Fig. 201.—Different views of Nautilus Danicus. Faxöe Limestone (Upper Cretaceous), Denmark. 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 Ammonitidœ, 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 Ammonitidœ, 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,
Fig. 202.—Ancyloceras Matheronianus. Gault. 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 Ammonitidœ; 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 Ammonitidœ, to the Orthoceras in the series of the Nautilidœ. 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 Ammonitidœ constitute one of the most conspicuous features in the life of the Chalk period.
The Dibranchiate Cephalopods are represented partly by
Fig. 203.—Turrilites catenatus. The lower figure represents the entire shell; the upper figure represents the base of the shell seen from below. Gault.
Fig. 204.—a, Ptychoceras Emericianum, reduced—Lower Greensand; b, Baculites anceps, reduced—Chalk; c, Portion of the same, showing the folded edges of the septa; d, Crioceras cristatum, reduced—Gault; e, Scaphites œqualis, natural size—Chalk; f, Hamites rotundus, restored—Gault. 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
