"P. sinensis" is readily distinguished from all other species by numerous features of the skull. Adult skulls are smaller than those of "P. mongoliensis" and have less teeth. Uniquely, the premaxillary bone contacts the jugal (cheek) bone on the outside of the skull. The jugals flare out sideways, forming 'horns' proportionally wider than in any other known "Psittacosaurus" species except "P. sibiricus" and "P. lujiatunensis". Because of the flared cheeks, the skull is actually wider than it is long. A smaller 'horn' is present behind the eye, at the contact of the jugal and postorbital bones, a feature also seen in "P. sibiricus". The mandible (lower jaw) lacks the hollow opening, or "fenestra", seen in other species, and the entire lower jaw is bowed outwards, giving the animal the appearance of an underbite. The skull of an adult "P. sinensis" can reach 11.5 centimeters (4.5 in) in length.