70 WISCONSIN PHYTOPLANKTON 



6. Staurastrum alternans De Brebisson. PL 68, Fig. 4. 



in Ealfs, Brit. Desm. 132, pi. 21, figs. 7A-7C. 1848; W. & G. S. West, Monogr. 

 Brit. Desm. 4: 170, pi. 126, figs. 8-9. 1912. 



Cells small, length about equal to the breadth, deeply constricted, 

 sinus acute-angled and with apex subacute, isthmus narrow; semicells 

 transversely elliptic, with the dorsal margin more flattened than the 

 ventral, lateral angles broadly rounded ; cell body with scattered 

 granules that are arrangetl in concentric rings near the angles. Ver- 

 tical view triangular, sides concave and angles rounded, the angles 

 of one semicell alternating with those of the other. (Tychoplanktont.) 



Zygospores spherical, furnished with spines that are bifurcate at 

 their apices (Ralfs). 



Cells 35 fx long ; 37 /t broad ; isthmus 10 fi broad. 



Speese (rrr). 



7. Staurastrum denticulatum (Nageli) Archer. PI. 68, Figs. 5-7. 



in Pritchard, A History of Infusoria, 4th. Ed. 738. 1861; W. & G. S. West, 

 Trans. Eoy Irish Acad. 33: 103, pi. 11, fig. 11. 1906. 



Phycastrum denticulatum Nageli non Kiitzing, Gatt. einz. Algen 128, pi. 8, 

 fig. C3. 1849. 



Cells small, breadth slightly greater than the length, fairly deeply 

 constricted, sinus widely open and with apices fairly acute, isthmus 

 narrow; semicells broadly triangular-ellipsoid, with ventral mar- 

 gin more strongly rounded than the dorsal, angles truncate and 

 with two very short divergent spines, angles of semicells with two to 

 four concentric rings of conical granules or short spines, the profile 

 of the cell being distinctly scalloped between the spines ; median por- 

 tion of cell body smooth. Vertical view triangular, with sides strongly 

 retuse and angles truncate and bearing two short divergent spines; 

 central portion of cell body smooth, angles with two to four trans- 

 verse rings of denticulations. Chloroplast Avith a small central mass 

 and two laminate plates running to each angle of the cell; pyrenoid 

 single and central. (Facultative planktont.) 



Zygospores unknown. 



Cells 26-29 fi long ; 32-34 fx broad with spines, 28-32 fi broad with- 

 out spines; isthmus 10 ju broad ; spines 1.5-2 ^ long. 



Beaverdam (rr), Lac Court Oreilles (sss). 



Differing from S. avicuhi De Brebisson in the fewer concentric rings 

 of granules at the angles of the cells and the shape of the semicells. 

 The angles in the vertical view are distinctly truncate and bear two 

 spines (four in an end view), while in the vertical view of 8. avicula 

 the angles are sharp and bear a single spine (two in an end view). 



Irish specimens examined in the G. S. West collection have one 

 or two more rings of granules at the angles of the semicells than do 

 the Wisconsin specimens, but this difference is of little importance. 



