FOSSIL INVERTEBRATED ANIMALS. 437 



sei^ved in the chalk, where so many other bodies have disap- 

 peared ; but in some places, as in the Alleghany Mountains 

 and some parts of this country, the stems of encrini have dis- 

 appeared, and have left nothing but their impression. 



It must be admitted that the testa of some shell-animals 

 may, in certain strata, be changed into an irregular crystalli- 

 zation. Were it not so, we might believe that the bodies which 

 have the most exact form of shells, both univalvus and bivalvus^ 

 which are found in the environs of Caen and Bayeux, in an 

 oolitic bed, lower than the chalk, and which are often disen- 

 gaged from their crust, were not genuine shells. It appears 

 that tha testa of those which these moulds represent, after having 

 disappeared, must have been replaced by a crystallization which 

 assumed precisely all its forms. It is quite certain that, on 

 breaking them, instead of any fibrous testaceous substance, we 

 find that those bodies are composed of nothing but crystals. 

 The different species of pleurotomaria, the ammonites, the 

 modiolar cypricardise, the testa of which is very thick, and some 

 other shells of the same strata above mentioned, are in this pre- 

 dicament. 



The belemnites do not seem ever to disappear in this way, 

 and they are met with even in the chalk, and in localities where 

 all soluble shells have disappeared. On breaking them, they 

 are always found composed of a sort of crystallization, con- 

 sisting of needles, or sharp lines, radiating from the centre to 

 the circumference ; but, as they have never been found except 

 in the fossil state, we cannot be assured that they were not 

 organized in this manner, previously to having passed into that 

 state ; and we cannot form the same supposition for them as 

 for the radiated echinodoemes and encrini. What, however, 

 appears very certain is, that, before they passed into the fossil 

 state, they were of a solid and calcareous matter ; since some 

 are found which were pierced and inhabited by {)holades, and 

 to others serpulse are found adherent. 



