128 BRITISH DESMIDIACE^l. 



Division I. Cell- wall smooth, punctulate or scrobiculate, outline 

 never granulate, margins sometimes undulate or crenate. 



SECTION A. Seniicells semicircular, semi-elliptical, or subsenii- 

 circular in outline. 



* Seniicells semicircular or subsemicircular in outline ; cells 

 about as long as broad or a little longer than broad 

 rarely 1| times longer than broad. 

 t Margin of seniicells entire. 



J Semicells with a blunt tooth or subniamillate 

 thickening at the basal angles. 



1. C. obsoletum. 



2. C. Smolandicum. 



JJ Semicells without any niamillate thickening at the 



basal angles. 



With no prominent scrobiculations in the 

 centre of the seuiicells. 



3. C. taxicliondriforme. 



4. C. circular e. 



5. C. Lundelln. 



6. C. pachydermum. 



7. C.Ealfsii\ 



With several prominent scrobiculations in the 

 centre of the seniicells. 



8. C. perforatum. 



9. C. ocellatum. 

 tt Margin of seniicells undulate. 



10. C. cyclicum. 



11. C. fontigenum. 



12. C. undulatum. 



** Seniicells semi-elliptical in outline ; cells 1^ 2 times longer 



than broad. 

 t Margin of seniicells undulate. 



13. C. subundulatum. 

 ft Margin of seniicells entire. 



J Constriction moderately deep. 



14. C. Cucumis. 



15. C. subcucumis. 

 %% Constriction slight. 



Sinus linear. 16. C. microspliinctuin. 

 Sinus open. 17. C. morsum. 



SECTION B. Semicells transversely elliptical, oblong-elliptical, 

 or reiiiforni in outline. 



Semicells elliptic -renif or m ; sinus closed. 



t Isthmus narrow ; vertical view with a protuberance on 

 each side. 



Apex of semicell entire. 



18. C. Pliaseolus. 



19. C. tumidum. 



