ARTIIRODESMUS. 101 



Staurastrum, and some individuals were observed in which one 

 semicell was of the normal elliptic form whereas the other 

 was triangular (v'nlf. PI. CXV, fig. 5). 



The recently described A. granulatus Larsen, which, by 

 the way, is scrobiculate and not granulate, is merely an 

 arctic form of this variety in which the spines are a little 

 reduced. 



3. Arthrodesmus quiriferus W. & Gr. S.. West. 



(PI. CXV, figs. 6, 7.) 



Arthrodesmus quiriferus W. & G. S. West, Scott. Freshw. Plankton, I. 

 1903, p. 542, t. 17, f. 9, 10; Brit. Freshw. Phytoplankton, etc., 1909, 

 p. 181. 



Cells of medium size, a little longer than broad 

 (without spines), fairly deeply constricted, sinus very 

 widely open and rounded ; semicells obversely sub- 

 triangular with convex sides and a widely concave 

 apex, each angle furnished with a strong, straight, 

 divergent spine of great length. Vertical view elliptic 

 with a strong, long spine at each pole. Cells usually 

 twisted at the isthmus. 



Zygospore unknown. 



Length without spines 28-31 p. ; breadth without 

 spines 21-23 /x; length of spines 31-44 /x; breadth of 

 isthmus 6-9*5 /A; thickness 10-1 1 p.. 



SCOTLAND.- -In the plankton of Loch Shin, Suther- 

 land ! In the plankton of Loch Laxadale, Harris, 

 Outer Hebrides ! 



In the plankton of Loch Shin this species was very 

 abundant. It is distinguished from the biradiate form of 

 Staurastrum jaculiferum by the outward form of the cells, 

 with retuse apices, and by the longer spines. The cells are 

 almost invariably twisted at the isthmus. 



Forma compacta. (PL CXV, fig. 18.) 



Semicells stouter, sinus not quite so widely open and 

 rather narrowed towards the extremity; spines not 

 quite so long. 



Length without spines 30 ^ ; breadth without spines 



