THE FRESH-WATER ALGAE 



131 



♦S (46) Valves showing a wave-like margin in girdle view. 



Cymatopleura W. Smith. 



This is a large diatom which is 

 easily recognized by the undulat- 

 ing outline of the girdle side. 



The genus is rather small, and 

 Wolle reports but seven species. 



Fig. 93. CyTnatopleura apiculata 

 W. Smith, a. Valve side. 6. girdio 

 side. X 600. (Original.) 



46 (45) Girdle view without wave-Uke margins Surirella Turpin. 



^.jx This genus is widely distributed 



s^ ) -^ and of frequent occurrence in all re- 

 gions where diatoms are found. Some 

 species are very large and conspicu- 

 ous, especially in the plankton. 



R Fig. 54. Surirella sp. Smith. a. 



Valve side. 6. girdle side. X 585. 



(Original.) 



47 (42) Valves without keels 48 



48 (59) CeUs without deep inner partitions sometimes with imperfect septa. 49 



49 iSS) Valves with transverse costae 50 



50 (54) Valves symmetrical with reference to a transverse axis. 



Family Diatomaceae 



SI 



Cells symmetrical with reference to both axes, borne in long chains; transverse striations 

 distinct and uninterrupted except in some cases by a longitudinal plain band. 



51 (52, 53) Valve side oval or linear, transverse markings uninterrupted, 

 girdle side rectangular, cells mostly in zig-zag chains, some- 

 times in short filament Diatoma de CandoUe. 



A <G2^ 



„ Fig. 95. Diatoma elongatum PLgzrdh, a. Valve 

 " side. i. girdle side. X about 300. (After W, 

 Smith.) 



