Shares of Sirius Satellite Radio ( SIRI) and rival XM ( XMSR) tumbled Wednesday after the chairman of the Federal Communications Commission said agency rules wouldn't allow the two to merge. A potential merger of the companies has been the subject of repeated speculation in recent months, but Bloomberg quoted FCC Chairman Kevin Martin as saying that existing regulations prohibit a single owner for the two satellite-ratio operators. Martin made the comments following an FCC meeting. Sirius was dropping 5.5% to $3.92. XM was slumping 7.6% to $15.83.