What you're looking at is a shell of a species of brachiopod that would do well in home aquaria. Just like clams have hinged shells, brachiopods , major difference is brachiopods are related to Phoronida, the featherless feather dusters (example picture attached, also a critter that would do great in home aquaria, usually found as a hitchikker and left to die). They oftentimes do not have identical shells on both sides, so that makes them more interesting. They attach to rocks with and their hinges are attached with flash, unlike clams which have calcified hinges and use something like a beard to attach to rock. Furthermore, they physical turn themselves from side to size on the rock to find the area of most flow. They would require supplemental phytoplankton.
In short, brachiopods look like clams, but aren't at all clams.