354, 
'I'UK GHOIiOtJlST. 
and to sliattei" causes tlieni to be soon ejecled from (he cabinet. This 
halter case is, howevci", tlic rarest, and the Lirge part of the shell is 
usually passed by and disregai'dcd by collectors, which is to be re- 
gretted, as from the adult forms the best information is to be 
obtained of the stracture of tlic animsd, and particularly of the fomi 
of the mouth, or orifice, of the shell. 
This, well preserved in many specimens fi'om other clay-strata, is 
a part unknown in many species, even amongst the commonest, from 
tlio Gault. Its outline is very distinct from those foliated svitm-cs 
which marlv 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 gi'owth. Such, however, is not 
the case. At tlie mouth, or opening of the shell the actual gi'owth 
of course took place ; it was there the shell-matter was 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 living nautilus, because the 
lower extremity of that animal is simple and bag-like ; while in the 
Ammonite and others of the extinct cephalopods, as the Ilamltes' 
Tiirrilites, Gonlatites, &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 ccnti-al part of the septa 
being smooth and undulating, while slightly varied striations, a 
nai'row flat band, or a te^ency 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- 
i3ut these indications of the former mouth oecm" solely on the out- 
side of the shell ; the foliations, being, in fact, the end-sections 
of the septa, are seen only in the casts, where, to use a familiar 
simile, they may be compared to the ends of the rafters of the 
