150 



middle, forming in front, just above the eye, an obtuse angle, and curving 

 behind evenly to the posterior extremity, ventral margin deeply sinuated in the 

 middle, anterior extremity broadly rounded off, posterior a little narrower and 

 likewise evenly rounded; seen dorsally, broadly ovate or somewhat rhomboid 

 in outline, with a slight constriction in the middle bounded anteriorly by an 

 obtuse prominence, greatest width behind and somewhat less than the height, 

 both extremities acutely pointed, the anterior narrower than the posterior. 

 Valves thin and pellucid, with the edges quite smooth, surface faintly reticulated 

 and provided at each extremity with scattered hairs. Anterior antennae with 

 the distal segment of the basal part rather dilated and finely ciliated in front, 

 terminal part almost twice as long, with the last joint very narrow and pro- 

 longed, linear, bearing on the tip 3 setae, the innermost one bifurcate. Posterior 

 antennae with the apical claws rather slender and successively diminishing in 

 length posteriorly. Caudal rami directed downwards and somewhat club-shaped, 

 with the apical bristle scarcely longer than the lateral one. Genital lobes heart- 

 shaped, slightly lobular below. 



Male of rather larger size than female, and having the shell narrower 

 and more elongated, with the ventral margin more deeply sinuated and the 

 posterior extremity more expanded. Copulative appendages very large, with 

 the basal part oval pyriform in shape, produced in front to an obtusely conical 

 prominence, and exhibiting inside several variously formed highly chitinised 

 processes; terminal part issuing from about the middle of the lower side of the 

 basal one, and having the form of a thin plate divided into 3 unequal lobules. 



Colour of shell in both sexes light yellowish brown, antennae and legs 

 bright yellow. 



Length of adult female 0.79 mm, of male 0.82 mm. 



Remarks. This species was first described in the year 1869 by Brady 

 and Robertson, and has subsequently been observed by several other authors. 

 It is easily recognised from the other species of the present genus by its com- 

 paratively large size and by the shape of the shell in the 2 sexes. 



Occurrence. The only place where I have hitherto met with this form, 

 is in our greatest lake, Mjesen, at Hamar. It occurred here on a woody 

 bottom, at a depth of 2 3 fathoms. Males specimens were almost as frequent 

 as femals, and could readily be distinguished from the latter by the rather different 

 shape of the shell. The animals crawl slowly on the bottom in the usual 

 manner of other Cytheridae. 



Distribution. Sweden, British Isles, Bohemia, Hungary, Switzerland. 



Fossil. Sweden, England. 



