ARTHRODESMUS. 103 



either at the sides or the apex; the vertical view has also 

 more acute poles. 



In its general form A. crassus agrees fairly well with A. 

 controversns W. G. 8. West, but the latter is very much 

 smaller and more delicate, with a broadly elliptic vertical 

 view. 



This species sometimes occurs in great abundance in the 

 plankton, the cells secreting a large quantity of mucus. 

 The fibrillar structure of this mucus Avas described by 

 Huitfeldt-Kass in 1906 as an armature of spines. 



5. Arthrodesmus controversus W. & G-. S. West. 



(PL CXV, figs. 12-14.) 



Arthrodesmus ? glaucescens Wittr. forma convexa West, Alg 1 . AY. Ireland, 

 1892, p. 170, t. 22, f . 10 ; Alg\ Engl. Lake Distr. 1892, p. 730, t. 9, f . 27 ; 

 Larsen, Ferskvandsalg. Vest-Gronl. 1907, p. 324. 



A. controversus W. & Gr. S. AYest, New Brit, Freshw. Alg. 1894, p. 9 ; Nordst. 

 Index Desm. 1896, p. 81 ; AY. & G. S. West, Alg. N. Ireland, 1902, p. 59. 



Cells minute, a little longer than broad (without the 

 spines), moderately constricted, sinus obtuse-angled 

 and very Avidely open, with an acuminate apex; semi- 

 cells obversely subtriangular or cuneate, sides and 

 apex convex, angles furnished Avith a A T ery minute 

 short spine. Side vie AY of semicell subcircular. 

 Vertical A r iew broadly elliptic, Avith a very minute 

 spine at each pole. Cell- wall smooth. 



Zygospore globose, furnished \vith long, simple 

 spines. 



Length 10' 5-1 2 //, ; breadth without spines 1011*5 /x, 

 with spines ll-lo'6/t; breadth of isthmus 5-5*9 /x ; 

 thickness 5'5-6*5//,; diam. zygosp. AYithout spines 

 16* 5 />t, with spines 29 ^. 



ENGLAND. - HaAA'kshead, Lancashire ! Wastdale, 

 Cumberland (with zygos pores) ! 



IRELAND.- -Near Lough Mag-rath, Donegal ! SlieA^e 

 Donard, DoAYn ! Creggan Lous;*li, Mavo ! Adrigole, 

 Cork ! 



The minute plant originally described by Wittrock as 

 " Art! a- ode sinus? glaucescens ' lias since been found to 

 belong to the genus Tetrapedia. As this minute alga is a 

 typical Desrnid in every respect, and conjugates to form 



