...invert?... ID help

[MrTreat]

can anyone ID the critter that owns the hard, tear-drop shape thing on this rock? when I moved the rock, I noticed that this piece closed tight where it was ...I poked at it and it's hard like bone.

Sorry, my camera kept focusing in on the rock, so it's a little out of focus.

Oh, is the critter good or bad?
Thanks!!

thingy.jpg

[MrTreat]

...anyone? I've pulled out the rock and i'm letting it dry in a bucket right now. I wanna see if this thing drops out

[MrTreat]

I should probably mention that this is also about the size of my pinky nail