PROSOBRANCHIATA. 101 
of recent mollusks, where the differences are much greater than 
those which paleontologists, 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. 
