A subclass of the class Bryopsida. Most genera of true mosses (Bryopsida) belong in the 16 orders of the Bryidae. The most characteristic feature is the peristome consisting of one or two series of teeth, derived from parts of cells rather than whole cells, as in the Tetraphididae, Dawsoniidae, and Polytrichidae. (The Buxbaumiidae have some resemblance in peristome structure to Bryidae.) The stems may be erect and merely forked, or prostrate and freely branched, with sporophytes produced terminally or laterally, respectively. The leaves are inserted in many rows, though sometimes flattened together and appearing two-ranked, but only rarely actually in two rows. The costa may be single or double, sometimes very short, and rarely lacking. The cells are short or elongate, thin- or thick-walled, and often papillose-roughened. The basal and especially the basal angular cells are often differentiated. The setae are generally present and elongate. The capsules dehisce by means of an operculum except in a few genera that show extreme reduction. The number of peristome teeth and segments of the inner peristome are usually 16. The spores are produced by two layers of spore mother cells; they are generally small. The calyptra may be cucullate or mitrate. See also: Archidiidae; Bryales; Bryophyta; Bryopsida; Bryoxiphiales; Dawsoniidae; Dicranales; Encalyptales; Fissidentales; Funariales; Grimmiales; Hookeriales; Hypnales; Isobryales; Mitteniales; Orthotrichales; Polytrichidae; Pottiales; Seligeriales; Splachnales; Tetraphididae