NGS Style Manual‎ > ‎- M -‎ > ‎

mid-

Follow Webster's.  If not there, hyphenate:
             mid-ocean
but      Mid-Ocean Ridge
             mid-Atlantic, mid-Atlantic states
but      Mid-Atlantic Ridge
             mid-1950s, the mid-fifties, but not mid-'50s
but      mid-40s for a person's age
                                                       

  mid-continent
  mid-island
  mid-January
  mid-part
mid-19th century
mid-19th-century wall
mid to late 19th century
mid to late 19th-century wall
early to mid 1900s