195
| 1. | Frond a single cell (its division being followed by separation), or cells fasciculated | 2 |
| Frond composed of several cells | 13 | |
| 2. | Segments, in the front view of the frond, lobed, incised, sinuated, or notched | 3 |
| Segments, in the front view, undivided | 4 | |
| 3. | Frond fusiform; segments notched at the end, otherwise entire. | Tetmemorus - key |
| Frond oblong, compressed; segments sinuated | Euastrum - key | |
| Frond lenticular; lobes of segments incised or bidentate at the apex | Micrasterias - key | |
| 4. | End view of frond angular or radiate | 5 |
| End view of frond neither angular nor radiate | 6 | |
| 5. | Processes geminate from each angle, one external to the other | Didymocladon - key |
| Processes, if present, with their divisions on the same plane | Staurastrum - key | |
| 6. | Cells short, rarely more than twice as long as broad | 7 |
| Cells elongated (cylindrical, fusiform or lunately curved) | 9 | |
| 7. | Frond with elongated, simple or branched spines | 8 |
| Frond not spinous | Cosmarium - key | |
| 8. | Segments with two spines | Arthrodesmus - key |
| Segments with four or more spines | Xanthidium - key | |
| 9. | Cells straight | 10 |
| Cells curved | 12 | |
| 10. | Endochrome spiral | Spirotaenia - key |
| Endochrome not spiral | 11 | |
| 11. | Frond moderately elongated, if linear not constricted at the middle, and its segments not inflated at the base | Penium - key |
| Frond much elongated, linear, generally much constricted at the middle, and its segments inflated at the base | Docidium - key | |
| 12. | Cells not fasciculated | Closterium - key |
| Cells fasciculated | Ankistrodesmus
- key
| |
| 13. | Frond an elongated jointed filament | 14 |
| Frond of few cells not forming a filament | 18
| |
| 14. | Filament plane or triangular, with junction-glands or foramina between the joints | 15 |
| Filament never plane; neither glands nor foramina between the joints | 16
| |
| 15. | Joints with junction-glands; their margins incised or sinuated | Sphaerozosma. - key |
| Joints with foramina between them; their margins bicrenate or entire | Aptogonum - key
| |
| 16. | Filament triangular or quadrangular | Desmidium - key |
| Filament cylindrical or subcylindrical | 17
| |
| 17. | Joints surrounded by a groove | Hyalotheca. - key |
| Joints with two opposite bicrenate projections | Didymoprium.
- key
| |
| 18. | Cells arranged in the form of a flattened star | Pediastrum. - key |
| Cells placed side by side in one or two rows | Scenedesmus. - key |