160 Mr. J. Rail's un the British Desmidiere. 



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 tenninate 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. S. octocorue: a, front view; h, frond dividing; c, end 

 view. 



Analysis. 



{P'ront view with straight, diverging processes 2 

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

 verging 4 



{Processes smooth, deeply divided at the apex bifidum. 

 Processes rough with minute granules, entire or ter- 

 minated by three minute points 3 



{End view compressed and having an entire process at 

 each extiemi ty tetracerum. 

 End view with tliree or four rays, each terminated by 

 three minute points paradoxum. 



{End view compressed and having a spine or mucro at 

 each extremity 5 

 End view with five or more angles or rays 6 



, r Segments with four spines octocorue. 



' \ Segments with two spines 5* 



{Segments transversely elliptic ; spines incui'ved convergens. 

 Segments with truncate ends; spines generally directed 

 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 i-ounded or else terminated by minute spines. 8 



„ f Each ray terminated by three minute spines 



■ \ End view with rounded and entire angles 10 



Q f Fronds spinulose aculeatum. 



'\ Fronds rough with minute granules gracile. 



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



• t Fr 



onds smooth 12 



id view with convex sides imiricatu. 



End view with concave sides tricorne. 



,„ r Angles inflated, mucronate mucronalum. 



'\ Angles neither inflated nor mucronate orhiculare. 



,„ r End view with broad, toothed lobes Jenneri. 



■ \ End view with entire rays 14 



. . / Processes short, stout margaritaceujn. 



'\ Processes long, slender Arachne. 



