CHAMBERED SHELLS. 311 
a view of illustrating certain points that have 
relation to the object of the present Treatise. 
I select these, first, because they afford proofs 
of mechanical contrivances, more obviously 
adapted to a definite purpose, than can be found 
in shells of simpler character. Secondly, be- 
cause 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, be- 
cause many of these chambered shells can be 
shewn, 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 sub- 
ordination 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 con- 
chology, which relate to the limitation of species 
to particular geological Formations ;* and affords 
striking proofs of the curious fact, that many 
* Thus, the Nautilus multicarinatus 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 themselvei. 
