Bythnia, is but one possible spelling of this area of Asia Minor / Turkey. Other possibilites might be Bithinia, Bythynia, etc. Its exact location is rather vague due to the lack of geographic mention in ancient accounts. But, in general modern scholarship has placed it upon the Asian side of the Bosphorus, to the south and east of Istanbul, Turkey, with a large coastal frontage upon the Black Sea. For a better idea please look at this modern representation? http://en.wikipedia.org/wiki/File:Anatolia_Ancient_Regions_base.svg