Identify that alga:
Web-based key to genera -12: planktonic colonies with chloroplasts-no sheath

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 flat plate
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planktonic,
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no chloroplast:
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Planktonic colony  no mucilage sheath

Not in a mucilage sheath Click on the thumbnail for a larger image  
  forming a flat disc, with the outer cells incised or bearing projecting horns

 
  Pediastrum: species are distinguished by type of ornamentation of the outer cells and the presence of intracellular spaces

 

  
  forming a spherical colony consisting of a hollow ball of cells


  Coelastrum

 

  
  forming a spherical coenobium; the cells are linked to the centre by short, thick stalks; the outward-facing side has 1-4 spines


  Sorastrum

 

  
  cylindrical cells linked to form a network

 
  Hydrodictyon

 

  
  cells spindle-shaped, single or linked in groups of 2-4 (occasionally more), with long axes parallel; sometimes with apical spines

 
  Scenedesmus: many species

 

  
  cells elongated spindle-shaped, often curved, in bundles of 2,4,..16, linked in centre. Single parietal chloroplast almost filling the cell: may have a mucilage sheath.

 
  Ankistrodesmus

 

 
  cells somewhat elongated, spindle-shaped or curved, solitary or in randomly arranged coenobia  
  Pseudokirchneriella subcapitata (formerly Raphidocelis contorta / Selenastrum capricornutum) 		 
       
  Genera not included: (no firm sheath)
Actinastrum, Dimorphococcus, Euastropsis, Gloeotaenium, Lobocystis, Micractinium, Neglectella, Nephrochlamys, Nephrocytium, Oonephris, Pseudodictyosphaerium, Pseudoquadrigula, Pseudotetrastrum, Radiococcus, Tetrastrum, Tetrachlorella, Tetradesmus		    

 

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John Kinross