Collemopsidium foveolatum forme sur Mytilus.
Form found on mussels (Mytilus), it is currently possible to distinguish 2 forms: one with a greyish to pale brown thallus and another without an apparent thallus, the perithecia and the thallus being endolithic, they are currently under investigation.
See: Evolution and taxonomy of the marine Collemopsidium species (lichenized Ascomycota) in north-west Europe. MORH Fiona, EKMAN Stefan, HEEGAARD Einar. Mycol. Res. 108 (5) : 515-532 (May 2004). The British Mycological Society.