Siphuncle.
It remains to consider the mechanism of the Siphuncle,
that important organ of hydraulic adjustment, by means of
which the specific gravity of the Ammonites was regulated.
Its mode of operation as a pipe, admitting or rejecting a
fluid, seems to have been the same as that we have already
considered in the case of Nautili.[1]
vatus; d, is the dorsal lobe enclosing the siphuncle, and e. f. the auxiliary ventral lobes, which open to receive the inner whorl of the shell. Pl. 49. Fig. 3. represents a cast of three chambers of A. catens, having two transverse, plates still retained in their proper place between them. The foliated edges of these transverse plates have regulated the foliations of the calcareous casts, which, alter the shell has perished, remain locked into one another, like the sutures of a skull.
The substance of the casts in all these cases is pure crystalline carbonate of lime, introduced by infiltration through the pores of the decaying shell. Each species of Ammonite has its peculiar form of air-chamber, depending on the specific form of its transverse plates. Analogous variations in the form of the air-chambers are co-extensive with the entire range of species in the family of Nautili.
- ↑ In the family of Ammonites, the place of the Siphuncle is always upon the exterior, or dorsal margin of the transverse plates. (See Pl. 36. d. e. f. g. h. i., and Pl. 42, Fig. 3. a, b.) It is conducted through them by a ring, or collar, projecting outwards; this collar is seen, well preserved, at the margin of all the transverse plates in Pl. 36. In Nautili, the collar projects uniformly inwards, and its place is either at the centre, or near the inner margin of the transverse plates. (See Pl. 31, Fig. 1. y. and Pl. 42. 1.)
The Siphuncle represented at Pl. 36, is preserved in a black carbonaceous state, and passes from the bottom of the external chamber (d.) to the inner extremity of the shell. At e. f. g. h. its interior is exposed by section, and appears filled, like the adjacent air-chambers, with a cast of pure calcareous spar. At Pl. 42. Fig. 3. b. a similar cast fills the tube of the Siphuncle, and also the air-chambers. Here again, as in Pl. 36, its diameter is contracted at its passage through the collar of each transverse plate, with the same mechanical advantages as in the Nautilus.
The shell engraved at Pl. 4-2. Fig. 4., from a specimen found by the Marquis of Northampton in the Greensand of Earl Stoke, near Devizes, and of which Figs. 5. 6. are fragments, is remarkable for the preservation of its Siphuncle, distended and empty, and still fixed in its place along
