evidence, which taken in conjunction with Mr. Owen's representation
of its termination in a large sac surrounding
the heart of the animal, (P. 34, p, p, a. a.) appears sufficient
to decide this long disputed question. If we suppose
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 Pl. 33. The Lias which fills this pipe, must have entered it in the state of liquid mud, to the same extent that the 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 calcareous spar, subsquently introduced by gradual infiltration, and at successive 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, Pl. 32, Fig. 3, d. e. £, show the edges of the calcareous sheath surrounding the oval casts of three compartments of the expanded siphuncle. This calcareous sheath was probably flexible, like that surrounding the membranous pipe of the recent Nautilus Pompilius. (Pl. 31, Fig. 1, b. d. e.) The continuity of this sheath across the air-chambers, (Pl. 32, Fig. 2, d. e. f. Fig. 3, d. e. £ and Pl. 33,) shows that there was no communication for the passage of any fluid from the siphuncle into these chambers: had any such existed, some portion of the line earthy matter, which in these two fossils forms the casts of the siphuncle, must have passed through it into these chambers. Nothing has entered them, but pure crystallized spar, introduced by infiltration through the pores of the shell, after it had undergone sufficient decomposition to be percolated by water, holding in solution carbonate of lime.
The same argument applies to the solid casts of pure crystallized carbonate of lime, which have entirely filled the chambers of the specimen Pl. 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 Pl. 42, Fig. 1, 2, 3, and Pl. 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 shells were lodged, finding through these fractures admission to the air chambers, has filled them with clay, or sand or limestone.
