_329_ 



all the segments fringed at the posterior edge with delicate spinules; last segment 

 about the length of the preceding one, and having the anal opercle finely spinulose 

 at the edge. Caudal rami about as long as they are broad, and transversely 

 truncated at the end, outer corner armed with a slender spine, middle apical seta 

 exceeding half the length of the body. Eye large and very conspicuous in the 

 living animal. Anterior antennae much less than half the length of the cephalic 

 segment, and distinctly 7-articulate, tapering gradually towards the end, and 

 densely clothed with setse, some of which are coarsely ciliated, 1st joint much the 

 largest, terminal part exceeding half the length of the proximal one. Posterior 

 antennse with the outer ramus much shorter than the distal joint, and carrying 

 3 setse, one lateral and 2 apical. Natatory legs with the basal part very broad 

 and flattened; inner ramus of 1st pair slightly longer than the outer, that of the 

 3 succeeding pairs a little shorter, middle joint of this ramus rather large and 

 expanded, in the 2nd and 3rd pairs carrying 2 setae inside. Last pair of legs 

 forming each a broad, rounded, quadrangular lamella edged with 9 comparatively 

 short setae. Ovisac large, oval in form, and projecting far beyond the caudal rami. 



Male somewhat smaller than female, and having the urosome narrower 

 and more elongated. Anterior antennas very powerful, subcheliform, 4th joint of 

 considerable size and globularly inflated, terminal part short, unguiform. 2nd 

 and 3rd pairs of legs comparatively more strongly built than in female, inner 

 ramus of 2nd pair with a conical deflexed process issuing from the end of the 

 middle joint inside, outer ramus of 3rd pair of very coarse structure, with the 

 setse of the inner edge much reduced in size. Last pair of legs smaller than in 

 female, with only 7 marginal setse. 



Body of whitish colour, with a slight yellow or orange tinge. 



Length of adult female 0.60 mm. 



Remarlcs. I do not find it necessary to reject the specific name Irevi- 

 corn'is under which the present form was first described. It may be that Lillje- 

 borg's identification of this species with Cyclops Irevicornis of 0. Fr. Mtiller 

 is untenable, but any difficulty in this respect will be avoided by simply annexing 

 to the species the author-name of Lilljeborg instead of that of Miiller. 



Occurrence. I have met with this form very abundantly in the neigh- 

 bourhood of Christiania in shallow creeks of the Fjord, sometimes in brackish water. 

 It also occurs under similar circumstances in many other places both on the south 

 and west coasts of Norway, and Th. Scott also records it from East Finmark. 

 It is a very active little animal, being almost constantly in motion, and running 

 about with considerable speed. Males and young females are often found tied 



44 Crustacea. 



