Incidentally, 12 ounces is still a pound in apothecary weights. The apothecaries' ounce is also known as the troy ounce.
You are correct that when we say "ton" in the U.S. we mean 2000 lbs. This is also known as the short ton, while the British ton of 2240 pounds is called the long ton over here.
There is a nice table of units of measure in the dictionary.com entry on measurement: