592 BULLETIN 159, UNITED STATES NATIONAL MUSEUM • 



79. Metasome spherically swollen and five times as long as urosome; 



third and fourth pairs of legs widely separated, fifth pair 



lacking; in lamellibranchs 9 Obesiella Ridewood, 1903 



Metasome elongate and cylindrical, or flattened and ovate 80 



80. Metasome passing insensibly into urosome; each fifth leg a 



small knob with 3 setae; second antenna 3-segmented, with 



1 stout apical claw; in common mussel cf 9 Mytilicola Steuer, 1903 



Metasome abruptly narrowed into urosome; each fifth leg 



replaced by 1 small seta; second antenna 4-segmented, with 



2 short apical setae; dredged, moderate depths. 



9 Thoostoma Wilson, 1924 



81. Urosome 4-segmented; body fifteen times as long as wide; first 



antenna 5-segmented; second antenna 3-segmented, with 

 stout claw; fifth leg replaced by 1 spine; in a gastropod. 



9 Trochicola Dollfuss, 1914 

 Urosome 2-segmented; body five times as long as wide; first 

 antenna 4-segmented; second antenna 3-segmented, with 

 slender apical spine; fifth leg lacking; in a terebellid. 



d^ 9 Entobius Dogiel, 1908 



82. Rami of first 3 pairs of legs 3-segmented, fourth pair diflferent 83 



Rami of second, third, and fourth legs 3-segmented, first pair 



different 109 



Endopods and exopods differing in segmentation; one or more 



pairs of legs often lacking * 114 



83. Exopod of fourth leg 3-segmented, endopod with less than 3 



segments 84 



Exopod of fourth leg 2-segmented, endopod 3-segmented 102 



Exopod of fourth leg 1-segmented, endopod 1-segmented or lack- 

 ing 104 



Both rami of fourth leg lacking 105 



84. Endopod of fourth leg made up of 2 segments 85 



Endopod of fourth leg made up of 1 segment 94 



Endopod of fourth leg replaced by a seta or spine, or lacking 98 



85. Urosome of female 2-segmented; first antenna 5-segmented; 



second antenna 4-segmented, with 1 apical claw, no exopod; 



fifth leg replaced by 3 small setae; pelagic, at surface. 



9 Vettoria Wilson, 1924 



Urosome of female 4-segmented, of male 5-segmented 86 



Urosome of female 5-segmented, of male 5-segmented 92 



86. Second antenna 4-segmented, with 3 apical setae and a tiny 



1-segmented exopod; metasome and genital segment with 



wide epimeral plates; dredged, 20 fathoms. 



9 Arctopontius G. O. Sars, 1915 



Second antenna 4-segmented, without an exopod 87 



Second antenna 3-segmented, without an exopod 91 



87. Fifth leg 2-segmented, basal segment partly fused with thorax 88 



Fifth leg 1 -segmented , sometimes very rudimentary 89 



88. Second antenna with 2 small apical claws and 3 filiform setae; 



metasome wider than long, depressed; fifth leg with 2 apical 



setae; parasitic on pennatulid 9 Stellicola Kossmann, 1877 



Second antenna with 7 or 8 curved apical setae; metasome much 

 longer than wide; fifth leg with long apical seta and short 

 spine; fresh water d" 9 Cyclops (part) O. F. Mliller, 1776 (p. 322) 



