180 BRITISH DESMIDIACE.E. 



165. Staurastrum arcuatum Nordst. 

 (PL CLV, fig. 8.) 



Staurastrum arcuatum Nordst. Norges Desm. 1873, p. 36, t. 1, f. 18 ; Cooke, 

 Brit. Desm. 1887, p. 169, t. 51, f. 2 ; Boldt, Desm. Gronland, 1888, p. 40 ; 

 De Toni, Syll. Alg. 1889, p. 1207 ; West, Freshw. Alg. Maine, II, 1891, 

 p. 4, t. 15, f. 13 ; Alg. W. Ireland, 1892, p. 181 ; Turner, Alg. E. India, 

 1893, p. 120, t. 14, f. 20 ; Roy & Biss. Scott. Desm. 1893, p. 17 ; West 

 & G. S. West, Alg. S. England, 1897, p. 493; Liitkem. Desm. Mill- 

 stattersees, 1900, p. 19; Comere, Desm. de France, 1901, p. 156, t. 11, 

 f. 16 ; Bohlin, Flor. alg. d^ores, 1901, p. 64, t. 1, f. 26 ; Cushman in. 

 Bull. Torr. Bot. Club, 1907, p. 164. 



Cells small, about \\ times broader than long, deeply 

 constricted, sinus acute and opening widely; semicells 

 elliptical, diverging or externally lunate, angles slightly 

 produced, ending in two stout diverging spines which 

 lie in the same vertical plane; dorsal margin straight, 

 ventral margin convex, cell-wall rough with granules, 

 arranged in concentric series round the angles. Vertical 

 view triangular, sides concave, angles produced and 

 tapering, with a pair of small bifid processes at the 

 base of each ; cell- wall smooth in the centre. 



Zygospore unknown. 



Length, without processes, 20 \i ; breadth, without 

 processes, 29 JJL ; including processes, 32 \L ; breadth of 

 isthmus 9 JJL . 



ENGLAND. Brandreth, Westmoreland ! Epping 

 Forest, Essex ! New Forest, Hants ! Gunwen Moor, 

 Cornwall ! 



SCOTLAND.- -Pool beside Loch Dawan, Dalbagie, and 

 in Glen Clunie, Aberdeen (Roy & Biss.). 



IEELAND.- -Lakes east of Lough Bofin, Gal way ! 

 Near Lough Brin, Kerry ! Dublin and Wicklow (Arch.). 



Gedgr. Distribution.-- France. Germany. Austria. 

 Servia. Greenland. Central China (var.). Japan. India. 

 Azores. United States. Colombia. 



Var. Guitanense West. (PL CLV, fig. 9.) 



St. arcuatum var. Guitanense West, Alg. W. Ireland, 1892, p. 181, t. 23, f. 10. 



Differs from the type in its relatively greater length 

 and broader isthmus, angles produced into shorter 



