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Bulla gouldiana
From Wikipedia: "Bulla is a genus of medium to large hermaphrodite sea snails, shelled marine opisthobranch gastropod molluscs. These herbivorous snails are in the suborder Cephalaspidea, headshield slugs, and the order Opisthobranchia."
I've moved this snail from the Arthropod category to the Other category for you. Snails and slugs are Mollusks. Arthropods have an exoskeleton and jointed limbs in at least one phase of their life. Think dragonfly, bee, crab, and ant.