288 U.S. NATIONAL MUSEUM BULLETIN 236 



Body not compressed dorsoventrally. 1st antennular segment without 

 spine; 2nd segment with spine. Internal furcal border not produced; 

 only one of setae lengthened and thickened, usually reaching considerable 

 length. Exopodite and baso-endopodite of leg 5 separate 2 



2. 3rd exopodal segment of legs 2 to 4 with 2 external marginal spines. 



O. wallini Lang 

 3rd exopodal segment of legs 2 to 4 with 3 external marginal spines ... 3 



3. 1st endopodal segment of legs 3 and 4 with 1 internal seta. 



O. propinquus Monard 



1st endopodal segment of legs 3 and 4 without internal seta (such a seta 



seems to occur on leg 4 in O. rugosus) 4 



4. 1st endopodal segment of leg 1 with internal seta O. sarsi Klie 



1st endopodal segment of leg 1 without internal seta 5 



5. Internal furcal border with pectinate or denticulate lamella 6 



Internal furcal border nude, spinulose or haired, no lamella 8 



6. External furcal border proximally with short crenulate lamella; internal 



furcal border with broad lamella with double fringe of denticules. 1st 



endopodal segment of leg 4 with internal seta . . . O. rugosus NichoUs 



External furcal border smooth, internal border with pectinate lamella with 



teeth arranged in one row 7 



7. Insertion of furca on anal somite, and anal flap bordered with fine teeth. 



Internal furcal lamella with 7-9 heavy teeth. 2nd endopodal segment of 



legs 2 and 3 with 3 setae O. pectinicauda, new species 



Anal somite smooth; no teeth along furcal insertion or on anal plate. In- 

 ternal furcal lamella with many fine teeth. 2nd endopodal segment of 

 legs 2 and 3 with 2 setae O. similis Nicholls 



8. 2nd exopodal segment of leg 2 with 1 internal seta; 2nd and 3rd exopodal 



segments of legs 3 and 4 each with 1 internal seta . O. littoralis Nicholls 



2nd exopodal segment of leg 2 without internal seta; 2nd and 3rd exopodal 



segments of legs 3 and 4 without internal seta 9 



9. Terminal antennular segment externally with short spine, inserting halfway 



along external margin O. linearis (Claus) 



Terminal antennular segment externally with 1 or 2 setae 10 



10. Terminal antennular segment with 1 external marginal seta. 3rd anten- 



nular segment about twice as long as terminal segment. Apex of baso- 

 endopodite reaches apex of exopodite of leg 5; exopodite 3 times as long 



as wide O. major Klie 



External margin of terminal antennular segment with 2 curved, denticulated 

 spiniform setae. Baso-endopodite of leg 5 does not reach apex of exop- 

 odite; exopodite twice as long as broad 11 



11. Anal plate with 6 teeth. Furcal rami internally spinulose, externally with 



short, denticulated proximal lamella. Two seta at external furcal border. 



Small species (1.15 mm.) O. agnathus Klie 



Anal plate crenulated. Furcal rami internally haired, no external lamella, 

 no setae at external border. Big species (1.54 mm.). 



O. linearis f. bulbosus Noodt 



