144 Illinois State Laboratory of Natural History. 

 Diaptomus pallidus Herrick. (PL XXVII., Fig. 3.) 



Diaptomus pallidus, Herrick, *79, p. 91, PI. II. 

 Diaptomus pallidus, Herrick, '83a, p. 3S3, PL VII., Fig. 1-6. 

 Diaptomus pallidus, Herrick, '84, p. 142, PI. Q, Fig. 17. 

 Diaptomus pallidus, de Guerne et Richard, '89b, p. 62, Fig. 34. 

 Diaptomus pallidus, Marsh, '93, p. 196, PI. III., Fig. 6, 7, 9. 

 Diaptomus pallidus. Herrick and Turner, '95, p. 73, PI. IV, Fig. 1-6; 

 PI. V., Fig. 10; PL XIII., Fig. 17. 



Of medium size, slender; cephalothorax widest near the 

 middle ; bead partially divided by a suture ; suture between 

 bead and thorax distinct. Fifth and sixth thoracic segments 

 confluent ; last thoracic segment produced laterodorsally, 

 bearing a small spine on each side. First abdominal seg- 

 ment unarmed but dilated laterally (not dilated in the male), 

 about as long as the remainder of the abdomen ; second 

 segment the shortest. Furcal rami hairy within. 



Antennae 25-segmented, reaching about to the tips of the 

 furca or slightly beyond. Male prehensile antenna moder- 

 ately swollen ; no special armature on the last three seg- 

 ments ; segments 19 and 20 ankylosed, armed with a process 

 and a long seta; 21, 22, and 23 ankylosed, armed with two 

 long setae; 24, with two long setae; and 25 with four long 

 setae and a sense-club. Some of the antenna! setae are 

 very minutely and sparsely hairy. 



First basal segment of right fifth foot of male (PI. XXVII., 

 Fig. 3) with large tubercle bearing a small spine on the pos- 

 terior aspect ; second basal segment as usual, about equal in 

 length to the first. First segment of the outer ramus sub- 

 quadrate, about as long as wide ; second segment about as 

 wide as the preceding and about one and a half times as 

 long, bearing on the inner margin, at the end of the proxi- 

 mal third, a small sharp-pointed cuticular projection. The 

 outer margin of this segment is almost straight to the begin- 

 ning of the distal third, where a sharp angle is made from 

 which springs the marginal spine. This spine is sharp, 

 slender, slightly curved, a little more than one third the 

 length of the segment. Terminal hook slender, about^ one 

 and a half times as long as the second segment ; not regularly 



