THE CRETACEOUS PERIOD. 



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and B acuities. In the genus Crioceras (fig. 204, d), the shell 

 consists of an open spiral, the volutions of which are not in 



Fig. 202 . A ncyloceras 



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 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 Ammonitidce; 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 Ammonitidce, to the Orthoceras in the series of the Nau- 

 tilidce. 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 



