Americauna is a fancy and misleading name for Easter Egger. Both are just a cross of an Ameraucana (no "I") or an Araucana with something else to produce a bird that will lay lots of blue or green eggs.
Ameraucanas are a true breed with muffs, a beard, and slate blue legs that lay only blue eggs and will breed true. Easter Eggers are hybrids produced by crossing blue egg layers with brown egg layers to produce offspring that lay eggs in colors other than just blue (olive, green, beige, and sometimes even pale pink). Unlike true Ameraucanas, EEs have no set standards and will not breed true. Hatcheries and feed stores often and incorrectly market their EEs under the label Ameraucana (frequently misspelled as Americana or Americauna). There is a good article at http://www.the-chicken-chick.com/2011/09/ameraucana-easter-egger-or-araucana.html explaining the differences between Ameraucanas, Araucanas, and Easter Eggers.