130 Illinois State Laboratory of Natv/ral History. 

 Diaptomus leptopus Fokbes. 



Cyclops lonyicornis (?), Derrick, 77, p. 238, Fig. 1. 



Diaptomus kentuckyensis (?), Chambers, '81, p. 48, PI. A, Fig. 12-1S; 



PI. B, Fig. 19-23. 

 Diaptomus leptopus, Forbes, '82a, p. G46, PI. VIII.. Fig. 17-1 U. 

 Diaptomus castor (?), Herrick, '82, p. 221, Pi. I., Fig. 1-7; PI. II., Fig! 



12, 1G. 

 Diaptomus longicornis var. leptopus. Herrick. '84, p. 140. 

 Diaptounis leptopus. tie Guerne et Richard, '89b, p. 21, PI. II.. Fig. 19; 



PL III., Fig. 9. 

 Diaptomus leptopus, Marsh, '93. p. 195, PI. III., Fig. 4. 5. 

 Diaptomus leptopus, Herrick and Turner, '95, p. 64, PI. II.; PI. IX., 



Fig. 9. 



Body long and slender, widest a little before the middle. 

 Head rather noticeably narrower than thorax, suture between 

 them distinct. Fifth and sixth thoracic segments confluent, 

 the last produced dorsally on each side into a triangular 

 process with a bluntly rounded apex armed with a single blunt 

 spine. The last thoracic segment of the male and the first 

 abdominal segment of both sexes unarmed. First abdominal 

 segment short, a little more than half as long as the succeed- 

 ing segment. Furcal rami about one and a half times as 

 long as wide, hairy within. 



Antennre 25-jointed, extending to the tip of the furcal rami. 

 The male prehensile antenna rather thickly swollen, the first 

 segment without armature, the other segments armed as fol- 

 lows : 2, with a short seta and a sense-club ; 3, short seta and 

 sense-club ; 4 and G, long spine ; 5 and 7, long seta and sense- 

 club ; 8, long spine and very short spine; 9, long seta, long 

 spine, and sense-club ; 10 and 11, process and long spine; 

 12, long spine, very short spine, and sense-club ; 13, process, 

 long spine, and sense-club; 14, long seta, long spine, and 

 sense-club; 15, process, short seta, long spine, and sense- 

 club; 16, process, long spine, long seta, and sense-club ; 17, 

 process and short thick spine; 18, process; 19,. 20, and 21 

 (completely ankylosed), a process, a long seta, and a very 

 short spine; 22 and 23 (completely ankylosed), a narrow 

 hyaline lamina produced into a hook which extends but little 

 beyond the end of the segment, and two long setae ; 24, two 



