please help identify shell

[thuyvan]

I picked the shell in the image below/attached from seabed during a dive in Gulf of Thailand.
I asked both the diver master and a fisherman there and none of them had a clue what this was.

It's so strange a shell. I really wanted to know what it'd looked like when it'd been alive!
Thanks

[thuyvan]

attached is how the shell's bottom looked

[wolfi]

Hi,
it's a sea urchin  , but I don't know the species...... .
Kind regards: wolf

[thuyvan]

thanks for the great tip
I looked up further on internet
and I think they're called heart urchins, FYI!