Bryum caespiticium Hedwig, Sp. Musc. Frond., 180. 1801; Ptychostomum imbricatulum (Müller Hal.) D. T. Holyoak & N. Pedersen

Plants large. Stems 0.1-2(-3) cm. Leaves weakly to moderately concave, 0.5-2(-3) mm; margins plane to strongly revolute, limbidium distinct to weak, of 1 or 2 rows of elongate incrassate cells; apex occasionally hyaline with age; costa short- to long-excurrent, awn brown, yellow-brown, or sometimes hyaline, smooth or rarely somewhat denticulate, 1/4-3/4 length; proximal laminal cells abruptly quadrate to rectangular, 1-2:1 away from costa, 2-4:1 along costa; medial and distal cells long-hexagonal, 12-18(-20) µm wide, (3-)4-6:1. Specialized asexual reproduction rare, by deciduous brood branchlets in distal leaf axils, and rhizoidal tubers, red, spheric, 100-200 µm, cells smooth. Capsule with endostome segments pale tan, brown, red-brown, or rarely pale yellow. Spores 8-14(-16) µm.

Capsules mature Apr-Aug (spring-summer). Disturbed soil, earth banks, rotten wood, rock, disturbed habitats; low to high elevations (0-4000 m); Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Idaho, Ill., Ind., Iowa, Kans., Ky., Md., Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico (Baja California, Hidalgo); s South America; Europe; Asia; Africa; Atlantic Islands; Pacific Islands; Australia.

The combination of small caespitose plants with ovate-lanceolate concave leaves, strong awns, and inflated subalar cells on fertile stems is diagnostic of Gemmabryum caespiticium. The last character is also a diagnostic character of sect. Cladodium of Ptychostomum and known from a few other species in other genera (for example, Bryum dyffryense Holyoak).