Weissia species with a short seta (so capsules are immersed) and well-differentiated (often longer and wider) perichaetial leaves belong in subgenus Astomum. A revision of this group in 2019 produced an article with a new key in the Journal of Bryology.
What we now recognise as Weissia angustifolia was formerly one of two varieties of W. longifolia; the other, var. longifolia, has also been elevated to species status as W. longifolia.
As with other Weissia species, the only time that species can be reliably identified is when mature capsules are present, typically in winter or early spring. W. angustifolia can often be provisionally recognised by its distinctive perichaetial leaves, whose upper margins are inrolled like scrolls (involute). All other similar-looking species (W. longifolia, W. wilsonii, W. levieri and W. sterilis) have plane or loosely incurved margins. Under the microscope, it is sensible to confirm the identification by checking that an abscission zone of small, clearly differentiated cells is present at the top of the capsule where the lid will detach.Read the Field Guide account