237 



tracted behind, greatest height about equal to 3 /5 of the length, dorsal margin 

 forming anteriorly a bold and quite even curve, with no trace of angle in the 

 ocular region, and followed by a slight concavity defined behind by a distinct 

 angular corner, ventral margin slightly sinuated in front and evenly bowed 

 behind, anterior extremity somewhat obliquely rounded, posterior produced in 

 the middle to a well-defined blunt protuberance; seen dorsally, broadly fusi- 

 form in outline, with the greatest width about equal to half the length, side- 

 edges quite evenly curved, both extremities abruptly contracted and acuminate, 

 the posterior somewhat more produced than the anterior. Valves thin and 

 pellucid, with the surface smooth and only marked with small distant pits, 

 edges unarmed, lateral expansions but slightly prominent and obtusely rounded. 

 Eyes absent. Antennae of a similar structure to that in B. constricta. Legs 

 very slender and narrow, last pair with the 1st joint of the terminal part about 

 the length of the other 2 combined. 



Male resembling the female in the general shape of the shell. Copulative 

 appendages distinguished from those in the other species by the short cordi- 

 form shape of the terminal part. 



Colour of shell whitish pellucid. 



Length of adult female 0.78 mm. 



Remarks. This species is easily recognised from those described in the 

 preceding pages, by the peculiar shape of the shell, as seen laterally, the 

 gibbously arched dorsal face being particularly very characteristic, and having 

 indeed given rise to the specific name proposed. 



Occurrence. Some few specimens of this form were taken, many years 

 ago, in the upper part of the Oslo Fjord, from a depth of about 50 fathoms, 

 muddy bottom, and I have subsequently also met with it occasionally in some 

 other places on the Norwegian coast. 



Distribution. As yet not recorded out of Norway. 



131. Bythocythere insignis, G. O. Sars. 



(PI. CIX, fig. 1). 

 Bythocythere insignis, G. O. Sars, Chr. Vid. Selsk. Forhandl. 1869, p. 173 



Specific Characters. Female. Shell remarkably expanded laterally, seen 

 from the side, oval subrhomboid in shape, about equally high in front and 

 behind, greatest height about equal to half the length, dorsal margin nearly 

 straigth, though exhibiting in front a slight convexity defined from the anterior 

 edge by a short sinus, ventral margin faintly sinuated anteriorly and rather 



