If you just go by what's under the limelight, U.S. billionaires are a boys club composed of trendy young tech-heads and wily old investors who have been plying their trade for decades. But look on the margins and you can find female billionaires in the U.S.

They are, however, an exclusive and low-profile group -- perhaps even more so than "average" billionaires. Of the 158 U.S. citizens listed in the Bloomberg Billionaires Index of top 500 billionaires, only 30, or 19 percent, are women. Only three members of the top 15 richest U.S. women could be considered self-made billionaires, with the rest inheriting their wealth from fathers, grandfathers and husbands of family dynasties that have seen billions pass through several generations.

Indeed, the family dynasties associated with just three companies -- Walmart, Mars and Cox Enterprises -- produced the wealth for the majority of the top 15 of U.S. female billionaires, with the top person on the list, Alice Walton, being one of three living heirs to the family fortune derived from Walmart. The Mars candy and food company, meanwhile, has five women in the top 15, including four sisters.

The following 15 women, in ascending order, are the nation's richest. The number in paranthesis is where they placed on the overall Bloomberg Billionaire Index as of March 20:

15. (296) Margaretta Taylor, $5.90 billion
Taylor owns 16 percent of Cox Enterprises, a share she inherited from her mother, Anne Cox Chambers, the daughter of Cox Enterprises founder James M. Cox. Neither Margaretta nor her siblings or mother have ever had an active role in the company, according to Forbes.