Archidium (Hedwig) W. P. Schimper, 1826.

Archidium has, so far, been encountered only once in California. It is one of our smallest mosses, and it grows as a winter ephemeral. (Note here that the majority of mosses considered ephemeral seem to perennate near the soil surface rather than dying back and regenerating primarily from spores). In the field, Archidium will appear as a very small Pleuridium-like moss but the capsules are difficult to view because they are essentially sessile and overtopped by the leaves. We suggest that the genus is probably overlooked in the state, and will be found by careful scrutiny of sterile clones of tiny acrocarpous mosses with narrowly lanceolate to subulate leaves. Often such searching requires the combing of one's finger over the surface of the clone to reveal the capsules hidden beneath.
see key to Acaulon Etc.

The plants included in this section have narrowly lanceolate, often subulate leaves on a stem so short as to seem almost absent. The sporophytes are cleistocarpous and immersed on a short seta, and those sporophytes are almost spherical and contain only a few spores so huge as to be visible with the naked eye. Among the mosses of California, only Lorentziella has sporophytes of similar size.

Species included in this key are in Archidiaceae, except Lorentziella (Gigaspermaceae):
Archidium alternifolium (Dickson ex Hedwig) Mitten
Archidium donnellii Austin, not known from CA
Lorentziella imbricata (Mitten) Brotherus

A. Leaves broadly ovate, abruptly contracted into a narrow concolorous awn; sporophyte nearly sessile with a broadly elliptic capsule .....Lorentziella: L. imbricata
A. Leaves lanceolate, often narrowly so; gradually contracted to the acute apex; sporophyte nearly sessile with a spherical capsule .....B

B. Median cells of larger leaves extremely variable in size and shape, often with quadrate cells adjacent to long elliptic cells; perigonia on lateral branches with fully defined bracts .....Archidium donnellii, not known from CA
B. Median cells of larger leaves uniformly short rectangular to rhombic; antheridia usually naked in axils of lower perichaetial leaves .....Archidium alternifolium