448 FOSSIL INVERTEBRATED ANIMALS. 



all the shells and other marine bodies of which these 

 marbles are composed ? Simple desiccation could never have 

 produced an effect like this, or divided into such small parts 

 shells or polyparia, as we witness in the specimens alluded to. 

 These shells themselves are even sometimes cleft, and filled 

 with spath, while the paste or stratum surrounding them is 

 not so. — Facts like these have perhaps not been sufficiently 

 studied, and assuredly they merit every degree of attention. 



It appears that it is more rare to find in the strata anterior 

 to the chalk localities in which marine bodies, which have dis- 

 appeared, have left their place empty, than in the strata poste- 

 rior to that substance. 



M. de France, without disputing the reasons which have 

 determined geologists to give to the strata posterior to the pri- 

 mitive rocks, the names of intermediate or transition, secondary 

 and tertiary, thinks that he is justified in making three dif- 

 ferent divisions of those in which organic fossil bodies are 

 found ;^-namely, the strata anterior to the chalk, those of the 

 chalk itself, and those posterior to the formation of that sub- 

 stance. 



According to this arrangement of M. de France, we find 

 that the strata anterior to the chalk contain forty-seven genera 

 of polyparia, seven of echinodse, five genera of Crustacea, one 

 genus of annelides, three of scapulae, one of cephalopodes 

 monothalami, one of cirrhipedes, forty-four genera of bivalve 

 shells, one of phyllidise, fifteen of univalve shells, ten genera 

 of partitioned shells, three genera of marine bodies little 

 known, three genera of reptiles, eleven genera of fish, and 

 twelve of vegetables. 



In the strata anterior to the chalk, univalve and bivalve 

 shells are found in a proportion, the difference of which is not 

 very remarkable. In the lower strata of the chalk, univalve 

 shells are still to be found ; but this is not the case in the upper 

 chalk. There we scarcely ever find any univalve unilocular 

 shells, such as cerites, volutse, and other soluble shells ; a^d 



