Atrina affinis was a pen shell, a large wedge-shaped bivalve that lived partly buried in the London Clay seabed. It is interpreted as a semi-infaunal suspension feeder: the pointed end of the animal was anchored within the sediment while much of the broad shell projected above it and the gills filtered suspended food from the water.

Living pen shells often anchor themselves with strong byssal threads, and the same general life position is a reasonable model for A. affinis. Its exposed shell would have provided a small patch of hard surface in an otherwise soft-bottom habitat, potentially offering attachment space or shelter to other small organisms.