210 On the Fossil Shells of the Paris Basin. 



secondary, which constitute the chalk formation, are perfect- 

 ly distinguished from the tertiary strata by means of geolo- 

 gical observations, as they are by those of the zoologist. Ob- 

 jections have been made to this result, founded on the exis- 

 tence of beds in which there is an intermixture of species of 

 the chalk with those of tertiary fossils. But we are convinced 

 that this is an error, arising from incomplete observations, 

 and we doubt not, will so appear when these same beds 

 shall have been examined by competent persons without 

 regard to theoretical opinions. Every where in short, not 

 alone in the Pyrenees, geologists agree in the obvious dis- 

 tinctions between the chalk formation and the tertiary beds. 



The Paris basin, placed in a geological series between 

 the chalk and the upper tertiary beds, presents to the re- 

 searches of the learned a deep interest, from the hope of 

 its affording a solution to questions of great importance. It 

 was natural at first, to compare the species which these 

 more ancient deposits afford with those which are now alive. 



But if it be true, as we believe, that the whole of the 

 species of the secondary beds have been destroyed in 

 Europe, at least before the establishment in the same 

 countries of those of the tertiary, we must conceive the 

 chain of succession to have been violently broken, from 

 whatever cause is difficult of explanation. If after a great 

 cataclysm, all the races of marine animals were to be des- 

 troyed, how are we to explain the sudden reappearance of 

 the whole of the zoology of the Paris basin, which we have 

 proved to consist of nearly 1200 species, belonging alone to the 

 class of molluscs ? These have been well examined in a suc- 

 cession of species and individuals, in order to establish their 

 modifications, and define the limits of the species ; but that 

 which we have been unable to comprehend, and which is 

 yet inexplicable to us in the present state of our know- 

 ledge, is the extinctions and remodelments of races of ani- 

 mals which have frequently taken place during long geo- 



