552 BULLETIN 159, UNITED STATES NATIONAL MUSEUM 



102. Head distinctly separated from first segment 103 



Head completely fused with first segment 105 



103. Right fifth leg 3-segmented, very short; left leg 5-segmented, 



elongated; fifth segment with rounded posterior corners; 



open ocean cf Cornucalanus Wolfenden, 1905 (p. 74) 



Right fifth leg 4- or 5-segmented, left leg 5- or 6-segmented 104 



104. Each fifth leg 5-segmented, right leg reaching middle of third 



segment of left leg; metasome with rounded corners; caudal 



rami very short; tropical marine plankton cT Phaenna Claus, 1863 



Left fifth leg G-segmented, right 4-segmented, hardly reaching 

 second segment of left leg; metasome with acute corners 

 (Mediterranean) cf Xanthocalanus Giesbrecht, 1892 (p. 68) 



105. Fifth legs 5-segmented, not reaching caudal rami, basal portion 



not enlarged; second segment sometimes with rudimentary 



endopod; open ocean cf Chiridius Giesbrecht, 1892 (p. 47) 



Fifth leg not reaching caudal rami, basal portion not enlarged; 

 right fifth leg 5-segmented, left 6-segmented; end segments 

 acuminate; littoral cf Pseudophaenna G. O. Sars, 1902 



Fifth legs reaching beyond tips of caudal rami, their basal por- 

 tion considerably enlarged 106 



106. Enlarged base of fifth legs with pectinated spines; right fifth 



leg 3-segmented, left 4-segmented, the second segment with 



several outer processes; bottom, littoral cf Diaixis G. O. Sars, 1902 



Enlarged base of fifth leg without spines; right fifth leg 5- 

 segmented, left 3-segmented; end segment of right leg 

 cultrate, of left leg lamellate; open ocean. 



c^ Scolecithricella G. O. Sars, 1902 (p. 83) 



107. Second basipod of right fifth leg much swollen proximally 108 



Second basipod of right fifth leg swollen little if at all 109 



108. Exopods of fifth legs 2-segmented, right one bifurcate at apex, 



left one with apical tuft of hairs and a spine; endopods long 



and 1-segmented; open ocean cf Macandrewella A. Scott, 1909 



Exopods of fifth legs 3-segmented; right endopod 2-, left 3- 

 segmented; urosome 4-segmented; exopod of second antenna 

 6-segmented; head without a median crest; open ocean. 



d^ Scaphocalanus G. O. Sars, 1900 (p. 76) 



109. Head with low median crest; exopods of fifth legs 2-segmented, 



apical segment with 4 small spines; endopods 1-segmented, 

 each with one spine; open ocean. 



cf Brachycalanus Farran, 1905 (p. 75) 

 Head without median crest; fifth legs not segmented as above 110 



110. Exopods of fifth legs 3-segmented, endopods 1- to 3-segmented, 



left one longer than left exopod; third segment of latter 



much narrowed; open ocean cT Lophothrix Giesbrecht, 1895 



Right fifth exopod 1- to 3-segmented, endopod 1- or 2-seg- 

 mented; left exopod 2-segmented, endopod 1-segmented, 

 shorter than exopod; latter not narrowed; open ocean. 



cf Scottocalanus G. O. Sars, 1905 (p. 80) 



111. Fifth metasome segment with acute lamellar processes at 



posterior corners; rami of second antennae subequal, the 

 exopod 7-segmented; bottom, littoral- . ? Undinopsis G. O. Sars, 1884 

 Fifth metasome segment with rounded posterior corners 112 



