Monographs Details: Rhynchostegium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants small to rather robust, in ± lustrous, soft, pale- to yellow-green, rarely dark-green, often extensive mats, usually on soil. Stems creeping, irregularly to subpinnately branched, the branches prostrate or ascending; in cross-section with small thick-walled cells surrounding larger thin-walled cells, central strand present, small; paraphyllia none; pseudoparaphyllia foliose; axillary hairs with 1-2 short brown basal cells and (1-)2-4 elongate hyaline or brown distal cells. Stem and branch leaves not or slightly differentiated, spreading to wide-spreading, rarely erect, typically ± complanate, ovate-lanceolate to ovate, usually ± abruptly acuminate, the apex often twisted, concave, not plicate; margins serrulate throughout, usually plane above, reflexed below; costa single, relatively slender, tapering distally, ending above midleaf, often projecting as a small spine; cells long-hexagonal to linear, smooth, thin- to firm-walled, not porose, not becoming shorter in the acumen; alar cells not or gradually differentiated, then quadrate to short-rectangular, often short-rectangular and lax in a broad band across the insertion. Asexual propagula none. Autoicous. Perichaetia inconspicuous; leaves wide-spreading from an erect base, oblong, abruptly long-acuminate; margins serrulate above, subentire below, mostly plane; costa none or rarely single; cells oblong-linear, smooth, firm- to thick-walled, ± porose, becoming rectangular toward the insertion; alar cells not differentiated. Setae elongate, smooth, reddish, ± twisted; capsules inclined to horizontal, ± arcuate, asymmetric, cylindric to long-cylindric, often with a distinct neck, often constricted below the mouth when dry; exothecial cells quadrate to short-rectangular, firm-to thick-walled, not or scarcely collenchymatous, stomata round-pored; annulus well developed, broad; operculum obliquely long-rostrate; exostome teeth reddish brown, triangular, shouldered, bordered, on the front surface cross-striolate below, coarsely papillose above, trabeculate at back; endostome mostly finely papillose, with a high basal membrane, segments keeled, perforate or sometimes gaping, ca. as long as the teeth, cilia in groups of 1-3, nodose, mostly well developed. Spores spherical, finely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Rhynchostegium Bruch & Schimp. in Bruch, Schimp. & W. Gümbel, Bryol. Eur. 5(fasc. 49-51, Monogr. 1): 197. 1852; Hypnum sect. Rhynchostegium (Bruch & Schimp.) Sull. in A. Gray, Manual, ed. 2, 670. 1856; Hypnum subgen. Rhynchostegium (Bruch & Schimp.) Hobk., Syn. Brit. Mosses 157. 1873.
Steerecleus H. Rob., Mem. New York Bot. Gard. 45; 680. 1987.
Discussion. Rhynchostegium has mostly complanate, autoicous plants without branch and stem leaf differentiation. The leaves are typically ovate-lanceolate, acuminate, and serrulate all around with a relatively weak, distally narrowing costa. The setae are smooth. Certainly the boundary between Rhynchostegium and Eurhynchium is not striking, but the species can usually be sorted with a relatively high degree of confidence. Thus, synonymy of the two genera (as Rhynchostegium) seems impetuous and unnecessary. I am impressed by the large number of tropical species of Rhynchostegium, whereas Eurhynchium is basically temperate and high elevation in distribution.
Robinson (1987) segregated most of the tropical species of Rhynchostegium into the segregate genus Steerecleus. As nice as it would be to have a genus in our flora named for Bill Steere, I am unable to justify separation of Steerecleus from Rhynchostegium since I see no significant differences between the European species of the genus, including the type, R. confertum (Dicks.) Bruch & Schimp., and the tropical members.