MANNER OF ACTION OF THE SIPHUNCLE. 247 



presentation of its termination in a large sac (P. 34, p, p,) 

 surrounding the heart of the animal, (a. a.,) appears suffi- 

 cient to decide this long disputed question. If we suppose 

 this sac (p, p.) to contain a pericardial fluid, the place of 

 which is alternately changed from the pericardium (p, p.), 



oval beads, connected at tlieir 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 mate- 

 rial 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. 3 J. The 

 Lias which fills this pipe, must have entered it in the state of liquid mud, 

 to the same extent that tlie pericardial fluid entered, during- the hydraulic 

 action of the siphuncle in the act of sinking-; not one of the air-chambers 

 has admitted the smallest particle of this mud; they are all filled with cal- 

 careous spar, subsequent I ij introduced by gradual infiltration, and at succes- 

 itive periods which are marked by changes in the colour of the spar. In 

 both these fossil Nautili, the entire series of the earthy casts within the 

 siphuncle represents the bulk of fluid which this pipe could hold. 



The sections, F'l. 32, P'ig. 3, d. e. f, show the edges of the calcareous 

 sheath surrounding the oval casts of three comiiartmeiits of the expanded 

 siphuncle. This calcareous sheath was probably flexible, like that sur- 

 rounding 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. c. f. and PI. 33,) shows that there was no 

 communi cation for the ])assage of any fluid from the siphuncle into these 

 chambers: had any such existed, some portion of the fine earthy matter, 

 which, in these two fossils foi ms the casts of the siphuncle, must have passed 

 through it into these chambers. Nothing has entered them, hnt pure crys- 

 tallized spar, introduced by infiltration through the pores of the shell, after 

 it had undergone sufficient decomposition to be percolated by water, hold- 

 ing in solution carbonate of lime. 



The same argument applies to the solid casts of pure crystallized car- 

 bonate of lime, which have entirely filled the chambers (>f the specimen 

 Fl. 32, Fig. 1; and to all fossil Nautih and Ammonites, in which the air- 

 chambers are either wholly void, or partially, or entirely filled with pure 

 crystallized carbonate of lim.e. (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 the 

 pipe was ruptured, or the external shell broken, the earthy sediment, in- 

 which such broken shtlls were lodged, finding through these fractures ad- 

 mission to the air-chambers, has filled them with clay, or sa.nd or limestone 



