;Ti5() COMPLEX FORM OF AIR CHAMBERS. 
The form of the air-cliainbers in Ammonites is 
much more complex than in the Nautili, in 
consequence of the tortuous windings of the 
foliated margin of the transverse plates.* 
Siphuncle. 
It remains to consider the mechanism of the 
Siphuncle, that important organ of hydraulic 
adjustment, by means of which the specific 
gravity of the Ammonites was regulated. Its 
* PI. 42, Fig. 1, represents the cast of a single chamber of N. 
Hexagonus, convex inwards, and concave outwards, and bounded 
at its margin by lines of simple curvature. In a few species only 
of Nautilus the margin is undulated, (as in PI. 43, Fig. 3, 4,) but 
it is never jagged, or denticulated like the margin of the casts of 
the chambers of Ammonites. 
In Ammonites, the chambers have a doTiblc curvature, and 
are, at their centre, convex outwards (see PI. 36. d. and PI. 39. 
d. V.). PI. 42, Fig. 2, represents the front view of the cast of 
a single chamber of A. excavatus; d, is the dorsal lobe enclosing 
the siphuncle, and e. f. the auxiliary ventral lobes, which 
open to receive the inner whorl of the shell. PI. 42. Fig. 3. re- 
presents a cast of three chambers of A. catena, having two trans- 
verse plates still retained in their proper place between them. 
The foliated edges of these transverse plates have regulated the 
foliations of the calcareous casts, which, after the shell has 
perished, remain locked into one another, like the sutures of a 
skull. 
The substance of the casts in all these cases is pure crys- 
talline carbonate of lime, introduced by infiltration through the 
pores of the decaying shell. Each species of Ammonite has its 
peculiar form of air-chamber, depending on the specific form of 
its transverse plates. Analogous variations in the form of the 
air-chambers are co-extensive with the entire range of species m 
the family of Nautili. 
