Zygodon W. J. Hooker & Taylor, 1818.

Throughout the world the genus Zygodon can usually be identified by the densely placed elliptical and multicellular axillary gemmae. Zygodon rupestris is widespread, especially on Quercus, in the more humid regions of lowland coastal areas of the state.
see Keys to Gymnostomum, Orthotrichum Etc.

Key to Zygodon

In the key that appears below we deal with two very different plants traditionally considered congeneric. Both are acrocarpous with entire leaf margins, and with dense clusters of multicellular rhizoidal gemmae in the axils of keeled leaves. Sporophytes are uncommon, but both species have sulcate gymnostomous capsules exserted on short setae.

Species included in this key are all in Orthotrichaceae:
Zygodon menziesii (Schwägrichen) Arnott
Zygodon rupestris W. P. Schimper ex Lorentz

Only a single species of Zygodon seems to be native to California, but several recent collections of the austral Zygodon menziesii have been gathered from the bark of planted trees within the city of San Francisco. This genus belongs in the Orthotrichaceae but the two species are very different. The native Z. rupestris combines pluripapillose cells and plane leaf margins with the result that it may be placed near the Pottiaceous genus Gymnostomum. In contrast, Z. menziesii has smooth leaf cells and basally recurved leaf margins. Both species, however, have the Orthotrichaceae characteristic of rapid water uptake when moistened.

A. Leafy stems hamate when dry, not significantly spirally twisted; median leaf cells pluripapillose; leaves ending in a uniseriate apex .....Zygodon rupestris
A. Leafy stems not at all hamate at apices but with leaves clockwise twisted around stem when dry; all cells of leaf smooth; leaf apices obtuse to blunt .....Zygodon menziesii