BRITISH DESMIDIE^. 205 



10. Ends of frond slightly curved upwards ; striae distinct .... turyidum. 

 Ends of frond straight ; striae none or indistinct 11 



1 1 . Frond linear-lanceolate ; ends conical, obtuse acerosum. 



Frond semilanceolate ; ends subacute lanceolatum. 



1 2. Frond smooth, crescent-shaped 13 



Frond either not crescent-shaped or else distinctly striated 17 



13. Vesicles numerous, scattered Ehrenbergii. 



Vesicles in a longitudinal row 14 



14. Empty frond colourless ; ends romided 15 



Empty frond usually reddish ; ends subacute 16 



15. Lower margin of frond inflated at the middle moniliferum. 



Frond not inflated at the middle Jenneri. 



16. Frond inflated at the middle Leibleinii. 



Frond slender, not inflated at the middle Dianee. 



1 7. Lower margin of frond inclined upwards at the trimcate 



ends ; striae none or indistinct didymotocum. 



Ends of frond inchned downwards ; striae distinct 18 



18. Striae three to seven, prominent 19 



Striae numerous, fine 20 



19. Frond semilunate or crescent-shaped costatum. 



Frond huear angustatum. 



20. Frond narrow-linear, nearly straight juncidum. 



Frond tapering, curved 21 



21. Striae crowded ; sutures one to three striolatum. 



Striae not crowded ; sutures usually more than three .... intermedium. 



17. Spirot^nia. 



Spire solitary, broad, conspicuous condensata. 



Spires several, narrow, obscure , . . . obscura. 



18. Ankistrodesmus. 

 falcatus. 



19. Pediastrum. 



1 . Lobes of marginal cells bifid or truncato-emarginate 2 



Lobes of marginal cells entire 3 



2. Cells four Tetras. 



Cells seven, circularly disposed round a central one Heptactis. 



Cells eleven, circularly disposed round five or six central ones, biradiatum. 



3. Marginal cells crescent-shaped ; connecting substance coloured. Selenoea. 

 Cells not crescent-shaped ; connecting substance hyaUne 4 



