Half buried black clam
(From Aquascope|Facts|Black clam|More facts 1)
I've seen better pics of half-buried clams (heck, I've seen better examples with my own eyes) that showed the "soft tissue" better (the stuff that lines the insides of both shell halves), but this should give you an idea of what that picture reminds me of (as does the tiny one in the "grind" example from yesterday).
