INSTITUTE OF SCIENCE, PHILADELPHIA. 53 



" An apparently anomalous condition of affairs existed, when this fossil 

 bed was being deposited, which promoted at the same time such a diversity 

 of action among several different genera. The following suggestion is 

 offered as a possible explanation for it. 



" No species has been found to be constant or permanent during a long 

 period of geological time ; and there appear to have been periods of rest 

 and periods of activity in the transmutation of species. 



" Surviving from the miocene age, the Fiilgnr contrarms may have been 

 ripe for a change, which was stimulated into action by a cause that would 

 not affect other species, especially such as had not been in existence long. 

 For the same reason the Melongena corona and the Strombus pugilis may 

 be active in their inconstancy now, as they have survived from a former 

 geological period." 



