1. Thelia Sullivant in A. Gray, Manual ed. 2. 660. 1856. • [Greek thele, nipple, alluding to prominent leaf papillae].
Plants small to medium-sized. Stems 1-pinnate or irregularly branched, branches erect-ascending, simple or irregularly branched; epidermal cells elongate; rhizoids reddish brown, densely branched, smooth. Leaves deltoid-ovate; base decurrent; apex broadly acute to obtuse, abruptly contracted, acumen long, apiculate or piliferous; costa single or sometimes double, 1/2-3/4 leaf length, sometimes spurred or 2-fid; alar cells quadrate or subrectangular; laminal cells rhombic to fusiform, stoutly 1-papillose abaxially, papillae columnar, simple or 3-6-branched. Seta yellowish when young, red when mature, flexuose when dry, smooth. Capsule cylindric to ovoid-cylindric, symmetric to somewhat curved, smooth to weakly wrinkled when dry; exothecial cells at mouth oblate to quadrate, proximal cells quadrate to short-rectangular, enlarged, red, walls thin to thick; stomata on neck; annulus rudimentary, persistent, 2- or 3-seriate, cell walls thin; operculum conic-subulate; exostome teeth united at base on short, smooth membrane, white, linear, external surface smooth basally, finely papillose distally, trabeculae and median line thickened, internal surface lightly papillose, trabeculae weak; endostome rudimentary, white, lightly papillose, basal membrane low or high, segments reduced to stubs or to 1/2 exostome length, cilia 1 or 2, nublike or absent. Spores smooth to lightly papillose.
Species 3 (3 in the flora): North America, ne Mexico, West Indies (Dominican Republic).
Thelia is remarkable in its glaucous, bluish green color, thick, densely tomentose mats, and tumid to julaceous stems and branches. The stem leaves are distinctively deltoid-ovate with rhombic cells, simple or 3-6-branched columnar papillae, and frequently long-ciliate margins. The sporophyte has a long seta and typically erect, symmetric, cylindric to ovoid-cylindric capsules. The diplolepidous peristome is yellowish white. The endostome segments and cilia are often reduced to nublike projections on a low or high basal membrane.
In the field, Thelia could be confused with Myurella (Pterigynandraceae) because both genera have terete-foliate stems and branches that occur in mats or cushions, radiculose stems, and a similar color. Myurella rarely occurs in dense mats, and its leaves have short, double costae, serrate or sinuate-dentate margins, and lower, unbranched papillae.