PROSOBRANCHIATA. 101 



of recent mollusks, where the differences are mucli greater than 

 those which palnsontologists, seeking distinctive characters for 

 their periods or formations, have been accustomed to consider 

 as of specific and even generic value. 



Bronn has also prepared a table of the number of species of 

 each genus of prosobranchiates appearing in the various forma- 

 tions, with the totals of species, fossil and recent, appertaining 

 to each. As in his table just quoted, much allowance must be 

 made for increase of species made known since his publication. 

 The genera are within the Lamarckian limits. 



