A REVISION OF THE NORTH AMERICAN SPECIES 
OF DIAPTOMUS. 
C. DWIGHT MARSH. 
(With Plates XV-XXVIII) 
INTRODUCTION. 
The genus Diaptomus is of special interest to the student 
of plankton, because, in so many cases, it forms the major por¬ 
tion of that part of the plankton which is available as food for 
fishes. It is true that in plankton-rich bodies of water, the 
largest collections consist, for the most part, of vegetable materi¬ 
al, but a large proportion of the vegetable material is waste so 
far as concerns its serving directly as food for animals. It is 
true, too, that in some cases other Copepoda or Cladocera may 
form the bulk of the animal part of the plankton, but com¬ 
monly it is Diaptomus that is most prominent. 
The genus occurs the world over, and is found not only in 
lakes, but in running streams and in temporary pools. While 
it is found so widely, however, but few of its species are widely 
distributed. Most of the species of Cyclops in America are 
identical with those of the other continents, but this is true of 
none of the species of Diaptomus. All of our species are peculi 
ar to this continent, and some of them have a habitat of rela¬ 
tively narrow limits. It would appear that Diaptomus is quite 
susceptible to the influences of its environment, and this fact 
makes the study of the genus of peculiar interest. 
In connection with lake work the author has been studying 
Diaptomus , as a side issue, for some years, and has at length 
accumulated enough material, apparently, to warrant publica- 
