At Plate 41. we have a rare and most beautiful example of the preservation of the transverse plates of the Ammonites giganteus converted to chalcedony, without 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 further 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 combined 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.[1]
gives greater strength (as in A. obtusus, Pl. 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 Log 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 Pl. 44, Figs. 4. and 5.)
- ↑ Pl. 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 undulated, (as in Pl. 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 Pl. 36. d. and Pl. 39. d. V.). Pl. 42, Fig, 2, represents the front view of the cast of a single chamber of A. exca-
