Bryologist 14: 44. 1911.
Plants small, in lax to dense tufts, bright green, rufous proximally. Stems 1–3 cm, lax, simple or 2-fid, tomentose proximally. Leaves laxly erect to erect, sometimes secund when dry, erect-spreading when moist, narrowly triangular-lanceolate, 1–2.5 mm; margins narrowly revolute, serrulate nearly throughout, teeth single or paired; apex acuminate; costa excurrent, abaxial surface weakly papillose; laminal cells long, narrow, more than 9:1, prorulose, prorulae pointed, projecting at extreme distal ends of cells; basal cells rectangular, shorter, broader than distal, to 45 × 12 µm; distal cells linear, 30–65 × 5–7 µm. Specialized asexual reproduction absent. Sexual condition autoicous; perigonia gemmiform. Seta 1.5–3.5 cm, straight or flexuose. Capsule 1.8–2 mm. Spores reniform, 26–33 µm.
Phenology: Capsules mature Feb–Dec.
Habitat: Soil, rock, wet embankments
Elevation: low to moderate elevations
Ala., Ark., Fla., Ga., Iowa, Kans., La., Miss., Mo., N.Y., Ohio, Okla., Pa., S.C., Tex., Va., Mexico, West Indies (Puerto Rico), Central America (Costa Rica, Guatemala), n South America (Venezuela).
Philonotis longiseta can be recognized by its flat, narrowly triangular leaves with forward projecting prorulae at the distal ends of laminal cells throughout the leaf. The perigonia are lateral and found just proximal to the perichaetia.