for Platypus by Chris Riddell
Platypus decides it's the perfect day to go collecting. He finds a big rock, an old shoe and a broken umbrella, but none of these are quite what he's looking for. Then he finds a curly shell - just right But it keeps going missing from his special box. The 'thief' turns out to be a little hermit crab living inside the shell so Platypus takes it back to the sea where it belongs. Luckily there are plenty of unoccupied shells for Platypus to collect - and keep
Publisher description retrieved from Google Books.