195 



ANALYSIS OF THE GENERA. 



1 . Frond a single cell (its division being followed by separation), 



or cells fasciculated 2 



Frond composed of several cells 13 



2. Segments, in the front view of the frond, lobed, incised, si- 



nuated, or notched 3 



Segments, in the front view, undivided 4 



3. Frond fusiform ; segments notched at the end, otherwise 



entire Tetmemoriis. 



Frond oblong, compressed ; segments siauated Euastrum. 



Frond lenticular ; lobes of segments incised or bidentate 



at the apex Micrasterias. 



4 . End view of frond angular or radiate 5 



End view of frond neither angular nor radiate 6 



5. Processes geminate from each angle, one external to the 



other Didymocladon. 



Processes, if present, with their divisions on the same 



plane Staurastrum. 



6. Cells short, rarely more than twice as long as broad 7 



Cells elongated (cylindrical, fusiform or Innately curved) 9 



7. Frond with elongated, simple or branched spines 8 



Frond not spinous Cosmarium. 



8. Segments with two spines Arthrodesmus. 



Segments with four or more spines Xanthidium. 



9. Cells straight 10 



Cells curved 12 



10. Endochrome spiral Spirotcenia. 



Endochrome not spiral 11 



1 1 . Frond moderately elongated, if linear not constricted at the 



middle, and its segments not inflated at the base Penium. 



Frond much elongated, linear, generally much constricted at 



the middle, and its segments inflated at the base Docidium. 



o 



