FAMILY 2. SIPHONOIDEA. 301 
from any adhesion ; but from its being furnished with a certain degree of 
elasticity, it advances by a series of periodical slips, the suddenness of 
which is undoubtedly counteracted by the attachment of the central 
membrane. The growth of the shell then proceeds in a circular direc- 
tion, and serves to buoy up its inhabitant in the water by having the 
vacated portion chambered in to meet its specific gravity. The geometri- 
cal increase of it arises simply thus. The natural position of the Nauti- 
lus, 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 animal 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 arrives at maturity, and the membrane being no longer wanted, pro- 
bably decays. Such is the manner in which our observations lead us to 
suppose the Nautilus grows; the chambers have certainly no communica- 
tion with the surrounding fluid. The camerated portion of the shell of 
Nautilus is evidently a simple, mechanical construction, (though planned 
by the wisest intelligence,) to assist the specific gravity of its inhabitant 
whilst under the different mutations of pressure that it is lable to at differ- 
ent periods of growth in its passage through the element ; and it is, more- 
over, a contrivance that could only be effected by the aid of this adjust- 
ing membrane upon the simple geometry of motion above described*. 
* The principle here advanced, of the geometrical formation of the Nautilus shell round its 
axis by the aid of an adjusting membrane, and of its camerated construction being accom- 
modated to the specific gravity of the inhabitant, will, perhaps, 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 struc- 
ture 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 appears 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.” 
VOL. II. 2R 
