IOWA ACADEMY OF SCIENCES. 181 



My thanks are due to Mr. C. P. Sigerfoos for the loan of 

 series of sections of two undetermined species of Sphaerium, 

 with which some comparisons were made. 



EXPLANATION OP PLATES. 



PLATE I. 



Fig. 1, A reconstruction of an adult specimen from serial sections, seen 

 from the left side. Median, and the paired organs of one side 

 shown. The liver and reproductive organs of older specimens 

 are more extensive. 



