202 BRITISH DESMIDIEiE. 



17. Segments in front view with a forked spine on each side, 



otherwise smooth Avicula. 



Segments in front view having many spines or none 18 



18. Frond smooth ; end view acutely triangular, with two acute 



accessory spines to each angle 19 



Frond rough or spinous ; angles in the end view not acute 20 



19. Segments in the front view with a forked spine on each side, monticulosum. 

 Segments in the front view with all their spines slender, 



simple, and directed outwards punc/ens. 



20. Frond rough with large granules ; end view with five or six 



rounded and toothed lobes sexcostatum. 



Frond either minutely granulate or spinous ; angles in end 



view either entire or elongated into rays 21 



2 1 . Frond rough with puncta-like granules ; angles or rays in 



end view entire 22 



Frond either spinous or having the angles or rays tipped with spines 28 



22. End view with romided or truncate short rays or angles 23 



End view with elongated slender rays or processes 27 



23. End view vdth slightly concave sides and broadly rounded 



angles punctulatum. 



End \dew either with very concave sides or with angles 



elongated into short rays 24 



24. End view triangular „ 25 



End view with four or more angles or rays 26 



25. Segments rounded on each side alternans. 



Segments tapermg on each side into a short process tricorne. 



26. Segments twice as broad as long ; end view 4-rayed dilatatum. 



Segments as long as broad ; end view 5- to 7-rayed . . margaritaceum. 



27. Segments with converging processes Arachne. 



Segments with diverging processes tetracerum. 



28. Frond rough with minute granules ; spines, besides those 



that tip the rays, few and inconspicuous, or none 29 



Frond with evident, more or less forked or emargiuate spines .... 33 



29. Processes much elongated 30 



Processes short 31 



30. Processes divergent in the front view paradoxwn. 



Processes parallel in the front view gracile. 



31. Processes convergent in the front view , cyrtocerum. 



Processes nearly straight in the front view 32 



32. Segments fusiform tricorne />. 



Segments ventricosc, not fusiform pohjmorj)/ium. 



