DOUBLY VAULTED INTERNAL ARCHES. 349 
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 shew 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 tlie 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 in- 
strument than this for affording universal resist- 
ance to external pressure, in which the greatest 
possible degree of lightness is combined with the 
greatest strength. 
similar increase of support ; whilst in those species to which the 
more circular form of the sides gives greater strength (as in A. 
obtusus, PI. 3.5.) 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 trans- 
verse 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.) 
