382 Wisconsin Academy of Sciences, Arts , and Letters. 
tion. The collections studied have covered a wide range of 
country, from the Saskatchewan district on the north to Mexico 
and Cuba on the south, and from Long Island on the east ta 
Washington and California on the west. It must not be as¬ 
sumed, however, that the collections give any complete idea of 
this wide extent of territory. Most of the collections are of a 
sporadic character. Outside the states of Wisconsin, Michi¬ 
gan, Minnesota and Illinois, nothing like any complete explora¬ 
tion has been made. Within the limits of these states, fairly 
complete work has been done. In the rest of the country, much 
less is known of the older states of the East than of the new 
states and territories of the West. 
A study of this material shows a number of new species, and 
throws new light on the relationships of those already known. 
It has been the ambition of the author to monograph the 
genus, so far as ETorth American species are concerned, but this 
will not be possible for some years, for very extensive collec¬ 
tions must be made before such a work can be possible. Mean¬ 
time the important part played by the genus in plankton makes 
it desirable that what is already known should be put in such 
shape that the student of plankton may be able to identify his 
species. This is very difficult to do at the present time. The 
only papers, ostensibly covering all the Horth American spe¬ 
cies, are those of Herrick and Turner, and Schacht, and 
neither of these, for various reasons, can be easily used for the 
determination of species. It is very difficult, even for one 
who is acquainted with the genus, to recognize species without 
figures, and for the average student of fresh water forms, wlm 
presumably has no special training in the study of Entomostra- 
ca, it becomes a discouraging task even to guess at the species. 
With suitable figures, however, any one, who has the patience 
to make the necessary dissections, ought to be able to determine 
correctly the species he has in hand. The present revision is 
undertaken with the hope that it may not only be a contribu¬ 
tion to our knowledge of the genus, but that it may prove a 
distinct help to those who are studying the problems of lim¬ 
nology, but have no special knowledge of Entomostraca. It 
