418 Illinois State Laboratory of Natural History. 



general structure between the various host species. The 

 evidence therefore seems to indicate that the capacity for 

 infestation in each host species is to a large extent a specific 

 characteristic. 



I am indebted to Prof. S. A. Forbes, Director of the Illinois 

 State Laboratory of Natural History, for opportunities of 

 study and of publication; to Dr. C. A. Kofoid, Superin- 

 tendent of the Illinois Biological Station, for many sugges- 

 tions of value; and to Mr. C. A. Hart, of the Laboratory and 

 Station staff, Dr. H. A. Pilsbry, of the Philadelphia Academy 

 of Sciences, and Mr. C. T. Simpson, of the United States 

 National Museum, for assistance in the determination and 

 nomenclature of the Unionidce. 



Just before going to press the nomenclature and systematic 

 arrangement of the host species have been revised by Mr. 

 Hart to bring them into accord with the more natural classi- 

 fication which is now being elaborated for this group. 



Cornell College, Mt. Vernon, Ia. 

 January, 1899. 



