This is Liebea, a Permian bivalve mollusc similar to modern mussels. They were epibyssate suspension feeders, meaning that they anchored themselves to rock or seaweed and filtered food particles out of the water. Liebea commonly formed clumps containing only that species.
brachiopods found there, the Liebea fossils appear to be quite small (about half the size they should be) but not to the same extent as the brachiopods. This could be interpreted as having a high infant mortality rate (due to overcrowding or a soft substrate) or small adult size due to overcrowding.