68. Cells round or, if shaped otherwise, not attached side by side 



in one plane 69 



69. Cells fusiform or trapezoidal in face view, adjoined by their inner 

 faces in groups of 4 about a common center; long axes of the 



cells parallel Tetradesmus 



69. Cells differently shaped, not attached about a common center 70 



70. Cells lunate or crescent-shaped, arranged in groups of 4, two cells 

 in one plane, and two in a plane vertical to the other pair ( PI. 66, 

 Figs. 4, 5 ) , one pole of each of the latter pair adjoined to a pole 

 of the cells in the former; cells usually inclosed 



in mucilage Tetrallantos ( in part) 



70. Cells fusiform or needle-shaped, twisted about one another to 

 form a definitely shaped knot of individuals, or a somewhat ir- 

 regular cluster Ankistrodesmus ( in part ) 



71. Cells held together by fragments of mother cell walls of previous 

 generation (Pi. 53, Fig. 14; PI. 55, Fig. 8) .72 



71. Cells not held together by fragments of mother cell walls 73 



72. Cells spherical, arranged in groups of 4 or 8, fragments of older 

 mother cell walls forming loops which help to hold the groups, 

 forming multiple colonies Westella 



72. Cells both ovoid and reniform, in groups of 4, arranged at the 

 ends of branching strands formed by old mother 



cell walls Dimorphococcus ( in part) 



73. Cells pyriform, semi-lunate, or reniform, bearing spines on their 

 outer faces, each cell at the end of a short, thick, gelatinous stalk 

 which radiates from a common center where it adjoins stalks of 



other cells in the colony _ Sorastrum 



73. Cells of different shapes, not arranged as above ____ 74 



74. Cells spherical or broadly elliptic, closely arranged in 2's and 4's 

 within the old mother cell wall, the cells separated from one 

 another by semi-opaque masses of mucilage, sometimes with a 

 plug of dark mucilage in the outer free wall of 



each cell Gloeotaenium ( in part ) 



74. Cells of different shapes, not separated by dark bands of mucilage 75 



75. Cells spherical, bearing spines; loose and temporary clusters 

 formed by entangling of spines — Golenkinia (in part) 



75. Cells not bearing spines 76 



76. Cells inclosed by old mother cell walls 77 



76. Cells not inclosed by old mother cell walls 78 



77. Cells reniform or broadly elliptic, inclosed by old mother cell 

 walls (often gelatinized, thin, and appearing as a mucilaginous 



sheath ) , Nephrocytitim ( in part ) 



77. Cells elliptic, moniliform, with nodules at the poles in some 

 species, or subcylindrical, or panduriform (retuse lateral margins); 

 old mother cell wall much expanded, 



sometimes gelatinizing Oocystis ( in part ) 



78. Cells spherical, with several polygonal chloroplasts; arranged in 

 spherical clvimps or distributed throughout the gelatinous enve- 

 lope ( which is sometimes wanting, or 



not apparent) ..Planktosphaeria ( in part) 



78. Cells more loosely arranged, sometimes solitary; chloroplasts 



not polygonal 79 



[577] 



