204 BRITISH DESMIDIE.E. 



7. Sporangium orbicular ; conjugated fronds deciduous Jenneri. 



Sporangium at first quadrate ; conjugated fronds persistent. Brebissonii. 



15. DOCIDIUM. 



1 . Frond neither constricted nor inflated ? asperum. 



Frond constricted ; segments mostly inflated at the base 2 



2. Suture inconspicuous ; segments not inflated minutiim. 



Suture distinct ; segments inflated at their base 3 



3. Frond stout ; suture projecting at the sides . . . , , 4 



Frond slender ; suture not projecting 5 



4. Segments with one inflation ; extremities attenuated truncatum. 



Segments with several inflations ; extremities not attenuated, nodulosum. 



5. Extremities clavate clavatwn. 



Extremities not clavate 6 



6. Ends bordered with tubercles Ehrenbergii. 



Ends destitute of tubercles Baculum. 



16. Closterium. 



1 . Frond suddenly constricted at each end into a conical point, attenuatum. 

 Frond not constricted at the ends 2 



2. Frond striated, tapering into a beak at each end, lower 



margin prominent at the middle 3 



Frond not rostrate ; if striated, its lower margin not pro- 

 minent at the middle 6 



3. Beaks setaceous, as long as or longer than the body 4 



Beaks linear, much shorter than the body 5 



4. Beaks much longer than the body setaceum. 



Beaks about as long as the body rostratum. 



5. Frond much inflated at the middle, rapidly tapering at each end. Ralfsii. 

 Frond slightly inflated at the middle, gradually tapering at 



each end lineatum. 



6. Frond minute, acicular ; sporangium cruciform 7 



Frond not acicular ; sporangium orbicular 8 



7. Ends obtuse Cornu. 



Ends acute acutimi. 



8. Frond semilunate or semilanceolate, the lower margin in- 



clined upwards at the ends 9 



Frond having either truncate ends or its lower margin 



inclined downwards at the ends 12 



9. Vesicles numerous, scattered Lunula. 



Vesicles in a longitudinal row 10 



