8 ILLINOIS BIOLOGICAL MONOGRAPHS [8 



original drawings which were found useful in presenting the characters 

 and structures of the species have been copied and used in the new 

 account. Possibly it may seem that a consideration of these species 

 might have been omitted without apparently having great effect on the 

 value of the work. The writer found however that serious errors had 

 been made in determination and upon these mistaken determinations 

 identification of other species had been made dependent. The only 

 way of escape from the tangle was to consider every species in the family. 



In order that the work might be of more value to many of the 

 American investigators to whom large numbers of the older works of the 

 European writers are unavailable and for the purpose of comparison 

 large extracts from these older writings have been quoted verbatim. 

 For these reasons also the number of drawings copied is larger than at 

 first glimpse seems necessary. The writer has been compelled however 

 to omit because of lack of space many extracts and many drawings 

 which would add to the value of the work. In working over the large 

 literature on the subject an attempt was made to secure every important 

 paper. Some less important papers are known to have been omitted. 



Thanks are due to the following investigators who at the request 

 of Professor Ward so kindly sent prepared slides or alcoholics from their 

 valuable collections for study and comparison: Professor Fritz 

 Zschokke, University of Basle, Professor Corrado Parona, University of 

 Genoa, Professor Max Braun, University of Konigsberg, Professor K. M. 

 Levander, Helsingfors, Finland, Professor Anton Collin, University of 

 Berlin, Professor A. E. Shipley, Christ's College, Cambridge, Professor 

 Fr. Sav. Monticelli, University of Naples, Professor 0. Fuhrmann, Uni- 

 versity of Neuchatel, Dr. C. W. Stiles, Hygienic Laboratory, Washing- 

 ton, D. C, Dr. B. H. Ransom, Bureau of Animal Industry, Washington, 

 D. C, Professor Edwin Linton, Washington and Jefferson College, 

 Professor A. J. Smith, University of Pennsylvania, Professor L. T. 

 Hankinson, Charleston, 111. Mr. E. G. Davis of Lincoln, Nebr., and 

 Mr. Herman Douthitt of Sulphur, Oklahoma, have also very kindly 

 furnished material for study. Thanks are due Mr. S. Fred Prince for 

 re-drawing many of the figures from other works and for making my 

 pencil sketches into the finished drawings. 



To the United States Bureau of Fisheries and to its corps of scien- 

 tific workers I am indebted for encouragement and assistance in many 

 ways, especially in securing valuable material from various sources. 



To Professor Henry B. Ward my sincerest thanks are due for the 

 use of his extensive collections, for the use of his library, and also for 

 securing for my use many rare specimens and rare books. His interest 

 and co-operation have given me the inspiration to complete the work. 



