THE FRESH-WATER ALGAE 133 



S 8 (S 6 , 57) Cells arranged in the form of a star. . . . Asterionella Hassall. 



The radial arrangement of the cells is due to the 

 presence at the inner ends of small mucous cushions 

 which unite the cells in this manner. The cells are 

 linear, unequally enlarged at the ends, capitate in the 

 valve view and truncate in the girdle view. The 

 valves are marked with delicate striations. 



Asterionella is common in ponds, lakes, and water 

 reservoirs. It is especially frequent in the plankton, 

 probably on account of the radial arrangement of the 

 cells, which would make it easily buoyed up by the 

 water. 



FlG. 101. AsterionellagraciUimaHeiberg. X 188. 

 (After Schroter.) 



59 (48) Cells with interrupted inner partitions. 



Family TABELLARIACEAE . 



60 



Valves linear, oblong, or elliptical, inflated at the center, 

 or more longitudinal partitions perforated at the center. 



Girdle side rectangular, with two 



60 (61) Cells slender, valves with only punctate striations. 



Tabellaria Ehrenberg. 



The inner partitions appear in the girdle view 

 as distinct lines which are not always equally 

 developed or opposite each other at the two ends 

 of the cell. At the interruption of the partitions 

 at the center the valve sides show an inflation. 



The zig-zag chains of Tabellaria are conspicu- 

 ous in almost all collections of algae. 



FIG. 102. Tabellaria fenestrata Kutzing. a. Valve 

 side. b. girdle side. X 600. c. showing characteristic 

 arrangement of cell. X about 150. (Original.) 



61 (60) Cells broader, with distinct transverse costae. . . Tetracydus Ralfs. 



Aside from the interrupted inner par- 

 titions there are also transverse septa 

 which appear on the valve sides as costae, 

 between which are very faint striae. The 

 septa are more numerous, and the cells 

 more cruciform than in Tabellaria; they 

 occur also in bands instead of in zig-zag 

 chains. 



Tetracydus lacuslris Ralfs. a. 

 b. girdle side. X 300. (After 



FIG. 103. 

 Valve side. 

 Smith.) 



62 (39, 41) Valves asymmetrical with reference to a longitudinal axis, the 

 cells being more or less arcuate. 



Family EPITHEMIACEAE . . 63 



Valves curved, usually with dotted transverse striations, sometimes also with transverse 

 costae. 



