7. Polar nodules conspicuously projecting 



inward from the cell wall O. nodulosa 



7. Polar nodules projecting outward ( sometimes 



with a slight inner swelling also) 8 



8. Cells elongate-elliptic to nearly cylindrical, 2—3 times their 



diameter in length, with subparallel margins O. submarina 



8. Cells ellipsoid or ovate, 1 1/2 times their diameter in length O. lacustris 



9. Chloroplasts 1—3— (4) laminate discs or plates 10 



9. Chloroplasts 4— 20— (30) parietal discs or plates 



which are oval or star-shaped . 14 



10. Cells narrowly elliptic, with sharply rounded to apiculate 



poles; 1 laminate chloroplast O. gloeocystiformis 



10. Cells oval or more broadly elliptic, with rounded or bluntly 



pointed (not apiculate) poles; chloroplasts 1—3— (4) 11 



11. Cells narrowly elliptic, with pointed poles O. parva 



11. Cells broadly elliptic or oval, with rounded poles 12 



12. Cell walls relatively thick; poles very slightly pointed in 



some individuals; cells often egg-shaped O. novae-semliae 



12. Cell walls thin; poles rounded 13 



13. Cells small, 3.8-7.5m. in diameter, 6-12^ long O. pusilla 



13. Cells larger, 9-13ai in diameter, 9-19^^ long O. Borgei 



14. Chloroplasts 4-8 star-shaped plates O. natans 



14. Chloroplasts (4)— 10— 20 parietal discs or small plates 15 



15. Cells 29— 40ju, in diameter; chloroplasts numerous; 



old mother cell wall symmetrically inflated O. gigas 



15. Cells ll-15.6u in diameter; chloroplasts 10-20; old 



mother cell wall irregularly inflated O. elliptica 



Oocijstis Borgei Snow 1903, p. 379 

 PI. 51, Fig. 10 



Unicellular or crowded in groups of 2-8, inclosed by the old 

 mother cell wall; ellipsoid or ovate cells with the poles broadly 

 rounded and smooth; chloroplasts 1 or as many as 4 parietal plates, 

 each with a pyrenoid; cells (9)-12-13;li in diameter, (9)-10-19/x 

 long; colony of 8 cells, up to 31/x in diameter, 46/i, long. 



Lemmermann (1903) has assigned this species, as a variety, to 

 O. gigas. 



Common in the plankton of soft water lakes; often appearing 

 among filamentous algae in shallow water. Mich., Wis. 



Oocystis crassa Wittrock in Wittrock & Nordstedt 1880, p. 117 



Pk51, Fig. 9 

 Unicellular or in colonies of 2-8, inclosed by a much swollen and 

 gelatinized mother cell wall; cells ovate, the poles broadly rounded 

 and furnished with a nodular thickening; chloroplasts several (as 

 many as 10) large parietal discs, with pyrenoids usually present; 

 cells 10-20;a in diameter, 14-26/* long. 

 Rare; in plankton. Mich., Wis. 



[243 ] 



