f 



158 



P. T. CLEVE, SYNOPSIS OF THE NAVICULOID DIATOMS. 



The speeies of Cymbella are fi'equently very variable and transitorial. ]\Iany names have 

 been proposed for forms, whieh differ only in trifling characteristics. The descriptioiis and figures 

 in the literature leave very much to desire; and in many cases, where original specimens are not 

 accessible, it is impossible to decide what the names given by the authors actually denote. Under 

 such circumstances it is extremely difficult to treat the numerous forms of this genus. 



Most Cymbellse live in fresh water, a few only in slightly brackish water, and none 

 is marine. They occiir in all parts of the world, from Spitsbergen and arctic America to the 

 Tropics. Many species live, attached to gelatinous stalks, in enormous masses, in rivulets, on moist 

 rocks, and water-plants. 



In the following artificial key some iinperfectly known forms, marked * in the text are not 

 inclnded. The key must not be regarded as absolutely exact, its definitions being somewhat elastic, 

 as indeed the forms themselves are. 



1. 



2. 



3. 



4. 



5. 



6. 



7. 



8. 



9. 

 10. 

 11. 

 12. 

 13. 

 14. 

 15. 

 16. 

 17. 



18. 



10. 

 20. 



Artificial hey. 



( Val ve cymbiform 2. 



\ — uaviculoid 33. 



j Central nodule witli a stigma 3. 



I — — withont stigma 7. 



j Stigma below the nodule 4. 



1 — iu — 5. 



I Striffi coarsely punctate C. awstralica A. S. 



I — tinely — i. . C. tumida Bréb. 



I Ends broad, rounded C. Janisckii A. S. 



I — subtruncate, obtnse 6. 



f Central area small C. mexicana Ehb. 



I — — large C puiictifera Cl. 



Median ventral strise ending with isolated puncta 8. 



— — — not — — — 12. 



Punctum one C. cymhiformis Kt;TZ. 



Puncta two or more 9. 



Puncta two C. turgidvla Geun. (C. Cistula var.) 



— several . . . .''.'"; 10. 



Median line dilated towards the ends C. Stnrii Grun. 



— — not — — 11. 



Stris coarse C. Cistula Hempe. 



— fine C. Stuxhergii Cl. 



Median line straight, approximate to the ventral margin 13. 



— — arcuate, distant from — — — 22. 



( Terminal nodules, distant from the ends 14. 



\ — — near the ends 15. 



) Striai coarse C. protracta Berk. 



\ — fine C. helvetica monstrosity. 



j Striae coarsely punctate 16. 



1 — iinely — 17. 



f Axial area very narrow C. Triangulum Ehb. 



t — — distinct C. turgida Greg. 



I Median line close to the margin 18. 



) — — at some distance from the margin 19. 



f Ends capitate C. Jordani Gkdn. 



■j — obtuse, rertexed C. itielegans Cl. 



I — — incurved C. ventricosa KuTZ. 



) Valve lunate, broad C. hfbridica Gkun. 



\ — — narrow 20. 



( Acute 21. 



\ Obtuse C. nori-egica Grun. 



