The parent of the New York Stock Exchange unveiled a plan yesterday for a landmark transaction that would create the first major trans-Atlantic stock market and force significant changes on the Big Board’s operations and identity.

The proposed deal, valued at $10.2 billion at Friday’s closing prices, calls for NYSE Group Incorporated (NYX) to merge with Euronext NV (24151.AE), a pan-European exchange that deals in stocks and derivatives. The merged company, to be known as NYSE Euronext, would be headquartered in New York with the NYSE’s John Thain as chief executive.

NYSE Euronext would boast the world’s largest stock market,with a combined $26.9 trillion in market value for its listed companies, more than the next four largest markets combined, and a market capitalization of more than $20 billion. In addition to stock trading in America and Europe, it would add a derivatives-trading arm in Europe to complement the options-trading business the NYSE obtained as part of its March acquisition of Archipelago Holdins.

The transaction would deepen the dramatic transformation Mr. Thain has brought to the exchange over the past two and a half years. Since arriving from Goldman Sachs Group Incorporated (GS) in 2004, Mr. Thain has pushed the NYSE to swap its old membership seats for publicly traded shares, merge with smaller electronic competitor Archipelago and, now, to reach well beyond its Wall Street roots.

The changes may be creating the prospect for faster growth, but they are also quickly eroding an identity built around the NYSE’s clubby trading floor in lower Manhattan.

Jamie Selway, a former Archipelago official who now is an executive at White Cap Trading in New York, said he wouldn’t be surprised to see less relevance for the NYSE’s storied trading floor as the exchange comes to rely heavily on Euronext’s trading technology, which Mr.Thain praised.

“They’ve really opened up a lot here,” Mr. Selway said. “If there were doubters out there, this underscores the point in technicolor that the NYSE’s world will be electronic.”

Making the deal work will test the NYSE’s willingness to make difficult choices. The NYSE says the deal will boost its earnings by 14% in 2007 and 21% in 2008. The plan calls for the exchanges to squeeze out $275 million in cost savings, with $250 million of that coming from technology.

Reaching those targets means Mr. Thain and his lieutenants could have to choose among trading platforms and methods as they knit together the operations of the markets under the NYSE Euronext umbrella.