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Polinices duplicatus
I believe the current binomial nomenclature for the Shark Eye is Neverita duplicata though Polinices duplicatus is a synonym or at least that's what I learned in Marine Bio. After looking at the third photo I see this is actually a spotting of a hermit crab. Great spotting either way!
The shell you are holding is a Shark Eye. Some people also call it a Cat's Eye. :)