384 Wisconsin Academy of Sciences , Arts , and Letters . 
fornia, for collections from Indiana, Colorado and California; 
to Professor Jacob Beighard 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. P. Baker of the Stacion Agronomica, 
Santiago, Cuba, for collections from Nevada, California, Mexi¬ 
co, Cuba and the southern states; to Professor B. H. Brown of 
Whitman College for collections from Washington; and to his 
former pupil, ftussell 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 SPECULATION IN REGARD 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, the affinities of the species and the re¬ 
lationships 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 T 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. 
