Finally, I wish to express my special thanks to Dr. Robert T. 

 Hatt, Director and the Board of Trustees of the Cranbrook Insti- 

 tute of Science for the release of Copyright, thus permitting the 

 publishing of a second edition of this volume. 



G. W. Prescott 

 Michigan State University 



SYMBOLS AND ABBREVIATIONS USED 



CO., cubic centimeter 



/x, micron ( 0.001 mm. ) 



mg., milligram 



mm., millimeter 



pH, measure of free hydrogen ions in a solution. ( Soft water or acid 

 lakes have a pH below 7.0, the neutral point; hard water lakes 

 give readings of pH 7.1 to pH 9.8. ) 



ppm, parts per million 



* — Used in the keys, to indicate orders, families, genera, or species 

 that are likely to be found in the central Great Lakes region but 

 have not been reported there to date. 



Xll 



