ADYERTISEMEXT. 



This memoir gives the results of an investif^ation relative to 

 the structure of a group of small and little-known Gasteropods, 

 which Dr. Stimpsoi' has undertaken with a view to their classifi- 

 cation, and arrangement in the museum of the Institution. It 

 is thought that these results may also be useful in the arrange» 

 ment of other collections of this group of Mollusks. 



JOSEPH HEXRY, 



Secretary S. I. 



S.MITnSOXIAX lySTITtTTION, 



WASHI^XTTON, August 30, 18G4. 



PHILADEI.PniA: 

 COLLINS, PRINTER. 



