Some
of these (_Conulus priscus_) are true Land-snails, resembling the
existing _Zonites_; whilst others (_Pupa vetusta_, fig. 128) appear
to be generically inseparable from the "Chrysalis-shells" (_Pupa_)
of the present day. All the known forms--three in number--are of
small size, and appear to have been local in their distribution
or in their preservation. More important, however, than any of
the preceding, are the _Cephalopoda_, represented, as before,
exclusively by the chambered shells of the Tetrabranchiates.
The older and simpler type of these, with simple plain septa,
and mostly a central siphuncle, is represented by the straight
conical shells of the ancient genus Orthoceras, and the bow-shaped
shells of the equally ancient _Cyrtoceras_--some of the former
attaining a great size. The spirally-curved discoidal shells
of the persistent genus _Nautilus_ are also not unknown, and
some of these likewise exhibit very considerable dimensions.
Lastly, the more complex family of the _Ammonitidoe_, with lobed
or angulated septa, and a dorsally-placed siphuncle (situated on
the convex side of the curved shells), now for the first time
commences to acquire a considerable prominence. The principal
representative of this group is the genus _Goniatites_ (fig.
129), which commenced its existence in the Upper Silurian, is well
represented in the Devonian, and attains its maximum here.
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