lateral margins convex; sides of the cell concave between the lobes; 

 cells 25-33- ( 40 )yLi in diameter. 

 Tychoplankter. Mich., Wis. 



Tetraedron trigonum ( Naeg. ) Hansgirg 1888a, p. 130 

 PI. 61, Figs. 11, 12 

 Cells flat, 3-angled, the angles tapering to sharply rounded, spine- 

 tipped apices; margins convex; sides of the cell body concave or 

 straight; cells 19-29.8/x in diameter. 



Common in the tychoplankton of several lakes. Mich., Wis. 



Tetraedron trigonum var. aracile (Reinsch) DeToni 1889, p. 598 



PL 61, Figs. 14-16 



Cells flat, triangular, the angles narrower and more produced 

 than in the typical plant, sometimes curved, tapering acutely and 

 ending in a spine; cells 25-40/a in diameter, including the spines. 



Euplankter. Wis. 



Tetraedron trigonum var. papilliferum ( Schroed. ) 



Lemmermann ex Brunnthaler 1915, p. 149 



A variety with only slightly concave margins, the angles tipped 

 with a blunt, wart-like papilla; cells 12-15)U in diameter. 

 Mich. 



Tetraedron tumidulum (Reinsch) Hansgirg 1889, p. 18 

 PL 61, Figs. 17, 18 

 Cells tetragonal, pyramidal, the margins straight, concave, or 

 convex; the angles bluntly rounded and sometimes with knob-like 

 extensions; cells 30-53/i, in diameter. 

 Plankter; in lakes and sloughs. Mich., Wis. 



Tetraedron verrucosum G. M. Smith 1918, p. 632 

 PL 61, Figs. 24, 25 

 Cells tetragonal, pyramidal, sometimes with lobes cruciately 

 arranged; margins of the cell convex, the angles ending in a stout, 

 blunt spine; wall coarsely verrucose; cells 65-80/t in diameter 

 including the spines, which are 13-18/a long. 

 Rare; in euplankton. Wis. 



Tetraedron Victoriae Woloszynska 1914, p. 203 



Cells deeply lobed to form 2 semicells which are subcruciform in 

 arrangement, the semicells bilobed, with the walls emarginate 



[270 1 



