| Plants with flat leaf blades | ..... | 1 |
| Plants with round or cylindrical leaf blades | ..... | Syringodium filiforme |
| |
| 1a. | Leaves paddle shaped, each associated with 2 scales at the base | ..... | 2 Halophila species |
| 1b. | Leaves in pseudowhorl, each with 2 scales at the base and two scales halfway
up the petiole or leaf stem | ..... | 3 Halophila engelmannii |
| 1c. | Leaves strap-like or at least linear | ..... | 4 |
| |
| 2. | Leaves with a rounded, oval in shape, leaf margins with minute serrations | ..... | Halophila decipiens |
| 2. | Leaves with a pointed tip, elogated, entire | ..... | Halophila Johnsonii |
| |
| 4. | Leaf blades greater than 3 mm in width, generally about 1 cm wide, plant rhizomotous | ..... | Thalassia testudinum |
| 4. | Leaf blades generally less than 3 mm in width | ..... | 5 |
| |
| 5. | Leaves clustered from a distinct node on a rhizome, leaf tip is truncated | ..... | Halodule wrightii |
| 5. | Leaves, threadlike from a branched stem, leaf tip is pointed | ..... | Ruppia maritima |
