444 FOSSIL INVERTEBRATED ANIMALS. 



and form a whole, with the mould of the whorl, or turbination, 

 which serves them as a support ; although, before the petrifac- 

 tion, there existed in this place a double thickness of the testa 

 — namely, that of the interior of the last whorl, and that of the 

 exterior of the preceding one. 



The great and small oysters, which constitute the stratum by 

 which the environs of Paris are covered, have been preserved 

 untouched, with the balance, serpulse, and flustree, with which 

 they are often charged, while shells of another genera, found 

 along with them, have only left their mould ; as may be 

 remarked at Montmartre, Pontenai-aux-Roses, and other 

 localities. 



We have seen that the belemnites never disappear : it is not 

 so, however, with their partitions, which appear to be of a sub- 

 stance different from that of the shell. They are preserved in 

 some strata more ancient than the chalk ; but as yet, we believe, 

 no example has been found of their preservation in this last. 

 When they are preserved, they are found either totally filled 

 with crystallizations, or with a paste which has a tendency to 

 separate between each partition ; or, in fine, some partitions 

 alone are crystallized, and others filled with a fine petrified 

 paste. In no case, however, has the substance with which 

 they are filled any relation with the contexture of the singular 

 shell on which they depend ; and what fills the alveolus does 

 not entirely resemble the substance of the stratum, except when 

 it has been filled after the partitions had been destroyed, either 

 in consequence of their fragility or solubility. 



It is very remarkable that, in the chalky mountain of St. 

 Pierre, near Maestricht, the claws of Crustacea are commonly 

 found not referable to any other genus than that of pagurus, 

 and yet the shells, in which those crustaceous animals must 

 have lodged, are not to be found ; while, on the other hand, 

 in the strata of the Placenza, where there has been no petri- 

 faction, and nothing has disappeared, univalve shells are often 

 observed, covered with a polyparium ; the presence of which, 



