200 BRITISH DESMIDIEiE. 



2 1 . Segments reniform, three times broader than long .... commissurale. 

 Segments not reniform, about twice as broad as long 22 



22. End margin straight Broomeii. 



End margin not straight 23 



23. Inflation reaching to or beyond the end margin ornatum. 



Inflation confined to the centre of the segment . . biretum. 



24. Frond rough with pearly granules which give a denticulate 



appearance to the margin 25 



Frond smooth ; margin entire 26 



25. Segments spherical orbiculatum. 



Segments subquadrate cylindricum. 



26. Segments spherical , moniliforme. 



Segments not spherical 27 



27. Segments broader than long (hemispherical or semi-oval) . . connatum. 

 Segments as long as or longer than broad 28 



28. Frond (oval) large, distinctly visible to the naked eye turgidum. 



Frond minute, scarcely visible to the naked eye 29 



29. Frond fusiform or oblong 30 



Frond not tapering ; ends broadly rounded 31 



30. Puncta distinct Cucurbita. 



Puncta wanting, or very indistinct Thwaitesii. 



31 . Endochrome in longitudinal fillets curtum, 



Endochrome not in fillets attenuatum. 



9. Xanthidium. 



1 . Spines divided at the apex armatum. 



Spines subulate 2 



2. Segment without central projections, its spines usually four . ? octocorne. 

 Segment with central projections ; spines eight or more 3 



3. Spines scattered aculeatum. 



Spines marginal, in pairs 4 



4. Segments with a solitary basal spine on each side, the rest 



in pairs cristatum. 



Segments with basal spines in pairs 5 



5. Central projections minute and smooth in end view .... fascieulatum. 

 Central projections prominent and denticulate in end view . . JBrebissonii. 



10. Arthrodesmus. 



Segments elliptic coiivergens. 



Segments with truncate ends Incus. 



