4U 



MANUAL OF THE MOLLUSCA. 



III. RANGE OF GEXERA IN TIME. 



I \ 



The genera of the older rocks are believed to be nearly all extinct ; for 

 although the names of many recent forms appear in the catalogue? of Palaeo- 

 zoic fossils, it must be understood that they are only employed in default of 

 more exact information. Buccinum, Jilelauia, and Mi/a, have been lonjj 

 since expunged ; and Modiola, Nucula, and JYatica, are only retained until 

 the characters which diatinguish them are better understood. 



