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