This is an internal cast of a fossil shell from the Portland Limestone. The shell itself has been dissolved away, leaving the impression of the inside of the shell complete with the hinge and muscle scar. The folklore name for these is ‘’osses ‘eads’, because they do resemble a horse’s head. The ‘teeth’ are the hinge between the shells, and the ‘eye’ is the muscle scar.
Find out more about bivalves on the molluscs pages.