Shares changing hands in private venues such as dark pools accounted for 40.4 percent of total share volume on June 10, according to data compiled by Bloomberg. The high came after Securities and Exchange Chair Mary Jo White last week voiced concerns about the level of trading taking place on venues where bids and offers are kept private, masking the true depth of demand for shares. The rise in off-exchange trading came as calmness pervaded markets, with the Chicago Board Options Exchange Volatility Index, also known as the VIX, sliding to a seven-year low last week.