926 SUPPLEMENT 
composition is infinitely more slow, and their colours are pre- 
served fora much longer time. The corneous fibres of the liga- 
ments are sometimes preserved for a great number of ages, and 
still more their laminated or fibrous structure, to such a de- 
eree, that they have often lost none of the parts which serve 
as generic, and even as specific characters. Finally, when 
the colours have disappeared, as well as the animal gluten, 
they arrive at a point, when white, and grating to the tongue, 
they may resist the inroads of time for such a number of years 
as it is impossible to calculate. However, in the long run, the 
pressure, determined by the new depositions which cover them, 
tends to break them, by approximating the molecules together. 
The diminution and disappearance of the animal matter, which 
retained the inorganic substance in forms, as it were, acci- 
dental to it, and determined by the living principle, facilitate 
the tendency which these molecules have to approximate, 
according to the simple laws of the inorganic kingdom. The 
shell then tends to disappear altogether, by the successive 
removal of the calcareous molecules which constitute it; but 
as its cavity was filled by the pressure in all directions of the 
terreous or argillaceous molecules which surrounded it, when 
the true testa has disappeared, it is as it were represented and 
prolonged for a lengthened period, by what is named its 
mould, which exhibits all the forms of its cavity. Itis equally 
possible to conceive, what indeed happens, that the calcareous 
_molecules, although having obeyed the laws of crystallization, 
do themselves preserve the form of the shell. The structure, 
it is true, is lost, but not the form. 'This is what constitutes a 
spathified shell, and prolongs, as it would appear, almost for 
an indefinite period, the proof of the existence of the organized 
being, through a series of ages, until at last it is converted, as 
it were, by continual pressure, by the moleculary movement of 
the parts which surround it, into the rock itself, which it con- 
tributes to form. 
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