GENERA OF DESMIDS. 79 



/. Cell constricted in the middle; no arms nor spines (/). 



/. " " " with arms or spines (/). 



/". Cell not constricted; no arms nor spines (m). 



j. Cell cylindrical, ends simply notched .... Tetmemorus, 18 



j. " " ends rounded, truncate or divided (k). 



j. Cell more or less dumb-bell, or hour-glass shaped (/). 



k. Cell 6 to 30 times longer than broad Docidiuiu, 14 



k. Cell 2-5 times longer than broad; ends rounded, Calocylindrus, 16 



/. Arms 2, 3 or more, radiating Staurastrum, 23 



/. Arms none; semi-cells with a central, rounded, truncate or dentic- 

 ulate tubercle; spines usually numerous and marginal, 



Xanthidium, 1 9 



/. Arms none; no central tubercle; spines 4 to 8, two on each end, ) 

 /. " " " spines 16, four on each end, j 



Arthrodesmns, 20 



m. Chlorophyll in one or more spiral bands . . . Spirot&ma, 1 1 

 m. " not in spiral bands (n). 



n. Surface rough with tooth-like or rounded elevations, Triploceras, 15 

 n. Surface without tooth-like elevations; ends rounded (o). 

 o. Cells in mucus, short, cylindrical or oval . . . Mesot&mum, 10 



o. Cells not or rarely in mucus Penium, 12 



p. End view 3-6 or more angular (r). 



p. End view not angular (s). 



r. Angles obtuse, acute, or with horn-like prolongations, 



Staurastrum, 23 

 s. Margins smooth, dentate or crenate; no spines . CosmaHum, 17 



