DISTANCES OK TRANSVEliSE PLATES. ;]25 
A fifth point of structure, producing mechanical 
advantage, is exhibited in the Distances at which 
these successive transverse Plates are set from 
one another. See PI. 3], 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 
remedy is provided in the simple contrivance 
of 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, I shall here no- 
tice, is that which regulates the ascent and des- 
cent of the animal by the mechanism of the Si- 
phuncle. The use of this organ has never yet 
been satisfactorily made out ; even Mr. Owen's 
most important Memoir leaves its manner of 
operation uncertain ; but the appearances it oc- 
casionally presents in a fossil state, (See PL 32, 
Fig. 2, 3,* and PI. 33,) supply evidence, which 
taken in conjunction with Mr. Owen's represen- 
tation of its termination in a large sac (PL 34, 
p, p.) surrounding the heart of the animal (a, a.), 
appears sufficient to decide this long disputed 
question. If we suppose this sac (p, p.) to con- 
* PI. 32, Fig. 2, represents a fractured portion of the interior 
of a Nautilus Hexagonus, having the transverse plates (c. c.) 
encrusted with calcareous spar ; the Siphuncle also is siniilaily 
