34 BRITISH 



strongly so, angles very slightly produced (almost 

 imperceptibly) and provided with two stout spines of 

 varying length, projecting obliquely outwards and lying 

 in the same vertical plane, the two spines either parallel 

 or converging. Vertical view triangular, sides slightly 

 concave and angles broadly rounded. Cell- wall very 

 thick and distinctly punctate. Chloroplasts numerous, 

 in the form of parietal bands running longitudinally, 

 with numerous scattered pyrenoids. 



Zygospore unknown. 



Length 90-1 20 ^ ; breadth, without spines, 73-1 00 ui ; 

 breadth of isthmus 36-41 ^ ; length of spines 9'o-32'5 pi. 



ENGLAND.- -Plankton of Ennerdale Water, Cumber- 

 land, and Grasmere, Westmoreland ! 



WALES. Capel Curig ! (Cooke & Witts) and Llyn-y- 

 cwm-ffynon, Carnarvonshire ! 



SCOTLAND.- -Plankton of Loch Shin, and Rhiconich, 

 Sutherland ! Plankton of Loch Shiel, Inverness ! Plank- 

 ton of Lochs Fadaghoda and Stranabhat, Lewis, and 

 Loch nan Eun, N. Uist, Outer Hebrides ! 



IRELAND. Near Lough Magrath, Donegal ! Lough 

 Aunierin, Co. Galway ! Plankton of Lough Currane, 

 Kerry ! Adrigole, Co. Cork ! 



Geoyr. Distribution. Norway. Sweden. India. Aus- 

 tralia. United States. 



St. longispinum is a large and characteristic species which 

 could not easily be confused with any other. It is very 

 abundant in plankton. The spines are variable in length, 

 and all stages occur between the typical long-spined form and 

 the form known as var. bidentatum, in which they are very re- 

 duced. 



Var. bidentatum (Wittr.) West. (PI. CXXXIV, figs. 2, 3.) 



St. bidentatum Wittr. Skand Desm. 1869, p. 16, f. 7. 

 St. (Pleurenterium) longispinum Lund. Desm. Suec. 1871, p. 73, t. 5, f. 1. 

 N/. longispinum Cooke, Brit. Desm. 1887, p. 164, t. 56, f. 1. 

 tit. longispinum, var. bidentatumWeat & G. S. West, Scott. Freshw. Plankt. I, 

 1903, p. 546 ; Cushman in Rhodora, 1905, vol. 7, p. 263, t. 64, f. 13. 



Similar to the type in every way except that the spines 



