9^ 



rm-"^ REVISION OF THE NORTH AMERICAN SPECIES 



OF DIAPTOMUS. 



C. DWIGHT MARSH. 



(With Plates XY— 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 eases other Copepoda or Cladocera may 

 form the bulk of the aniuia] part of the plankton, but com- 

 monly it is Diaptomus that is most prominent. 



The genus occurs the world o^er, 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 oi 

 none of the species of Diapfonius. 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 

 accumuiated enough material, apparently, to warrant publica- 



