158 Mr. J. Ralfs on the British Dcsmidiese. 



entire process which is generally somewhat incurved. The end 

 view is elevated in the centre, and has from five to seven short, 

 narrow, obtuse marginal rays. 



In the front view this species differs from all the preceding by 

 its segments being of a subglobose form and attenuated at their 

 junction, and by the short, linear, obtuse, lateral processes which 

 also appear more distinct from the body of the segment ; its end 

 view may be distinguished from that of their many-rayed varie- 

 ties by the elevation of the centre, and the short, narrow, entire 

 rays. 



Plate XL fig. 7. a^. margaritaceum : a, front view j h, end view. 



13. 5^. Jenneri. Segments in the front view with a toothed angle at 

 each side ; end view circular, with five or six broad, short, toothed 

 lobes or processes. 



Mayfield, Sussex, Mr. Jenner. 



Fronds large, rough, with conic granules which give a den- 

 tate appearance to the outline ; segments about as broad as long, 

 produced into a toothed angle on each side, where also a trian- 

 gular sinus is formed between the angles. The end view is cir- 

 cular and elevated in the centre, and has five or six broad, short, 

 toothed marginal lobes. The transverse view has a large central 

 opening surrounded by a row of large granules. 



Plate XI. fig. 8. S. Jenneri: a, front view; b, end view; c, transverse 

 view of empty frond. 



**** Front view without diverging processes ; end view compressed, 

 and having a process or mucro at each extremity. 



14. S. convergens, Mgh. Fronds smooth ; in the front view the seg- 

 ments are elongated at each side into a conic spine which is curved 

 inwards ; end view compressed with a spine at each extremity. 

 Mgh. I. c. p. 228. Arthrodesmus convergens, Ehr. Infus. p. 152. 

 tab. 10. fig, 18; Pritch. Infus. p. 190. figs. 112 and 113. Eu- 

 astrum, Bailey, American Bacil. pi. 1. fig. 11. 



In pools. Brambletye near East Grimstead, and Rackham Com- 

 mon near Pulborough, Sussex, and Rusthall Common near Tunbridge 

 Wells, Mr. Jenner ; Dolgelley and Penzance. 



Fronds smooth, deeply constricted in the middle ; segments 

 broader than long ; their spine4ike processes, by curving inwards, 

 converge towards each other on the same side. 



This plant was placed by Ehrenberg in Arthrodesmus {Scene- 

 desmus) ; but it has no affinity with the true species of that genus. 



Plate XII. fig. I. ^S*. convergens : a, front view; h, end view. 



15. S. Incus, Mgh. Fronds smooth ; in the front view the segments 

 are lunate and have a mucro at each angle ; end view elliptic, with 



