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Found several of these dead on the beach in Crescent City. One had funky orange stuff in his shell but most were barely the outer shell broken among the surf.
These are so neat!! My dad called them "mud buddies". When I was a kid I remember sitting at the edge of the water & digging after them every time the water would roll back out. (This was on the east coast though.) Sorry you only saw deceased ones.
Looks like a Sand Flea, but there are different types. I don't know the family that this one belongs to.