Probably not an urchin but....

[bluemon]

Maybe an abalone

 

[Jay York]

This is mine

View attachment 20190204_205832.jpg

I'll try to get a better picture of mine.

 

[KJ]

Hard to tell. Thorny oysters maybe Cowrie?

 

[DSC reef]

This has got me curious. Almost looks like a brittle star in a bivalve shell.

 

[Matt Carden]

Mine finally came out of hiding.

 

[bluemon]

Mine finally came out of hiding.

Yours is just a cerinth snail with algae growing on its shell

 

[Matt Carden]

Yours is just a cerinth snail with algae growing on its shell

That's what I thought too but it has a feeding tube like a Nessarious.

 

[bluemon]

That's what I thought too but it has a feeding tube like a Nessarious.

Well cerinths have tube like mouths too. They are longer snouts than other snails like nerites or turbos.

The shell is 100% a Florida cerinth