Updated from 4:08 p.m. EDTShares of Matrix Bancorp. ( MTXC) plunged Friday after the Denver-based thrift and Southwest Securities Group ( SWS) announced that they had called off their merger plans. Matrix ended Friday regular trading down 36%, dropping 3 13/16 at 6 13/16 after reaching a disappointing 52-week low of 5 1/2. The news comes two years after Matrix was forced to abort efforts to merge with Fidelity National Financial ( FNF) of California. That announcement in August 1998 had an equally harsh affect on Matrix's stock, which plummeted about 39% in one day.