15. Cells with transverse whorls of spine-bearing protuberances; the 



poles lobed or divided and bearing straight or curved spines Triploceras 

 , . PI. 2, Fig. 5 



15. Cells without transverse whorls of spiny protuberances 16 



16. Cells folu" to six times longer than wide; median constriction very 

 slight; semicells not or but very little swollen at their bases Penium 



PI. 2, Fig. 14 



16. Cells more! than six times as long as wide; elongate; median constric- 

 tion prominent; bases of semicells distinctly swollen; lateral margins 

 often undulate, 'at least for a short distance beyond the basal swell- 

 ings of the semicells 17 



17. Base of semicells bearing a transverse whorl of teeth of folds in the 

 wall just above and below the median sinus; (focus at the margin 



of the incision ) Docidium 



PI. 2, Fig. 11 



17. Base of semicell without a whorl of teeth or folds in the cell 

 wall , ., Pleurotaenium 



PI. 2, Fig. 6, 7 



18. Cells lunate or curved (crescent-shaped) or sometimes nearly straight 

 with one side slightly curved, and with the cells tapered toward 

 the poles (seta-like in some), or at least distinctly reduced in dia- 

 meter at the poles 19 



18. Cells straight, cylindrical or nearly so (if slightly curved or bent, 

 then with both lateral walls equally curving); not at all or scarcely 

 tapering at the apices. (See Genicularia however, Pi. 2, Fig. 15) 20 



19. Cells bearing a single stout spine at the poles Spinoclosterium 



PI. 1, Fig. 15 



19. Cells without spines at the poles Closterium 



PI. 2, Fig. 9, 10, 12 



20. Chloroplasts in the form of spiral ribbons 21 



20. Chloroplasts plate-like or band-like, usually axial (in the midregion 



of the cell), rarely parietal (along the wall of the cell) 22 



21. Cells cigar-shaped, rounded at the poles Spirotaenia 



PI. 2, Fig. 3 



21. Cells cylindrical (sometimes bent), with squarely truncate 



poles Genicularia 



PI. 2, Fig. 15 



22. Cells elongate-cylindric, with parallel margins and squarely truncate 

 poles; wall spiny Gonatozygon 



PI. 2, Fig. 1, 2 



22. Cells sh(Vt-cylindric or elongate-oval, lateral margins shghtly con- 

 vex; wall not spiny 23 



23. With a star-shaped chloroplast in each half of the cell; some species 

 shghtly constricted Cylindrocystis 



PI. 1, Fig. 16 



23. Chloroplasts not star-shaped 24 



24. Chloroplast a thin, laminate plate, one in a cell, usually lying along 

 the wall; cytoplasin generally purplish, with numerous oil drop- 

 lets Mesotaenium 



PI. 1, Fig. 17 

 24. Chloroplast otherwise; cytoplasm not purplish 25 



[937] 



