Caloplaca luteoalba. This is a species which has suffered dramatic decline following the loss of its main habitat, which was the the bark of elm trees through Ducth Elm Disease. It is now rare, while it was relatively abundant in the early 1970s. Now it should be searched for on sycamore, or ash especially where the bark has been wounded. This photograph is of a specimen collected prior to the disease in Sussex. It has a white thallus, and rather crowded apothecia which are bright orange.