The skull of a Xindi-Avian).
The skull is the framework of the head in most animals including humanoids. The skull protects the brain and houses sockets for the eyes. It is part of the skeletal system.
Gallamite have transparent skulls. (DS9: "Penumbra")
Data's skull was composed of cortenide and duranium. (TNG: "The Chase")
Cranial implant are installed into the skull. (DS9: "The Wire")
Quark once had damaged bones in the back of his skull repaired. (DS9: "Take Me Out to the Holosuite")
Terran Empire MACOs used the skull as a symbol. (ENT: "In a Mirror, Darkly")