72 BRITISH DESMIDIACE2E. 



* Cells attenuated towards each end, never cylindrical. 



1. P. LiMlul'. 



2. P. Nai-icnla. 

 * * Cells cylindrical or ellipsoidal. 



t Poles rounded. 



3. P. Jen a <' ri. 



4. P. spinospermum. 



5. P. didymocarpum. 



6. P. Mooreanwm. 



7. P. subocta/ngulare. 



8. P. minutissimum. 

 t t Poles truncate. 



9. P trintcatitnt. 



SECTION B. (Sphinctopenium Gay). Cells either conspicuously 

 constricted in the middle or very slightly constricted ; always with 

 a distinct demarcation between the newer and the older parts of 

 the cell-wall. 



* Cell-wall minutely or coarsely granulate. 



t Cell-wall granulate over its entire surface. 



10. P. margaritaceum. 



11. P. cyliiidrits. 



12. P. cuticulare. 



13. P. exiynum. 



14. P. granulatum. 

 ft Granules restricted to the poles. 



15. P. Clevei. 



* * Cell-wall striated. 



t Cells large, striations very distinct, spirally disposed. 



16. P. spirostriolatum. 

 1 1 Cells small, striatious longitudinal and faint. 



17. P. polymorpJium. 



18. P. phymatosporwm. 

 if Cell-wall smooth, punctate, or finely scrobiculate. 



t Cells minute, rounded, 1^ times longer than broad. 



19. P. s-ubtlU: 

 1 1 Cells 2-4 times longer than broad. 



a. Cell-wall scrobiculate or punctate. 



20. P. adelochondrum. 



21. P. lagenarioides. 



22. P. Cucurbitinwm. 



23. P. crassiusculum. 



24. P. curium. 

 (3. Cell-wall smooth. 



25. P. rufescens. 



26. P. cruciferum. 



27. P. inconspicuum. 

 1 1 1 Cells elongated, up to 30 times longer than broad. 



28. P. miniifum. 



