130 Cincinnati Society of Natural History. 



segment is more than half the cephalo-thorax, and is broad 

 and thick in front, the greatest width being about the middle of 

 this segment. The antennae are seventeen-jointed, tapering 

 from the broad first joint to the last two, which are four times 

 as long as broad. The fifth foot is two-jointed, each longer than 

 broad. The first bears a ring of fine spines near its base and 

 a row near its distal end. It bears a very small seta, plumose 

 only near the end. The second joint bears two serrate spines 

 laterally and medianly a seta, plumose at the tip, like that of 

 the first joint. 



The abdomen is short and thick. The last three segments 

 are not as long as the preceding. The furcae are short, but 

 nearly as long as the two preceding segments, or twice their 

 own breadth. The setae are closely plumose. The median 

 setae are as three to four. 



The two varieties, coronatus and tenuicornis, I have found 

 together, but coronatus is always in small numbers. In one 

 case tenuicornis was alone. The real differences between them 

 are confined to the seventeenth joint of the antenna and the 

 caudal setae. The fifth feet are alike to the minutest detail, 

 and the difference between their first cephalo-thoracic seg- 

 ments and their furcae is hardly distinguishable. The last 

 antennal joint bears a serrate hyaline plate in coronatus and 

 not in tenuicornis. In coronatus the caudal setae from the inner 

 out are to each other as 2:4:3:1 ; in tenuicornis as 2:4:3:%. 



The armature for the terminal joints of the swimming feet 



for both is as follows : 



Outer ramus. Inner ramus. 



Firstfoot.... ex. 3 spines 1 seta. 



ap. 1 spine, 1 seta. 1 spine, 1 seta. 



in. 3 setae 3 setae. 



Second and ex. 3 spine 1 seta. 



Third feet.. . ap. 1 spine, 1 seta 1 spine, 1 seta. 



in. 4 setae 3 setae. 



Fourth foot., ex. 2 spines 1 seta. 



ap. 1 spine, 1 seta 2 spines. 



in. 4 setae 2 setae. 



Though this species is not marked as appearing many times 

 on the table, yet I found it in a good many places always in 

 small numbers. It was commonest in the deepest pools. 



12 



