124 



MANUAL OF THE MOLLTJSCA. 



III. EANGE OF GENEEA IN TIME. 



The genera of the older rocTcs are believed to be nearly all 

 extinct ; for altbougb the names of many recent forms appear 

 in the catalogues of Palaeozoic fossils, it must be understood 

 that they are only employed in default of more exact infor- 

 mation. Buccinum, Melania, and Mya have been long since 

 expunged ; and Modiola, Nucula, and Natica, are only retained 

 until the characters which distinguish them are better under- 

 stood. 



