North American Species of Diaptomus. 117 



14 (15). Inner ramus armed with very short subequal spines 



and hairy at apex. Second segment of outer ramus 

 curved, denticulate on inner margin ; third segment 

 represented by two spines, the inner the longer and 

 about half as long as the second segment. Last tho- 

 racic segment produced, armed with two large spines on 

 each side ; penultimate thoracic segment produced into 

 dorsal hump. First abdominal segment as long as 

 remainder of abdomen, armed with large spine on 

 each side ; second segment shorter than the third, 

 which is about equal to the furcal rami. Antennae 

 extending about to base of abdomen. sanguineus. 



15 (11). Inner ramus armed with long conspicuous spines. 



16 (17). Second segment of outer ramus straight, about as 



long as the first, armed with a spine at base of third 

 segment. Inner spine of third segment slightly the 

 longer. Inner ramus hairy at apex ; terminal spines 

 straight, hairy, about a third as long as the ramus. 

 Last thoracic segment armed on each side with a spine. 

 First abdominal segment short, unarmed. Antennas 

 extending to end of thorax. leptopus. 



17 (16). Second segment of outer ramus straight, slightly 



shorter than first ; third segment represented by two 

 subequal spines. Inner ramus barely as long as first 

 segment of outer ramus, hairy at apex, armed with 

 two rather long spines. Antennae extending to end of 

 furca. birgei. 



18 (1). Inner ramus clearly reaching end of first segment 



of outer ramus or beyond. 



19 (22). Outer ramus 3 -segmented. 



20 (21). Third segment of outer ramus small but distinct ; in- 



ner of the two spines slightly the longer. Second seg- 

 ment curved, shorter than first ; denticulate on inner 

 margin ; armed with small spine at base of third seg- 

 ment. Terminal spines of inner ramus very long, 

 almost as long as the ramus. Last thoracic segment 

 armed with two minute spines. First abdominal 



