WISCONSIN PHYTOPLANETON 



111 



isthmus narrow; semicells broadly campanulate, with apices slightly 

 tumid and with pairs of minute denticulations between the processes, 

 body of semicells smooth; angles continued in long slightly tapering 

 processes that terminate in three minute denticulations, dorsal mar- 

 gin of processes iinely crenulate, ventral margin smooth or slightly 

 crenulate, processes strongly convergent. Vertical view five-radiate 

 (rarely six-radiate), the processes of one semicell alternating 

 slightly with those of the other semicell; body of cell pentagonal (or 

 hexagonal) with the sides somewhat emarginate, ornamented with a 



Fig. 13. Staurastrum arachne EaKs (x625). From plankton material collected 

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



pair of intramarginal denticulations between each pair of processes, 

 sometimes with two more delicate denticulations, one on each side 

 towards the base of the processes; central area without ornamenta- 

 tion; processes tapering slightly, straight or curved in the same 

 direction, lateral margins finely crenulate and with a single median 

 row of minute denticulations. Chloroplast axial, fairly massive, with 

 two laminate lobes running into each process; pyrenoid single and 

 central. 



Zygospores unknown. 



Cells 32 fi long ; breadth with processes 60-65 /i, without processes 

 15-18 {ji ; breadth at isthmus 7.5 /x. 



Not found in the plankton of Wisconsin lakes. 



Resembling 8. pentacerum (WoUe) G. M. Smith in the five-radiate 

 character of the semicells but much smaller and with a totally differ- 

 ent shape of the semicells in front view. In this species the con- 

 vergence of the processes is a constant feature (in the type) whereas 

 in 8. pentacerum the processes are parallel. The ornamentation of 

 the cell body, as seen in the vertical view, is subject to some variation; 

 the two denticulations lying midway between the processes are always 

 present but the accessory denticulations flanking them are frequently 

 lacking. The processes may be straight or all may be curved in the 

 same direction as in 8. cyrtocerum De Brebisson. 



