VI 



PAGE. 



Nymphs 25 



Images 26 



Family Petalurida 27 



Family ALschnidee 27 



Key to Genera 30 



Imagos 30 



Nymphs 32 



Nasiaeschna 33 



Epiaeschna 35 



Boyeria 36 



Basiaeschna 38 



jEschna 39 



Key to nymphs 40 



Anax 46 



Family Gomfihidce 50 



Keys to Genera 52 



Imagos 5 2 



Nymphs 53 



Aphylla 54 



Progomphus 55 



Diastatatomma and Herpetogomphus 57 



Key to nymphs 58 



Hagenius 60 



Lanthus 62 



Dromogomphus 64 



Gomphus 65 



Key to nymphs 66 



Species , 68 



Literature 90 



ARTICLE II.— PLANKTON STUDIES. IV. THE PLANKTON OF 

 THE ILLINOIS RIVER, 1894-1899, WITH INTRODUCTORY 

 NOTES UPON THE HYDROGRAPHY OF THE ILLINOIS 

 RIVER AND ITS BASIN. PART I. QUANTITATIVE INVES- page 

 TIGATIONS AND GENERAL RESULTS. BY C. A. KOFOID.... 95 629 



Introduction 95 



Acknowledgments 96 



The Geological and Hydrographic Features of the Illinois 



River Basin 98-25 1 



Introductory 98 



Location 99 



Geological Features of the Illinois River Basin 99 



Elevation 99 



Glacial and valley Drift 100 



Relief 101 



The Illinois River Sy stein 102 



Length and area 102 



