Plants forming dense clones. Roots arising at nodes. Petioles 1--14 cm, sparsely erect-pilose to glabrous. Pinnae 2--17 × 1--16 mm, densely pilose abaxially, sparsely pilose adaxially. Sporocarp stalks erect, unbranched, attached at base of petiole, or rarely up to 3.8 mm above it, not hooked at apex, 1.7--6.7 mm. Sporocarps nodding or perpendicular, 2.4--5 × 2--3 mm, 1.3--1.7 mm thick, ovate in lateral view, covered with needlelike hairs with spreading tips when young but soon glabrate; raphe 0.6--1.4 mm, proximal tooth 0.2 mm, blunt, or absent, distal tooth 0.2 mm or absent. Sori 10--14.

Sporocarps produced spring--fall (May--Oct). On mud and in shallow water, in ponds and marshes; 1300--2000 m; Ariz., Tex.; Mexico; South America.

The name Marsilea mexicana A. Braun has sometimes been misapplied to this species.

The red or brown streaks on the pinnae, reported as characteristic of Marsilea mollis by a number of authors, are found on floating leaves of nearly all species in the genus (D. M. Johnson 1986).