Monographs Details: Helicodontium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Family:Myriniaceae
Scientific Name:Helicodontium
Description:Genus Description - Plants small and slender, in mostly dull, brownish green or dark-green, dense to lax, ± wiry mats. Stems creeping, irregularly to subpinnately branched; in cross-section with small thick-walled cells surrounding larger thinner-walled cells, central strand often present; rhizoids not tufted, arising all along undersides of stems; pseudoparaphyllia broadly foliose; axillary hairs difficult to demonstrate, with 1-2 short hyaline basal cells and a single somewhat elongate hyaline distal cell. Stem and branch leaves similar, crowded, appressed to spreading, lanceolate to broadly ovate, obtuse to acuminate, somewhat concave, slightly decurrent; margins serrulate above, entire below, plane; costa single, usually ending ca. 3/4 the leaf length, rarely shorter, sometimes projecting as a spine or prickle at apex; cells rounded-rhombic to long-hexagonal, smooth, firm-walled, becoming longer toward the insertion; alar cells quadrate to subquadrate in large areas, not extending over the costa. Asexual propagula none. Autoicous. Perichaetial leaves sheathing, oblong-lanceolate, sometimes broadly so, acuminate; margins serrulate above, entire below, plane; costa single, ending above midleaf; cells long-hexagonal, smooth, becoming rectangular toward the insertion. Setae elongate, slender, smooth or roughened, reddish; capsules erect, symmetric, ovoid to short-cylindric, slightly contracted below the mouth when dry, the mouth often darkened, especially with age; exothecial cells short-rectangular, ± thick-walled, becoming smaller and thinner-walled toward the mouth; annulus not differentiated; operculum obliquely rostrate; columella cylindric, not expanded at apex, ending ca. 3/4 the urn length; peristome double, attached at the mouth, exostome teeth narrowly triangular, yellow, often becoming brownish gray with age, shouldered, bordered, on the front surface with a zig-zag median line, cross-striolate below, papillose above, trabeculate at back, usually unornamented; endostome with a moderately high basal membrane, segments narrow, keeled, perforate, ca. as long as the teeth, cilia none. Spores spherical, papillose. Calyptrae cucullate, naked, smooth.

Discussion:Helicodontium Schwägr., Sp. Musc. Frond. Suppl. 3(2): 293. 1830; Hypnum sect. Helicodontium (Schwagr.) Mitt., J. Linn. Soc., Bot. 12: 545. 1869. Discussion. Helicodontium is characterized by small plants with ± ovate leaves in which the single costa extends to ca. 3/4 the leaf length, although when the leaves are narrower, the costa is proportionately shorter. The leaf cells are smooth and mostly about 3-4:l. The capsules are erect and often distinctively darkened around the mouth, especially on older capsules. The peristome is hypnoid except for the absence of endostomial cilia. As in several genera of the Myriniaceae, the teeth often become brownish gray with age. Helicodontium was revised for northern South America to Mexico, including the West Indies (Buck, 1980a). Only a single species was recognized for the area. Helicodontium differs from Austinia in the slightly larger stature of the plants, the stronger costa, and most importantly by the presence of an exostome. From Schwetschkeopsis it differs in the smooth leaf cells and single costa.