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 number of other allied forms, which cannot be noticed here.  Not a single one of these genera, further, has hitherto been detected in any strata higher than the Cretaceous.  We may therefore consider that these wonderful, varied, and elaborate forms of Ammonitidoe constitute one of the most conspicuous features in the life of the Chalk period.

[Illustration:  Fig. 201.—­Different views of Nautilus Danicus.  Faxoee Limestone (Upper Cretaceous), Denmark.]

[Illustration:  Fig. 202.—­Ancyloceras Matheronianus.  Gault.]

The Dibranchiate Cephalopods are represented partly by the beak-like jaws of unknown species of Cuttle-fishes and partly by the internal skeletons of Belemnites.  Amongst the latter, the genus Belemnites itself holds its place in the lower part of the Cretaceous series; but it disappears in the upper portion of the series, and its place is taken by the nearly-allied genus Belemnitella (fig. 205), distinguished by the possession of a straight fissure in the upper end of the guard.  This also disappears at the close of the Cretaceous period; and no member of the great Mesozoic family of the Belemnitidoe has hitherto been discovered in any Tertiary deposit, or is known to exist at the present day.