Identify that alga:
Web-based key to genera -13: attached cushions with chloroplasts

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planktonic,
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 attached
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 attached,
 flat plate
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  Attached; round cushion of cells: (with a well-defined outline - distinguish from a flat disk of irregular shape)
    Click on the thumbnail for a larger image
 

Forming a tight disc with cells angular from mutual compression; each cell with a sheathed bristle; colony initially a single layer, a second layer later developing in the centre.
  Coleochaete: the sheathed bristle is indicative; it is actually pseudoparenchymatous; other species are more obviously so, or filamentous.
[Bristle, synonymous with seta = a stiff projecting fibre]
 

Forming a round or oval colony, cells close but not tightly packed together; each with 1 or 2 pseudocilia. Chloroplast parietal, 1 pyrenoid.
  Chaetopeltis
[Pseudocilium = flexible, but not motile, projecting fibre]
       
  (pseudo) parenchymatous plate   Parenchymatous:
Psudoparenchymatous:
 

parenchymatous plate or sheet, 1 cell thick.
  Monostroma
       
 

Pseudoparenchymatous plate growing attached to surfaces: usually only sampled if the surface itself can be removed for examination (e.g. microscope slides)

  
 

Prostrate system of branching filaments


  Protoderma: may be confused with Pseudendoclonium and the basal system of young Stigeoclonium plants, both of which also have erect systems of branches
    
 

Prostrate system of branching filaments, with extensive system of upright branching filaments, each ending in a pointed cell or multicellular hair.
  Stigeoclonium: other spp. have a less well-developed basal system than the one illustrated. This (S. farctum) also appears to have concentric rings in the basal disc, due to the chloroplasts in adjacent filaments being aligned. See branched filaments
     
 

Prostrate system of branching filaments, with limited system of upright branching filaments

 Pseudendoclonium
       
       
       

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John Kinross