The skull, an obvious symbol of (human) death, has been given strong religious connotations in several cultures. Among the people of India and neighboring regions, the skull necklace is an iconographic memento mori worn by certain gods and goddesses,
Among many Europeans and North Americans of European descent, the skull now has less of a religious meaning than it does a "spooky" one. The frightening old European skull-and-crossbones -- used as a memento mori in European-style Freemasonry, and as the "Jolly Roger" flag of pirates -- has been tamed over the centuries. These days the skull is trotted out for display at Halloween -- along with jack-o-lantern pumpkins and black cats -- as a conventional symbol of mild and amusing fright.
  Bikers and low-riders, as well as some members of the military, utilize the skull as a signification of reckless machismo barely protected from disaster or of doomed bravery that accepts its fate. When seen in this context, the skull (often called a "death's head") is frequently accompanied by a coiled rattlesnake or pierced through by a dagger. The teeth of such "death's head" skulls may be exaggerated in number and size, the eyes may be painted or lit up so as to glow in the dark, and recently there has been a trend to combine this extra-toothy skull with the contemporary slanted eyes of the popular culture "extra-terrestrial alien" face.
  The skull and crossbones motif has been used among Europeans as a symbol of both piracy and poison.
The crossed lines or "X" would signify the invocation of spiritual forces. The purpose of the invocation would be communicated by the use of the accompanying symbol of the skull to signify death.
The skull is symbolic of wisdom and retained knowledge. When coupled with the teachings of the Mysteries of Occultism, such as the Fermentation Mysteries, the skull signifies that the inner nature or core of the person has been stripped down through initiation. Here the skull's death symbol can mean physical death, the dying of the flesh, or psychological death, the dying of the self. For this reason, among others, the skull was present during the initiation ceremonies of the Old Religion to signify the death of the old personality and the birth of a new consciousness.
  The skull with crossbones stood for the God in old Pagan religions. The crossed bones beneath the skull symbolized the Slain God, and his resurrection from death. In Witchcraft when the skull is displayed on the front of the cauldron it symbolizes renewal through the transformation powers .
  Their descendents, both Indians and people of mixed European and Native American heritage, continue to venerate the skull and the skeleton during the national Day of the Dead ceremonies, when skulls made of sugar are eaten and offerings are placed on the graves of deceased relatives and friends. In Nepal, the use of skull-beads on mallas (prayer beads, sometimes called "rosaries" by Americans) continues to this day. The story is that it had been 'sent out' via a reed boat from the island, to  South/Central America long ago, to await the 'right person' who would eventually come for it and take it elsewhere. The tribe reveres skulls and has other crystal skulls in their possession at this time.