Tetraedron arthrodesmiforme var. contorta Woloszynska 1914, p. 203 



PI. 59, Figs. 9, 10 



A variety diflFering from the typical by having the 4 lobes twisted 

 and extending in at least 2 planes, a character which is particularly 

 evident when seen from the 'side'; cell 25-32- ( 50 );ii in diameter, 

 up to 60|Lt in the longest dimension. 



Rare; in euplankton. Wis. 



Tetraedron asymmetricum Prescott 1944, p. 357 



PI. 59, Figs. 11-13 



Cell quadrangular in outline, unsymmetrically incised to form 2 



major lobes with an isthmus bordered by a widely open sinus, the 



2 major lobes slightly bilobed, the lobules tipped with 2 or 3 short 



spines; cells 10-18/x in their longest dimension. 



This species should be compared with T. irregulare (Reinsch) 

 DeToni. 



Rare; in euplankton and tychoplankton. Wis. 



Tedraedron bifurcatum (Wille) Lagerheim 1893, p. 160 



PI. 59, Fig. 14 

 Cells irregular tetrahedrons, the lobes tipped by 2 short spine- 

 bearing processes, the margins of the cell concave between the 

 apices of the lobes; maximum diameter of the cell 55-60/a. 

 Rare; in plankton of lakes; tychoplanktonic in swamps. Wis. 



Tetraedron bifurcatum var. minor Prescott 1944, p. 358 

 PI. 59, Figs. 15, 16 

 A form differing from the typical by its small size and in having 

 the lobes bifurcated, the lobules rather stc it and tipped with a 

 short spine; cells up to 22.5/a in their maximum diameter. 

 Rare; in tychoplankton. Wis. 



Tetraedron caudatum (Corda) Hansgirg 1888a, p. 131 

 PI. 59, Figs. 17, 24, 25 



Cells flat, 5-sided, the angles rounded and tipped with a short, 

 sharp spine, the sides between the angles concave, but with one 

 margin narrowly and deeply incised; cells in their longest dimension 

 8-15-(22)/.. 



Tetraedron caudatum var. incisum Lagerheim, reported from 

 Michigan by Taft (1939), does not seem to be separable from the 

 typical form. The sinus of the incision on the margin is very narrow. 



In tychoplankton; in shallow water of lakes and in swamps. Wis. 



[ 263 ] 



