96 WISCONSIN PHYTOPLANETON 



33. Staurastrum tetracerum (Kiitzing) Ralfs. 



Ann. & Mag. of Nat. Hist. 15: 150, pi. 10, figs. lA-lC. 1845; Brit. Desm. 137, 

 pi. 23, figs. 7A-7F. 1848; LundeU, Nova Acta Eeg. Soe. Sci. Upsaliae, 3 Ser, 8: 

 68. 1871. 



Cells verj'- small, length (with processes) about equal to the 

 breadth, deeply constricted, sinus obtuse-angled and with apices sub- 

 acute, isthmus narrow; semicells obversely triangular, with apices 

 very slightly retuse; angles continued in delicate slightly tapering 

 processes with sharply crenulate margins; ends of processes without 

 teeth ( ? ) . Vertical view compressed and body of semicells not much 

 broader than the processes. Semicells almost invariably twisted at 

 the isthmus. 



C 



Fig. 11. Staurastrum tetracerum Ealfs (x800). Specimens collected at Moidart, 

 Scotland, by W. & G. S. West. 



Zygospores spherical, with about sixteen processes that are dilated 

 at the base and furcate at the apex (LundeU). 



Cells 20-23 {x long with processes, 10 fi long without processes ; 

 breadth with processes 22.5 ix ; breadth at isthmus 5 /x. 



Zygospores 30 fx. in diameter with processes, 16 /a without processes 

 (LundeU). 



Not found in the plankton of Wisconsin lakes. 



var. VALIDUM W. & G. S. West. PI. 76, Figs. 9-10. 

 Jour. Eoy. Micr. Soc. 1897: 495, pi. 6, fig. 25. 1897. 



Cells somewhat larger, deeply constricted and with sinus rectangu- 

 lar; semicells obversely triangular and with point of origin sharply 

 defined; length of processes somewhat greater than the breadth of 

 the semicells ; processes with margins subparallel and finely crenu- 

 late, terminating in four minute spines ; body of semicell smooth but 

 with a finely crenulate outline; semicells not twisted at the isthmus. 

 (Facultative planktont.) 



Cells 36-43 /x long with processes, 16-20 fx long without processes; 

 breadth with processes 46-52 /n ; without processes 14-17 /x ; breadth 

 at isthmus 6-7 /x ; thickness 7.5 fi. 



Duck (r). 



The variety as observed in our plankton has the processes more 

 finely crenulate than W. & G. S. West show and does not have the 

 smooth outline of the body of the semicell. It might possibly be a 

 small variety of 8. chaetoceras (Schroder) G. M. Smith but has been 

 excluded since no granules were observed within the margins of the 

 body or the processes of the cells. 



