350 COMPLEX FORM OF AIR CHAMBERS. 
The form of the air-chambers in Ammonites 
is much more complex than in the NautiH, 
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 double 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. represents 
a cast of three chambers of A. catena, having two transverse 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 crystalline 
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 trans- 
verse plates. Analogous variations in the form of the air-cham- 
bers are co-extensive with the entire range of species in the 
family of Nautili. 
