43. Cells strongly curved so that the poles nearly touch, scattered 

 throughout the colonial mucilage in groups of 4, generally with 

 the convex walls apposed Kirchneriella 



43. Cells crescent-shaped, arranged in groups of 4, 2 of which are 

 in one plane facing each other, and 2 at right angles to this 

 plane, each of the latter adjoined to one pole of the cells in 



the other pair Tetrallantos (in part) 



44. Cells fusiform _ 46 



44. Cells ovate, reniform, or oblong __. _45 



45. Cells ovate, arranged in 4's within a gelatinous sheath, forming 



a flat plate Dispora 



45. Cells reniform or oblong, reproducing by autospores usually re- 

 tained within the old mother cell wall which may gelatinize 



and appear as a mucilaginous sheath Nephrocytium (in part) 



46. Cells sohtary or in linear pairs within a fusiform, gelatinous enve- 

 lope, multiplying by cell division.,.. Elakatothrix (inpart^ 



46. Cells arranged in parallel bundles of 4, reproducing by auto- 

 spores — .... ... .... Quadrigula 



47. Cells, or some of them, in clusters, bearing gelatinous bristles 



or setae 48 



47. Cells not bearing gelatinous bristles with or without spines 49 



48. Cells compactly arranged to form an adherent, dome-shaped 

 cluster within the old mother cell wall which bears a dichoto- 

 mously branched seta. (This cluster of cells results from the 

 formation of zoospores and not from division to form successive 

 generations of vegetative cells.) Dicranochaete (in part) 



48. Cells globose, epiphytic, loosely arranged side by side to form a 

 cluster (often soUtary however), each bearing a sheathed, vm- 

 branched seta Chaetosphaeridium (in part) 



49. Cells attached at the ends of branching gelatinous stalks, epizoic 



on microscopic animals 50 



49. Cells not so arranged, not epizoic 51 



50. Cells ellipsoid or spindle-shaped, chloroplasts 1 or 2 parietal 



bands Chlorangium (in part) 



50. Cells ovate or oblong, with numerous disc-like chromatophores 

 (sometimes referred to as chloroplasts) Colacium (in part) 



51. Cells broadly fusiform or elliptic, attached end to end to form 



loose, branching chains.. Dactylococctis (in part) 



51. Cells not so arranged 52 



52. Cells globose or angular from mutual compression; aerial; forming 

 clumps on moist substrates ....Protococcus (in part) 



52. Cells differently shaped, not forming expansions on moist substrates 53 



53. Cells acicular or slightly lunate 54 



53. Cells differently shaped 55 



54. Cells strongly curved, arcuate or lunate, not entangled but closely 

 aggregated in famihes of crescent-shaped individuals Selenastrum 



54. Cells straight or slightly crescent-shaped, loosely entangled or 

 twisted about one another; often solitary Ankistrodesmus (in part) 



55. Cells adjoined by their lateral or end walls to form definite pat- 

 terns or arrangements 56 



55. Cells grouped otherwise, not adjoined by their lateral walls or, 



if so, not forming definite patterns 71 



[ 575 ] 



