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