30 BRITISH DESMIDIACE^E. 



centre, apex of semicell almost smooth. Chloroplast 

 axile with a central pyrenoid in each semicell. 



Zygospore unknown. 



Length, without spines, 35-39 y. ; breadth, without 

 spines, 35-43 y. ; breadth of isthmus 10-13 (JL ; length of 

 spines 3 '5-1 2 pt. 



ENGLAND. Cumberland ! Westmoreland ! W., N., 

 and E. Yorks ! Essex ! Oxford ! Hants ! Warwicks ! 

 Dartmoor, Devon (Harris). Cornwall (Rolfs}. 



SCOTLAND.- -Near Tain, Ross ; near Brin, Inverness ; 

 near Alford, S. of Birsemore, Dalbagie, Aberdeen (Roy 

 & Biss.). Craig an Lochan, Perth ! Mull in Argyle 

 (0. Borge). 



IRELAND. --Foxford, Co. Mayo! Derryclare Lough 

 and Oorid Lough, Co. Galway ! Near L. Brin, Co. 

 Kerry. 



Geogr. Distribution.- -France. Germany. Roumania. 

 Norway. Finland. Sweden. Faeroes. Greenland. 

 Siberia. Azores. United States and Alaska. 



The granules in the vicinity of the angles often tend to 

 become developed into small spicules in this species, so that 

 the angular spines may seem to be duplicated. 



Var. planctonicum W. & G. S. West. (PL CXXXIII, 



figs. 20-22.) 



Staurastrum lunatum var. planctonicum West & G. S. West, Scott. Freslnv. 

 Plankton, I, 1903, p. 546, t. 16, f. 11, 12; Freshw. Alg. Orkneys and 

 Shetlands, 1905, p. 24 ; Comp. Study Plankton Irish Lakes, 1906, p. 103 ; 

 Brit. Freshw. Phytoplankton, 1909, p. 174. 



Cells relatively broader and attaining a larger size 

 than in the type, angles of semicells more acute and 

 ending in a shorter spine. Vertical view triangular, 

 angles gradually tapering to a short spine, lateral mar- 

 gins broadly retuse. 



Length 40-44 \L ; breadth, without spines, 42-50 \L ; 

 breadth of isthmus 14'5-16 fi ; length of spines 3-5'5 \L. 



ENGLAND.- -Plankton of Crummock Water, Ennerdale 

 Water, Bassenthwaite Water and Wast Water, Cumber- 



