me 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. Pompilius_); but the _Orthocerata_
and _Goniatites_ of the Trias have finally disappeared; and the
great majority of the Tetrabranchiate forms are referable to
the comprehensive genus _Ammonites_, with its many sub-genera
and its hundreds of recorded species. The shell in _Ammonites_
is in the form of a flat spiral, all the coils of which are in
contact (figs. 170 and 171). The innermost whorls of the shell
are more or less concealed; and the body-chamber is elongated
and narrow, rather than expanded towards the mouth. The tube or
siphuncle which runs through the air-chambers is placed on the
dorsal or _convex_ side of the shell; but the principal character
which distinguishes _Ammonites_ from _Goniatites_ and _Ceratites_ is
the wonderfully complex manner in which the _septa_, or partitions
between the air-chambers, are folded and undulated. To such an
extent does this take place, that the edges of the septa, when
exposed by the removal of the shell-substance, present in an
exaggerated manner the appearance exhibited by an elaborately-dressed
shirt-frill when viewed edgewise. The species of _Ammonites_ range
from the Carboniferous to the Chalk; but they have not been found
in deposits older than the Secondary, in any region except India;
and they are therefore to be regarded as essentially Mesozoic
fossils. Within these limits, each formation is characterised by
particular species, the number of individuals being often very
great,