cleaning old shells

[NilyahEyakam]

I was cleaning out a room and found a lot of old shells I picked up when I was younger. I found two that may be usable for the crabs, and a LOT of huge food dishes it's like a nice little reward for doing the work I've been pushing off.
but one of the shells had that black old stuff on it. I'm not that great at explaining things so I'll put a picture.
I got some of it off but it's still extremely dirty. does anyone know if there's a way to clean that out (on the inside too, I'm sure it's way back there) or if I have to just throw it away. if I can't use it it's no big deal, I don't even know if they would like this kind or if its the right size but I figured it would be worth a shot. they might enjoy a new shell option.

[NilyahEyakam]

I boiled it for a little less than an hour and tried using wet paper towels and q-tips. sorry I forgot to say that

[dinky]

Set it in bucket, put half water and half vinegar, let it set few minutes, if it doesn't come off add more vinegar...best and safest way I know.....rinse and air dry

[NilyahEyakam]

for anyone who might have this problem, I tried a lot of other stuff but dinky was right. it's been sitting in a little more than 50/50 vinegar and water for about 4 hours and almost all of it came off! there's even little flakes of the black stuff sitting on the bottom of the tub. so THANK YOU.