Mystery Critter, snail, slug

[Howieytown]

Ok now that I figured out how to post pics your all going to suffer

Anyhow does anyone know what this guy is? I found him at the top of the tank, attached like a snail, holding tight. I've seen plenty of other snails that have smooth oval shells not much bigger than a pinky fingernail but this guy looks more prehistoric with the pointy serrations on his back.
Hes not in the same spot today, lost in the rock work for another surprise visit.

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[Howieytown]

I forgot to say it's size is about two of my thumb nails!

 

[Sk8r]

If that shell is segmented, it's a chiton, ancient species and algae-eating.

 

[Kreeger1]

I agree on the Chiton

 

[Howieytown]

cool, Yes its segmented, thanks