Whats this?

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Was about 3/4" long 5 months ago. Now almost 3 inches. My guess is some sort of clam or oyster or barnacle. I noticed it opens about 1/8" and I can see dark soft tissue inside. It's sitting on an old snail shell for better pic view.. Any thoughts??

 

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Found out it's a mussel. Good news if that's living and growing.