42 BRITISH DESMIDIACEJ1. 



semicells subcuneate, with, slightly diverging sides, 

 apex truncate, straight, or very slightly convex. Side 

 view of semicell semi-elliptic, slightly narrowed towards 

 the base. Vertical view elliptic, ratio of axes 1 : 1*3. 

 Cell- wall smooth and often yellowish. 



Zygospore unknown. 



Length 16-18 /i ; breadth 12-15/x; breadth of 

 isthmus 10-13 /x; thickness 11-12 /x. 



SCOTLAND.- -Braes of Gright, Aberdeen ; Muchalls, 

 Kincardine; Clova Table-land, Forfar (Roy Bissett). 



Geogr. Distribution.- -Germany (var.). Galicia in 

 Austria. Poland. Xova Zembla. Franz Joseph 

 Land. Spitsbergen. Greenland. New Zealand. Porto 

 Rico (var.). 



The broad, flattened apices of this species are very charac- 

 teristic. Nordstedt has observed a triangular variety (var. 

 trigonum JSTordst.) from Spitzbergen. C. arctoum is an 

 essentially alpine and arctic species. 



Forma minor AYest. (PI. LXIX, figs. 3, 4.) 



C. arctoum fornia minor West, Alg. W. Ireland, 1892, p. 162, t, 24, f. 24. 



Cells similar to the type, but only two-thirds the size. 

 Length 12*5ju; breadth 10 ^ ; breadth of isthmus 

 8*5 p. ; thickness 7'5/x. 



IEELAXD.- -Ballynahinch, Galway ! 



Var. tatricum Racib. (PL LXIX, fig. 4.} 



C. arctoum Jsordst. var. tairicinn Racib. Norm. Desm. Polon. 1885, p. 78, 



t, 11, f. 6. 

 C. bicuneatiim (Gay) Nordst. var. tatricwn (Eacib.) Schmidle, Lappniark 



Susswasseralgen, 1898, p. 27. 



Cells much smaller than in the type and more 

 deeply constricted; angles of semicells not quite so 

 rouDcled; vertical view more narrowly elliptic. 



Length 9-14 p ; breadth 8-10*5^; breadth of 

 isthmus 5*2-7 /x ; thickness 5*6-7 /x. 



ENGLAND.- -New Forest, Hants ! 



Geor/r. Distribution. X. Sweden. Poland. Silesia 

 in Austria. Xew Zealand (var.). 



