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 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_. Faxöe Limestone (Upper Cretaceous), Denmark.] [Illustration: Fig. 202.--_Ancyloceras Matheronianus_. Gault.]