The Collaborative International Dictionary of English v.0.48:

Rictus \Ric"tus\, n. [L., the aperture of the mouth.]
   The gape of the mouth, as of birds; -- often resricted to the
   corners of the mouth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rictus
    n 1: a gaping grimace

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "rictus":
   bob, bobble, bounce, bump, didder, dither, falter, grimace, jar,
   jerk, jig, jigget, jiggle, jog, joggle, jolt, jostle, moue, mow,
   pout, quake, quaver, quiver, shake, shiver, shock, shudder, snarl,
   tic, tremble, tremor, twitch, twitter, wobble, wry face,
   wry mouth