166 BULLETIN 15 8, UNITED STATES NATIONAL MUSEUM 



segment rounded, but each armed on its dorsal surface with a deli- 

 cate spinule; genital segment as long as the abdomen, protruding 

 ventrally, armed on the sides and at the posterior margin with scat- 

 tered spinules; caudal rami much longer than wide, slightly asym- 

 metrical, the right one the larger; end segment of fifth legs very 

 slender and elongate, definitely longer than the seta on the preceding 

 segment and without hairs or teeth. Total length, 0.9-1.1 mm. 



Male. — Smaller than female; metasome narrower, with similar 

 spinules on the dorsal surface of the posterior corners ; end segment 

 of right fifth leg strongly swollen through the center, not much 

 curved, with a smooth outer margin ; end segment of left leg consid- 

 erably swollen, fringed with long hair on its inner margin, and tipped 

 with two stout spines. Total length, 0.8-1 mm. 



Remarks. — This species may be recognized in the female by the 

 exceptional slenderness of the last segment of the fifth legs and the 

 fact that it is longer than the seta. In the male the dorsal spinules 

 on the corners of the metasome afford a means of identification. In 

 company with clausii this species forms the chief constituent of the 

 plankton of Chesapeake Bay. Fish found it most abundant at 

 Woods Hole from January to May, but it occurs also during summer. 



Family TORTANIDAE 



Genus TORTANUS Giesbrecht, 1898 



Head separated from the first segment; fourth and fifth segments 

 fused, with pointed processes in the female, without them in the 

 male ; caudal rami very unlike in the female, to a lesser degree in the 

 male ; exopods of the first four pairs of legs 3-segmented, endopods 

 2-segmented ; fifth legs uniramose in both sexes, the right leg in the 

 male tipped with a chela; endopod of first leg often 3-segmented in 

 male; urosome 2- or 3-segmented in female, o-segmented in male, 

 conspicuously asymmetrical. 



KETX" TO THE SPECIES 



FEMALES 



1. End segment of fifth leg three or four times as long as penulti- 

 mate segment, curved and acutely pointed, with smooth 



margins discaudatus (p. 167) 



End segment of fifth leg shorter than penultimate segment and 

 blunt, 1 apical spine, 2 at inner corner, 1 at outer cen- 

 ter setacaudatus (p. 1G8) 



MALES 



1. Fifth legs about same length ; end segment of left leg no longer 

 than penultimate segment ; thumb of chela on right leg 



conical discaudatus (p. 167) 



Left fifth leg much longer than right, its end segment twice as 

 long as penultimate segment ; thumb of chela swollen into a 

 sphere setacaudatus (p. 168) 



