North American Species of Diaptomus. 103 



6(5). Abdomen of female 3- or 4-segmented ; sometimes 

 unsymmetrical. Antennae never with less than 24 seg- 

 ments. In the male, segments 19-21 and generally 22 

 and 23 are confluent. Abdomen 5-jointed ; either right 

 or left antenna prehensile. The fifth pair of feet are 

 grasping organs and both always present, but with 

 inner ramus normal, or degenerate to complete dis- 

 appearance. Family Centropagims. 



7 (8). Thorax 6-jointed. All the feet of female with 3-seg- 



mented rami. Abdomen 3-jointed. Antenme 25- 

 jointed, segments 24 and 25 confluent. Eight male 

 antenna prehensile. Outer ramus of left fifth foot 

 2-jointed ; of right, subchelate. 



Subfamily Centeopagina. 



8 (7). Thorax 5-jointed. Fourth and fifth thoracic seg- 



ments confluent. 



9 (16,17.) Abdomen of female 3-jointed, sometimes unsym- 



metrical. Antennae 23- or 24-jointed. Four anterior 

 pairs of feet generally with 3 -segmented rami. Fifth 

 pair of feet degenerate, with inner ramus wanting or 

 small and 1-jointed, outer ramus 1-3 jointed. Pre- 

 hensile antenna generally the right; segments 19-21 

 and 22 and 23 confluent. Subfamily Temorina. 



10 (11). Furca with but three large terminal setae to each 



ramus. Abdomen of male unsymmetrical, provided 

 with lateral prehensile apparatus. Fifth pair of legs 

 of female uniramose, 3-jointed, not terminating with 

 a long spine. Genus Epischura. 



11 (10). Furca with four large terminal setae to each ramus. 



12 (13). Inner ramus of first pair of legs 2-jointed; of the 



following three pairs 3-jointed. Fifth pair of legs in 

 both male and female biramose, inner ramus rudi- 

 mentary. Genus Diaptomus. 



13 (12). Both inner and outer rami of the first four pairs of 



legs 3-jointed. Fifth pair of legs in both sexes biramose, 

 those of the female differing from the other legs only 

 by the presence of a strong inner hook on the second 



