Crossidium Juratzka, 1882.

Crossidium is especially common on thin soil over boulders. It often grows as linear colonies in cracks in rock outcrops. Our species all have an hyaline awn on leaves with recurved margins. Even with a hand-lens, the cushion of photosynthetic filaments can often be seen as a dark green pad in the leaf center.

Key to Crossidium

Here we present a single genus of acrocarpous mosses of semiarid to arid habitats. Hand-lens observation of the adaxial cushion of cells will allow the observer to think of a rather limited suite of pottiaceous species: Pterygoneurum spp. have the adaxial cushion arranged into sheets (photosynthetic lamellae) and these sheets are sufficiently separated as to be individually seen with a hand-lens; Aloina bifrons has adaxial photosynthetic filaments but it differs from Crossidium by the inflexed leaf margins; Tortula guepinii may have a very small adaxial cushion, but its leaves are rather flat, never concave; Tortula atrovirens has a cushion of adaxial cells along the costa, but the cells of the cushion are not arranged into filaments and the leaves never have a hyaline awn.

Species included are in Pottiaceae:
Crossidium aberrans Holzinger & E. B. Bartram
Crossidium crassinerve (De Notaris) Juratzka
Crossidium seriatum H. Crum & W. C. Steere
Crossidium squamiferum (Viviani) Juratzka

A. Median leaf cells mostly papillose or mammillose; terminal cell of photosynthetic filaments nearly globose; photosynthetic filaments often so low as to be inconspicuous .....B
A. Median leaf cells smooth (ignore any papillosity that may be on cells near the leaf apex); terminal cell of filaments markedly longer than wide; photosynthetic filaments so high as to obscure almost all of the leaf lamina .....C

B. Upper leaf cells mammillose or with a single central papillae; perichaetial leaves broadest near the apex; photosynthetic filaments separate to the base .....Crossidium aberrans
B. Upper leaf cells papillose with several C-shaped papillae; perichaetial leaves widest near the base to near the middle; photosynthetic filaments partially fused into lamellae that may be up to 4 cells high .....Crossidium seriatum

C. Lumen/wall ratio of upper and median cells mostly more than 4:1, the lumen therefore obvious; leaf margin recurved to revolute from base to above the leaf middle; papillae solid on terminal cell of photosynthetic filaments .....Crossidium crassinerve
C. Lumen/wall ratio of upper and median cells mostly less than 1:1; the lumen therefore inconspicuous; leaf margin plane at base, sometimes recurved near leaf middle; papillae hollow on terminal cell of photosynthetic filaments .....Crossidium squamiferum