Anyone have any idea what this is;

http://www.io.com/~lawrence/snail2.jpg

(scanner-eye view)

I was walking along when I noticed about a dozen of them creeping across
a sidewalk. They almost seem like some sort of snail with a wooden
shell, but the shell actually feels somewhat soft and organic.

It looks a little like the Doomsday machine from the original Star Trek,
though I'm pretty sure it didn't have a snail head...