FOSSIL INVERTEBRATED ANIMALS. 439 



substance which is no longer found in the mould of the testse 

 of mollusca, they must have carried it into places at a greater 

 depth, or, perhaps, they may have formed new crystallizations*. 



We are informed that, in the environs of Amberg, a very 

 considerable quantity of the alveoli of belemnites was found ; 

 while the exterior envelope of those fossils was extremely rare, 

 and scarcely ever was found in an entire state. 



M. de Ranee, from whom the substance of our observations 

 on this subject is borrowed, not having seen those alveoli, 

 does not venture to pronounce on their nature and origin. He 

 thinks, however, that the alveoli have been preserved, only 

 because they were seized by a petrifaction, which filled the 

 cavity of the belemnites at the same time that it enveloped their 

 bodies, and formed, in all probability, the stratum in which 

 they are found. If they alone exist at present^ it must be 

 because the shells, which contained these alveoli, have been 

 dissolved subsequently to the petrifaction. Still, even supposing 

 this to be the case, we ought to find the mould of their external 

 forms. 



In certain localities, as at Montmartre, are found, in the 

 marly strata, models or moulds, in marble, of marine shells and 

 of Crustacea, without any appearance of an external mould 

 having existed of a different nature from the model. In breaking 

 the marl, these models become detached from the rest of the 

 mass, and represent exactly the external forms of the shells 

 and Crustacea. They are covered with a yellowish sort of 



* It may be as well to explain here the sense in which the word mould 

 is used by naturalists. The external mould is the vacancy left by the 

 testaceous covering of the fossil body which has disappeared in the 

 localities where petrifaction has taken place. The internal mould is the 

 name given to the paste or substance of the stratum which is moulded 

 and petrified in univalve or bivalve shells. The word model might, per- 

 haps, be used with more propriety to designate whatever has filled these 

 shells before their dissolution ; though, strictly speaking, it is a model 

 not of the animal, but of the place which it occupied. 



