•2<S Illinois State Laboratory of Natural History. 



l)een used for the sei)aratioii of species by our most eminent 

 workers. 



I have made a vigorous effort to obtain authentic- examples 

 of all the species considered; and while I have been in 

 general successful, I have been forced in the case of Herriek's 

 species to depend wholly upon the collections to which I have 

 had access myself, and the identifications of other men who 

 have collected in Herriek's localities. 



The collections which I have had an opportunity to study 

 in the preparation of this i)aper are those of the Illinois 

 State Laboratory of Natural History, exteiuling over a period 

 of more than twenty years ; several made under the auspices 

 of the Y . S. Fish Commission in Wisconsin and in the far 

 "West ; collections made in Florida by Mr, Adolph Hempel and 

 in ]\Ianitoba l)y Prof. L. S. Eoss, of Drake University, Iowa, 

 and kindly loaned me by these gentlemen ; and all of the col- 

 lections made at the Illinois Biological Station at Havana, 

 Illinois. This material is from the following states: Massa- 

 chusetts, Florida, Ohio, Indiana, Michigan, Wisconsin, Illi- 

 nois, Kentucky, Iowa, Missouri, Minnesota, Idaho, Wyoming, 

 Washington, and Oregon, and from Manitoba, in the Domin- 

 ion of Canada. 



The localities represented ])y these collections are widely 

 distributed and of great variety. They extend from the 

 New England states in the northeast to Florida in the south- 

 east, to Manitol)a on the north and to Washington and Oregon 

 in the northwest, and from the sea-level in Massachusetts 

 and Florida to some of the highest lakes in the liocky and 

 Sierra Nevada Mountains. In these collections are ( 'iidojis 

 from small temporary ponds of a feAv weeks' duration, from 

 the greatest lakes and rivers in the' world, and from a great 

 number and variety of situations intermediate in character. 

 They have l)een made in every season of the year ; and 

 although the southwestern part of the United States is not 

 represented, they probably contain nearly all (^f the American 

 species of the genus. 



The results published in this ))aper do not, however, repre- 

 sent an exhaustive study of the distrilmtion of the species 



