398 THE MICROSCOPIST. 



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4. Docidium. Cell straight, constricted, truncate at 

 ends. 



5. Spirotcenia. Cell straight, not constricted ; endo- 

 chrome spiral. 



IT. COSMARIE^:. Cells single, distinctly constricted in 

 the middle ; segments seldom longer than broad; sporan- 

 gia spinous or tuberculated. 



6. Micrasterias. Lobes of the segments incised or bi- 

 dentate. 



7. Euastrwn. Segments sinuated, generally notched at 

 ends, and with inflated protuberances. 



8. Cosmarium, Segments neither notched nor sinuated, 

 end view elliptic, circular, or cruciform. 



9. Xanthidiam. Segments compressed, entire, spinous. 



10. Arthrodesmus. Segments compressed, each with 

 only two spines. 



11. Staurastrum. End view angular, radiate, or with 

 elongated processes which are never in pairs. 



12. Didymodadon. End view angular, each angle with 

 two processes, one inferior and parallel with that of other 

 segment, the other superior and divergent. 



III. DESMIDIE.E. Cells united into an elongated jointed 

 filament ; sporangia spherical, smooth. 



13. Hyalotheca. Filament cylindric. 



14. Didymoprium. Filament cylindric or subcylindric ; 

 cells with two opposite bidentate projections. 



15. Desmidium. Filament triangular or quadrangular ; 

 cells with two opposite bidentate projections. 



16. Aptogonum. Filament triangular or plane, with 

 foramina between the joints. 



17. Spk&rozosma. Filament plane; margins incised or 

 sinuate ; joints with junction glands. 



