556 BULLETIN 159, UNITED STATES NATIONAL MUSEUM 



138. A triangular lappet on inner margin of second exopod segment 



of second antenna and on posterior margin of basal segment 



of maxilliped; Antarctic plankton ? Mesogaidius Wolfenden, 1911 



Inner margin of second exopod segment of second antenna and 

 posterior margin of basal segment of maxillipeds smooth, 

 without lappets 139 



139. Rostrum divided to its base, or lacking; fourth and fifth meta- 



some segments completely fused, with spines at their poste- 

 rior corners; open ocean 9 Chiridius Giesbrecht, 1892 (p. 47) 



Rostrum short, undivided; fourth and fifth segments distinctly 

 separated, the latter produced into strong spines at its 

 corners (South Pacific) 9 Gaidiopsis A. Scott, 1909 



140. Fifth legs wholly lacking, females only 141 



Fifth legs present, both sexes included 152 



141. Fifth metasome segment with long, sharply pointed processes 



at its posterior corners, extending backward 142 



Fifth segment rounded or only slightly angular at its corners 146 



Fifth segment produced laterally into short spines with broad 

 bases; end spines of second to fourth exopods with 50-60 

 teeth (North Atlantic) 9 Faroella Wolfenden, 1904 



142. Rostrum strong and bifurcate, with stout rami 143 



Rostrum weak, rami minute or usually lacking 145 



143. Fourth and fifth metasome segments fused, the processes at 



their corners reaching behind the distal margin of the genital 



segment; open ocean 9 Aetideus Brady, 1883 (p. 44) 



Fourth and fifth segments distinctly separated, the processes 



shorter 144 



144. Posterior processes of fifth segment reaching center of genital 



segment; exopods of first and second legs each made up of 



3 segments; open ocean 9 Aetideopsis G. O. Sars, 1903 (p. 46) 



Posterior processes of fifth segment not reaching genital seg- 

 ment; exopods of first and second legs each made up of only 

 2 segments; open ocean 9 Valdiviella Steuer, 1904 



145. Exopods of first legs 3-segmented; exopod of second antenna 



one-half longer than endopod; fifth body segment with short 

 spines at its corners; open ocean. _ 9 Chiridius Giesbrecht, 1895 (p. 47) 

 Exopod of first legs 2-segmented; exopod of second antenna 

 scarcely longer than endopod; fifth body segment with long 

 spines at its corners; open ocean__ 9 Gaidius Giesbrecht, 1895 (p. 52) 



146. Ventral protuberance on genital segment large and promi- 



nent; inner seta of caudal rami hairlike, lengthened, 



geniculate 147 



Protuberance on genital segment small or lacking; no genicu- 

 late setae; metasome and genital segment symmetrical 148 



Protuberance on genital segment small; no geniculate setae; 

 corners of metasome and genital segment asymmetrical; 

 head with crest; Antarctic plankton. 



9 Mesundeuchaeta Wolfenden, 1911 



147. Spines on distal segments of second maxillae with dense fringe 



of short spinules; posterior corners of metasome with long 



hairs; open ocean 9 Paraeuchaeta A. Scott, 1909 (p. 65) 



Spines on distal segments of second maxillae with long scat- 

 tered spinules; posterior corners of metasome usually 

 without hairs; open ocean 9 Euchaeta Philippi, 1843 (p. 62) 



