GENERA OF ALG^E. 29 



b. Aerial, on moist objects; gelatinous, cells globose or angular, be- 



coming red Porphyridium, 57 



c. Family in jelly mass, thallus pyriform, irregular, tubular, clustered, 



branched, etc. (*'). 

 c. Family globose (d). 



c. Family plane, discoid or stellate, often perforated; cells polygonal,, 



Pediastrum, 40 



c. Family cubical, cells 4-8-16, rhomboidal . . . Staurogenia, 44 

 c. Family of laterally united, often spinous or crescentic cells (e). 



c. Family of cylindrical cells (/). 



d. Cells in a single stratum; ccenobium hollow . . Ccelastrum, 42 

 d. Cells 4-8-16-32, wedge-shaped, apices sinuate or bifid; ccenobium 



solid . Sorastrum, 43 



d. Cells 2-4-8, globose or oblong Gl&ocystis, 60 



e. Cells cylindrical to ovate, ends often spinous . Scenedesmus, 45 



e. Cells not spinous, fusiform, cylindrical, straight or variously curv- 



ed, single or united Rhaphidium, 62 



f. Upper cells spreading, contracted at base .... Sciadium, 46 



g. Cells globose or pear-shaped (h). 



h. Neck not cylindrical, aperture with a lid ... Chytridium, 68 



h. Neck cylindrical, elongated Olpidium, 69. 



/. Cells connected by fine filaments (*). 

 *". Cells not connected by filaments (/). 



* Filament attached to the convex side of the cells, 



Dimorphococcus, 63 



* Filament attached to the concave side of the cells, 



Dictyosph&rium, 5 1 



Families free-swimming (not attached), (k). 

 Families not free-swimming (attached), (/). 

 k. Cells oblong, reniform, 2-4-8-16; tegument oval or reniform; 



variable Nephrocytium, 61 



k. Cells oval, small, in grape-like clusters; tegument thin, difHuent, 



Botrycoccus, 59 

 k. Cells globose, large, cell walls firm, border hyaline; chlorophyll 



sometimes radiate Eremosphczra, 65 



/. Cells fusiform, crescentic or straight, cylindrical, ends cuspidate or 



acuminate, tegument thin Rhaphidium, 62 



L Cells globose, elliptical or elongate (m). 



m. Cells red, large; tegument thick, concentrically lamellate, stem 



ringed Urococcus, 66 



m. Cells green; tegument not lamellate (n). 



