gg WISCONSIN PHYTOPLANKTON 



4. Staurastrum brevispinum De Brebisson, 



in Ralf s, Brit. Desm. 124, pi. 34, figs. 7A-7D. 1848 ; W. & G, S. West, Monogr. 

 Brit. Desm. 4: 145, pi. 123, figs. 1-3. 1912. 



Cells fairly small, length and breadth equal, deeply constricted, 

 sinus open and with apex sublinear to acute, isthmus fairly narrow; 

 semicells broadly elliptic, with ventral side more rounded than the 

 dorsal, median portion of the strongly tumid ventral side truncate 



Pig. 7. Staurastrum brevispinum De Brebisson (x600). Plankton specimens 

 from Irish lakes. Figs. A-B from Lough Corrib, Fig. C from Lough Leane, Figs. 

 D-E from Lough Neagh. All from the G. S. West collection. 



to very slightly retuse, lateral angles rounded and terminating in 

 a small mucro. Vertical view triangular, sides of the cell body some- 

 what retuse and angles subacute to rounded; angles with a single 

 rounded mucro. Cell wall smooth. Chloroplast axial, with a small 

 central mass and two laminate arcuate arms running to each cell 

 angle ; pyrenoid single and central. 



Zygospores unknown. 



Cells 35-42 ju, long ; 35-42 fx. broad ; isthmus 10-13 {i broad. 



Not found in the plankton of Wisconsin lakes. 



The type of the species varies in the amount of curvature of the 

 upper and lower halves of the semicells and the incurving of the sides 

 of the cells when viewed from above. The mucro at the angles dis- 

 tinguishes the species from all others found in the state. 



var. BoLDTii Lagerheim, PI. 67, Figs. 16-17. 



Nuova Notarisia 8: 163. 1893. 



Staurastrum brevispinum forma Boldt, Ofvers. kgl. Sv. Vet.-Ak. Forh. 42, 

 No. 2: 113, pi. 5, fig. 30. 1885. 



Length of cells one and a quarter times the breadth, apical portion 

 of sinus more linear than the type; apex of semicells more tumid. 

 Vertical view triangular, with sides retuse. (Facultative planktont.) 



Cells 40 jx long ; 36-38 /x broad ; isthmus 9 [i broad. 



Shell (r). 



Approaching the variety alatum W. & G. S. West, but with the 

 curvature of the two sides of the semicells about the same. 



