THE CRETACEOUS PERIOD. 



281 



teristic 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. Be- 



Fig. 202. Ancyloceras Matheronianus . Gault. 



sides these pre-existent types, the Cretaceous rocks have 

 yielded a great number of entirely new forms of the Ammoni- 

 tidcc, which are not known in any deposits of earlier or later 

 date. Amongst the more important of these may be men- 

 tioned Crioccras, Turrilites, Scaphites, Hamites, Ptychoceras, 

 and Baculites. 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 ele- 

 vated turreted shell, the whorls of which are in contact with 

 one another. In the genus Scaphites (fig. 204, tf), 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 volution 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. 201, b 

 and c) the shell is simply a straight elongated cone, not bent 

 in any way, but possessing the folded septa which characterize 

 the whole Ammonite family. The Baculite is the simplest of 

 all the forms of the Ammonitida; and all the other forms, how- 



