Consolidated U.S. equity volumes are at four-plus year lows, according to Raymond James Research.

Volume, by its count, was 6.1 billion shares a day. That was 11% down from June and 13% from a year ago.

Volume in July was 11% down from June and 13% from a year ago.

Volume at the New York Stock Exchange was down 10% from a month ago and 16% from a year ago.

At the same time, "Nasdaq underperformed industry-wide volumes over both comparable periods,'' analyst Patrick O'Shaughnessy said, in a report to clients. This "although July was just the second slowest month for the exchange since January 2008 due to fluctuations in market share.''

Nasdaq's July volumes declined at a faster pace due to lost market share at its price-size exchange, known as PSX, he said. That venue, which tries to favor the size in shares of an order over the speed that an order takes to reach an exchange, fell to 0.4% of all equities trading in July. That is compared to 1.4% in June and 1.2% in July 2011.

PSX was launched in October 2010. Nasdaq OMX CEO Robert Greifeld heralded the size over speed approach as a "fundamental change" to the basic approach to trading on U.S. markets.

Stock trading has been plunging since before PSX launched, however. Market volatility spawned by the credit crisis gave trading a boost in 2009, but then it began falling, as the nation's economy suffered. Plus, investor confidence has been rattled as exchanges have suffered through troubles such as the May 2010 Flash Crash and this year's Facebook IPO order miscues, BATS Global Markets IPO technical failure and Knight Capital's flood of erroneous orders on August 1.

Share trading actually peaked in 2009, with an average daily volume of 6.9 billion shares traded, according to the Securities Industry and Financial Markets Association (see chart). That fell to 4.8 billion shares on average in the first half of 2012.

Tom Steinert-Threlkeld writes for Securities Technology Monitor.