Plants turf-forming, yellowish green distally, medium brown to light brown basally. Stems to ca. 3 cm, hyalodermis present or occasionally absent, sclerodermis usually weak, often of substereid cells, central strand present or absent; axillary hairs with proximal 1-3 cells thicker walled or all hyaline. Leaves incurved, often catenulate, often reflexed at top of appressed base when dry, weakly to widely spreading when moist; oblong, elliptical or ligulate to long-lanceolate, adaxial surface flat or shallowly channeled, 1.5-2.5(-5) mm; base elliptic to rectangular, occasionally not differentiated in shape; distal margins plane or somewhat erect but never sharply incurved, entire to crenulate-notched or occasionally dentate in the distal ¹/2-³/4, occasionally with a narrow, less papillose border; apex narrowly to broadly acute or rounded, occasionally broken or reflexed; costa usually excurrent as a smooth, sharp mucro, adaxial outgrowths absent, adaxial cells quadrate but occasionally elongate near apex, in 2-6(-8) rows; cross section semicircular, reniform or ovate, adaxial epidermis present, adaxial stereid band present, occasionally weak, guide cells 2-4(-6) in 1 layer, hydroid strand usually absent, abaxial stereid band present, crescentlike in section, abaxial epidermis weak or absent; basal cells differentiated across leaf or medially, occasionally rising weakly along margin, rectangular, seldom bulging, 3-5:1; distal cells rounded-quadrate, occasionally transversely elliptical along margins, 6-12(-18) µm wide, papillae usually 2-fid, crowded, 2-6 per lumen, occasionally single and multiplex and covering the lumens, superficial cell walls equally thickened and convex on both sides. Specialized asexual reproduction absent [rare, of gemmae on rhizoids or adaxial surface of costa, of several cells, vermiform to irregular in shape, occasionally branching]. Sexual condition dioicous or occasionally autoicous. Perichaetia terminal, interior leaves weakly sheathing at base or not sheathing, little different from cauline leaves. Capsule stegocarpic [cleistocarpic]. Seta 0.4-1.5 cm. Theca cylindric, ovate or elliptic ca. 1-3 mm, annulus of 1-4 rows of vesiculose cells, persistent or seldom revoluble; operculum [when differentiated] long-conic to rostrate; peristome teeth 16, usually rather short, occasionally rudimentary or absent, ligulate to filamentous, entire or occasional

Trichostomum species differ from similar Barbula species by the short peristomes and plane-margined leaves, and from Tortella by the basal cells differentiated approximately across the leaf or only occasionally running up the margins in 1-3 rows, as opposed to rising distally along both leaf margins from near the costa in a distinct V; Tortella also usually has a twisted peristome. When differentiated basal cells do rise marginally in Trichostomum they form a differentiated basal area in the shape of a W, and there is sufficient morphological variation that some specimens will be run through keys to both Trichostomum and Tortella; these plants should then be compared to illustrations. The genus is complex and heterogeneous. H. N. Dixon (1924) treated all species of both genera under the name Trichostomum, which, after the present floral study of Trichostomoideae, probably could be extended, instructively, to include Weissia. The similar genus Weissia differs from Trichostomum by its naviculate distal portion of the leaves, the often narrowly incurved or tightly involute distal leaf margins, sometimes bulging adaxial laminal cell walls, and commonly monoicous sexual condition (R. H. Zander 1993). Possible parallel reduction series in peristome development between the two genera was discussed by Zander (1985).

Excluded Species:

Trichostomum brittonianum R. H. Zander, Bull. Buffalo Soc. Nat. Sci. 32: 92. 1993 [new name for Hymenostomum flavescens E. Britton in N. L. Britton and C. F. Millspaugh, Bahama Fl., 485. 1920; Weissia flavescens (E. Britton) W. D. Reese]

The report of this species from the flora area (Key West) by W. D. Reese (1991) remains doubtful. The sterile Key West specimens agree with the Bahamian type in the several-celled apiculus, and the clear, nonpapillose quadrate to short-rectrangular cells running up the proximal leaf margins in 1-3 rows. Gametophytically, the eperistomate T. brittonianum is indistinguishable from the peristomate T. crispulum, so presence of the former in the flora area is not yet established.

Trichostomum subdenticulatum Austin, Bot. Gaz. 3: 29. 1878, not Müller Hal. 1851

The type of this forgotten name could not be found by curators of BM, BUF, FH, MANCH, and MO.