19. Processes not divided and without secondary lobes 20 



19. Processes divided, bi- or trifurcate 23 



20. Processes very short, angles scarcely produced 



at all, tipped with 2 spines T. armatum 



20. Angles markedly produced .__ 21 



21. Cells cruciately lobed, the lobes in 1 plane, 



deeply bifurcate at the apices T. pusillum 



21. Cells not cruciately lobed; lobes not all in the same plane 22 



22. Cells tetrahedral, pyramidal, the lobes long and 



tapering, tipped with 3 spines... T. hastatum 



22. Cells with short, asymmetrical lobes, not at all or scarcely 



tapering, tipped with 2-3 blunt spines T. asymmetricum 



23. Lobes all in the same plane 24 



23. Lobes not all in the same plane 27 



24. Cells definitely 4-lobed, the lobes forming 



an H-shaped figure T. constrictum 



24. Cells not definitely H-shaped 2.5 



25. Lobes and lobules of the cell long and slender, tipped with 2 spines 26 



25. Lobes short and broad, rarely with secondary lobes T. cruciattim 



26. Processes long and slender, much branched T. gracile 



26. Processes short, rarely with secondary lobes T. lobulatum 



27. Angles extended into very short, broad processes 



which have short secondary lobes T. enorme 



27. Angles extended into long, tapering processes 



with long, narrow secondary lobes 28 



28. Processes much branched 



to the fifth order T. lobulatum var. polyfurcatum 



28. Processes with 3 or less series of divisions 29 



29. Body of the cell narrow, processes longer 



than the diameter of the cell T. limneticum 



29. Body of cell thick; processes shorter than 



the diameter of the cell T. planctonicum 



Tetraedron armatum (Reinsch) DeToni 1889, p. 611 

 PI. 59, Figs. 6,7 

 Cells irregularly triangular and lobed, the angles scarcely or not 

 at all produced, but furnished with a pair of widely separated 

 spines; the angles of the cell sometimes not all in the same plane, 

 so that 1 or 2 pairs of spines are seen one above the other when 

 viewed from the side; cell 30-47.6/* in diameter. 

 Rare; in euplankton. Mich., Wis. 



Tetraedron arthrodesmiforme (G. S. West) Woloszynska 1914, p. 203 



PI. 59, Fig. 8 



Cells quadrate in outline, symmetrically incised to form 2 lobes; 

 a spine on each of the 4 angles; 2 sides of the cell subparallel, 

 convex, the isthmus bordered by a widely open sinus; cells up to 

 56/A in diameter. 



Typical form not reported from our region. 



[ 262 ] 



