The Medium Shell can be obtained uncommonly from Diving with the Oxygen Tank, but not from the Row Boat or Canoe Boat, with a 1/20 chance. It can be sold to the Museum for 10,000 Coins.
After adding the Medium Shell to the Museum, it can be examined to show the following message:
A sea shell found on the ocean floors. Would be pretty cool if I could carry my house on top of me.
1/20
