OF THE SIPHUNCLE. 327 
siphuiicle, in the act of admitting or rejecting 
the pericardial fluid. 
The principle to which we thus refer the rising 
and sinking of the living Nautilus, has been al- 
ready stated (P. 318) to be the same which regu- 
lates the ascent and descent of the Water Balloon : 
the forcing of a quantity of water into the single 
air chamber of the balloon compresses the air, 
and increases the quantity of matter in this cham- 
ber, without enlarging the magnitude of the bal- 
iri the colour of the spar. In both these fossil Nautili, the entire 
series of the earthy casts within the siphuncie represents the bulk 
of fluid which this pipe could hold. 
The sections, PI. 32, Fig. 3, d. e. f., shew the edges of the cal- 
careous sheath surrounding the oval casts of three compartments 
of the expanded siphuncie. This calcareous sheath may have 
been flexible, like that surrounding the membranous pipe of 
the recent Nautilus Pompilius. (PI 31, Fig. 1, b. d. e.) The 
continuity of this sheath across the air chambers, (PI. 32, 
Fig. 2, d. e. f. Fig. 3, d. e. f. and PI. 33), shows that there 
was no communication for the passage of r.ny fluid from the 
siphuncie into these chambers: had any such existed, some 
portion of the fine earthy matter, which in these two fossils 
forms the casts of the siphuncie, must have passed through it 
into these chambers. Nothing has entered them, hut pure crys- 
tallized spar, introduced by infiltration through the pores of the 
shell, after it had undergone sufficient decomposition to be per- 
colated by water, holding in solution carbonate of lime. 
The same argument applies to the solid casts of pure crys- 
tallized carbonate of lime, which have entirely filled the cham- 
bers of the specimen PI. 32, Fig. 1 ; and to all fossil Nautili and 
Ammonites, in which the air chambers are either wholly void, or 
partially, or entirely filled with pure crystallized carbonate of 
lime. (See PI. 42, Fig. 1, 2, 3, and PI. 36). In all such cases, 
it is clear that no communication existed, by which water could 
pass from the interior of the siphon to the air chambers. When 
