264 COMPLEX FORM OF AIR-CHAMBERS. 



At Plate 41. we have a rare and most beautiful example 

 of the preservation of the transverse plates of the Am- 

 monites giganteus converted to calcedony, v^^ithout the 

 introduction of any earthy matter into the area of the air- 

 chambers.* 



This shell is so laid open as to show the manner in which 

 each transverse plate forms a tortuous partition between the 

 successive air-chambers. By means of these winding plates, 

 the external shell, being itself a continuous arch, is farther 

 fortified with a succession of compound arches, passing 

 transversely across its internal cavity; each arch being 

 disposed in the form of a double tunnel, vaulted not only at 

 the top, but having a corresponding series of inverted arches 

 along the bottom. 



We can scarcely imagine a more perfect instrument than 

 this for affording universal resistance to external pressure, 

 in which the greatest possible degree of lightness is com- 

 bined with the greatest strength. 



The form of the air-chambers in Ammonites is much 

 more complex than in the Nautili, in consequence of the 

 tortuous windings of the foliated margin of the transverse 

 plates.* 



gives greater strength (as in A. obtusus, PI. 35.) the sinuosities of the septa 

 are proportionately few. 



It seems probable that some improvement might be made, in fortifying 

 the cylindrical air-tube of Massey's Patent sounding machine, for taking 

 soundings at great depths, by the introduction of transverse plates, acting on 

 the principle of the transverse plates of the chambered portion of the shells 

 of Nautili and Ammonites, or rather of Orthoceratites, and Baculites, (see 

 PI. 44, Figs. 4. and 5.) 



* PI. 42, Fig. 1, represents the cast of a single chamber of N. Hexagonus, 

 convex inwards, and concave outwards, and bounded, at its margin by lines 

 of simple curvature. In a few species only of Nautilus the margin is undu- 

 lated, (as in PI. 43, Fig. 3, 4,) but it is never jagged, or denticulated like 

 the margin of the casts of the chambers of Ammonites. 



In Ammonites, the chambers have a double curvature, and are, at their 

 centre, convex outwards (see Pi. 36. d. and PI. 3D. d. V.) PI. 42, Fig 

 2, represents tlie front vievir of the cast of a single chamber of A. exca- 



