One of the limestones of the Jura,
believed to be of the age of the Coral-rag (Middle Oolite) of
Britain, abounds to such an extent in the turreted shells of
_Nerinoea_ as to have gained the name of "Calcaire a Nerinees."
In addition to forms such as the preceding, we now for the first
time meet, in any force, with the Carnivorous Univalves, in which
the mouth of the shell is notched or produced into a canal, giving
rise to the technical name of "siphonostomatous" applied to the
shell. Some of the carnivorous forms belong to extinct types,
such as the _Purpuroidea_ of the Great Oolite; but others are
referable to well-known existing genera. Thus we meet here with
species of the familiar groups of the Whelks (_Buccinum_), the
Spindle-shells (_Fusus_), the Spider-shells (_Pteroceras_), _Murex,
Rostellaria_, and others which are not at present known to occur
in any earlier formation.
Amongst the Wing-shells (_Pteropoda_), it is sufficient to mark
the final appearance in the Lias of the ancient genus _Conularia_.
[Illustration: Fig. 170.--_Ammonites Humphresianus_. Inferior
Oolite.]
[Illustration: Fig. 171.--_Ammonites bifrons_. Lias.]
Lastly, the order of the _Cephalopoda_, in both its Tetrabranchiate
and Dibranchiate sections, undergoes a vast development in the
Jurassic period. The old and comparatively simple genus _Nautilus_
is still well represented, one species being very similar to the
living Pearly Nautilus (_N.
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