help ID this clam-like thing

Sorry, I don't have a camera so I'll describe it the best I can. It was a hitchiker on some live rock I got about a week ago. Until this morning I thought it was just a piece of the rock. When I turned the lights on, I noticed that it had opened up. It's covered with Corraline, so I don't know what the outside looks like, but it's shaped like a football, rounded on the bottom, coming to a point at the top, and the side facing the glass is flat (looks to be the backside??). It's approx. 1 1/4" tall. When it's open, I can just make out some thin black and white stripes. Any ideas???
 
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