|Micromitrium tenerum at a drying pond edge. Permission pending from
Okayama University of Science, http://www1.ous.ac.jp/garden
They are similar to Ephemerum crassinervium, which is in the same family, but that species has leaves with distinct midribs, more ovoid capsules, and papillose cells near the tip of the leaf.
The ranges of the three species overlap, as they all occur throughout eastern North America. M. tenerum has been also reported from British Columbia, and M. synoicum from Oregon. In Florida, they are all found primarily in the panhandle region, with reports from as far south as Polk County. None are collected very often, however, as they are short-lived and hard to see.
In M. megalospermum the leaves are broadly ovate and the spore capsule opens irregularly, while in M. synoicum and M. tenerum the leaves are lance-shaped, and the capsules open along a distinct ring around the middle or above.
In M. synoicum the leaves are erect, with smooth, somewhat incurved margins and have distinct stems below the crown of leaves, while in M. tenerum the leaves are spreading, with flat margins, and teeth at the tips, and the plants are stemless.