THE FRESH-WATER ALGAE 



127 



8 (6, 7) Valves circular, with radial rows of dots, between which are clear 

 spaces; center either punctate or hyaline; on the margin 

 a circle of acute spines; girdle view with undulating ends. 



Stephanodiscus Ehrenberg. 



The length of the spines on the margins of 

 the cells varies greatly; in some species they 

 are short and acute, while in others they may 

 exceed the diameter of the cell many times. 

 Stephanodiscus occurs frequently in the plank- 

 ton, but usually not in great quantities. 



FIG. 73- 



Stephanodiscus niagareoe Ehrenberg. 

 X 606. (Original.) 



A B 



9 (i, 10) Valves more or less cylindrical, often in chains, ends greatly ex- 



tended, usually forming long spines. 



Family RHIZOSOLENIACEAE. 

 Only one genus Rhizosolenia Ehrenberg. 



FIG. 74. Rhizosolenia eriensis H. Smith. X 190. (After 

 Schroter.) 



10 (i, 9) Valves not circular or cylindrical, of different shapes, symmetrical in 



reference to a longitudinal or transverse axis; surface marked 

 by costae or punctate lines making definite angles with a 



middle raphe or a median line 1 1 



n(34>38) A middle nodule present on the raphe of both valves. ... 12 



See also 40 and 65. 



12 (32, 36) Girdle view symmetrical with reference to both a transverse and 



a longitudinal axis 13 



13 (26) Valves not arched or keeled; usually symmetrical with reference to a 



straight or a sigmoid raphe. Family NAVICULACEAE . . 14 



Valves symmetrical with reference to a straight or curved middle line; girdle symmetrical 



with reference to both axes; a straight or curved raphe; a central and two end nodules present. 



14 (15) Cells without inner partitions; raphe and valves straight. ... 16 



15 (14) Raphe and valves sigmoid Pleurosigma W. Smith. 



FIG. 75. Pleurosigma attenuatum W. 

 Smith. X 300. (After Smith.) 



16 (19) Cells linear, oblong, with rounded nodules, the two end ones turned 



toward one side, the prominent costae not punctate. . . 17 



17 (18) The costae interrupted by a plain band at the center. 



Stauroptera Ehrenberg. 

 18(17) The costae not interrupted at the center. . Pinnularia Ehrenberg. 



FIG. 76. Pinnularia viridis 

 Smith. X6oo. (Original.) 



19 (16) Cells more lance-shaped, the end nodules not turned toward one 

 side. Striations composed of lines of individual dots. . 20 



