544 BULLETIN 159, UNITED STATES NATIONAL MUSEUM 



31. Middle segment of fifth exopod in female with a group of large 



blunt spines on distal margin; first antennae twice as long as 



the body; open ocean plankton ? Heterostylites G. O. Sars, 1920 



Middle segment of fifth exopod in female with 3 tiny spines at 

 outer distal corner; first antennae little longer than body; 

 open-ocean plankton cf 9 Disseta Giesbrecht, 1889 



32. Middle segment of fifth exopod in female with a smooth inner 



spine, end segment with 4 setae; caudal seta on left ramus 



only thickened 33 



Middle segment of fifth exopod in female with inner plumose 

 seta; end segment with 3 setae; 1 seta on each caudal ramus 

 thickened; open-ocean plankton- _ 9 Augaptilus Giesbrecht, 1889 (p. 135) 



33. Caudal rami symmetrical, outer seta at center of outer margin; 



second maxillae ending in 2 equal stout, thickly toothed 



spines; open-ocean plankton c? 9 Mesorhabdus G. O. Sars, 1905 



Caudal rami asymmetrical, left longer than right 34 



34. Outer seta of caudal rami close to apical setae; second maxillae 



ending in long slender setae fringed with cilia; Antarctic 



plankton cf 9 Alloiorhabdus Wolfenden, 1911 



Outer seta of caudal rami removed from apical setae; second 

 maxillae ending in stout, sickle-shaped, toothed claws; Ant- 

 arctic plankton c?" 9 Hemirhabdus Wolfenden, 1911 



35. First antennae asymmetrical in both sexes, left larger than 



right; fifth legs uniramose, 3-segmented in female, 4-seg- 



mented in male; Norwegian fiords d^ 9 Scottula G. O. Sars, 1902 



Females only, first antennae symmetrical, fifth legs not as above 36 



Males only, first antennae asymmetrical, fifth legs not as above 49 



36. Fifth legs biramose and symmetrical, with plumose setae 37 



Fifth legs uniramose or asymmetrical or both, often without 



setae 47 



37. Both rami of fifth legs 3-segmented 38 



Both rami of fifth legs 2-segmented 41 



Exopods of fifth legs 3-segmented, endopods 2-segmented 44 



Exopods of fifth legs 3-segmented, endopods 1-segmented 45 



38. Middle segment of fifth exopod with large inner spine, end 



segment with denticulate terminal spine; exopod of second 

 antennae 6-segmented, short; pelagic, at surface. 



9 Centropages Kr0yer, 1849 (p. 85) 

 Middle segment of fifth exopod with inner awl-shaped or 

 plumose seta, or with a naked inner margin; no denticulate 

 terminal spines 39 



39. Terminal segment of fifth endopod with 5 plumose setae; exopod 



of second antennae 8-segmented; fourth and fifth metasome 

 segments fused and rounded; open-ocean plankton. 



9 Lucicutia Giesbrecht, 1898 (p. 128) 

 Terminal segment of fifth endopod with 6 or more setae; exopod 



of second antennae with 7 segments or less 40 



40. Middle of forehead with knob or spiniform process; endopod of 



second antenna 3-segmented, four times as long as exopod; 

 first basipod without hook; pelagic at surface. 



9 Haloptilus Giesbrecht, 1898 



