ric·tus 
(rĭktəs)
(rĭktəs)n. pl. rictus or ric·tus·es
1. A gaping grimace: "his mouth gaping in a kind of rictus of startled alarm" (Richard Adams).
2.
a. The expanse of an open mouth or a bird's beak.
b. The corner of the mouth or the fleshy area where the upper and lower mandibles of a bird meet.
[Latin, from past participle of ringī, to gape.]
rictal adj.