Confidence: I'm as sure as I can be.
Notes: The strong ridge down the back, and the fingerprint pattern just visible on the mantle (the flap of skin behind the head) makeit one of the Limacidae keeled slugs. Large size and dark colour rule out all but Leopard Slug Limax maximus, and Ash-black L. cinereoniger. It is very dark for Leopard, and the whitish keel is typical of Ash-black. The paler stripes on the sides suggest it's a 'teenager' (probably about a year old).