It's possible that the seller has sent you a Canon version - they have physically larger mounts, and often use the APS-C designation (as opposed to DX)!
Does it look more like this http://en.wikipedia.org/wiki/File:Canon_EF_lens_mount.jpg as opposed to your other Nikon lenses?
Yep -- that's exactly what it looks like. Thank you. I thought the description said "Nikon", but I'll have to look back through my coorespondence. It may have been my fault that I ordered the wrong version.