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. 



Stephanodisciis Ehrenberg. 



The length of the spines on the margins of 

 the cells varies greatly; in S(jme species they 

 are short and acute, while in others they may 

 exceed the diameter of the cell many times. 

 Stephanodisciis occurs frequently in the plank- 

 ton, but usually not in great quantities. 



Fig. 73- 



Suphanodiscus niagareoe Ehrenberg. 

 X 606. (Original.) 



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

 tended, usually forming long spines. 



Family Rhizosolexiaceae. 

 Only one genus Rhizosolenia Ehrenberg. 



Fig. 74. Rhizosolenia eriensis H. Smith. 

 Schrdter.) 



X 190. (After 



10 (i, 9) Valves not circular or cyhndrical, 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 11 



11 (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. 



Stauroptcra Ehrenberg. 



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



^■M 



E^ilBi« 



Fig. 76. Pinnularia viridis 

 Smith. X600. (Original.) 



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

 side. Striations composed of lines of individual dots. . ^"o 



