114 PAPERS, ETC. 
than one Aptychus; a fact which, I think, will help to 
establish the connexion of this body with the ammonite. 
Some of the ammonites containing Aptychi are so small as to 
be nearly microscopic, and it is not likely that shells so dimi- 
nutive and young, would be selected by any parasitic 
creature for a habitation ; or that they would by accident be 
washed into the deserted chambers of these shells, which 
with larger specimens might have happened. 
Iam still further strengthened in the conclusion that 
the Aptychus must belong to the ammonite, by the fact 
that I have been able to make out about ten forms or 
species of Aptychi from the upper lias; and I find that a 
particular form of Aptychus is always allied to a particular 
species of ammonite, a circumstance which cannot be 
accidental, and which has not hitherto been noticed. This 
latter fact will I think be considered conclusive, that this 
body performs some as yet unascertained functions in con- 
nexion with it. That it is an operculum I am inclined to 
doubt. The principal reason for its being so considered is, 
that when the valves of the Aptychi are expanded, they 
would cover about a transverse section of the chamber. 
We know that the nautilus, the animal nearest allied to 
the ammonite, does not possess an operculum. This animal 
has continued through most of the world’s changes to the 
present time, and consequently its organization can be 
studied. The ammonite, although presented to us as 
“ Medals of Creation,” of many hundred species in the 
rocks which surround us, has no living representative ; for 
which reason we can only speculate as to the office this 
body had to perform in its economy. Both these shells 
are chambered, and possessed of a beautiful provision for 
regulating their specific gravity in their native element. 
By means of the syphuncular tube, which. passes through 
