Mesa offered $1.67 billion for Diamond Shamrock.

The energy firm headed by T. Boone Pickens Jr. proposed buying all the oil company’s common stock for $15 a share in cash. Earlier, Diamond Shamrock had rejected as inadequate Pickens’ offer to exchange cash and Mesa securities for Diamond Shamrock’s 111.1 million total common shares outstanding. On Monday, Diamond Shamrock countered with a dramatic restructuring proposal that calls for breaking Diamond Shamrock into two pieces, buying back stock and placing a block of preferred shares in friendly hands. It had no comment on Pickens’ latest move.