Seems to have too many segents in the carapace.
To be a horseshoe crab (they tend to have 1 large fontal segement and then a second shorter one at the back). I think it's more likely to be one of the Bathynomus species (a sea dwelling woodlouse) as the carapace and legs fit and they can grow up 2ft long. Which is an alltogether more awsome find!
[/Marine Biologist]
Edit: After spotting one of the replies above, it's most probably a spider crab underside.
(, Wed 9 Jan 2008, 18:30, Reply)