5. Semicells transversely elliptic or oval, the median incision (sinus) 

 of the cell deep and forming a narrow isthmus between the semi- 

 cells Spondylosium 



PI. 1, Fig. 2, 12, 13, 18, 19 



5. Semicells not transversely elliptic; median incision not deep, some- 

 times only a slight invagination of the lateral walls in the mid- 

 region 6 



6. Cells with lateral margins smooth, without angles or processes; cells 

 cylindrical, or barrel-shaped, or sometimes nearly quadrate 7 



6. Cells with lateral margins lobed or possessing angles, teeth or short 

 projections; cells short-cylindric, only slightly if at all longer than 

 wide. (See, however, Desmidium Baileyi which has no median con- 

 strictions; has quadrate cells with rather long, truncate processes 

 from the poles which adjoin those from neighboring cells and so 

 leave relatively large intercellular spaces). See Pi. 1, Fig. 3, 4, 10 



7. Chloroplasts spiral ribbons; ends of cells squarely truncate Genicularia 



PI. 2, Fig. 15 



7. Chloroplasts not spiral ribbons 8 



8. Cells elongate, many times their diameter in length, cylindrical, 

 without a median incision; walls usually spiny Gonatozygon 



PI. 2, Fig. 1, 2 



8. Cells shorter; walls not spiny 9 



9. Cells barrel-shaped, with a slight notch in the midregion where 



the cell is somewhat bulged Bambusina 



PI. 1, Fig. 7, 8 

 9. Cells short-cylindric to almost quadangular in front view, the median 



notch in the form of a shallow depression or invagination Hyalotheca 



PI. 1, Fig. 9, 10 

 10. Cells wider than long, or as wide as long, the median incision often 

 a shallow notch, or without a median incision at all; walls at the 

 poles of the young semicells infolded or replicate; cells in vertical 



view oval, quadrangular, or triangular Desmidium 



PI. 1, Fig. 3, 4, 11 



10. Cells longer than wide, rectangular in outline, with a prominent 

 median incision; walls at poles of young cells not infolded; cells in 



end view quadrately lobed Phymatodocis 



PI. 1, Fig. 1 



11. Cells with a median incision, dividing it into semicells (half-cells) 12 



11. Cells without a median incision 18 



12. Cells cylindrical or elongate-oval, three to many times longer than 



their diameter in length 13 



12. Cells oval, rounded, or rectangular; length always less than two times 



the diameter (except in a few species of Cosmarium) 26 



13. Cells with a star-shaped chloroplast in each semicell Cylindrocystis 



PI. 1, Fig. 16 



13. Cells with chloroplasts other shapes 14 



14. Cells with bluntly rounded apices in which there is a vertical cleft 



or notch Tetmemorus 



PI. 2, Fig. 4 

 14. Cells without an apical incision 15 



[936] 



