Marsilea fournieri C. Christensen; M. mucronata A. Braun; M. tenuifolia Engelmann ex A. Braun; M. uncinata A. Braun; M. vestita subsp. tenuifolia (Engelmann ex A. Braun) D. M. Johnson

Plants forming diffuse or dense clones. Roots arising at nodes. Petioles 2--20 cm, sparsely pubescent. Pinnae 4--19 × 4--16 mm, pubescent to glabrous. Sporocarp stalks erect, unbranched, attached at base of petiole (occasionally up to 3 mm above it), not hooked at apex, 0.5--25 mm. Sporocarps perpendicular or slightly nodding, 3.6--7.6 × 3--6.5 mm, 1.5--2 mm thick, elliptic to nearly round in lateral view, pubescent but soon glabrate, scars left by fallen trichomes often appearing as purple or brown specks; raphe 1.1--1.7 mm, proximal tooth 0.3--0.6 mm, blunt, distal tooth 0.4--1.2 mm, acute, often hooked at apex. Sori 14--22.

Sporocarps produced spring--fall (Apr--Oct). Widespread and variable; in ponds and wet depressions and on river floodplains; 0--2300 m; Alta., B.C., Sask.; Ariz., Ark., Calif., Colo., Idaho, Iowa, Kans., La., Minn., Mont., Nebr., Nev., N.Mex., N.Dak., Okla., Oreg., S.Dak., Tex., Utah, Wash., Wyo.; Mexico; South America in Peru.

A number of segregate species have been named and recognized in regional floras in North America: Marsilea mucronata A. Braun (less hairy, found east of Rocky Mountains), M . uncinata (glabrous, sporocarp stalks long, distal tooth of sporocarp hooked, south central United States), M . tenuifolia (pinnae very narrow, central Texas), and M . fournieri (small plants and pinnae, southwest). The features upon which these species are based intergrade into one another. The species are therefore best treated as conspecific with M . vestita (D. M. Johnson 1986).

Putative hybrids between Marsilea macropoda and this species are discussed under the former.