DESMIDS, DIATOMS, AND FRESH-WATER ALG.E. 75 



f. Cell constricted in the middle ; no arms nor sharp 

 spines (g). 



f. Cell constricted in the middle, with arms or sharp 



spines (A). 



f. Cell not constricted in the middle ; no arms nor 

 sharp spines ("). 



g. Ends notched, cell cylindrical. Tetmemorus, 9. 



g. Ends not notched, cell cylindrical. Docidium, 



10. 

 g. Ends not notched ; cell more or less dumb-bell or 



hour-glass shaped (I). 

 h. Arms three or more, radiating, tipped with one or 



more points. Staurastrum, 12. 

 h. Arms none, spines four, two on each end. Ar- 



throdesmuSj 14. 



h. Arms none; spines several, on the edges; a round- 

 ed, truncate, or denticulate tubercle in the centre 



of each semi-cell. Xantladium, 13. 

 *. Chlorophyl in a spiral band, cell cylindrical. Spi- 



rotcenid, 15. 



*. Chlorophyl not in a spiral band, cell cylindrical (&). 

 k. Surface roughened by tooth-like elevations. Tri- 



ploceras, 16. 

 ~k. Surface smooth, ends rounded, neither divided nor 



notched. JPenium, 17. 



L End view three to six or more angular (m). 

 I. End view not angular, margins smooth, dentate, 



or crenate, without spines; ends always entire. 



Cosmdriwn, 11. 



