Young Brunette Girl Wearing A Seashell Necklace

Other more atypical kinds
A few other categories of marine animals leave remains which might be considered "seashells" in the widest possible sense of the word.
• Vertebrate "shells": chelonians
Sea turtles have a carapace and plastron of bone and cartilage which is developed from their ribs. Infrequently a turtle "shell" will wash up on a beach.