Physcomitrium (Bridel) Bridel, 1827.

Of the four genera of the Funariaceae with exserted capsules, Physcomitrium is the one that can have either immersed or exserted capsules depending on the species. We key the one member of the genus with immersed capsules (Physcomitrium immersum) under Acaulon. The four remaining species of Physcomitrium must instead be distinguished from those generic members of the Funariaceae with exserted capsules (Entosthodon, Funaria and Pyramidula). Funaria is easily distinguished by its inclined and mostly sulcate capsules. Entosthodon is distinguished by the erect capsules, mostly with a reduced endostome. Pyramidula has a seta that is barely emergent, and the 4-angled, very large calyptra is completely diagnostic.

Key to Physcomitrium

Species included are in Funariaceae:
Physcomitrium californicum E. Britton
Physcomitrium collenchymatum Gier
Physcomitrium hookeri Hampe
Physcomitrium immersum Sullivant, not known from CA
Physcomitrium pygmaeum T. P. James, not known from CA
Physcomitrium pyriforme (Hedwig) Hampe

The genus Physcomitrium must be identified by a suite of characters, both gametophytic and sporophytic. A preparation of a specimen for viewing under the compound microscope must include the mounting of one of the more developed leaves. It should also include a longitudinal section of a capsule arranged so that one can view the exothecial cells and an unfragmented peristome.

A. Capsules immersed .....Physcomitrium immersum not known from CA
A. Capsules exserted .....B

B. Leaves without a differentiated limbidium; dry capsule broadest at the mouth, globose or nearly so; suboral cells of capsule poorly differentiated or in few rows .....C
B. Leaves with a clear limbidium at least near middle; dry capsule not flaring, pyriform to turbinate; exothecial cells at capsule mouth rectangular, mostly in more than 8 rows .....E

C. Leaves essentially entire; costa ending a few cells before leaf apex; annulus vesiculose .....Physcomitrium hookeri
C. Leaves serrulate distally; costa percurrent to excurrent; annulus of small and essentially quadrate cells .....D

D. Operculate capsule orbicular with little definition of a neck; costa ending within 2-4 cells of apex .....Physcomitrium collenchymatum
D. Operculate capsule pyriform with neck nearly as long as urn; costa mostly shorter .....Physcomitrium pygmaeum, not known from CA

E. Leaves serrulate distally; capsule erect and pyriform .....Physcomitrium pyriforme
E. Leaves essentially entire; capsule erect and turbinate .....Physcomitrium californicum