56. Cells cylindrical, adjoined at each end by 2 other similar cells -in 

 successive order to form a network of coenocytic units, the meshes 

 of which are 5- or 6-sided Hydrodicttjon 



56. Cells attached by their lateral walls, or clustered to form other 



types of thalli 57 



57. Cells forming flat, circular or rectangular plates - 58 



57. Cells not forming flat plates 61 



58. Cells forming circular or subcircular plates which may be per- 

 forate or continuous; marginal cells with a deep or shallow 

 incision or sinus; cells within the plate usually somewhat different 



in shape from those at the margin Pediastrum 



58. Cells not arranged to forjn circular plates 59 



59. Cells triangular or ovoid, the walls bearing 1 or more spines, 

 forming quadrate plates of 4 cells each Tetrastrum 



59. Cells not bearing spines 60 



60. Cells rectangular or trapezoidal, the outer free walls smooth and 

 entire, cruciately arranged to form quadrate plates of 4 cells or 

 multiples of 4 Crucigenia 



60. Cells trapezoidal, the outer free walls deeply incised, cruciately 

 arranged to form subquadrate plates of 4 or 8 Pediastrum tetras 



61. Cells bearing spines -- 62 



61. Cells without spines - 65 



62. Cells ellipsoid, bearing many needle-like spines, adjoined side by 

 side to form clusters of indefinite number (usually 2 to 8), some- 

 times solitary Franceia ( in part ) 



62. Cells with few spines (1 to 4, rarely as many as 7); cells defi- 

 nitely arranged __ 63 



63. Cells ovoid, adjoined side by side, forming a linear series (single 

 or double ) ; spines usually few in number and mostly confined to 

 the poles of the cells or with 1 or 2 on the lateral free walls 

 (many minute spines in Scenedesmus hystrix) Scenedesmus (in part) 



63. Cells spherical, arranged in groups of 4 and multiples of 4 to 

 form compound colonies; outer free walls bearing 1-7 very long 

 slender spines --- 64 



64. Compound colony triangular in outline; spines 1 to 7 in num- 

 ber Micractinium 



64. Compound colony always pyramidal in shape, outer free wall of 



cells bearing a single, stout long spine "Errerella bornhemiensis 



65. Cells spherical or trapezoidal, arranged to form hollow, spherical, 

 or polyhedral colonies, the cells adjoined by interconnecting 

 protuberances Coelastrum 



65. Cells not arranged in hollow colonies.— — - — 68 



66. Colony composed of narrowly fusiform cells radiating from a 

 common center where they are adjoined at their poles Actinastrum 



66. Colony not composed of fusiform cells radiating from a common 



center 67 



67. Cells elliptical or fusiform, attached end to end to form branch- 

 ing, chain-like series Dactylococciis (in part) 



67. Cells not forming such branching chains 68 



68. Cells ovoid, elliptical, or fusiform, adjoined by their lateral walls 

 to form a series of 4 or more cells lying side by side in one 



plane (rarely in a curved plate) Scenedesmus (in part) 



[576] 



