326 MANNER OF ACTION 
tain a pericardial Jiuid, the place of which is 
alternately changed from the pericardium (p, p.) 
to the siphiincle (n.), we shall find in these organs 
an hydraulic apparatus for varying the specific 
gravity of the shell ; so that it sinks when the 
pericardial fluid is forced into the siphuncle, and 
becomes buoyant, when the same fluid returns 
to the pericardium. On this hypothesis also the 
chambers would be permanently filled with air 
alone, the elasticity of which would admit of 
the alternate expansion and contraction of the 
encrusted, and distended in a manner which illustrates the 
action of this organ. (PI. 32, Fig. 2, a. a^ a-, a^. d. e. f, 
and Fig. 3, d. e. f ). The fracture at Fig. 2, b. shews the dia- 
meter of the siphuncle, where it passes through a transverse 
plate, to be much smaller than it is midway between these Plates 
(at d. e. f.). The transverse sections at Fig. 2, a. and b., and 
the longitudinal sections at Fig. 2, d. e. f. and Fig. 3, d. e. f., 
shew that the interior of the siphuncle is filled with stone, of the 
same nature with the stratum in which the shell was lodged. 
These earthy materials, having entered the orifice of the pipe at 
a in a soft and plastic state, have formed a cast which shews the 
interior of this pipe, when distended, to have resembled a string 
of oval beads, connected at their ends by a narrow neck, and 
enlarged at their centre to nearly double the diameter of this 
neck. 
A similar distension of nearly the entire siphuncle by the stony 
material of the rock in which the shell was imbedded, is seen in 
the specimen of Nautilus striatus from the Lias of Whitby, 
represented at PI. 33. The Lias which fills this pipe, must have 
entered it in the state of liquid mud, to the same extent that the 
pericardial Jiuid entered, during the hydraulic action of the si- 
phuncle in the act of sinking ; not one of the air chambers has 
admitted the smallest particle of this mud ; they are all filled 
with calcareous spar, subsequently introduced by gradual infil- 
tration, and at successive periods which are marked by changes 
