Centrohelida Kühn, 1926
Key to the genera (should be revised):
| 1 | Without siliceous elements in periplasts | 2 |
| – | Periplasts with siliceous elements | 3 |
| 2 | Cell surface is naked | Oxnerella |
| – | Cell surface is covered by a mucous coat and may bear tangential or radial organic spicules | 3 |
| 3 | Organic spicules absent | Chlamydaster |
| – | Organic spicules present | 4 |
| 4 | Spicules tangential | Sphaerastrum |
| – | Spicules radial | Heterophrys |
| 5 | Periplast comprised of tangential elements only | 6 |
| – | Periplast with tangential plate scales and radial elements | 9 |
| 6 | Periplast comprised of tangential rod-like spicules | Parasphaerastrum |
| – | Periplast comprised of elliptical or spindle-shaped plate-scales | 7 |
| 7 | All scales with a broad marginal rim | 9 |
| – | Two types of plate scales, both lacking broad marginal rim | Pseudoraphidiophrys |
| 8 | Scales with smooth or reticulate texture | Raphidocystis |
| – | Plate scales with radial slits | Raphidiophrys |
| 9 | Radial elements tube-shaped, trumpet-shaped or funnel-shaped | 10 |
| – | Radial elements spine-like; plate scales without broad marginal rim | 11 |
| 10 | With one type of funnel-shaped radial elements; plate scales without broad marginal rim | Pseudoraphidocystis |
| – | Two types of radial elements: funnel-shaped and trumpet-shaped or tube-like; plate scales with broad marginal rim | Raphidocystis |
| 11 | Spicules are bilaterally symmetrical and consist of a shaft and a base formed as a continuation of a membranous flange to the shaft | 12 |
| – | picules consist of a shaft and a discrete baseplate | 13 |
| 12 | The membranous base continues as two lateral wings extending along the shaft, and a further basal wing forms the base of the spicule extending perpendicularly to the shaft and lateral wingsshaft | Echinocystis |
| – | Without well-developed basal wing; spicules often with stalk | Pterocystis |
| 13 | Spicule base-plates are heart-shaped; the shafts attach centrically | Choanocystis |
| – | Base plates of spicule are circular; shafts attach centrally | Acanthocystis |
This key comes from The Illustrated Guide (2000), which I was one of the authors.