Tidepool Tim says,  "The name of this little fellow is dead on - it looks like a little cup with a saucer!  Of course, you have to kill him and pull out his body to fully appreciate it, but if you find just the shell washed up on the beach doing this is not necessary.  The top of the shell curls over to one side and it has delicate little ridges running down the sides. We find these when dredging for other specimens like sponges or shrimp.  Typically they are attached to blue mussel shells or scallop shells - sometimes a small stone or even a live whelk. Once only we had one attached to a green crab. I am not certain but believe these to feed on plankton or other detritus. Like slipper shells we often find them stacked one on top of another for reproduction purposes."