2. Sheaths lamellate, the lamellations diverging 3 



2. Sheaths not lamellate, or with lamellations parallel 5 



3. Filaments 18-30-(36)m in diameter; cells 6-12fi wide S. myochrous 



3. Filaments smaller— 4 



4. Forming free-floating clots with filaments more or less radiating 



from a common center, 10-15/i in diameter S. tolypothricoides 



4. Forming attached, wooly expanses; filaments 15-21/u in diameter... S. mirahile 



5. Cells very short, about % their width in length; filaments very 



coarse and wiry, seldom branching S. crispum 



5. Cells longer; branching frequent : 6 



6. Heterocysts ovate or subglobose; filaments 18-24/^ in diameter S. coactile 



6. Heterocysts quadrate to cylindrical; filaments 



12-1 6ai in diameter. S. Archangelii 



Scytonema alatum ( Carm. ) Borzi 1879, p. 373 

 PL 123, Figs. 4, 5 



Filaments forming dark olive-brown, wooly mats or tufts. Branches 

 of trichomes in pairs between the heterocysts, branches relatively 

 short. Cells short cylindric or barrel-shaped; 9-15/x in diameter, 

 8-15|a long. Heterocysts subglobose, tinged with yellow-brown. 

 Sheaths wide; much lamellated, with the layers diverging, forming 

 'wings'; with decided constrictions here and there, especially at 

 the heterocysts. Filaments 24-66/i, in diameter. 



Tychoplankter. Mich., Wis. 



Scytonema Archangelii Bornet & Flahault 1887, p. 92 

 PL 123, Figs. 6, 7 



Filaments in fascicles or tufts, forming brownish or gray mats and 

 cushions. Trichomes with long, gracefully curved branches arising 

 singly at the heterocysts, or more commonly in pairs between the 

 heterocysts. Cells quadrate; without constrictions at the cross walls; 

 12-18)U in diameter, 14-20/^ long. Heterocysts quadrate or cylindrical; 

 either colorless or ochraceous. Sheaths thin, close, hyaline. Filaments 

 12-16^ in diameter. 



Tychoplanktonic and on shore. Wis. 



Scytonema coactile Montague in Kuetzing 1849, p. 305 

 PL 124, Figs. 1-3 

 Filaments forming thick, blue-green skeins or film-like expansions. 

 Trichomes frequently branched, with the solitary habit predominat- 

 ing; false br-^nches long and spreading. Cells quadrate or com- 

 pressed barrel-shaped, 12-18/x in diameter, 5.8-7.5/x long; cell 

 contents a deep blue-green or yellow-green, especially in the distal 

 ends of the branches. Heterocysts scattered and infrequent; olive- 



[ 534 ] 



