North American Species of Diaptomus. 161 



seen from above, short and obtuse posteriorly ; armed \vitli 

 small spines. First abdominal segment (PL XXX., Fig. 1) 

 about equal to the rest of the abdomen, moderately dilated 

 anteriorly, rounded at the sides, and armed here with small 

 spines or mucros ; always destitute of all lateral processes. 

 An imperfect suture remains long after maturity in the pos- 

 terior part of this segment. Second segment of the abdomen 

 much shorter than the third, and also more slender and 

 easily pushed within the preceding joint.* Furcal rami short, 

 fully one and a half times longer than broad ; sparsely ciliate 

 within. 



"First pair of antenna of female, reflexed, extend about 

 to the furca ; composed of 25 articles. Antepenultimate 

 article of the male prehensile antenna (PI. XXX., Fig. 2) 

 provided with an unguiform process, slightly surpassing the 

 end of the penultimate article. 



"Outer ramus of the fifth pair of feet in the female (PI. 

 XXX.,' Fig. 4) three-segmented, the third segment very small 

 but distinct, and bearing two spines. The unguiform process 

 of the second segment of this ramus rather arcuate and finely 

 ciliate within at the lower part, the last spine thicker than 

 the rest. Inner ramus simple and equal to the first segment 

 of the outer ramus, bearing two long equal spines at the 

 apex, of which the outer is ciliate within at the base. 



"The right fifth foot of the male (PL XXX., Fig. 3) rather 

 robust. The second segment of the outer ramus almost rect- 

 angular and comparatively short ; the outer marginal spine 

 placed near the apex, and the inner margin armed with a mi- 

 nute spine. Terminal hook long, distinctly sigmoid or S- 

 shaped and tapering toward the apex. Inner ramus small 

 and barely reaching the middle of the second article of the 

 outer ramus ; either imperfectly two-segmented or one- 

 segmented and armed with an apical spine. 



"The second segment of the outer ramus of the left fifth 

 foot of the male lamelliform, almost triangular, and thinner 

 within. This segment on the thicker, outer, side bears a short 



* The meaning of the original at this point is rather obscure. It reads: ^'Seg- 

 mentum 2-dum caudoe 'S4io muUo brevius ejusque testa tenuior et facile adsirni- 

 genda.'' 



