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Take a look at this pic. The view is from the open end of the shell. It only opens about 1/4"-3/8" and closes when ever i open the hood. It is about 2" long X 1.5" wide X 1.5" tall.
Looks like a filter feeding clam. These are not like the tridacnids as they contain no zooxanthellae and require phytoplankton for all their nutritional needs.