Plants medium-sized, tufted or cespitose, glossy, dark green. Stems erect or repent, to 1.5 cm. Leaves crispate when dry, broadly acute to acuminate, 3--4 mm; margins coarsely serrate distally, erect or irregularly recurved proximally; apex plane or with erect margins but not cucullate. Gemmae lacking. Seta 1(--4) per perichaetium, 3--4 mm. Capsule cylindric, 2--2.4 mm, smooth to slightly or prominently wrinkled or ribbed when dry; peristome teeth divided into filiform segments, densely papillose. Calyptra lobes less than half length of calyptra.

Capsules mature Mar., Apr., Dec. Calcareous rock and concrete in forests; 0--2200 m; La., S.C., Tex.; Mexico; West Indies (Dominican Republic).

Ptychomitrium serratum grows in dark conspicuous tufts or cushions on basic rocks and concrete. Its habitat, size, prominently serrate leaf margins, and plane leaf tips make it easy to recognize. It is somewhat similar to P. gardneri but plants of the latter are much larger; the two species do not co-occur. This moss has been collected repeatedly in Texas, mostly in Culberson Co., in the Guadalupe Mountains, especially in McKittrick Canyon. It has also been found at a few sites in central and eastern Texas and in central and southern Louisiana, and at one site in South Carolina. Most of the specimens from eastern Texas, Louisiana, and South Carolina were taken from man-made calcareous substrates (concrete), but one colony from Louisiana grew on an old asphalt shingle roof.