"He turned over his card. Printed on it was the calavera"
Calavera de azúcar
Creative Commons Attribution Share AlikeCalavera de azúcar - Credit: El Comandante

Spanish for skull, the calavera is usually associated with Mexican Day of the Dead celebrations, such as the calaveras de azúcar ('sugar skulls'). The term calavera is also used to refer to any artistic rendering of skulls, such as the lithographs of José Guadalupe Posada or the one printed on the Lotería card given to Billy. 

The origins of the skull in Mexican tradition are somewhat obscure, although generally viewed as a result of the intermingling of Spanish and Mesoamerican cultures. This is perhaps most obvious in the calavera de azúcar tradition, which can trace its lineage back to the Aztec practice of decorating human skulls with jewels and mosaics.

 

'Gran calavera eléctrica' ('Grand electric skull'), by José Guadalupe Posada
Public Domain'Gran calavera eléctrica' ('Grand electric skull'), by José Guadalupe Posada - Credit: Library of Congress
Mesoamerican decorated skull (c. 15th-16th century)
Creative Commons AttributionMesoamerican decorated skull (c. 15th-16th century) - Credit: Beesnest McClain