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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 having 

 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 contain- 

 ing air, the shell, with the animal, would float, and would only sink 

 upon the admission of water into the siphuncle, or upon its close con- 

 traction. 



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 w r hich, with 

 the various beautiful hues with which they are blended, 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 an equal degree, nor 

 indeed a similar kind, of brilliancy with that of the former shell. I have 



