Van der Moolen ( VDM) tumbled Monday after the Dutch brokerage warned it would post a third-quarter loss.

The New York Stock Exchange specialist firm says it expects principal trading revenue from its U.S. operations to decline by 30% from the second-quarter of this year. The company also said it expects to make staff reductions in light of the Big Board's move towards more electronic trading as part of its so-called "hybrid market." Shares were falling 76 cents, or 11.2%, to $6.04

Santander Bancorp ( SBP) gained after the San Juan, Puerto Rico-based bank was upgraded by Sandler O'Neill to buy from hold. Shares were trading up $1.86, or 10%, to $19.81.

Mercantile Bankshares ( MRBK) surged after PNC Financial ( PNC) said it way buying the Baltimore-area lender for about $6 billion.

The deal values Mercantile at $47.24 a share, a 28% premium over Friday's closing price of $36.78. PNC expects the transaction, slated to close in the first quarter of 2007, to add to earnings in 2008. Shares of Mercantile Bankshares gained $8.10, or 22%, to $44.88. PNC fell $3.43, or 4.6%, to $70.17.

Affordable Residential Communities ( ARC) gained after the Englewood, Colo.-based REIT announced a deal to buy NLASCO, a privately held property and casualty insurer for $117.5 million.

Affordable Residential says the insurer had pretax income of $26.6 million and net income of $17.4 million last year. The company also said Flexpoint Fund, an investment fund managed by Flexpoint Partners, invested $20 million to buy up shares of the REIT. Shares gained 58 cents, or 5.9%, to $10.48.

Ventas ( VTR) slipped after the Louisville, Ky.-based health care REIT was downgraded by Stifel Nicolaus to hold from buy. Shares were falling $1.26, or 3.2%, to $37.57.

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