Folks, this Thanksgiving, remember,
it's scalloped potatoes, not 'scallop potatoes'.
To scallop (v), past tense scalloped, is a verb derived from Old English collop, meaning to slice thinly.
It is a parallel spelling and pronunciation to scallop (n) i.e. the bivalve mollusk harvested as seafood, but that word derives from old French escalope which itself derives its spelling and meaning from old Germanic and Middle Dutch words such as skalpr or schelpe, meaning a sheath, shell, or carapace
Thus, scalloped here describes the method of preparation - thinly sliced.
And remember - if you were to bake scallops in the oven for as long as you would a potato gratin casserole, they'd turn out rubbery, dry and gross. :)