160 BRITISH DESMIDIACE.i;. 



22. Staurastrum retusum Turn. 

 (PI. CXXV, figs, 6, 7.) 



Staurastrum retusum Turn. Freshw. Alg. E. India, 1893, p. 104, t. 13, f. 13; 



Nordst. Index Desm. 1896, p. 224. 

 St. retusum var. punctulatum Eichl. & Gutw. Nonn. spec. alg. nov. 1894, 



p. 174, t. 5, f . 45 ; W. & G. S. West, Fresliw. Alg. Ceylon, 1902, p. 178 ; 



Freshw. Alg-. Burma, 1907, p. 216, t. 15, f. 30-32. 



Cells small, as long as broad, very deeply constricted, 

 sinus narrowly linear with a slightly dilated extremity ; 

 semicells shortly pyramidate-trapeziform, angles a 

 little rounded, lateral margins straight, convex, or 

 concave, apex very slightly concave. Vertical view 

 triangular, angles rounded, sides concave. Cell- wall 

 finely punctate, punctulations much more pronounced 

 at the angles. 



Zygospore unknown. 



Length 15'5-30/x; breadth 15*5-30 p ; breadth of 

 isthmus 3*6-10 p.. 



Geogr. Distribution. Poland. India. Ceylon. Burma. 



We have mentioned on two previous occasions that the var. 

 punctulatum should most prohahly be regarded as identical 

 with the original form. -All the tropical forms of this species 

 which have been found since the publication of Turner's 

 description are punctulate, and Turner himself,, judging by his 

 expression " glabras ? ", was doubtful concerning this point. 

 It should also be remembered that the figure given by Turner 

 is from an old drawing of Wallich's, and is probably far from 

 good. 



The typical form of this species is not known to occur in 

 the British Islands. 



Var. boreale W. & GL S. West. (PI. CXXV, fig. 8.) 



St. retusum var. boreale W. & G. S. West, Freshw. Alg. Orkneys and 

 Shetlands, 1905, p. 25, t. 2, f. 30. 



Cells smaller ; semicells more rounded, lateral 

 margins convex, apices very slightly retuse ; cell- wall 

 smooth. 



Length 17'5-19/x; breadth lG'2-17'5 JJL; breadth of 

 isthmus 5' 5 ^. 



SCOTLAND.- -Hoy, Orkneys ! In bog near Lerwick, 

 Shetlands ! 



