Formerly Hygrohypnum eugyrium. Dark green or purple-green mats of a pleurocarp on rocks and other surfaces in the splash zone of fast-flowing upland streams and rivers may be this species. It is a familiar sight in the hill country of the north and west, but virtually absent from lowland districts.
Examine it with a hand-lens in the field, and its wide leaves turned to one side are certainly reminiscent of Hygrohypnum and its relatives. Often, the reddish patches of enlarged alar cells are visible with the leaves in situ, ruling out e.g. Hygrohypnum luridum and Hygrohypnella ochracea. To be sure, collect some and look at the leaves microscopically – the alar cells are very conspicuous and striking.
In the most oceanic areas, such as in parts of western Scotland, the rarer P. subeugyrium is also a possibility and then microscopic checking of plants is essential. Fortunately, Tom Blockeel write a paper about the differences between the two species which was published in the Journal of Bryology.Read the Field Guide account