The series of skulls are based on the poem called "Whispers of Immortality" by T.S.Elliot.
Whispers of Immortality
WEBSTER was much possessed by death
And saw the skull beneath the skin;
And breastless creatures under ground
Leaned backward with a lipless grin.
Daffodil bulbs instead of balls
Stared from the sockets of the eyes!
He knew that thought clings round dead limbs
Tightening its lusts and luxuries...