Hyriopsis bialatus, sometimes referred to as the Sailfin Clam, is a large freshwater mussel with a distinct flange on the seam of its shell. These filter feeders require a tank with lots of suspended particulate matter to thrive, so a well stocked planted tank is ideal. They will bury most of their shell under the sand, leaving one tip out to feed. They're an odd creature for sure, but make a unique addition to a tank and can help clarify water.
These are adults around 4-5" long.