100 lUinois State Lahoratori/ of Xatnral History. 



but little above the freezing point, or even, as in tlie case of 

 D. minutus, water lioAving from the foot of a glacier. Although 

 this would seem to indicate that these crustaceans are quite 

 hardy, I have repeatedly found that in jars contaming living 

 specimens of Cyclops, Dlaj^tomiis, and Osphr antic urn, those 

 of Diaptonivs were the first to succumb to unfavorable con- 

 ditions. 



The genus Diaptomus is the most cosmopolitan of its 

 family, species having been reported from North and South 

 America, Europe, Asia, Africa, and Australia. No species, 

 however, is known to be common to the mainlands of Europe 

 and of America. This fact is the more remarkable since 

 almost the direct opposite is true of the companion genus, 

 Cyclops, only one or two species of which are, so far as known, 

 peculiar to this continent. Even D. minutus, which is found 

 in Oregon, Illinois, Wisconsin, Minnesota, Michigan, New- 

 foundland, Greenland, and Iceland, has not as yet been found 

 in northern Europe or even in Great Britain, although the 

 expanse of salt Avater between Iceland and Scotland or be- 

 tween Iceland and Scandiilavia is but little greater than that 

 between Greenland and the mainland of North America. But 

 few species of this genus have been described from tropical 

 regions, most of them having thus far been found in the north 

 temperate zone ; a fact to be attributed doubtless in large 

 measure to the greater attention paid to zoological studies in 

 these northern latitudes. 



In this paper the plan followed by de Guerne and Eichard 

 in their " Revision " has been adopted, separate keys being 

 made for males and for females. When females are so nearly 

 alike as in I), sicilis, siciloides, pallidus, and ashlancli, it is 

 som.ewhat difficult to find distinguishing characters, and 

 differences not usually taken into account must be seized 

 upon. The males are much more easily separated, since they 

 offer a larger number of peculiarities. Giesbrecht and 

 Schmeil have paid considerable attention to the armature of 

 the entire male prehensile antenna instead of regarding only 

 that of the last three segments, and in one or two cases I 

 have done the same. In this connection a fact became 



