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.
The top of two Depressed Slippersnails (S. Hutcinson Island 2021)
Inside of Depressed Slippersanils
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Bailey-Matthews blog:
Shell of the Week: The Depressed Slipper Snail
Crepidula depressa Say, 1822 is a local member of the family Calyptraeidae. It is one of the “flat white slipper snails” whose identification is difficult without a glimpse at the soft parts. The species may attain 40 mm (1.6 inch) at most, but is most often found in the 25 mm (about an inch) size range. Unlike the Black-foot Slipper Snail, (Crepidula atrasolea, the “other” local flat, white slipper snail), the Depressed Slipper Snail has a yellowish-cream color on the foot and mantle. Their white shells, however, are virtually indistinguishable.