Secondary stems pinnate and frondiform distally; paraphyllia present; pseudoparaphyllia dissected-lanceolate to subfoliose. Stem leaves loosely imbricate when dry, spreading when moist, lanceolate to oblong-lanceolate; margins strongly recurved at base, plane at apex, entire or sometimes weakly serrulate at apex; apex acute to acuminate; costa variable on same plant, weak and almost ecostate, double and short, or strong and disappearing mid leaf; alar region filling basal angles, extending up margins to 1/2 leaf length; medial laminal cells oval-oblong. Branch leaves smaller, narrower. Sexual condition dioicous; perichaetial leaf apex filiform-acuminate. Seta 3–5 mm. Capsule erect-symmetric or nearly so, barely exserted beyond perichaetial leaves, oblong-cylindric; stomata basal, sunken; operculum oblique-rostrate; exostome teeth free, broadly subulate, punctulate-scabrous; endostome well developed. Calyptra naked. Spores 20 µm, light brown.
w North America, nw Mexico.
Alsia is strikingly similar to Forsstroemia trichomitria, but the two taxa are completely distinct geographically. Alsia is endemic to the west coast of North America, while F. trichomitria is broadly distributed in eastern North America, occurring no further west than Oklahoma. The stems of Alsia are densely invested with paraphyllia, while paraphyllia are absent in F. trichomitria.