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Found occasionally on limestone fragments and pebbles sheltered from direct rain and disturbance, P. byssophila is very like P. linearis. What distinguishes this species is the combination of grey-green-brown thallus, specialised habitat and the three- (to seven)-septate spores. Often the spores show little variation from the three-septate kind, but it is worth persisting, especially when a collection has a clear, visible thallus. In P. linearis the thallus is always immersed; P. chlorotica favours siliceous rock. In Ireland, the species is known from the Burren, County Clare and from County Westmeath.
Original text submitted by Vince J. Giavarini