Hey John,
Have you touched it? Does it have a hard or soft texture? and has it been growing outward from the center the way a snail would or is the entire thing growing at once? Have you ever noticed any extensions or movement from it?
I was thinking Cowpie myself. I've never seen anything like it, but might guess it's a hard housing for something like a snail, worm, or crustacean. But that's just a wild guess.
-Rogue
