Schneiderman, in a press conference on Wednesday, argued Barclays falsely marketed its dark pool as being safe from what its internal marketing materials described as "predatory," "aggressive," and "toxic" activity by high-frequency traders.

The complaint cited internal emails in which Barclays employees deliberately withheld or changed information about its dark pool to make it appear safer and more attractive to prospective clients such as  mutual funds .

5. -- GoPro, the maker of wearable sports cameras, said late Wednesday it sold 17.8 million shares at $24 each in its initial public offering of stock.

The IPO was priced at the high end of GoPro's expected range and raised $427 million, valuing the whole company at about $3 billion. 

The stock will begin trading on Nasdaq Thursday under ticker symbol " ."

6. -- Hedge fund Paulson & Co. has amassed a large stake in  Allergan  of more than 6 million shares and supports a deal between the Botox maker and Valeant Pharmaceuticals , people familiar with the matter told Reuters.

The position is expected to make the U.S. hedge fund, led by John Paulson, one of the top 10 to 15 holders of Allergan, according to Thomson Reuters shareholder data as of March 31.

Paulson's new position in Allergan could give Valeant a boost, as the company tries to drum up support from at least 25% of Allergan shareholders so that it can call a special meeting, Reuters said. Valeant and Bill Ackman's Pershing Square Capital Management are seeking to call a meeting to elect new directors to Allergan's board, which could pave the way for a takeover of the company.

7. -- London Stock Exchange agreed to acquire  Frank Russell Co. , owner of Russell Investments, for $2.7 billion from Northwestern Mutual Life Insurance