WISCONSIN PHTTOPLANKTON 163 



KEY TO THE SPECIES. 



Spines on outer face of cells short (4-8 M) S. SPINTJLOSUM 



Spines on outer face of cells long (10-15 /*) S. AMEBICANUM 



SORASTRUM SPINULOSUM Nageli. PI. 44, Figs. 4-5. 



Gatt. einz. Algen 99, pi. 5, figs. Da-Dd. 1849; G. S. West, Brit. Freshw. Algae 

 215, figs. 89A-89B. 1904. 



Coenobe of 4^8-16-32 cells. Cells broadly renii'orm to cuneate, with 

 four short horns at the corners of the outer face. Stipe leading to 

 sphere at center of colony short. Chloroplast single, generally filling 

 the entire cell; pyrenoid single. (Tychoplanktont). 



Cells 8-20 ^ broad, 6-18 /*, long, 5-8 p, thick. Horns of adult cells 

 4-8 fji long. 



Hillman (rrr), Horse (rrr). 



SORASTRUM AMERICANUM (Bohlin) Schmidle. PI. 44, Fig. 1. 



Engl. Jahrb. 27: 230. 1900: Lemmermann, Arch. f. Hydrob. u. Planktonkde. 

 5: 310, fig. 5. 1910. 



Selenosphaerium americanum Bohlin, Bin. t. Kgl. Sv. Vet.-Ak. Handl. 23, 

 Afd. 3, No. 7: 40, pi. 2, figs. 38-41. 1897. 



Coenobia spherical, solitary, of 16-32-64-128 (generally 16 or 32) 

 cells. Cells cardiate to pyramidate, somewhat flattened, with the apex 

 towards the center of the coenobe and ending in a broad gelatinous 

 stalk as long as the cell. Stalk cylindrical, 5-6 faceted at the base; 

 with the facets in contact with those of other stalks and forming a hol- 

 low sphere. Outer face of cells emarginate, with a single long spine at 

 each of the four corners. (Facultative Planktont). 



Cells 7-20 ^ broad, 5-20 p. long (without stalk), 4-8 /* thick. Spines 

 of adult cells 10-15 /* long. 



Devils (1) (rr), Devils (2) (rrr), Nancy (rrr), Travis (rr). 



The longer spines and stalk, together with the more prominent central 

 sphere, separates this species from the foregoing. 



var. UNDULATUM G. M. Smith. PI. 44, Figs. 2-3. 

 Trans. Wis. Acad. Sci., Arts, & Lett. 19 1 : 640, pi. 13, fig. 1. 1918. 



Sides of facets at base of stipe undulate. Dimensions as in the typi- 

 cal form. (Facultative Planktont). 



Beaverdam (rrr). 



