THE FRESH-WATER ALGAE 



45 (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- 

 A ing outline of the girdle side. 



The genus is rather small, and 

 Wolle reports but seven species. 



FIG. 93. Cymatopleura apiculata 

 \ a W. Smith, a. Valve side. b. girdle 

 I side. X 600. (Original.) 



c 



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



This genus is widely distributed 

 and of frequent occurrence in all re- 

 gions where diatoms are found. Some 

 species are very large and conspicu- 

 ous, especially in the plankton. 



FIG. 94. Surirella sp. Smith. a. 

 Valve side. b. girdle side. X 585. 

 (Original.) 



47 (42) Valves without keels 48 



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



49 (55) Valves with transverse costae 50 



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



Family DIATOMACEAE . . 51 



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 Candolle. 





B FIG. 95. Diatoma elong alum Agardh. o. Valve 

 B side- * g^dle side. X about 300. (After W. 



