50 SYNOPSIS OF THE GENERA. 



8. Cosmarium. Segments in front view neither notched nor sinuated, 



in end view eUiptic, circular or cruciform. 



9. Xanthidium. Segments compressed, entire and spinous. 



10. Arthrodesmus. Segments compressed, and having only two spines 



or mucros. 



1 1 . Staurastrum. End view angular, radiate, or with elongated pro- 

 cesses, which are never geminate. 



12. Didymocladon. Segments angular, each angle having two processes, 

 one inferior, and parallel with the similar one of the other segment, 

 the other superior and divergent. 



*** Frond simple from complete transverse division, generally much- 

 elongated, never spinous, frequently not constricted at the 

 centre. Sporangia smooth. 



13. Tetmemorus. Frond straight, constricted at the centre and notched 

 at the ends. 



14. Penium. Frond straight, scarcely constricted at the centre. 



15. Docidium. Frond straight, much-elongated, constricted at the 

 centre, truncate at the ends. 



16. Closterium. Frond crescent-shaped or arcuate, not constricted at 

 the centre. 



17. Spirotsenia. Frond straight, not constricted at the centre ; endo- 

 chrome spirally twisted. 



**** Cells elongated, entire, fasciculated. 



18. Ankistrodesmus. Cells aggregated into faggot-like bmidles. 



***** Frond composed of few cells, definite in number, and not 

 forming a filament. (Sporangia unknown.) 



19. Pediastrum. Cells arranged in the form of a flattened star; their 

 outer margin bideutate. 



20. Scenedesmus. Cells oblong or fusiform, entire, placed side by side 

 in a single row, but during division in two rows. 



