. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. THE CRETACEOUS PERIOD. 273 and Baadites. In the genus Crioceras (fig. 204. d), the shell consists of an open spiral, the volutions of which are not in. Fig. 202.—A7tcyioceras Matherottianns. Gault. 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 Am?Honite in its structure; but instead of forming a flat spiral, it is coiled into an ele- vated
. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. THE CRETACEOUS PERIOD. 273 and Baadites. In the genus Crioceras (fig. 204. d), the shell consists of an open spiral, the volutions of which are not in. Fig. 202.—A7tcyioceras Matherottianns. Gault. 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 Am?Honite in its structure; but instead of forming a flat spiral, it is coiled into an ele- vated 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 volu- tions 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,/), 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 Ptycho- ceras (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 Anmionifidce; and all the other forms, how- ever complex, may be regarded as being simply produced by the bending or folding of such a conical septate shell in differ- ent ways. The Baculite, therefore, corresponds, in the series of the A?7i7nonitidce, to the Ofihoce?'as in the series of the N'ati- tilidce. All the above-mentioned genera are characteristically, or exclusively,
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