148 BRITISH DESMIDIACEJE. 



Forma EETUSA (Borge) nob. St. brevispinum var. ret mum 

 Borge, Siissw. Chlor. Archang. 1894, p. 36, t. 3, f . 42 ; W. & G. 

 S. West, Scott. Fresliw. Plankton, I. 1903, p. 546, t, 14, f. (i. 



Apex of semicell slightly retuse in the middle, angles of 

 semicells very broadly rounded. (PI. CXXIII, fig. 6.) 



Length 43-52 /n breadth 36-41 /< ; breadth of isthmus 

 13-14 ,u. 



SCOTLAND. Loch Nan Cuiime and Loch Rnar, Sutherland ! 

 (J. Murray']. Loch Fadaghoda, Lewis, Outer Hebrides f 



Geogr. Distribution. X. Russia. 



Var. altum W. & G. S. West. (PI. CXXIII, fig. 5.) 



St. brevispinum var. alt inn W. & G. S. West, Fm-ther Contrib. Fresh w. 

 Plankton Scott. Lochs, 1905, p. 502, t. 7,1 16 ; Comp. Study Plankton Irish 

 Lakes, 1906, p. 86 ; Brit. Fresh \v. Phytoplankton, etc., 1909, p. 181. 



Cells longer than broad, dorsal margin of semicells 

 very strongly convex (almost subsemicircular) ; in the 

 vertical view with the sides less concave and the 

 angles thicker. 



Length 57-G6//,; breadth 43-50 p. ; breadth of 

 isthmus 12'5-15'3 p.. 



SCOTLAND.- -Plankton of Lochs Xan Cninne and 

 Ruar, Sutherland ! (J". Mnrrrn/) ; Lochs Fadaghoda, 

 an Sgath, and an Tomain, Lewis ; and Loch Diracleet, 

 Harris, Outer Hebrides ! 



IRELAND.- -Plankton of Lough Corrib, Gralway ! 



Var. obversum ^Y. & G. S. West. (PL CXXIII, 



fig. 7.) 



St. brevispinum var. obversum W. & G. S. West, Further Contrib. Fresliw- 

 Plankton Scott. Lochs, 1905, p. 502, t. 7, f. 15 ; British Fresliw. Phyto- 

 plankton, etc., 1909, p. 181. 



Cells broader than long, sinus narrow and sub- 

 linear towards the extremity ; semicells obversely 

 semi-elliptic, with a prominent papilla at each superior 

 angle, dorsal margin slightly convex, ventral margin 

 very strongly convex. 



Length 38-42 p. ; breadth 45-49*5 JJL ; breadth of 

 isthmus 10'5-11/x. 



SCOTLAND. Plankton of Loch Fadaghoda, Lewis, 

 Outer Hebrides ! 



