354 



THE GEOLOGIST. 



and to shatter causes them to be soon ejected from the cabinet. This 

 latter case is, however, the rarest, and the large part of the shell is 

 iisnally passed by and disregarded by collectors, which is to be re- 

 gretted, as from the adult forms the best information is to be 

 obtained of the structure of the animal, and particularly of the form 

 of the mouth, or orifice, of the shell. 



This, well preserved in many specimens from other clay-strata, is 

 a part unknown in many species, even amongst the commonest, from 

 the Gault. Its outline is very distinct from those foliated sutures 

 which mark in the casts of these fossils the septal divisions of the 

 shell ; and which are often thought by amateurs, as they have been 

 by some unreflecting naturalists, to represent the successive edges of 

 the mouth at the various stages of growth. Such, however, is not 

 the case. At the mouth, or opening of the shell the actual gTowth 

 of course took ])lace ; it was there the shell-matter v\^as added layer 

 by layer to the edge or rim, as we see it done in other shells. But 

 this was effected by the upper part of the Ammonite -animal, while, 

 on the contrary, it was by the loiver part of the same animal that the 

 foliated septa were formed which divided off the unoccupied portions 

 of the shell into separate chambers. These septa or divisional plates 

 are smoothly concave and plain in the hving nautilus, because the 

 loiver extremity of that animal is simple and bag-like ; while in the 

 Ammonite and others of the extinct cephalopods, as the Samites^ 

 Turrilites, Goniatites, &c, it was concave and more or less highly 

 foliated, or zig-zag, as the ovaries on each side were more or less 

 elaborately constructed and sub-divided into small and separate 

 egg-bags. 



These foliations are entirely lateral, the central part of the septa 

 being smooth and undulating, while slightly varied striations, a 

 narrow flat band, or a tendency to prismatic colouring on the sides 

 of the shell-substance, may guide the experienced eye of the 

 naturalist to detect the outlines of the former cusps or undulations 

 which ornamented or characterized the mouth of the living shell. 

 But these indications of the former mouth occur solely on the out- 

 side of il)o slicll ; the foliations, being, in fact, the end-sections 

 of the scpla, arc seen only in the casts, where, to use a familiar 

 Niiiiile, they may bo compared to the ends of the rafters of the 



