I've been re-reading "Ulysses" and noticed this
Stephen totters, collapses, falls, stunned. He lies prone, his face to the sky
(emphasis mine) and
Stephen, prone, breathes to the stars.
and a little later, Bloom
bends again and undoes the buttons of Stephen's waistcoat
Full text of the novel (massively NSFW in case you don't know): http://www.gutenberg.org/files/4300/4300-h/4300-h.htm
So, according to everything I know, "prone" should mean he was lying face down, but these passages imply he's lying face up.