Andy Warhol's Skulls

I've been a huge fan of Andy Warhol for many years. I'm familiar with much of his work. I've even seen it in person while I lived in Philadelphia. I seem to have either missed his series of skulls or maybe I just stands out more because my artistic concentration lately has been skulls. I don't know much about the pieces, but this is one style (my favorite) of a few different ones.