Ol' THE SIPHUNCLE. 
im 
become buoyant, whenever this fluid returns 
to the pericardium. On this hypothesis also 
the chambers would be continually filled with 
air alone, the elasticity of which would readily 
admit of the alternate expansion and contrac- 
tion of the siphuncle, 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, is the same 
which regulates the ascent and descent of the 
^ater Balloon: the application of external pres- 
"'ith calcareous spar, subsequently introduced by gradual infil- 
tration, and at successive periods which are marked by changes 
■n the colour of the spar. In both these fossil Nautili, the entire 
Series of the earthy casts within the siphuncle represents the bulk 
fluid which this pipe could hold. 
The sections, PI. .32, Fig. 3, d. e. f., shew the edges of the cal- 
eareous sheath surrounding the oval casts of three compartments 
®f the expanded siphuncle. This calcareous sheath was pro- 
bably 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, 
Figs. 2, d. e. f. Fig. 3, d. e. f. and PI. 33), shows that there 
Was no communication for the passage of any fluid from the 
®*phuncle into these chambers : had any such existed, some 
portion of the fine earthy matter, which in these two fossils 
orms the casts of the siphuncle, must have passed through it 
"to these chambers. Nothing has entered them, hut qmre crys- 
^^llized spar, introduced by infiltration through the pores of the 
0 b 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- 
W lized carbonate of lime, which have entirely tilled the cham- 
ers of the specimen PI. 32, Fig. 1 ; and to all fossil Nautili and 
®mouites, in which the air chambers arc either wholly void, or 
