contrivances, more obviously adapted to a definite purpose, than can be found in shells of simpler character. Secondly, because the use of many of their parts can be explained, by reference to the economy and organization of the existing animals, most nearly allied to the extinct fossil genera and species with which we are concerned. And, thirdly, because many of these chambered shells can be shown, not merely to have performed the office of ordinary shells, as a defence for the body of their inhabitants; but also to have been hydraulic instruments of nice operation, and delicate adjustment, constructed to act in subordination to those universal and unchanging Laws, which appear to have ever regulated the movement of fluids.
The history of Chambered shells illustrates also some of those phenomena of fossil conchology, which relate to the limitation of species to particular geological Formations;[1] and affords striking-proofs of the curious fact, that many genera, and even whole families, have been called into existence, and again totally annihilated, at various and successive periods, during the progress of the construction of the crust of our globe.
The history of Chambered Shells tends further to throw light upon a point of importance in physiology, and shows that it is not always by a regular gradation from lower to higher degrees of organization, that the progress of life has advanced, during the early epochs of which geology takes cognisance. We find that many of the more simple forms have maintained their primeval simplicity through all the varied changes the surface of the earth has undergone; whilst, in other cases, organizations of a higher order preceded many of the lower forms of animal life; some of
- ↑ Thus, the Nautilus multicarinatis is limited to strata of the Transition formation; the N. bidorsatus to the Muschelkalk; N. obesus, and N. lineatus, to the Oolite Formation; N. elegans, and N. undulatus, to the Chalk. The divisions of the Tertiary formations have also species of Nautili peculiar to themselves.
