Plants in thin, loose mats, yellow-green to green. Stems to 40 × 2--3 mm, prostrate. Leaves close, complanate, wide-spreading to squarrose, smooth, usually wrinkled and contorted when dry, 1--1.5 × 0.4--0.7 mm, ovate-lanceolate to ovate, often curved and asymmetric, acuminate; margins plane to erect throughout, serrate to serrulate distally, serrulate proximally; median cells 71--141 × 5--7 µm; alar cells short-rectangular to quadrate, in small groups with 1--2 cells on margins, 19--33 × 9--19 µm. Specialized asexual reproduction lacking. Sexual condition autoicous. [Seta brown to reddish brown, 2--3 cm. Capsule light brown to reddish brown, horizontal to pendulous, sometimes nearly erect, 1--1.5 mm, ovoid to ellipsoid, contracted below the mouth when dry; operculum obliquely short-rostrate. Spores 9--14 µm.]

Capsules mature apparently fall (fully developed sporophytes not seen in the flora region). Low sandstone bluffs along river; altitude unreported; Miss.; Mexico; West Indies; Central America; South America.

Isopterygium tenerifolium is known from only one recent collection from Mississippi (Lauderdale Co., E shore of Chunky R. at Stukey Bridge, ca. 32° 15' N, 88° 52' W, 30 Sept. 1992, Buck 22129 CANM, NY). The species is morphologically close to I. tenerum, differing by the somewhat larger plants, stems often 2--4 cm, leaves 1--1.5 mm and usually wrinkled and contorted when dry, and seta 2--3 cm. The collection in CANM contains only one plant with an undeveloped sporophyte that has a seta of 2 cm. The species should be searched for elsewhere in the Gulf Coastal Region where other populations likely occur.