Amongst the
former, the long-lived genus _Nautilus_ (fig. 201) again reappears,
with its involute shell, its capacious body-chamber, its simple
septa between the air-chambers, and its nearly or quite central
siphuncle. The majority of the chambered _Cephalopods_ of the
Cretaceous belong, however, to the complex and beautiful family
of the _Ammonitidoe_, with their elaborately folded and lobed
septa and dorsally-placed siphuncle. This family disappears wholly
at the close of the Cretaceous period; but its approaching
extinction, so far from being signalised by any slow decrease
and diminution in the number of specific or generic types, seems
to have been attended by the development of whole series of new
forms. The genus _Ammonites_ itself, dating from the Carboniferous,
has certainly passed its prime, but it is still represented by
many species, and some of these attained enormous dimensions
(two or three feet in diameter). The genus _Ancyloceras_ (fig.
202), though likewise of more ancient origin (Jurassic), is
nevertheless very characteristic of the Cretaceous. In this genus
the first portion of the shell is in the form of a flat spiral,
the coils of which are not in contact; and its last portion is
produced at a tangent, becoming ultimately bent back in the form
of a crosier. Besides these pre-existent types, the Cretaceous
rocks have yielded a great number of entirely new forms of the
_Ammonitidoe_, which are not known in any deposits of earlier or
later date.
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