DOUBLY VAULTED INTERNAL ARCHES. 349 
At Plate 41. we have a rare and most beautiful 
example of the preservation of the transverse 
piates of the Ammonites gigantens converted to 
chalcedony, without tlie introduction of any 
earthy matter into the area of the air-chambers. 
This shell is so laid open as to shew the 
laanner in which each transverse plate forms a 
^ertuous partition between the successive air- 
chambers. By means of these winding plates, 
^^le external shell, being itself a continuous arch, 
further fortified with a succession of compound 
^I’ches, passing transversely across its internal 
cavity ; each arch being disposed in the form of 
^‘ double tunnel, vaulted not only at the top, but 
having a corresponding series of inverted arches 
along the bottom. 
can scarcely imagine a more perfect in- 
^hiiment than this for affording universal resist- 
ance to external pressure, in which the greatest 
possible degree of lightness is combined ivith the 
greatest strength. 
®'milar increase of support; whilst in those species to which the 
’'nore circular form of the sides gives greater strength (as in A. 
^^*018118, PI. 35.) the sinuosities of the septa are proportionately 
■ew. 
h seems probable that some improvement might be made, in 
ortifyiijg the cylindrical air-tube of Massey’s Patent Log for 
^King soundings at great depths, by the introduction of trans- 
''®>’se plates, acting on the principle of the transverse plates of 
® chambered portion of the shells of Nautili and Ammonites, 
lather of Orthoceratites, and Baculites, (see PI. 44, Figs. 4. 
and 5 .) 
