72 WISCONSIN PHYTOPLANKTON 



10. Staurastrum aristiferum Ealfs. 



Brit. Desm. 123, pi. 21, -figs. 2A-2B. 1848. 



Cells of medium size, length (without spines) slightly greater than 

 the breadth, deeply constricted, sinus widely open and with a rounded 

 apex, isthmus elongate and narrow; semicells obversely triangular, 

 with dorsal margins somewhat retuse and ventral margins indented 

 midway between the isthmus and the apex ; angles mamillate and 



Fig. 8. Staurastrum aristiferum Ealfs (x625). From plankton specimens col- 

 lected in Capel Curig lakes, Wales, by W. & G. S. "West. 



terminating in long delicately tapering spines with acute apices, spines 

 outwardly divergent. Vertical view quadrangular (rarely triangu- 

 lar), with sides strongly retuse, angles rounded and bearing a single 

 straight delicately tapering spine. Cell wall smooth. 



Zygospores unknown. 



Cells 58 jM long with spines. 27.5 ju, long without spines ; breadth 

 45 fx with spines, 25 /a without spines ; isthmus 6 fi broad ; spines 

 15-18 /x long. 



Not found in the plankton of Wisconsin lakes. 



var. indentatum var. nov. PI. 68, Figs. 14-17. 



Cells smaller than the type, isthmus narrower and not elongate ; 

 ventral margin of semicells slightly tumid and smooth in outline, 

 angles mamillate. Vertical view triangular, sides slightly concave 

 and with a sharp indentation midway between the angles. (Euplank- 

 tont.) 



Cells 35-44 /a long with spines, 20-22 ju, long without spines ; breadth 

 46-52 /x with spines, 22-25 /x without spines; isthmus 5-6.5 /a broad; 

 spines 15-18 jx long. 



Devils (r). 



The typical S. aristiferum differs from other Staurastra both in 

 the indentation of the ventral margins of the semicells in the front 

 view and in the indentation of the sides in the vertical view. This 

 variety has the indentation in the vertical but not in the front view. 

 It might well be placed with other species bearing a single spine at 

 each angle of the semicells but the indentation of the cell shows that 

 it belongs to S. aristiferum. 



