from the Arctic and Scandinavian Seas. 30 
pits, and having a sharply angular ridge or crest just within 
and parallel to the ventral margin. Colour white. Length 
z's inch. 
Two specimens only in the gathering from Davis’s Straits. 
In shape these agree very closely with Sars’s C. atra; but the 
sculpture and colour of the shell would seem to be different. 
The description “ valvule distincte et sat regulariter reticulate, 
areola mediana obsoleta. ‘Testa tota colore saturatissime atro 
insignis ’’ does not apply here. The sculpturing of C. rudis, 
is too decided to be called mere reticulation ; and there is no 
trace of coloration of any kind in our specimens. 
EXPLANATION OF THE PLATES. 
PuaTE LY. 
Fig. 1. Cythere borealis (female), from left side.) 
Fig. 2. The same, from above. | 
Fig. 3. The same (male), from left side. Lt 
Fig. 4. The same (male), from below. eaten’ 
Fig. 6. The same, outline of left valve (female). | 
Fig. 7. The same, outline of right valve (female). ; 
Fig. 5. Cythere Robertsoni (female), seen from left side. 
Fig. 8, The same, seen from above. 84 
Fig. 9. The same, seen from below. cigs 
Fig. 10. The same, seen from front. 
Fig. 11. Pontocypris attenuata, seen from left side. 
Fig. 12. The same, seen from above. 
Fig. 13. The same, seen from below. 
Fig. 14. The same, end view. 
[The description of Pontocypris attenuata (a southern species) 
will be given in a subsequent paper. | 
PLATE V. 
Fig. 1. Cytheropteron montrosiense (adult female), seen from 
left side. 
Fig. 2. The same, seen from above. 
Fig. 3. The same, end view. 
Fig. 4, The same (young ?), seen from left side. x40. 
Fig. 5. The same, seen from below. 
Fig. 6. Cytheropteron vespertilio, right valve, seen from outside. 
Fig. 7. The same, seen from above. ; 
‘ig. 8. Cytheropteron inflatum, right valve, seen from outside. 
Fug. 
Fig. 11. Cytheropteron pyramidale, seen from left side. 
Fig. 12. The same, seen from above. 
Fig. 13. The same, seen from below. 
Fig. 14. The same, seen from behind. 
Fig. 15. Cytherura rudis, seen from left side. 
Fig. 16. The same, seen from above. 
Fig. 17. The same, seen from the front. j 
Fig. 18. Cythere pulchella (female), seen from left Aids) 
Fig. 19. The same, seen from above. x 40. 
Fig. 20, The same, seen from the front. 3 
2 
3 
4 
5 
6 
if 
8 
§. The same, seen from ahove. 
Fig. 10. The same, end view. 
11 
12 
13 
14 
15 | 
) 
