Identification notes
This rather uncommon moss is often found growing with the much commoner Grimmia pulvinata, which looks very similar, so the challenge is telling them apart. There are a few pointers to spotting this needle in the haystack. It tends to favour older surfaces such as exposed base-rich rocks, tombstones or slightly disintegrating concrete, and generally forms flatter, spreading, untidy blackish hoary patches that tend to break up easily. Grimmia pulvinata forms denser and more compact grey-green cushions.
Fortunately, both species fruit freely. Grimmia orbicularis has shortly ovoid capsules with a mamillate lid and Grimmia pulvinata generally a more elongated capsule with a rostrate lid. Microscopic confirmation is really needed. The elongate basal cells are 4-8 times as long as wide (2-4 in G.pulvinata)
