310. Branches usually arising in unilateral pairs about midway be- 

 tween the heterocysts of the main filament; sheath firm and 

 lamellate, rarely homogeneous Scytonema 



310. Branches arising singly, just below a heterocyst or a series of them 311 



311. Filaments aggregated, somewhat radially arranged in a muci- 

 laginous layer; branching regularly dichotomous; heterocysts 

 sohtary Diplonema 



311. Filaments solitary or aggregated, sometimes forming free- 

 floating, cottony tufts; branching not dichotomous, arising from 

 beneath a heterocyst or series of heterocysts Tolypothrix 



[595] 



