102 BRITISH DESMtDIAOEyE. 



28 ja; length of spines 3 1-36 /x ; breadth of isthmus 

 9-5 ju. 



Hob.- -In the plankton of Loch Shin, Sutherland ! 



4. Arthrodesmus crassus W. & G. S. West. 

 (PI. CXV, figs. 8-11.) 



Arthrodesmus crassus W. & Gr. S. West, Scott. Freshw. Plankton, I. 1903, 

 p. 541, t. 14, f . 8, 9 ; Further Contrib. Plankton Scott, Lochs, 1905, p. 

 485 ; Comp. Study Plankton Irish Lakes, 1906, p. 85 ; Nordst. Index 

 Desmid. Supplem. 1908, p. 43 ; W. & G. S. West, Brit. Freshw. Phyto- 

 plankton, etc., 1909, p. 181 ; Phytoplankton Engl. Lake Distr. 1909 r 

 p. 288, fig. 5 B. 



Staurastrwn Sarsii Huitfeldt-Kaas, Plankton. Norske Yande, 1906, pp. 55, 

 156, t. 1, f. 11-17 [fio-ures incorrect]. 



Cells small, about as long as broad (without the 

 spines), moderately constricted, sinus very widely open 

 and slightly acuminate ; semicells obversely sub trian- 

 gular, sides very slightly convex, apex widely convex, 

 each angle furnished with a very short acute spine. 

 Vertical view broadly elliptic-fusiform, poles acute 

 and furnished with a very short spine. Chloroplasts 

 axile, one in each semicell with a single central 

 pyrenoid. 



Zygospore unknown. 



Length 19*5-24 p, ; breadth without spines 19-23 JJL ; 

 length of spines 1*5 JJL ; breadth of isthmus 9 '5-1 2* 5 JJL ; 

 thickness 11*5-13 ju. 



ENGLAND.- -Plankton of Ennerdale Water, Cumber- 

 land ; and Hawes Water, Westmoreland ! 



SCOTLAND.- -Plankton of Lochs Cuthaig, Fadaghoda, 

 Mor Bharabhais, and Roinebhall, Lewis, Outer 

 Hebrides ! 



IRELAND.- -Plankton of Lough Currane, Kerry ! 



Geogr. Distribution.- -Norway. 



This characteristic plankton-species is found in all the 

 British lake-areas except the Welsh, in which it Las not yet 

 been observed. 



It is of the same size as the Brazilian A. psilosporus Nordst. 

 & Lofgr. (in Wittr. & Nordst. <Alg. Exsic.' 1883, no. 558), 

 but is relatively shorter, and the semicells are never retuse 



