Plastered skulls, c.7000BC, from the Neolithic 'Ain Ghazal site near Amman, Jordan. The plaster-mix was applied to skulls, once cleaned of soft tissue, as part of a funerary process. The function was to preserve a likeness. It seems that it is the 'visage' which is rendered, rather than the entire volume, sometimes stopping above the mouth.
Cowrie shells were used for the eye, bitumen for the pupil. First saw and drew specimens in the British Museum. I hear there are others at the Ashmolean in Oxford.
Les Yeux Sans Visage (1960), directed by Georges Franju.
A toad's shed skin. Recourse to flatland, as seen in Edward R. Tufte's seminal Visual Explanations.
John Stezaker's Mask II (1991-2), via The Approach.