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some kind of jelly fish?
Atlantic ocean
no tentacles
Looks like jellyfish mesoglea. Basically the sea and surf pound away at the jellyfish body, pulling off the tentacles and obvious feature and just leaving this jelly-like substance behind.
Perhaps part of a Moon Jellyfish (Aurelia aurita) or Mushroom Cap Jellyfish (Rhopilema verrilli)
You can read more about mesoglea here: http://en.wikipedia.org/wiki/Mesoglea