2a. Outer half of valve with two types of lines, one long and one short ---- 3
2b. Outer half of valve with radial lines all alike---- Cyclotella meneghiniana H,S, salt, pH-6.7-9.0 opt-8.0-8..5, fall

3a. Outer valve zone constituting less than .5 the dia.--- Cyclotella bodanica clean
3b. Outer valve zone constituting more than .5 the dia.--- Cyclotella compta taste, lakes, eupl