[gordypoo]

I was wondering if someone could give me an idea of what i have on my rock. I have no picture so i'll try to explain. Most are round shells and one peanut shaped, maybe the size of a dime or nickel, attatched to various pieces of rock. Last night i observed one shell opening up a little at the tip and there was this jet black thing inside. Although i have no idea what it was because you could only make out the dark balck color as the shell was only opened a bit and then later the shell closed. The shells have jagged edges to them. For the life of me i can't figure out what kind of creatures would appear so black. Any ideas????? In fact another shell is open most of the time with the same jagged edges and if i pick up the rock or disturb it, it will close. What could be in these shells?