WISCONSIN PHYTOPLANKTON 123 



var. ARMiGERUM forma gracillinmm n. forma. PI. 83, Figs. 8-11. 



Staurastrum furcigerum var. armigerum (De Brebisson) Nordstedt Vidensk. 

 Medd. f. d. naturh. Foren. i Kiobenhavn 1888: 207. 1888. 



Staurastrum armigerum De Brebisson, Mem. de la soc. imp. d. Sc. nat. de Cher- 

 bourg 4: 136, pi. 1, fig. 22. 1856. 



Superior whorls containing six (not three) processes arismg in 

 pairs at the base of the processes of the inferior whorl and lying (in 

 the vertical view) at right angles to the sides of the cell. Processes 

 long and delicate and with their apices bifurcate. (Facultative 

 planktont.) 



Cells 63-68 /x long with processes, 30-32 [x long without processes ; 

 breadth with processes 67-73 /*, without processes 27-30 [x ; breadth 

 at isthmus 12-15 ix. 



Shell (r). 



Three very similar species have been described in which there are 

 three processes in the lower whorl and six in the upper. Nordstedt 

 (I. c.) has shown that two of these become the varieties eustephanum 

 (Ehrenberg) Nordstedt and armigerum (De Brebisson) Nordstedt of 

 S. furcigerum; while the third S. pseudofurcigerum Reinsch cannot 

 be recognized but belongs in part to the variety armigerum and in 

 part to the variety eustephanum. The variety armigerum is char- 

 acterized by long delicate processes with undulate margins and bifid 

 ends while the processes of the variety eustephanum are short and 

 have the ornamentation of the type. 



58. Staurastrum leptacanthum Nordstedt. PI. 83, Figs. 12-14. 



Vidensk. Medd. f. d. naturhist. Foren. i Kiobenliavn 1869; 229, pi. 4, fig. 46. 

 1870. 



Cells fairly large, length (with processes) about the same as the 

 breadth, fairly deeply constricted, sinus widely open and with the 

 apex subacute, isthmus narrow ; semicells subcircular to subhexagonal 

 in outline, with the ventral margins slightly tumid, dorsal margins 

 and apex tumid; cell body punctate and with processes arising in 

 two whorls; lower whorl arising midway between the isthmus and 

 the apex of the semicells, containing six processes (four visible in 

 front view) ; processes subparallel, long and very slightly attenu- 

 ated, terminating in a bifurcation that is continued in short spines, 

 processes without ornamentation; superior whorl of processes sub- 

 apical in insertion, processes four in number and similar to those of 

 the lower whorl but lying at an angle of 45 degrees with the vertical 

 axis of the semicell. Vertical view hexagonal, with sides slightly 

 concave and angles continued in straight processes that terminate in 

 a bifurcation, the two arms of the bifurcation lying in the same ver- 

 tical plane, processes without ornamentation ; central portion of cell 

 body with four similar processes. Chloroplast with a comparatively 

 small central mass and a single ray running into each process of both 



