Marsh — North American Species of Diaptomus. 383 



seems unfortunate that so many papers on plankton list the 

 Diaptomi simply under the generic name without an attempt 

 to distinguish species, thus detracting much from the value of 

 the observations, inasmuch as the Diaptomi have very distinct 

 individual peculiarities, and react very differently to their 

 environments. 



With this in view, the author has attempted to provide suffi- 

 cient figures of each species so that its identity cannot be a 

 questionable matter. The original plan involved personal ac- 

 quaintance with each species, and figures from specimens actu- 

 ally in the collections studied. It was found impossible, how- 

 ever, to carry this out in every case ; for example, it was found 

 impossible to get material of D. novamexicanus, as Herrick 

 had preserved none of the original material. The papers de- 

 scribing Pearse's species Wardi and spatulocrenatus were re- 

 ceived while this paper was in press and after the plates had 

 been made, so that it was impossible to provide figures, or to do 

 much more than to give the species their proper place in the 

 classification. So, in other cases, because of imperfect catalog- 

 ing of museum material, it was impossible to get specimens of 

 the animals originally studied. Therefore, in some cases, it has 

 been found necessary to reproduce the original figures of the 

 authors of the species. Most of the figures, however, are from 

 material in the collection of the author. 



As the result of studies on other collections, it is hoped, be- 

 fore many years, to supplement this work, so that we may get 

 more nearly a synopsis of all the ISTorth American species. 

 Meanwhile it is to be expected that intermediate forms will be 

 discovered which may change materially the specific limits as 

 they SLj^-peRT at the present time. 



The author would express his grateful acknowledgment to 

 all who kindly aided in this work by assisting iu making the 

 collections on which it is based. To the following he has been 

 especially indebted: to Professor E. A. Birge of the Univer- 

 sity of Wisconsin, not only for the exchange of material from 

 Wisconsin, but for extensive collections made in the southern 

 states ; to Professor Chauncey Juday of the University of Cali- 



