axes of the cells radiating from a common center; chloroplast a 

 parietal plate, with 1 pyrenoid; cells 3-5.6/a in diameter, 12-22/x long. 

 Common in plankton. Mich., Wis. 



Actinastrum Hantzschii var. elongatum G. M. Smith 1918, p. 636 



PI. 65, Fig. 2 

 Cells cylindrical, very slightly if at all narrowed toward the apices, 

 larger than in the typical plant; cells 4-5/a in diameter, 30-35/i, long. 

 Rare; in plankton. Wis. 



Actinastrum Hantzschii var. fluviatile Schroeder 1899, p. 20 



PL 65, Fig. 1 

 A variety differing from the typical in that the cells are sharply 

 pointed; cells 3.3-3.5//. in diameter, 39-42/x long. 

 Rare in a swamp. Wis. 



TETRADESMUS G. M. Smith 1913, p. 76 

 A 4-celled, free-floating colony; cells fusiform, or cylindrical, 

 arranged in 2 planes with their longitudinal axes parallel, tlie 

 adjoined walls straight, in contact throughout most of their length, 

 the outer free wall straight or concave, the poles of the cell directed 

 away from the center of the colony; cells in vertical view spherical, 

 arranged in a quadrangle; chloroplast a parietal plate with 1 

 pyrenoid. 



Key to the Species 



Cells fusiform, attached throughout the length 



of their lateral walls T. wisconsinense 



Cells crescent-shaped, cylindrical, attached along only a 



portion of their lateral walls T. Smithii 



Tetradesmus Smithii Prescott 1944, p. 360 

 PI. 64, Figs. 15-17 



Cells slightly arcuate or crescent-shaped or subcylindrical, slightly 

 narrowed toward the poles which are broadly rounded, in groups 

 of 4 with their long axes parallel and with the convex walls adjoined 

 in the mid-region only; outer free walls concave or nearly straight; 

 cells in end view spherical, arranged in a quadrangle; chloroplast 

 a parietal plate; cells 3.7-4/x in diameter, 27-29/a long. 



This species should be compared with Quadrigula spp. The cells 

 are not so straight as in that genus although they have about the 

 same proportions and are arranged in bundles of 4. Also Tetradesmus 

 Smithii, unlike Quadrigula, has no gelatinous investment inclosing 

 the quartets of cells. 



Rare; in plankton. Wis. 



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