A gregarious coelomycete on the dead stems of Marram grass (Ammophila arenaria). The pycnidia are immersed with just the neck showing

References: 
Sutton (1980) The Coelomycetes: Fungi Imperfecti with Pycnidia, Acervuli and Stromata
Ellis & Ellis (1997 new enlarged edition) Microfungi on Land Plants: An Identification Handbook
Dennis (1981) Fungi of Ammophila arenaria in Europe