DESMIDS, DIATOMS, AND FRESH-WATER ALG/E. 7 1 



I). Each cell with two teeth on each narrow end. 



Didymdpriuin,. i. 

 /', Each cell deeply divided almost into two parts. 



Sphcerozdsma, 2. 



b. Each cell without teeth and not divided. Hyalo- 



thdca, 3. 



c. Cells barrel-shaped, the band not twisted. Bajti- 



busina, 4. 



c. Cells not barrel-shaped, the band twisted. Dcs- 



m{dium, 5. 



d. Cells more or less crescent-shaped. ClostMuni, 



6. 

 d. -Cell cylindrical, spindle-shaped, hour-glass, or 

 dumb-bell shaped (/). 



d. Cell flattened, oblong, circular, or often divided 



into arms {e). 



e. Mostly circular or broadly elliptical, often cut 



and divided by slits and depressions; marginal 



teeth usually sharp. Micrasterias, 7. 

 e. Mostly oblong or elliptical, the margin wavy, the 



depressions rounded. Eudstrum, 8. 

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



spines (^). 

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



spines (Ji). 



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



sharp, spines. (?■). 



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

 g. Ends not notched, cell cylindrical. Dpddium, 10. 

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



hour-glass shaped (/). 

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

 or more points. Staurdstrum, 12. 



