These are two specimens of the bivalve Trigonia. As with most fossils from Portland only a mould of the shell remains. I was only able to find a small portion of the exterior of the shell which is shown below:

The knobbly bits on the surface of the shell have been shown to improve its ability to burrow into soft sediment. Although not as spectacular as the rediscovery of the Coelacanth, Trigonia was thought to be extinct until 1802 when Neotrigonia, the sole survivor was found alive and well, living in the warm seas round Australia.