20 BRITISH DESMIDIACE.E. 



and Loch Shiel, Inverness ! Plankton of Loch Tay, 

 Perthshire ! ; Loch Doon, Ayrshire ! ; Six lochs in Lewis, 

 Outer Hebrides ! Plankton of the Shetlands ! 

 Geogr. Distribution. Norway. Finland. 



St. curvatum is most commonly found in the western parts of 

 the British Isles, and is frequent in the plankton of nearly all 

 the British lakes, especially those in the area of the older palaeo- 

 zoic rocks. It is almost entirely a plankton species, but has 

 occasionally been reported from bogs. 



52. Staurastrum megacanthum Lund. 

 (PL CXXXI, figs. 7, 8.) 



Staurastrum megacanthum Lund. Desm. Stiec. 1871. p. 61, t. 4, f . 1 ; Wills in 

 Midi. Nat. 1881, p. 16, t. 5, f. 7 ; Wolle, Desm. U. S. 1884, p. 121, t. 51, 

 f. 10-12 ; Cooke, Brit, Desm. 1887, p. 142, t. 49, f . 7 ; De Toni, Syll. 

 Alg. 1889, p. 1141 ; Roy & Biss. Scott. Desm. 1893, p. 22 ; Schmidle, 

 Lappm. Susswasseralgen, 1898, p. 50, t. 2, f. 34 (forma) ; West & G. S. 

 West, Alg. N. Ireland, 1902, p. 44 ; Alga-fl. Yorks, 1902, p. 96. 



Cells of medium size, about as long as broad, not 

 including the spines, or a little shorter, very deeply 

 constricted, sinus acute- angled, sometimes almost rect- 

 angular ; semicells triangular, or more often fusiform 

 in outline, dorsal margin straight or slightly convex, 

 ventral margin rather more inflated, angles gradually 

 attenuated, each ending in a strong spine. Vertical view 

 triangular or quadrangular, sides concave, each angle 

 produced into a stout spine. Chloroplast axile, with 

 a central pyrenoid and a pair of lobes projecting into 

 each angle. 



Zygospore unknown. 



Length 43-50 p ; breadth, not including the spines, 

 48-57 pi ; length of spines 11-18 \i ; breadth of isthmus 

 1 2-14-5 [ju 



ENGLAND. Wastdale, Cumberland ! Plankton of 

 Stickle Tarn, Westmoreland ! Pilmoor, N. Yorks ! 

 Rice-all Common, E. Yorks ! 



WALES. Capel Curig, Carnarvonshire (Cooke & Wills) ! 

 Plankton of the Welsh Lakes ! 



SCOTLAND. Aberdeen, Kincardine, Perth and Argyle 

 (Roy & Biss.). Loch Ghriama, Sutherland ! Loch 



