o24 ADDITION OF CHAMBERS. 
We may next notice a fourth contrivance by 
which the apparatus that gives the shell its 
power of floating, is progressively enlarged in 
due proportion to the increasing bulk of the 
body of the animal, and increasing weight of the 
external chamber in which it resides; this is 
effected by successive additions of new trans- 
verse Plates across the bottom of the outer 
chamber, thus converting into air chambers that 
part of the shell, which had become too small to 
hold the body. This operation, repeated at in- 
tervals in due proportion to the successive stages 
of growth of the shell, maintains its efficacy as a 
float, enlarging gradually and periodically until 
the animal has arrived at full maturity.* 
fossil Nautili. As the internal transverse plates are convex in- 
luards, (see PI. 32, Vio;. 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 the 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. 35, 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. 
* In a young Nautilus Pompilius in the collection of Mr. 
Broderip, there are only seventeen Septa. Dr. Hook says that 
he has found in some shells as many as forty. A cast, expressing 
the form of a single air chamber, of the Nautilus Hexagonus is 
represented in PI. 42, Fig. 1. 
