Horse mussels (Modiolus) are true Jurassic survivors. You can still find their shells on the beach today.

They lived by clinging to rocks. In life, there would have been two half-shells joined by a hinge. Modiolus could lock itself away by keeping the hinge firmly closed, thanks to a strong set of interlocking teeth.

The picture shows a Modiolus found in a brick pit at Napton.