154 BRITISH DESMIDIACEJi:. 



obtuse. They may be horizontally disposed, but are more 

 often slightly turned upwards. The shortly cylindrical 

 isthmus is also characteristic. 



St. inelegans undoubtedly stands near to St. Clepsydra, but 

 differs in its more produced angles, its more elevated apices, 

 its elongated isthmus,, and in the form of the vertical view. 

 It is also a larger species. 



19. Staurastrum angulatum West. 

 (PL CXXIII, figs. 8, 9.) 



Staurastrum angnlatum West, Desm. Mass. 1SS9, p. 20, t. 3, f. 20 ; Nordst. 

 Index Desm. 1896, p. 44. 



Cells of medium size, about 1J times as long as 

 broad, deeply constricted, sinus widely open and acute- 

 angled ; semicells rhomboid with straight sides and 

 very slightly rounded angles. Vertical view triangular, 

 sides faintly retuse in the middle, angles a little inflated 

 and subacute. Cell-wall smooth. 



Zygospore unknown. 



Length 76-79 p, ; breadth 60-62 ^ ; breadth of 

 isthmus 17-18'5/x. 



Geocjr. Distribution.- -United States. 



The typical form of this species is not known to occur in 

 the British Islands. It was originally found in collections 

 from Amherst, Massachusetts. There is a faint indication of 

 an obscure mucro at the angles of the semicells. 



Var. planctonicum AY. & G. S. West. (PL CXXIII, 



fig. 10.) 



St. angulatum var. planctonicum W. & G. S. West, Scott. Freshw. Plank- 

 ton, I. 1903, p. 551; t. 16, f. 10; Brit. Freshw. Phytoplankton, etc., 

 1909, p. 181. 



Cells smaller, almost as broad as long, semicells 

 rhomboid-depressed, each angle furnished with a small 

 apiculus. 



Length 47 ^ ; breadth 44-46 /x; breadth of isthmus 

 9-5-10-5 /x. 



SCOTLAND. Plankton of Loch Shin, Sutherland ! 



