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, and the size which is sometimes attained being nothing short
of gigantic.
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