North American Sjiecies of Diaptomus. 119 



shorter than third and about equal to f ureal rami. 

 Antennae extending beyond base of furca but not be- 

 yond the tip. tyrrelli. 



26 (23). Second segment of outer ramus not armed with 



an additional spine. 



27 (32). Second segment of outer ramus longer than preced- 



ing segment. 



28 (29). Outer of the two spines representing the third seg- 



ment of outer ramus very small and inconspicuous ; 

 second segment shorter than the first, denticulate. 

 Inner ramus reaching slightly beyond end of first seg- 

 ment of outer ramus ; hairy on outer margin and at 

 apex ; armed with two rather long subequal spines. 

 Last thoracic segment produced laterodorsally, armed 

 with two spines on each side. First abdominal seg- 

 ment slightly shorter than remainder of abdomen, 

 dilated, armed on each side with a large spine ; second 

 segment shorter than third ; third segment and furca 

 about equal. Antennae reaching beyond tip of furca. 



clavipes. 



29 (28). Outer spine rather conspicuous. 



30 (31). Terminal spines of inner ramus smooth. Inner 



ramus reaching end of first segment of outer ramus ; 

 apex hairy ; spines small. Second segment of outer 

 ramus about as long as the first, denticulate, point 

 acute ; third segment represented by two spines. First 

 abdominal segment as long as remainder of abdomen, 

 dilated, armed with small spines ; second segment 

 shorter than third ; third about equal to the furca. 

 Antennae extending to tips of furcal setae. 



oregonensis. 



31 (30). Terminal spines of inner ramus hairy, subequal. 



Second segment of outer ramus about equal to the 

 first; third segment represented by two subequal 

 spines. Last thoracic segment armed on each side with 

 two small spines ; the penultimate thoracic segment 

 with a small hump. First abdominal segment slightly 



