COPEPODS OF THE WOODS HOLE EEGION 545 



Forehead smooth; enclopod of second antenna 2-segmented, 

 short; anterior surface of second basipod of first leg with a 

 stout hook curved downward; open-ocean plankton. 



9 Megacalanus Wolfenden, 1904 (p. 26) 



Forehead smooth; endopod of second antenna 3-segmented, not 

 twice as long as exopod, often no longer; no hook on basipod of 

 first legs; open-ooean plankton ? Euaugaptilus Sars, 1920 



41. Urosome 4-segmented; rami of second antennae subequal in 



length . 42 



Urosome 3-segmented; rami of second antennae very unequal in 



length 43 



42. Posterior corners of metasome with short, sharp spines; first 



and second segments of fifth exopod each with 2 or 3 spines 



at outer distal corner; open ocean ? Macrocalanus G. O. Sars, 1905 



Posterior corners of metasome rounded, without spines; first 

 and second segments of fifth exopod each with 1 spine at the 

 outer distal corner; open ocean 9 Pseudaugaptilus G. O. Sars, 1907 



43. Caudal rami usually as wide as long and obliquely truncated; 



masticatory lobe of first maxilla well developed, with stout 



spines; open ocean 9 Euaugaptilus G. O. Sars, 1920 



Caudal rami usually much longer than wide and rounded; 

 masticatory lobe of first maxilla rudimentary, with scattered 

 setae only; open ocean 9 Augaptilus Giesbrecht, 1889 (p. 135) 



44. End segment of second endopod with 4 setae; second basipod 



of first legs with rodlike process; second segment of fifth 

 exopod with 2 outer smooth spines; open ocean. 



9 Isochaeta Giesbrecht, 1889 

 End segment of second endopod with 8 setae; no process on basi- 

 pod of first leg; second segment of fifth exopod with a single 

 outer denticulate spine; pearl-oj'ster washings. 



9 Ridgewayia Thompson and Scott, 1902 



45. Fifth endopod reaching middle of second exopod segment; 



latter with a broad toothed inner spine; distal exopod 



segment with 4 inner setae; littoral, among algae 9 Isias Boeck, 1865 



Fifth endopod not reaching base of second exopod segment; 



latter with a slender and smooth inner spine 46 



46. End segments of second, third, and fourth exopods with 5 inner 



setae; posterior corners of metasome with sharp spines; geni- 

 tal segment longer; pelagic 9 Pachyptilus G. O. Sars, 1920 



End segments of second, third, and fourth exopods with 6 inner 

 setae; posterior corners of metasome without spines; genital 

 segment wider; Antarctic plankton. 9 Pseudhaloptilus Wolfenden, 1911 



47. Fifth legs often asymmetrical, rami 3-segmented, end segment 



with 1 terminal spine, second segment with 2 inner setae; 

 ramiof second antennae about equal; pelagic- 9 Arietellus Giesbrecht, 1892 

 Fifth legs always symmetrical, rami 1-, 2-, or 5-segmented 48 



48. Rami of fifth legs 1- or 2-segmented, terminal segment with 



plumose setae; exopod of second antenna only half as long as 



endopod, or even less; open ocean 9 Paraugaptilus Wolfenden, 1904 



Rami of fifth legs 5-segmented, terminal segment leaflike with 

 serrated apex; endopod of second antenna half as long as 

 exopod or less; open ocean 9 Phyllopus Brady, 1883 (p. 136) 



