126 FRESH-WATER BIOLOGY 



1 (9, 10) Valves circular, raphe lacking, markings radial 2 



2 (5) Cells cylindrical or ellipsoidal, united into filaments. Valve side circu- 



lar, either convex or flat. . . Family Melosiraceae . . 3 



3 (4) Cells with no spines or teeth; valves either smooth or punctate, usually 



convex; girdle side punctate Melosira Agardh. 



-J'.. 



Melosira is very common in ponds, rivers, lakes, and 

 reservoirs, and occurs in great quantities in the plankton. 

 The filaments are often very long. 



Fig. 69. Melosira varians Agardh. X 600. (Original.) 



4(3) 



Cells similar to those of Melosira, but with a circle of tooth-like pro- 

 jections between the valve and girdle sides. 



Orthosira Thwaites. 



Van Heurck and West include Orthosira under Melosira, while 

 many others make a separate genus. The Orthosira forms are 

 found in the same localities as Melosira but are much less 

 C abundant. 



Fig. 70. Orthosira orichalcea W. Smith. X 600. (Original.) 



5 (2) Cells single, disc-shaped, not forming filaments; valves flat, convex, or 



undulating, mostly with radial rows of punctulations. 



Family Coscinodiscaceae . . 6 



6 (7, 8) Valves circular or nearly so, with radiating rows of dots or areola- 



tions, the disc with a distinct edge, usually bearing a circle 

 of inconspicuous submarginal spines. 



Coscinodisciis Ehrenberg. 



The number of species of Coscinodiscus is verj' large, mostly 

 marine, although some occur in fresh water with other similar 

 centric forms. 



Fig. 71. Coscinodiscus apiculatus Ehrenberg. X330. (After Wolle.) 



7 (6, 8) Valves circular, showing a central smooth or punctate area, and an 

 outer margin of radiating striations. Girdle view with 

 undulating ends Cyclotella Kiitzing. 



The cells are disc-shaped and are distinguished from other disc- 

 shaped forms principally by the smooth or punctate center and the 

 undulating ends. It is found commonly in the plankton. 



Fig. 72. Cyclotella compta Kutzing var. affinis Grun. a. Valve side; b, 

 girdle side. X 408. (After Schiitt and van Heurck-Gninow.) 



