108 BULLETIN" 15 8, UNITED STATES NATIONAL MUSEUM 



mented in female, 4- or 5-segmented in male; caudal rami sym- 

 metrical and elongated; exopods of first four pairs of legs 3-seg- 

 mented; endopod of first leg 1-segmented, of second, third, and 

 fourth legs 2-segmented ; fifth legs uniramose and 4- or 5-segmented 

 in both sexes. 



Remarks. — This genus is peculiar in that it frequents water show- 

 ing every degree of salinity, from that of the ocean to absolutely 

 fresh water. One or two species are apparently confined to a cer- 

 tain degree of salinity, but by far the larger number seem capable 

 of adapting themselves to salt, brackish, or fresh water indis- 

 criminately. 



KEY TO THE SPECIES 



Upper surface of caudal rami and anal segment covered with 

 short spines or hairs ; terminal segment of fifth legs sub- 

 globular 2 



No hairs or spines on dorsal surface of anal segment ; terminal 

 setae of each fifth leg unequal in length and stout 4 



Fifth legs tipped with 2 exceptionally long, equal, plumose setae ; 

 inner margin of 2 distal segments fringed with long 

 hairs americana (p. 109) 



Fifth leg tipped with 2 nonplumose spines, inner one three 



times as long as outer; no hairs on distal segments 3 



Posterior corners of fifth segment turned backward ; penultimate 

 segment of fifth legs with 2 spines on outer margin hirundoides (p. 110) 



Posterior corners of fifth segment turned outward; penultimate 

 segment of fifth legs with 3 spines on outer margin aflBLnis (p. Ill) 



Fifth segment with pointed processes at posterior corners; 

 penultimate segment of fifth legs six times as long as 

 wide herdmani (p. 112) 



Fifth segment with rounded posterior corners; penultimate seg- 

 ment of fifth legs as wide as long or wider lacustris (p. 114) 



1. Right fifth leg 4-segmented, the 2 distal segments being fused 2 



Right fifth leg 5-segmented, the 2 distal segments distinctly sep- 

 arated 3 



2. Second (basipod) segment of right fifth leg as long as third seg- 



ment (first exopod) ; end segment with 1 outer and 2 inner 



spines hirundoides (p. 110) 



Second (basipod) segment of right fifth leg much shorter than 

 third segment; end segment with no outer and 4 inner 

 spines lacustris (p. 114) 



3. Fourth segment of each fifth leg four or five times as long as 



end segment; end segment of left leg much wider than 



long americana (p. 109) 



End segment of each fifth leg longer than preceding segment ; 



left fifth leg 4-segmented, the 2 distal segments fused 4 



