1. Aphanorrhegma serratum (Wilson & Hooker) Sullivant in A. Gray, Manual. 647. 1848.
Schistidium serratum Wilson & Hooker, Musc. Amer. S. States 2: no. 20. 1841
Plants gregarious to scattered; leaves slightly twisted when dry; perichaetial leaves erect-spreading when dry, exposing the capsule.
Capsules mature (Jul) Sep-Dec. Soil, clay, along creeks or trails in places subject to inundation, rarely among other mosses; low to moderate elevations; Ont., Que.; Ark., Conn., D.C., Fla., Ill., Ind., Iowa, Kans., Ky., La., Md., Mich., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., Tenn., Va., Vt., W.Va., Wis.
Aphanorrhegma serratum resembles the sympatric Physcomitrella patens, from which it differs in its capsule’s regular line of dehiscence and collenchymatous exothecial cells; it also grows occasionally with Physcomitrium immersum. The latter two species are small and both have an immersed operculate capsule. Physcomitrium immersum is distinguished by a distal line of dehiscence (versus equatorial in A. serratum), the presence of an annulus of small cells, the thin-walled (versus collenchymatous) exothecial cells, and an urceolate rather than a globose operculate urn. Aphanorrhegma serratum has been reported from Texas by H. A. Crum and L. E. Anderson (1981). I have seen only one collection (Duke) from that state, but the specimen lacks collenchymatous exothecial cells, bears a distinct annulus, and hence is here referred to Physcomitrium immersum.