m 



136 BRITISH DESMIDIE.E. 



The segments are very variable in form and often resemble those of Stau- 

 rastrum tricorne and S. margaritaceum, but in these species the processes 

 are never spinous. S. polymorphum is smaller and less spinous than S. 

 asperum. 



Length of frond ^iroo ^f an inch ; breadth -^^ ; breadth at constriction 

 .jJL_. ; diameter of sporangium -^-^ ; length of spine of sporangium -^-^. 



Tab. XXII. fig. 9. a, b, c, d. front views ; e, f, g. dividing fronds ; h. end 

 view of three-rayed segment ; i. transverse view of four-rayed segment ; h. end 

 view with endochrome ; /. end view of empty frond. 



Tab. XXXIV. fig. 6. a. front view of variety ; h. end view. 



26. /S. gracile (Ralfs) ; segments rough, elongated on each side into 

 a slender process which is terminated by minute spines ; end view 

 triradiate. 



Euastrum, No. 13, Bailey, Amer. Bacil. in Amer. Journ. of Science and 



Arts, V. 41. p. 296. t. 1. f. 2-5 (1841). 

 Staurastrum gracile, Ralfs, Annals of Nat. Hist. v. 15. p. 155. t. 11. f. 3 



(1845) ; Trans, of Bot. Soc. of Edinburgh, v. 2. p. 142. t. 14. 

 Goniocystis {Trigonocystis) gracilis, Hassall, Brit. Freshwater Alg. p. 352 



(1845). 



Dolgelley and Penzance, /. R. Ambleside, Mr. Sidebotham. Aberdeen, 

 Mr. P. Grant. Manchester, Mr. Williamson. 



New York and New England, Bailey. Falaise, Brebisson. 



Frond rough with minute granules, deeply constricted at the middle ; seg- 

 ments two or three times longer than broad and tapering, on each side, into a 

 slender, straight and colourless process, which is apparently terminated by 

 three minute points : Mr. Jenner however informs me that the processes 

 really terminate in four points, which are \isible only when a process pre- 

 sents its extremity to the observer. The granules are arranged in trans- 

 verse lines on the processes, and are here more conspicuous than they are on 

 the segment. The end view is triradiate ; the colouring matter is restricted 

 to the centre and forms three rays, which are frequently bifid. 



Staurastrum gracile differs from S. tricorne in its elongated processes 

 terminated by minute points. The end view somewhat resembles that of 

 S. paradoxum, but the latter species is easily known by the divergent pro- 

 cesses of its front view and its smaller size. 



Length of frond from ^ka ^^ 'sk'^ *^^ ^" '\i\c\\ ; breadth from 3^^ to -^\-^ ; 

 breadth at constriction axoT- 



Tab. XXII. fig. 12. a. front view; b, c. end views; d. transverse view. 



27. S. Arachne (Ralfs) ; segments rough with minute granules, sub- 

 orbicular, with elongated, slender, incurved processes ; end view 

 with five linear rays. 



Staurastru7n Arachne, Ralfs, Annuls of Nat. Hist. v. 15. p. 157. t. 11. f . 6 

 (1845) ; Trans, of Bot. Soc. of Edinburgh, v. 2. p. 143. t. 14. 



