General diagram of the diapsid skull. Note the two holes in the skull behind the eye

Allosaurus skull

This Massospondylus skull shows the two temporal fenestrae typical of diapsids, plus the antorbital fenestra in fron of the eyes.

A diapsid is a reptile with two fenestrae (holes) behind the eye on each side of its skull. Most reptiles are diapsids, and the condition does not occur in mammals. The function of the holes is to improve jaw movements, and to reduce the weight of the skull. The holes give places for muscle attachment; also, as jaw muscles contract, the holes allow space for the muscles to bulge.