gh 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. Amongst the more important of these may be mentioned
_Crioceras, Turrilites, Scaphites, Hamites, Ptychoceras_, and
_Baulites_. In the genus _Crioceras_ (fig. 204, d), the shell
consists of an open spiral, the volutions of which are not in
contact, thus resembling a partially-unrolled _Ammonite_ or the
inner portion of an _Ancyloceras_. In _Turrilites_ (fig. 203), the
shell is precisely like that of the _Ammonite_ in its structure;
but instead of forming a flat spiral, it is coiled into an elevated
turreted shell, the whorls of which are in contact with one another.
In the genus _Scaphites_ (fig. 204, e), the shell resembles that
of _Ancyloceras_ in consisting of a series of volutions coiled
into a flat spiral, the last being detached from the others,
produced, and ultimately bent back in the form of a crosier; but
the whorls of the enrolled part of the shell are in contact,
instead of being separate as in the latter. In the genus _Hamites_
(fig. 204, f), the shell is an extremely elongated cone, which
is bent upon itself more than once, in a hook-like manner, all
the volutions being separate. The genus _Ptychoteras_ (fig. 204,
a) is very like _Hamites_, except that the shell is only bent
once; and the two portions thus bent are in contact with one
another. Lastly, in the genus _Baculites_ (fig. 204, b and
c) the shell is simply a straight elongated cone, not bent
in any way, but possessing the folded septa which characterise
the whole Ammonite family. The _Baculite_ is the simplest of
all the forms of the _Ammonitidoe_; and all the other forms,
however complex, may be regarded as being simply produced by the
bending or folding of such a conical septate shell in different
ways. The _Baculite_, therefore, corresponds, in the series of
the _Ammonitidoe_, to the _Orthoceras_ in the series of the
_Nautilidoe_. All the above-mentioned genera are characteristically,
or exclusively, Cretaceous, and they are accompanied by a n