Extrado-back dominoes appear not to tessellate.
If we make a domino out of two equilateral, but concave pentagons and hook the back side of one to the external flank of another (see here for background), then try to tessellate with the resulting dominoes, it appears not to work. I can't quite prove it yet, but a few hours of playing around provide some confirmatory evidence. And I got some pretty pictures enroute.
