FOSSIL INVERTEBRATED ANIMALS. 477 



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to the operations of external agents, is altered by putrefaction, 

 or otherwise destroyed, it is only the remaining part which 

 forms the siliceous nodule found towards the point of the shell. 

 When the animal is totally decomposed, the shell remains 

 empty, or has been filled only by the chalk itself, when the 

 latter was of a pasty consistence. The testae of echini, and the 

 scales of shells, are most frequently found in their natural state, 

 or have been converted only into calcareous spath (as the belem- 

 nites, for instance) ; because these bodies contain too little ani- 

 mal matter, and it is too much screened by the calcareous 

 earth of which they are composed, to allow of the siliceous 

 petrifaction. But they may be easily changed into calcareous 

 spath by a water charged with carbonic acid, which operates 

 insensibly the crystallization of their molecules. 



Without pretending to decide at all between the different 

 hypotheses now laid before the reader, we may, without adopt- 

 ing, be permitted to say, that the least complicated is prefer- 

 able to all others, — that which does not admit, a priori, ima- 

 ginary phenomena of which no clear idea can be formed, but 

 presents marked relations with all that is already well known 

 concerning the laws of crystallization. 



We shall take the liberty of indulging in a few more general 

 observations, connected with the invertebrated fossils, before 

 we proceed to any notice of their principal families, as we are 

 anxious to collect as much interesting matter as we can, and 

 to weary our readers as little as possible by more specific 

 details. 



The fossils, as may have already been seen, which are in- 

 comparably more multiplied than all the rest, are shells, and 

 other marine productions. They form of themselves a con- 

 siderable portion of the calcareous matter of which the most 

 recent strata are composed, which caused Buffon, and other 

 writers, to imagine that all calcareous substances were derived 

 from the debris of marine bodies. But this hypothesis is com- 

 pletely destroyed by observation : for, independently of the 



