Monographs Details: Actinodontium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants small, in lustrous, greenish yellow to yellow-brown, small tufts. Stems erect to ascending, mostly simple, occasionally with a single, apparently dichotomous branch, scarcely complanate-foliate; in cross-section all cells ca. the same size, the outermost layer with thin exterior walls over several rows of firm-walled cells surrounding thin-walled cells, central strand none; rhizoids often numerous at base of stems, minutely roughened; pseudoparaphyllia none; axillary hairs 2-celled, with a short brown basal cell and an elongate hyaline distal cell. Leaves erect to wide-spreading, somewhat contorted when dry, crowded, uniform, narrowly lanceolate to ovate-lanceolate, acute to acuminate, rarely ± concave, not decurrent; margins indistinctly bordered by 1(-2) rows of cells or not, serrulate above, entire below or rarely throughout, plane or typically recurved, at least below; costa double, extending to midleaf or somewhat beyond, gradually divergent; cells long-rhomboidal to long-hexagonal, smooth, thin- to firm-walled, becoming rectangular and mostly colored toward the insertion; alar cells not differentiated. Asexual propagula sometimes in leaf axils, uniseriate. Heteroicous or dioicous. Perichaetia small, inconspicuous, in leaf axils; leaves much smaller than vegetative leaves but otherwise similar in shape, margin, and areolation, costa sometimes lacking or as in vegetative leaves. Setae elongate, smooth or roughened above, reddish, straight or curved near base; capsules erect to suberect, cylindric, symmetric; exothecial cells long-rectangular, the vertical walls ± thick-walled, the horizontal ones thin-walled; annulus weakly differentiated; operculum rostrate from a conic base, straight, the cells subquadrate, thin-walled; peristome double, attached at the mouth, whitish, exostome teeth narrowly triangular, bordered, not shouldered, on the front surface with a ± straight median line, papillose-spiculose throughout, only slightly trabeculate at back; endostome with a very low basal membrane, segments slender, ± keeled, narrowly or not perforate, with baffle-like crosswalls, ca. as long as the teeth, cilia none. Spores spherical, finely papillose. Calyptrae mitrate, lobed at base, ± plicate, naked, mostly smooth.
Discussion:Actinodontium Schwagr., Sp. Musc. Frond. Suppl. 2(2, 1): 75. 1826; Lepidopilum sect. Actinodontium (Schwagr.) Mitt., J. Linn. Soc., Bot. 12: 366. 1869.
Discussion. Actinodontium is characterized by its erect, essentially unbranched stems with doubly costate, indistinctly bordered leaves. The laminal cells are elongate. The setae are smooth or roughened above, the exostome teeth are pale and papillose-spiculose, and the lobed calyptrae are unadorned. Actinodontium is a small genus with 2-3 species on each of the three tropical continents.
Actinodontium, with its slender double costa, enlarged outer cortical cells (hyalodermis), and bordered exostome, is surely most closely related to Lepidopilum. However, its noncomplanate habit, erect stems, and mostly smooth setae justify generic segregation. Its erect habit, as well as acute to gradually acuminate leaf apices and papillose exostome teeth, separate Actinodontium from Stenodictyon.