51 



urring about in the middle. Lateral parts of the penultimate trunk-segment of 

 e same shape as those of the 2 preceding ones. Last trunk-segment only slightly 

 reduced laterally. Tail of moderate length, with the genital segment rather 

 large, though evenly contracted behind. Caudal rami almost attaining the length 

 of the last 3 segments combined, and very slightly divergent; seta of outer edge 

 somewhat remote from the apex ; middle apical setee slender, the inner one some- 

 what exceeding the tail in length; seta of inner corner extremely small and 

 rudimentary, that of outer corner normally developed. Anterior antennae about 

 equal in length to the cephalic segment, and composed of only 16 joints, the 

 3rd and 4th joints being confluent. Posterior antennae with the terminal joint 

 longer than the penultimate one. Maxillipeds rather short and stout, though 

 otherwise exhibiting the usual structure. Natatory legs to some extent imperfectly 

 developed, both rami of 1st pair being only biarticulate, and the inner ramus of 2nd 

 pair likewise biarticulate, outer ramus of this pair, on the other hand, as also 

 both rami of the 2 succeeding pairs, distinctly 3-articulate; terminal joint of 

 outer ramus in these pairs provided outside with 2 spines, inside with 3 setae; 

 apical spines of inner ramus in 4th pair comparatively short, the inner one being 

 the longer. Last pair of legs distinctly biarticulate, resembling in structure those 

 in C. pulchellus, the distal joint being rather slender, sublinear in form, with 

 the lateral spine attached close to the end. Ovisacs generally of very large size, 

 reaching in some cases far beyond the caudal rami, and somewhat divergent; 

 Seminal receptacle with the anterior part transversely elliptical in form, posterior 

 part very little produced. 



Colour uniformly whitish grey. 



Length of adult female about 1.00 mm. 



Remarks. This form was described by the present author as early as 

 the year 1863, and has subsequently been also observed by some other authors. 

 It is chiefly distinguished from the species described in the preceding pages by 

 the imperfect development of the 2 anterior pairs of natatory legs, agreeing in 

 this respect with the 2 succeeding species. From the latter it may be readily 

 recognised by its comparatively larger size, by the greater number of joints in 

 the anterior antennae, and by the normally developed last pair of legs. 



Occurrence. 1 have met with this form occasionally in shallow pools and 

 ditches near Christiania, especially in the spring. The movements of the animal, 

 when alive, are very slow and are not, as usual, effected by abrupt jumps, but 

 more resemble an even course through the water, during which the body turns now 



