Here is a butterfly flying fish plate from William Beebe's Arcturus Adventure, detailing his Pacific exploits.  Bit nice yeah?

Again, I realize this isn't ephemera (nor the above), but The Arcturus Adventure, pub. 1942, has delightful endpapers.

Book endpapers in no way qualify as paper ephemera.  But I'm rooting about my collections today, and this endpaper from The Cat Whose Whiskers Slipped demand posting and sharing despite not fitting the theme.  Other illustrations from the same book are likely to follow, as they're stunning.  Entirely wasted on children, with their lousy taste in everything.