Cowries are a huge family of mollusks whose shells are among the most popular for collectors. The shells were once used as money by many countries of Southeast Asia.
Scientific name: Lyncina leviathan
I'm not sure about the identification of this one, but it was the closest I could find. This was photographed at night off the coast of Kona, Hawaii.
Scientific name: Ovula ovum
The shell of the Egg Cowrie is actually pure white. It's the mantel that has the black patches on it.
Scientific name: Cypraea tigris
Tiger cowries are common throughout the Pacific and Indian oceans. The one pictured above is probably just sleeping, which is why it's wedged under some coral. At night, it will extend it's mantel and go on the hunt for food.
Scientific name: Cypraea erosa
Most cowrie references show just the shell as collected from the shore, rather than the live animal, so it's hard to be sure of any identifications.