1,1) 



occurring somewhat in front of the middle, Cephalic segment somewhat longer 

 than the 4 succeeding segments combined. Last trunk-segment very small, with 

 the lateral parts ohtusely rounded. Tail attaining vV, of the length of the anterior 

 division: genital segment about equalling in length the 2 succeeding segments 

 combined, and almost cylindrical in form. Caudal rami of moderate length and 

 somewhat diverging, seta of outer edge placed almost in the middle; apical seta? 

 comparatively shorter than in the preceding species, the inner mediate one 

 scarcely exceeding half the length of the tail ; seta of inner corner fully 3 times 

 us long as that of the outer, and only slightly shorter than the outer mediate 

 one; dorsal bristle unusually slender. Anterior antenna? much elongated, reaching, 

 when reflexed, even somewhat beyond the 3rd segment, and clothed with rather 

 slender setae, lateral ribs of outer joints very slight and quite simple. Posterior 

 antennse with the terminal joint less narrow than in M. obsoletus, and scarcely 

 longer than the penultimate one. Both pairs of maxillipeds largely developed, 

 the anterior ones without any crenulation of the hind edge. Natatory legs with 

 the rami very slender; apical spine of outer ramus in 2nd to 4th pairs much 

 elongated and coarsely denticulated; terminal joint of inner ramus shorter than 

 the other 2 combined; apical spines of this ranius in 4th pair very unequal, the 

 outer one being extremely small, the inner very much elongated and generally 

 somewhat bent inwards ; connecting lamella of same pair with the projections of 

 the hind edge obtuse at the tip. Last pair of legs resembling in shape those in 

 M. ol/.soletus, but having the apical seta? comparatively shorter. Ovisacs, as a 

 rule, small, rounded, and containing only a very limited number of ova. Seminal 

 receptacle resembling that of the preceding species, though having the anterior 

 lobes more obtuse and the posterior part narrower. 



Body highly pellucid, with a very faint yellow tinge. 



Length of adult female scarcely exceeding 0.90 mm. 



Remarks. This form is easily recognisable by its very slender and narrow 

 body and the long spreading setae clothing the anterior antenna?, these characters, 

 as also its habits, somewhat recalling the slender pelagic species of the marine 

 genus Oithona. It was indeed in allusion to this resemblance that the specific name, 

 oithaiioitli'*. was proposed by the present author. The Cyclops hyalinus of Rehberg 

 is unquestionably this species, as is easily seen from the figure he gives of the furca. 



Occurrence. I have found this form not infrequently in several of our larger 

 lakes, more rarely in small tarns. In habits it is a true limnetic form, keeping 

 itself always freely suspended in the water, and generally near the surface. 



Distribution. Sweden (Lilljeborg), Germany (Schrneil), central part of 

 Asia (G-. 0. Sars), North America (Herrick). 



