201 



Male conspicuously differing from the female in the shape of the shell, 

 which, seen laterally, is comparatively narrower, with the dorsal margin slightly 

 concave and the ventral more deeply sinuated; seen dorsally, rather swollen 

 behind, but gradually tapered anteriorly, without any trace of the lateral tubero- 

 sities occuring in female. Sculpture of valves very strongly marked in form 

 of a rather regular reticulation. Copulative appendages very large and com- 

 plicate, basal part rounded quadrangular in shape, and sending off at the 

 junction with the terminal part a slender posteriorly pointing process, some- 

 what elbowed in the middle and abruptly hooked at the extremity; copulatory 

 string of quite an extraordinary length, spirally convoluted at the base and 

 ascending upwards to the dorsale face of the basal part, terminating in a very 

 thin point; terminal part obtusely rounded off in front, but irregularly indented 

 behind, the lower corner being produced to a somewhat digitiform narrow 

 process, above which is a curved spine and 2 rounded lobules. 



Colour of shell very dark, almost blach, but with the tips of both extre- 

 mities and an oblique band across the back whitish. 



Length of adult female 0.56 mm. 



Remarks. This form has been recorded as early as the year 1785 by 

 O. Fr. Miiller, and ought thus more properly to be regarded as the type of 

 the present genus. Another supposed species, named by that author Cythere 

 gibbera, has turned out to be the male of the present form. It is an easily 

 recognisable species, being well distinguished from the other known members 

 of the present genus by the shape and sculpture of the shell, as also by its 

 peculiar colour. 



Occurrence. In habits the present species ought more properly to be 

 regarded as a brackish-water form, being found most abundantly in places where 

 the salinity of the water is much reduced. I have myself only met with it in the 

 upper part of the Christiania Fjord, near the town, where a few specimens were 

 taken, many years ago, close to the beach among Zostera. According to 

 Zencker this form is viviparous, a particularity not observed in any of the 

 other known species. 



Distribution. Kattegat, Baltic, Finland, British Isles, Holland. 



Fossil. Norway, Scotland. 



25 Crustacea. 



