2. Outer free walls of peripheral cells deeply incised or 



emarginate to form 2 to 4 processes 5 



2. Outer free walls of peripheral cells not incised or lobed; processes, if 



any, only prominent undulations or knob-like thickenings on the wall.. 3 



3. Peripheral cells quadrangular-rhomboid, 5-6 sided, the outer margins 

 bearing 1-2 stump-like projections P. integrum 



3. Peripheral cells not as above.- 4 



4. Peripheral cells with a single emargination, throwing the margin 



of the colony into a series of undulations P. muticum 



4. Peripheral cells with 2 shallow emarginations, forming 3 lobes which 



do not project beyond the periphery of the colony_ ____ P. Braunii 



5. Lobes of the peripheral cells not in the same plane, but one 



above the other as seen in surface view P. Kawraiskyi 



5. Lobes of the peripheral cells in one plane 6 



6. Lobes of the peripheral cells elongate and tipped 



by a globular swelHng ._.. __ p. glanduliferum 



6. Lobes of the peripheral cells without apical thickenings 7 



7. Colony entire, without perforations, or with but 



a few minute interstices 8 



7. Colony distinctly perforate 12 



8. Incision of the peripheral cells deep and narrow 



sometimes linear and completely closed 9 



8. Incision of the peripheral cells widely open 10 



9. Incision of the peripheral cells forming a closed sinus with the 



sides nearly parallel, peripheral cells definitely 4-lobed P. obtusum 



9. Incision of the peripheral cells forming a sinus with converging 

 margins, outwardly widely open; peripheral cells 

 not prominently 4-lobed P. tetras 



10. Cell wall smooth or granulate P. Boryanum 



10. Cell wall reticulate 11 



11. Cell wall with fine, close reticulations; processes 



of peripheral cell long P. sculptatum 



11. Cell wall with coarse reticulations; processes of peripheral cells 



short, or with a broad concave emargination between them .... P. araneosum 



12. Lobes of the peripheral cells bifurcate at their apices .._ P. hiradiatum 



12. Lobes of the peripheral cells not bifurcate at their apices. P. duplex 



Pediastrum araneosum (Racib.) G. M. Smith 1916, p. 476 



[P. angulosum var. araneosum Raciborski] 



PI. 47, Fig. 4 



Colony entire, or with minute interstices; cells 5-sided; peripheral 

 cells 2-lobed; margin concave between the lobes, which are short 

 and are about as far apart as are the lobes of two adjacent cells; 

 wall with reticulate ridges; cells 15-32;u, in diameter. 



Common in the tychoplankton and euplankton of many lakes; 

 generally distributed. Mich., Wis. 



[221] 



