Midcentury vs. mid-century vs. mid century

Calling all grammarians and spelling bee champions:

What is the correct spelling — or are there correct spellings:
Is it:
“Midcentury” … “mid-century” … or “mid century”?

As in (1): “midcentury modern sofa”, “mid-century modest house” or “mid century American history”. And would it change as in: (2) “in midcentury/mid-century/mid century America”.

Golly, over the past six+ years, we’ve spelled it every which way. Hey: Covering all our bases and google keywords, too, I guess. However, my Meade County High School English teacher Ivy B. Hawkins — a fantastic teacher who made us diagram sentences until we could do it in our sleep — would not be amused by this lax just get it onto the www who cares about spelling anymore 21st Century (21st-Century? 21st-century?) attitude of mine.

So which is it?

Finally, what exact parts of speech do you call the word/compound, in each usage? Ack! My brain explodes! As you respond, can you also provide your citations – add hotlink in your comment. Thank you!