13. Cell with a prominent dorsal flange, decidedly triangular when 



seen in end view (cf. P. Swirenkoi, which is somewhat triangular) 16 



13. Cell without a dorsal flange, not triangular in end view 14 



14. Margin of cell with prominent notches 15 



14. Margin of cell entire 17 



15. Cell with 1 or 2 lateral notches; 1 large 



circular paramylon body. - P. pseudoswirenkoi 



15. Cell with several notches in both sides; 1 large 



and several small paramylon bodies P. Birgei 



16. Cell with a strongly decurved caudus -._ P. orbicularis 



16. Cell with the caudus straight or only slightly 



curved away from the longitudinal axis P. triqueter 



17. Cells 15-25m in diameter 18 



17. Cells 30-50-(60)m in diameter 21 



18. Paramylon plates 2 large oval rings, at least 



J2 the cell body in length P. alattis 



18. Paramylon plates smaller, 2 or several - _ 19 



19. Cells distinctly twisted in the posterior part, just above the 

 caudus, where there is often a swelling; one side of 



the cell much thicker than the other P. Lemmermannii 



19. Cells not distinctly twisted in the posterior portion; 



without a swelling above the caudus 20 



20. Cell ovoid to ellipsoid, with a straight 



or only slightly cvuved caudus P. caudatus 



20. Cell broadly ovoid to orbicular, with a short 



caudus curving to the left P. curvicauda 



21. Cell distinctly twisted in the posterior part just above the 

 caudus; paramylon bodies usually 2 large 



and several small plates. P. Lemmermannii 



21. Cells not distinctly twisted above the caudus; 



paramylon bodies usually 2 large rings 22 



22. Cell orbicular, broadest below the transverse median line; 

 caudus deflexed mostly to the right; 



cell angular in end view 23 



22. Cell ovoid or nearly so, broadest at the 



transverse median line, or above it - 21 



23. Cell twisting slightly into the caudus, which is 



prominent and elongate, sHghtly deflexed P. pleuronectes 



23. Cell not twisting into the caudus, which 



is short and sharply deflexed P. Swirenkoi 



24. Cell broadly ovoid or orbicular, with a strongly 



curved caudus; cells 50-100^1 long — — - P. orbicularis 



24. Cells narrowly ovoid, somewhat unsymmetrical, with a nearly 



straight caudus; cells up to 48/^ long _ P. anacoelus 



Phacus acuminatus Stokes 1885a, p. 183 

 PI. 88, Fig. 4 

 Cells suborbicular in outline, broadly rounded posteriorly, with 

 a short, blunt apiculation; periplast longitudinally striated; paramy- 

 lon bodies 1-2 ring-like discs; cells 20-22/x in diameter, 23-25^ long. 

 Plankter; in cedar swamp. Wis. 



[ 396] 



