Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/269

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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.

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.