160 Mr. J. Ralfs on the British Desmidieae. 



In all three species the frond is compressed, and the acute 

 subulate spines seem more like those present in some species of 

 Xanthidium than the processes which terminate the angles in 

 Staurastrum. They may therefore form a section of Xanthidium, 

 or what is perhaps better, a distinct genus, for which Ehrenberg's 

 name Arthrodesmus should be retained. 



Plate XII. fig. 3. -5". octocorne : «, front view ; b, frond dividing; c, end 

 view. 



4 



{Front view with straight, diverging processes 2 

 Front view with the processes, if any, parallel or con- 

 verging 4 



r Processes smooth, deeply divided at the apex hijidum. 



2. < Processes rough with minute granules, entire or ter- 



L minated by three minute points 3 



End view compressed and having an entire process at 



each extremity tetracerum. 



End view with three or four rays, each terminated by 



three minute points paradoxum. 



r End view compressed and having a spine or mucro at 



4.-< each extremity 5 



L End view with five or more angles or rays 6 



K r Segments with four spines octocorne. 



* \ Segments with two spines 5* 



r Segments transversely elliptic ; spines incurved convergens. 



5* -< Segments with truncate ends; spines generally directed 



L outwards Incus. 



{End view with three or four angles or rays 7 

 End view circular, with five or more marginal rays or 

 lobes 13 



{End view with four broad, truncate, entire rays dilatatum. 

 End view with three angles or rays, each of which is 

 either rounded or else terminated by minute spines. 8 



^ r Each ray terminated by three minute spines 9 



' I. End view with rounded and entire angles 10 



Q J Fronds spinulose aculeatum. 



' "1, Fronds rough with minute granules gracile. 



,^ r Fronds muricated, or rough with minute granules 11 



* t Fronds smooth 12 



,, r End view with convex sides muricatum. 



'\ End view with concave sides tricorne. 



12 r Angles inflated, mucronate mucronatum. 



'\ Angles neither inflated nor mucronate orbicular e. 



,„ rEnd view with broad, toothed lobes Jenneri. 



* \ End view with entire rays 14 



,. J Processes short, stout tnargaritaceum. 



* \ Processes long, slender Arachne. 



