No key was found for the requested taxon, but its parent (Marsilea) is keyed as shown below.

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Keyed in multiple places:

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Key to Marsileaceae

Marsileaceae

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1 Leaves clover-like, the 4 cuneate, obovate or wedge-shaped leaflets borne at the summit of the petiole; sporocarps ovoid
1 Leaves grass-like, linear, the leaf blade absent, the petiole narrowly winged; sporocarps spherical
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Key A2: pteridophytes and lycophytes growing as floating or rooted aquatics

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1 Plant with at least some leaves palmately or pinnately lobed or 1-4× pinnately divided (‘fern-like’) and > 2 cm long
1 Plant either a floating aquatic with leaves <5 cm long, or with clover-like or linear leaves.
  2 Plant a floating aquatic
  2 Plant a rooted aquatic.
    3 Plant clover-like, with 4 leaf segments borne terminally
    3 Leaves linear.
      4 Plants cormose or with short rhizomes; leaves numerous, undivided leaves
      4 Plants with creeping rhizomes; leaves few, reduced to a winged petiole
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Key C2: rooted aquatics with basal leaves, compound or divided

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1 Leaves palmately 4-foliolate, with very clear differentiation of the long petiole from the 4 leaflets
  2 Plants usually attached to rocks in flowing water of streams and rivers; [mainly Piedmont, Mountains, Interior Low Plateau, and Interior Highlands]
  2 Plants in mud or soil of stagnant waters; [Coastal Plain]