maxillary_rictus

The membrane over the corner of an animal's mouth, specifically the soft-tissue, flexible, or membranous area where the upper (maxilla) and lower (mandible) jaws join. Widespread in birds, reptiles, and some mammals, this region is often glabrous (featherless) and rich in nerves. It is distinct from the bony structure of the skull.

It is most famously displayed on the mouth of the Tyrannosaurus Rex named Rexy (Jurassic Park) from Jurassic Park.