384 Wisconsin Academy of Sciences, Arts, and Letters. 



fornia, for collections from Indiana, Colorado and California; 

 to Professor Jacob Reighard of the University of Michigan, 

 for collections from the Great Lakes and from the inland lakes 

 of Michigan; to Professor II. B. Ward of the University of 

 Nebraska, for collections from Nebraska, Colorado and Cali- 

 fornia; to Professor C. F. Baker of the Stacion Agronomica, 

 Santiago, Cnba, for collections from jSTevada, California, Mexi- 

 co, Cuba and the sonthern states ; to Professor B. H. Brown of 

 Whitman College for collections from Washington; and to his 

 former pnpil, Russell T. Congdon, for collections from the 

 Saskatchewan district. 



Many others have collected for me more or less material, and 

 without this assistance, this revision would have been im- 

 possible. 



A SPECULATIOA' IN KEGAKD TO THE AFFINITIES OF THE NORTH 

 AMERICAN SPECIES OF DIAPTOMUS. 



I speak of this as a speculation, for the present condition of 

 our knowledge in regard to this genus hardly justifies the use 

 of the term theory. Until vastly more complete collections 

 have been made, tlie affinities of the species and the re- 

 lationshii)s of the genus to other genera can only be guessed at. 



Origin of the genus. 



Practically nothing is known of the origin of the genus. Of 

 course, its ancestors were marine, but there are no marine 

 forms at the present time very closely related to it. It is gen- 

 erally considered that its nearest relative is the genus Drepa- 

 nopus, of which three species are known. These are found in 

 the southern oceans in a few localities, the latitude varying 

 from 30 to 47 degrees. 



We have no geological knowledge of the history of Diapto- 

 mus. We may assume that it was separated long before the 

 glacial period, perhaps far back of that time, and that, prior to 

 the invasion of the ice, the species of the eastern and western 

 continents were distinct. 



