158 V. T. CLBVE, SYNOPSIS OF THE NAVICULOID DIATOMS. 



The species of Cymbella are frequently very variable and transitorial. Many names have 

 been proposed for forms, which differ only in trifling characteristics. The descriptions 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 v^hat the names given by the authors actually denote. Under 

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



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

 is marine. They occur 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 imperfectly known forms, marked * in the text are not 

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

 as indeed the forms themselves are. 



Artificial key. 



I Valve cymbiform : . . .... . . . .3. 



\ — naviculoid . ,...,. 33. 



/Central nodule with a stigma. ....]. .^ 3. 



\ — — withont stigma ... ..... . . .' .' .1 7. 



( Stigma below the nodule ■.....', . . 4. 



I — in — . . . ■■' 5. 



J Striae coarsely punctate . .... C. australica A. S. 



\ — finely — . . . . . . C. tumida BEiB. 



Ends broad, rounded C, Janischii A. S. 



subtruncate, obtuse . . . . ... 6. 



Central area small ... . . C. mexicana Ehb. 



large . . C.'punctifera Cl. 



_ j Median ventral striae ending with isolated puncta . . 8. 



• I — — — not — — — 12. 



„ ( Punctum one C. cymbiformis KtJTZ. 



\ Puncta two or more . . 9. 



J Puncta two . . . . C. turgidula Geun. (C. Cistula vat.) 



\ — several . 10. 



line dilated towards the ends C. Sturii Grun. 





..^ ( Median lin( 



I - — not - — ....'.'. 11. 



^j I Stri! 



\ — fine . C. Stuxbergii Cl 



..„ j Median line straight, approximate to the ventral margin 13. 



' \ — — arcuate, distant from — — — . . 22. 



Striae coarse . C. Cistula Hempr. 



— fine . C. Stuxbergii 



Median line straight, approximate to the ventral margin 13. 



— — arcuate, distant from — — — . . 22. 



..., /Terminal nodules, distant from the ends '. i . 14. 



' \ — — near the ends . . . , , . 15. 



.(^ J Striae coarse C. protraeia. Berk. 



fine . . . . . . C. helvetica monstrosity. 



... j Striae coarsely punctate . . . 16, 



{' 



finely — 17. 



area very narrow C. Triangulum Ehb. 



— distinct . . . C. turgida Greg. 



^_ I Median line close to the margin ... ... 18. 



1 — — at some distance from the margin 19. 



{Ends capitate C Jordani Grun. 

 — obtuse, reflexed C. inelegans Cl. 

 — — incurved C. ventricosa Kutz. 



.._ I Valve lunate, broad C. hebridica Gbun. 



■ \ — — narrow 20. 



20. i^""** ' • ■'••■•••• 21- 



\ Obtuse C. norvegica Grun. 



