian
deposits, all the known forms being still Tetrabranchiate. Besides
the ancient types _Orthoceras_ and _Cyrtoceras_, we have now
a predominance of the spirally-coiled chambered shells of
_Goniatites_ and _Clymenia_. In the former of these the shell is
shaped like that of the _Nautilus_; but the partitions between the
chambers ("septa") are more or less lobed, folded, or angulated,
and the "siphuncle" runs along the _back_ or convex side of the
shell--these being characters which approximate _Goniatites_ to
the true Ammonites of the later rocks.
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