324 
ADDITION OF CHAMBERS. 
an extraordinary mnnber of transverse beams 
and bulk heads.* 
We may next notice a fourth contrivance by 
which the apparatus that gives the shell its 
power of floating, is progressively maintained in 
due proportion to the increasing weight and bulk 
of the body of the animal, and of the external 
chamber in which it resides ; this is effected by 
successive additions of new transverse Plates 
across the bottom of the outer chamber, thus 
converting into air chambers that part, which 
had become too small for the body of the Sepia. 
This operation, repeated at intervals in due pro- 
portion to the successive stages of growth of the 
outer shell, maintains its efficacy as a float, 
* The disposition of the curvatures of the transverse ribs, or 
lines of growth, in a different direction from the curvatures of 
the internal transverse plates, affords an example of further con- 
trivance for producing strength in the shells both of recent and 
fossil Nautili. As the internal transverse plates are convex in- 
wards, (see PI. 32, Fig. 1, b. to c.) whilst the ribs of the outer 
shell are in the greater part of their course convex outwards, 
these ribs intersect the curved edges of the transverse plates at 
many points, and thus divide them into a series of curvilinear 
parallelograms; the two shorter sides of each parallelogram 
being formed by the edges of transverse plates, whilst its two 
longer sides are formed by segments of the external ribs. The 
same principle of construction here represented in our plate of 
Nautilus hexagonus, extends to other species of its family of 
Nautilus, in many of which the ribs are more minute ; it is also 
applied in other families of fossil chambered shells ; e. g. the 
Ammonites, PI. 3r>, and PI. 38. Scaphites, PI. 44, Fig. 15- 
Hamites, PI. 44, Fig. 8 — 13. Turrilites, PI. 44, Fig. 14, and 
Baculites, PI. 44, Fig. 5. 
