Trouble Selecting Correct Font-series

[Ben Strecker]

I'm using Lilypond version 2.19.25 on a mac running Yosemite, and I'm trying to
use the font family Iowan Old Style.  In addition to the regular styles (roman, 
bold, italic, bolditalic, etc.) this family comes with Black and BlackItalic 
styles.  in my small example below, the markup gets engraved using "Iowan Old 
Style Black" instead of "Iowan Old Style Roman", and I can't seem 
to find a way to fix it.

\paper {
  fonts = #
  (make-pango-font-tree
   "Iowan Old Style"
   "Optima"
   "Monaco"
   (/ (* staff-height pt) 2.5))
}

\relative c' {
  c^\markup{Test} d e f g a b c
}

My suspicion is that it may have something to do with the organization of the
.ttc file where the font resides, but I don't know enough on the subject to 
fix it.