Weissia perssonii is not included in the Field Guide but has an almost exclusively coastal distribution and is uncommon (although locally plentiful in some sites). The following description is taken from Alan Crundwell’s 1971 article in the Transactions of the British Bryological Society.
W. perssonii is very closely related to W. controversa. The two are distinguished thus:
W. perssonii. Nerve in lower part of leaf (45-)50-75µ wide, the upper surface uniform almost to the leaf apex and consisting of narrow elongate non-papillose cells; cells of upper part of lamina 9-10µ wide; capsule ellipsoidal or oval, strongly contracted at the mouth, smooth when dry and empty; peristome teeth up to 60µ long, but frequently shorter, ca 20µ.
W. controversa. Nerve in lower part of leaf 30-60(65) µ wide, the upper surface consisting in the lower part of the leaf of narrow elongate non-papillose cells, in the upper part of wider quadrate papillose cells similar to those of the lamina; cells of upper part of lamina 7-9 µ wide; capsule cylindrical or ellipsoidal, slightly contracted at the mouth, smooth or slightly grooved when dry and empty; peristome teeth up to 100 µ long, but sometimes very short.
It often grows with W. controversa var. controversa and looks similar, so should always be confirmed microscopically.