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 to 

 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- 

 gaUj 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 ISTorth 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 p[ipers, ostensibly covering all the North 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, who 

 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 



