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aware of the existence of this siphuncle, have yet supposed that this 
animal has been always obliged to remain, with its shell, at the bottom 
of the sea. But if this were the case, it would seem as if the animal 
had been supplied, in the siphunculus, with a useless organ. But it 
is most probable that, as in the Nautilus, the weight of the shell and 
of the animal, was so nearly balanced by the numerous cavities of the 
shell as to allow the animal, which, like the nautilus, filled the first 
chamber, to raise or sink itself at pleasure, by the alteration of the 
gravity of the mass, by occasionally filling the siphuncle with air, or 
perhaps with water. Those who doubted of the Cornu ammonis hav- 
ing possessed this power, have been chiefly misled by a mistake 
respecting the weight of the shell ; and seeing shells of this genus of 
the size of the fore -wheel of a chariot, and weighing upwards of a 
hundred pounds weight, have supposed that they must necessarily 
have always remained, whilst living, at the bottom of the sea. But 
from every specimen which I have examined, it appears, that the 
shells of this genus must have been so thin and light, as to give no 
difficulty to the supposition, that with so many closed cavities, and 
the siphuncle itself, containing air, the shell, with the animal, would 
float, and would only sink upon the admission of water into the 
siphuncle, or upon its close contraction. 
The shells of this genus, like those of Nautilus, had a covering of 
nacre, or mother-of-pearl, on their internal surface. But this nacre 
appears to have differed from the mother-of-pearl of those shells, of 
which recent analogues exist, in manifesting a much greater variety 
and brilliancy of colour in its mineralized state. It is this pearly 
coat of the Cornu ammonis which forms the brilliant flame-like spots, 
which render the marble of Carinthia (fire-marble) , so resplendent; 
and which, with the various beautiful hues with which they are blend- 
ed, enables that substance to vie in beauty with the opal itself. That 
the nacre of the Cornu ammonis differed from that of the Nautilus, 
is, I think, evident, from the fossil nacre of the latter never displaying 
