Are these some type of urchins?

[D4jack]

Hi there. I've been up and running for almost 2 years. I ordered live rock from G.L.R. in Florida when I first got set up. I've recently noticed a handful of these urchin/sand dollar looking things. They seem to stay on or in the live rock. I only notice them when the lights are off. They don't consume the coraline algae like the other urchins I have in my fuge. Any ideas what these are? They are basically flat on the bottom and conical.

 

[rwreef]

Look like limpets snails. If so, good guy CUC