NUMBER OF GENERIC FORMS CREATED. 



were not simultaneously extinguished, a certain number dis- 

 appearing for ever at the close of each period. 



558. The specific forms are far more numerous, and not so 

 exactly ascertained. Those of fossil Mollusca and Radiata have 

 been elaborately described and catalogued by M. D'Orbigny, who, 

 as we have already stated, has shown that each successive period 

 had its own assemblage of species. In the following table, 

 compiled from the Prodrome de Pal6ontologie of that eminent 

 naturalist, we have given a synopsis of the distribution of species 

 in the successive ages and periods. 



Table showing the number of species of Mollusca and Radiata ly the 

 fossil remains that have lived during the successive geological epochs 

 and periods. 



559. Since the date of the publication of this table, the number 

 of ascertained species of fossil Mollusca and Radiata has been 

 increased by farther discovery, and now amounts to 21000, to 

 which 3000 fossil species of Vertebrata and Annulata being added, 

 gives a total number of fossil specific forms, amounting to 24000. 



560. The most conspicuous condition which distinguishes the 



155 



