KEY TO THE GENERA OF THE DESMIDS. 



Cells united into filaments (a). 



Cells not united into filaments (g). 



a. In a transparent, jelly-like sheath (b). 



a. Not in a jelly-like sheath (d). 



b. Cells with 2 teeth on each narrow end Dcsmidium, 6 



b. Cells deeply constricted, almost into two parts (<:). 



b. Cells not deeply constricted, and without teeth . . Hyalotheca, 3 



c. With 'claspers' across the sutures Onychonema, 9 



c. Without 'claspers;' cells united by a narrow isthmus, Sph&rozosma, 8 



d. Band not twisted; cells with 'claspers' across the sutures, 



Onychonema, 9 

 d. " " cells without 'claspers' (e). . 



d. Band twisted; cells triangular or quadrangular . Desmidium, 6 



e. Cells barrel or hub-shaped, with i or 2 median bands, Bambusina, 4 

 <?. " " " without bands, the sutures projecting, 



Leptozosma, 5 

 e. Cells cylindrical, sometimes swollen at base (/"). 



e. Cells quadrangular, deeply constricted, often slightly twisted, 



Phymatodocis, 7 



f. Ten to 30 times longer than broad Gonatozygon, i 



f. Three to 6 times longer than broad Genicularia, 2 



g. Cell more or less crescentic Closterium, 13 



g. Cell cylindrical, fusiform, dumb-bell or hour-glass shaped (/). 



g. Cell flattened; orbicular, oblong, or elliptical (h). 



h. Mostly orbicular or broadly elliptical; centre deeply constricted, 



the semi-cells 3-5 lobed, the lobes entire or variously incised, 



Micrasterias, 22 

 h. Mostly oblong or elliptical; margins wavy, the depressions rounded; 



ends usually notched or incised Euastrum, 21 



