Mushroom "Rock"

[tebstan]

Check out jewel box clams. Spiny oysters generally have a more recognizable shell.

Thanks for the tip. I found Spondylus, a family of oysters that fits this guy, commonly called Jewel Box or Thorny or Spiny oysters. Looks most like the Spondylus varians, but no way to tell for sure. It was neat to find other pics that showed how pretty and unique these are. My bivalves not ugly.