Depressed Slippersnail
Crepidula depressa (Say, 1822)
Family Calyptraeidae.
These are different versions of the "flat" slippersnail. There are two very similar types: the Depressed Slippersnail and the Black-foot Slipper Snail. This is the Depressed Slippersnail (H. Lee, 2021). The depressed slipper snails below have very little space in the inner shelf (see second pic below) while other "Depressed" slippersnails have more space. The shells depicted below are almost flat.