Erpodium acrifolium Pursell, Bryologist. 69: 465. 1966.
Plants shiny, dark green, irregularly branched, branches ± erect, in loose mats. Leaves imbricate when dry, widely spreading when wet, not complanate, symmetric to ± asymmetric, 0.6--1.1 mm, ovate to oblong-lanceolate, subulate, subula hyaline, smooth to serrulate; laminal cells bulging, smooth, thin- but firm-walled, quadrate to oblate-hexagonal, 16--38 µm × 20--36 µm; marginal cells slightly smaller. Perichaetial leaves enlarged, sheathing. Seta short to virtually lacking, 0.04--0.08 mm. Capsule immersed to emergent, yellow-brown, ovoid-cylindric, 0.9--1.1 mm, stomatose; annuli broad, 2--3 rows of ± quadrate cells wide, persistent; peristome lacking; operculm erect-rostrate. Calpytra to 0.6 mm, smooth, plicate, plicae serrate. Spores 23--32 µm, minutely papillose.
Dry areas on bark of trees and rocks; low to medium elevations; c and s Tex. (Burnet and Live Oak cos.); n Mexico (Sonora, Tamaulipas).
In general aspect, E. acrifolium resembles E. beccarii G. Venturi, a wide ranging species found in Mexico but not known from the United States. Both species have subulate leaves. However, E. acrifolium has smooth laminal cells whereas the laminal cells of F. beccarii are pluripapillose. Three other species, E. coronatum (Hooker f. & Wilson) Mitten, E. pringlei E. G. Britton, and E. glaziovii Hampe, with smooth laminal cells are known from Mexico but not the United States. Plants of E coronatum and E. pringlei are yellow-green to green-brown, and the dry stems and branches are terete. Whereas the leaves of E. acrifolium are lanceolate-subulate, those of F. coronatum and F. pringlei are more or less ovate-apiculate. Moreover, E. coronatum has broad, persistent annuli and peristomes of irregular, pale, papillose segments. Erpodium pringlei has at best rudimentary annuli and lacks peristomes. Plants of E. gloziovii are dark green and the broadly acute to obtuse leaves are loosely complanate.