FOSSIL INVERTEBRATED ANIMALS. 449 



the marine bodies met with there belong to families which 

 resist dissolution in the localities where the others have dis- 

 appeared. 



It is extremely probable that, in the upper chalk, univalve 

 shells did exist, as well as in the preceding strata ; and that 

 they have disappeared, leaving no trace behind, because this 

 substance has not assumed a consistence, or crystallization, 

 capable of preserving the forms of the shells, or other marine 

 bodies which it contained, and which have been dissolved. 

 This we cannot avoid believing, when we find, as before in- 

 stanced, the supports or bases of the hipponyces in the chalk, 

 without uniting the shells which they sustained ; and when we 

 see that the oysters, the lower valves of the cranise, those 

 of dianchora, the spirorbes, and other adherent shells, which 

 are found in the chalk remote from all other bodies, bear the 

 traces of polyparia, and other testaceous marine bodies on 

 which they have adhered ; and yet we do not find any of 

 those said bodies in the same formation. 



In one specimen of the chalky substance of Mount St. 

 Pierre, in the possession of M. de France, and which was 

 sufficiently solid to preserve the external and internal mould 

 of a species of large cerithium, on which some oysters had 

 adhered, the testa of the univalve shell has disappeared, while 

 that of the oysters has remained untouched. 



Similar examples are found in the green sand in this coun- 

 try, which is under the chalk. The oysters are perfectly well- 

 preserved, and the stratum is of such a consistence, that the 

 form of the univalve shells, on which the oysters adhered, is 

 preserved also, but the testa has disappeared. 



In some localities the chalk acquired so much consistence, 

 previously to the dissolution of the shells and other marine 

 bodies which it contained, that their forms are still to be found, 

 and may be seen with the helemnites macronatus, and other 

 bodies which essentially characterize this substance, a prodi- 

 gious quantity of pectunculi, of bacuhtes, of gervillia3, of am- 



2 G 



