THE APTYCHUS. 115 
all their chambers, they were able to adapt themselves to 
any circumstances, and sink or swim at pleasure. The dif- 
ference in the two shells is not great, the prineipal being 
in the position of the syphuncular apparatus—that of the 
nautilus being central—that of the ammonite being situ- 
ated on the outer margin of the chambers. In the casts 
containing Aptychi to which I have referred, there are 
always some traces of this syphuncular apparatus ; and in 
some instances the tube is in nearly perfect preservation, 
passing along the outer edge of the whorls of the shell 
until it reaches near to the position where the Aptychus is 
imbedded, and it appears to be of the same horny tex- 
ture as is presented by the Aptychus. In the fractured 
sections of ammonites, where the Aptychus lies in the 
chamber of the shell, it is frequently with its dorsal 
margin pointed to, or near where the syphuncle passes ; 
and from this, combined with the preservation of the 
syphuncular tube with it, I am rather inclined to the 
opinion that the body has something to do with the curious 
but beautiful provision with which nature has furnished 
these shells, and which further observation will probably 
more clearly demonstrate. 
Since reading my paper at the Weston meeting, I have 
learnt that Von Buch has noticed the discovery of the 
Aptychus in a Scaphite, in a paper published in the Bulletin 
of the Geological Society of France; and T. Rupert 
Jones, Esq. the zealous assistant secretary of the Geologi- 
cal Society of London, mentions the occurrence of an 
Aptychus holding its proper place in an Orthoceras, in their 
museum. Thus it has been found in three genera of 
cephalopida, in each of which it was destined to perform 
the same office. 
