Rudists are a class of Pelcypods that have one valve that is much larger than the other. They are best known for being reef builders during the Cretaceous. Below is a specimen of Laperousia crateriformis from the Cretaceous (Cenomarian) near Magnac, France. It is just one of the valves from the animal.
In the top view you can see "veins" and patterning along the edges where the other valve (which would have been flat or slightly domed) would have rested.
This is a view into the body chamber where the animal lived.
Side view showing layered growth.
Bottom view of the specimen. This was not the very base but just a small portion of a much larger structure.