This is an Egyptian sarcofagus and below is an actual mummy. Story goes that a ship captain transporting goods to a museum stopped in St Pete, but could not pay the harbor fee, so offered these to the city instead.
This is what the museum calls its "unusual collection" including a large dinosaur bone, two headed cow, large replica Swiss army knife, and other oddities. The museums other displays all fit quite nicley into a set system depicting the history of the city. These... well.... don't.