9. Cells round in end view; the wall simple and in 1 piece, some- 

 times inclosed by a mucilaginous sheath Chlamydomonas 



9. Cells compressed when seen in end or side view; the wall 

 bivalved and laterally extended on either side of the protoplast to 

 form an expansion or flange, especially noticeable when seen 



from the side - 10 



10. Cells lenticular or elliptical in side view; valves of wall, when 

 viewed from the side, apparent in the vegetative condition; cells 

 often rectangular in front view, with horn-like processes at 

 the angles ^Phacotus 



10. Cells ovate in side view; the valves of the wall evident only 

 during cell division or release of swarmers; cells circular or 



ovoid in front view ___ ^Pteromonas 



11. Cells elliptical, ovoid, or somewhat heart-shaped, with 4 flagella 

 arising from the midregion of the anterior end Carteria 



11. Cells ovoid in front view, quadrate in end view, with 4 rounded 

 lobes; flagella 4, each attached in a depression of the anterior 



end - Pyramimonas 



12. Plant a motile colony; cells inclosed by colonial mucilage; swim- 

 ming by means of 2 or 4 flagella 13 



12. Plant non-motile in the vegetative condition; filamentous, or con- 

 sisting of a definitely or indefinitely formed colony or aggregate 



of cells (See Desmids: Appendix )-. — 20 



13. Colony composed of cells arranged to form a flat or twisted plate — 14 



13. Colony spherical, spheroidal, or ovoid 15 



14. Colony a circular or rectangular plate Gonium 



14. Colony an ovoid or horseshoe-shaped plate, broadly rounded 

 anteriorly, truncate posteriorly, with 3 prominent projections 



of the colonial investment " Platydorina 



15. Colony oblong, without a gelatinous investment; cells pyriform, all 

 directed toward the anterior end of the colony; flagella 4—Spondylomorum 



15. Colony ovoid or spherical, with a gelatinous investment; flagella 2- 16 



16. Cells with sharply pointed lateral extensions of the protoplast, 

 arranged to form a median girdle within a spheroidal colonial 



sheath _ Stephanosphaera 



16. Cells not arranged as above - 17 



17. Cells pyriform, broadest at the anterior end, compactly arranged 



in ovoid or ellipsoidal colonies - Pandorina 



17. Cells ovoid or spheroidal, not compactly arranged within the 

 colonial mucilage, colonies globular or ovoid 18 



18. Colony spherical, involving hundreds of cells (500 to 5000 in- 

 dividuals); all the vegetative cells the same size and sometimes 

 interconnected by protoplasmic strands ..Volvox 



18. Colony spherical or obovoid, involving but a few cells (usually 

 32-64, rarely up to 256), without intercellular connections 19 



19. Colony spherical, containing cells of 2 sizes which are evenly dis- 

 tributed at the periphery of the colonial mucilage Pleodorina 



19. Colony obovoid or spheroidal, containing cells all the same size 



and often arranged in tiers Eudorina 



20. Plant a pair of trapezoidal cells adjoined along their bases Euastropsis 



20. Cells otherwise arranged, or of different shape 21 



[572] 



