250 Illinois State Laboratory of Natural History. 



19 — 21, and 22 and 23 ankylosed ; antepenultimate segment 

 unarmed ; segments preceding geniculation very slightly 

 swollen. Second pair of antenna! about as in Diaptomus. 



All the swimming legs biramose, the outer ramus three- 

 segmented, the inner, one-segmented. Fifth pair of legs of 

 the female alike, uniramose, three-segmented, the first seg- 

 ment of the ramus being, however, really the second basal 

 segment ; armed at the outer apical angle with a hair or deli- 

 cate spine. Third segment armed with a varying number of 

 spines (5 — 7). 



Fifth pair of legs of the male unlike, uniramose, modified 

 into a grasping organ. Eight leg two- (lacustris, nevadensis) 

 or three-segmented (nordenskioldi); last segment almost 

 always flexed. Left leg three-segmented, the first segment 

 produced on the inner margin to form a strong hook-like 

 process ; last segment variously armed on the outer margin 

 with a number of spines, and provided on the inner margin 

 with fine long hairs. 



Female generally bears spermatophore, and does not carry 



eggs m egg- sac. 



Inhabits* deep fresh water lakes. 



As will be seen from the above, the doubtful E. fiuviatilis 

 Herrick has not been considered in this description, but only 

 the three recognized species, lacustris, nevadensis, and norden- 

 skioldi. 



The species of this genus seem not to be fully differentiated 

 from each other. This is illustrated by the variable armature 

 of the fifth pair of legs of the females, nevadensis having 

 sometimes one and sometimes two spines at the outer apical 

 angle of the second segment and either six or seven spines on 

 the last segment, and nordenskioldi also varying in the latter 

 respect, having sometimes five and sometimes six spines. 

 All female specimens of lacustris observed, were constant in 

 the armature of the fifth legs, but in the left fifth leg of the 

 male the second segment, although generally unarmed, was 

 sometimes provided at the outer apical angle with a small 

 spine. 



This genus, confined so far as known to North America, 



