DISTANCES OF TRANSVERSE PLATES. 325 
enlarging gradually and periodically until the 
nnimal has arrived at full maturity.* 
A fifth consideration is had of mechanical 
advantage, in disposing the Distance at which 
these successive transverse Plates are set from 
one another. (See PI. 31. Fig. 1. and PI. 32, 
Fig. 1 , 2). Had these distances increased in the 
same proportion as the area of the air chambers, 
the external shell would have been without due 
support beneath those sides of the largest cham- 
bers, where the pressure is greatest : for this a 
I’emedy is provided in the simple contrivance 
uf placing the transverse plates proportionally 
nearer to one another, as the chambers, from 
becoming larger, require an increased degree of 
support. 
Sixthly, The last contrivance, which I shall 
bere notice, is that which regulates the ascent 
and descent of the animal by the mechanism of 
the Siphuncle. The use of this organ has never 
yet been satisfactorily made out; even Mr. 
Owen’s most important Memoir leaves its man- 
ner of operation uncertain ; but the appearances 
"^hich it occasionally presents in a fossil state, 
(See PI. 32, Fig. 2, 3, and PI. 33, )t supply 
* III a young Nautilus Ponipilius in the collection of Mr. 
Broderip, there are only seventeen Septa. Dr. Hook says tliat 
has found in some shells as many as forty. A cast, expressing 
(lie form of a single air chamber, of the Nautilus Hexagonus 
•s represented in PI. 42, Fig. 1. 
t PI. 32, Fig. 2, represents a fractured portion of the interior 
’ll’ a Nautilus Hexagonus, having the transverse plates (c. c'.) 
