North American Species of Diaptomus. 139 



densely plumose. There is but little difference in the length 

 of the abdominal segments of the male. 



Antennae 25-segmented, reaching to the middle of the 

 abdomen. Prehensile antenna of the male (PI. XXVIII., 

 Fig. 2) thickly swollen anterior to the twelfth article, with 

 armature as follows: segments 1 and 5, long spine and 

 sense-club; 2, three long spines and sense-club; 3, short 

 seta; 4 and 6, long spine; 7, short seta and sense-club; 8 

 and 12, long spine and short spine; 9, long spine, short 

 seta, and sense-club; 10, 11, 13, and 17, process and 

 long spine; 14 and 16, long spine, short seta, and sense- 

 club ; 15, process, two long spines, and sense-club ; 18, proc- 

 ess ; 19, 20, and 21 (ankylosed, with the sutures indistinctly 

 indicated), two processes, a stunted spine, and a long seta; 

 22 and 23 (ankylosed), a broad hook-like process not reach- 

 ing the end of the penultimate segment, and four setae; 24, 

 two setae; and 25, four setae, a sense-hair, and a sense-club. 



Second basal segment of the right fifth foot of the male 

 subquadrate, about twice as long as wide ; on the posterior 

 surface a large smooth hyaline lamina occupying about a 

 third of the inner margin near the middle, and near the outer 

 distal angle a minute cuticular process bearing a delicate 

 hair. First segment of the outer ramus almost three times 

 as long as broad ; second segment about as long as the first 

 and for about the proximal third nearly as wide, but beyond 

 this considerably broader. Marginal spine near the outer 

 distal angle ; straight, very strong and thick, little less than 

 half as long as the segment. Terminal hook rather short 

 and very stout, irregularly curved, heavily and closely den- 

 ticulate at the distal half of the inner margin. 



Inner ramus of the right fifth leg spatulate, not nearly 

 reaching the middle of the first segment of the outer ramus ; 

 apex rounded, armed with a few strong spines. 



Second basal segment of the left fifth foot armed at the 

 outer margin, a short distance above the distal angle, with a 

 short, thick, pointed spine. First segment of the outer ramus 

 about three times as long as wide, armed at the distal third 

 of the inner margin with a few strong hairs. Second segment 



