100 BRITISH DESMIDIACE2E. 



Var. subtriangularis (Borge) W. & Gr. S. West. (PL 



CXV, figs. 1-3.) 



A. Incus var. subtriangularis Borge, Algologiska Notiser, 1897, p. 212, 



t. 3, f . 4. 

 A. triangularis var. hebridarum W. & G. S. West, Scott. Fresh AV. Plankton, 



I. 1903, p. 542. 

 A. triangularis var. subtriangularis (Borge) W. & G. S. West, Fresh w. 



Alg. Orkneys and Shetlands, 1905, p. 24, t. 2, f . 36 ; Further Contrib. 



Plankton Scott. Lochs, 1905, p. 485 ; Comp. Study Plankton Irish 



Lakes, 1906, p. 85 ; British Freshw. Phytoplanktou, etc., 1909, p. 181 ; 



Phytoplankton Engl. Lake Distr. 1909, p. 138. 

 A. granulatus Larsen, Ferskvandsalg. Vest-Gronl. 1907, p. 324, t. 7, f. 7- 



Slightly larger, with more robust semicells as in 

 var. inflatus ; lower margins of semicells indented just 

 above the isthmus ; apex much elevated, convex, but 

 retuse in the middle; cell- wall commonly possessing 

 a number of irregularly scattered scrobiculations. 



Length 30-42; breadth without spines 23-32 /x; 

 with spines 75-84*5 /*; breadth of isthmus 8-8'5/x. 



ENGLAND.- -In the plankton of Ennerdale Water 

 and Thirlmere, Cumberland ! Plankton of Codale and 

 Easdale Tarns, Westmoreland ! 



WALES.- -Plankton of Llynau Dywaunedd, Carnar- 

 vonshire ! 



SCOTLAND.- -Frequent in the Scottish plankton, 

 especially in the lakes of Sutherland, Ross, Inverness, 

 Argyll (Borge), and the Outer Hebrides ! Shetlands ! 



IRELAND.- -Lakes near Recess, and between Clifdeii 

 and Eoundstone, Galway ! 



Geogr. Distribution.- -West Greenland. 



This northern variety of A. triangularis appears to be 

 mostly confined to the plankton. In the " piuched-in 3 

 character of the semicells just above the isthmus it is strictly 

 analogous to the var. indentatns of A. Incus. The scrobicu- 

 lations on the cell-wall are variable both in number and 

 disposition,, and are sometimes very indistinct. 



A most interesting form with triangular semicells (in 

 vertical view) occurred in the plankton of Easedale Tarn, 

 Westmoreland. This we have named forma TEIQUETEA W. 

 & GL S. West, ' Phytoplankton Engl. Lake Distr/ 1909, 

 p. 288, and text-fig. 5 d. (PL CXV, fig. 4). It is another of 

 those connecting forms between the g'enera Arthrodesmus and 



