Marsupella marginata habitat, Corrwg Fechan Valley
Minimal torment here, and a species that all of you know well, but a nice record of it in one of the headstream cwms of the Afan Valley, near Glyncorrwg. The habitat is a dripping wet, north-facing sandstone rock face where it grows in a small area as pendant mats with Pellia epiphylla, Hyocomium armoricum and Diplophyllum albicans. Drier areas of rock face in the vicinity are plastered with Amphidium mougeotii.
It looks like var. aquatica to me, which is also found at Craig y Llyn (not far away as the crow flies).
The leaves have a very shallow notch, the shoots are relatively large and the habitat is right for var. aquatica. Apparently there can be some overlap between var. emarginata and var. aquatica.
Marsupella emarginata leaves with recurved margins
Watson's description draws attention to the rounded cell lumen and the the well-developed trigones, which can be seen in the photo below.
I'm not sure to what extent other Marsupella spp. show such a thick-walled cell structure. Perhaps Sam or Graham can comment on that.