and on the construction of its Shell. 123 



then proceeds in a circular direction, and serves to buoy up 

 its inhabitant in the water by having the vacated portion 

 chambered in to meet its specific gravity. The geometrical 

 increase of it arises simply thus. The natural position of the 

 Nautilus, like other Cephalopods, is with its head downwards, 

 the shell being consequently above ; and the periodical slip of 

 the belt of adhesion most probably takes place when the ani- 

 mal is in this supine position. It lets itself down, and round 

 and round, as it were, upon its axis, by the limited extension 

 of this membranous pulley ; the operation ceases when it ar- 

 rives at maturity, and the membrane being no longer wanted, 

 probably decays. Such is the manner in which our observa- 

 tions lead us to suppose the Nautilus grows ; the chambers 

 have certainly no communication Avith the surrounding fluid. 

 The camerated portion of the shell of Nautilus is evidently a «v 

 simple, mechanical construction, (though planned by the wisest u 

 intelligence,) to assist the specific gravity of its inhabitant 

 whilst under the dilferent mutations of pressure that it is liable 

 to at different periods of growth in its passage through the 

 element ; and it is, moreover, a contrivance that could only 

 be effected by the aid of this adjusting membrane upon the 

 simple geometry of motion above described*. 



M. Valenciennes regards the periodical introduction of a 

 partition in the shell of Nautilus as in some measure analogous 

 to the occasional deposit of a varix in the shell of Murex and 

 other Gastropods. The septa, like the varices, may undoubt- 

 edly be secreted by the mantle during a period of rest, but 

 there the analogy ceases. We would rather compare the ro- 

 tatory increase of the Nautilus to the horizontal growth of that 

 singular Gastropod, the Mugilus (Conch. Sy st. p. 231). The one 

 gravitates round a centre, increasing by a peculiarity of con- 



* The principle here advanced, of the geometrical formation of the Nau- 

 tilus shell round its axis by the aid of an adjusting membrane, and of its ca- 

 merated construction being accommodated to the specific gravity of the in- 

 habitant, will, perliaps, receive additional weight by a consideration of the 

 following passage from the ' Memoir ' of Professor Owen : — 



" In sections of recent shells, its [the membrane's] dried remains may 

 occasionally be seen of a black colour and pergameneous texture, continuing 

 from septum to septum as far as the central or first-formed chamber ; and a 

 further confirmation that this is the true structure of the parts, is afforded 

 by the fossil shells of this genus. In some polished sections of these remains, 

 not only is the continuation of the tube through all the chambers evident, 

 but it is seen to become slightly dilated in them, and in some instances ap- 

 pears also to have been reflected over the outer part of the testaceous tube 

 prior to being continued across the chamber to the next partition. There is 

 no indication, however, of the latter structure in the recent shells where the 

 membranous tube is preserved." 



