MORE U.S. BANKS JOIN IN PRIME RATE CUT Chemical Bank, the main bank subsidiary of Chemical New York Corp, and Marine Midland Banks Inc said they were raising the prime lending rate to 7-3/4 pct from 7-1/2 pct, effective immediately. In St Louis, Mercantile Bancorp said its Mercantile Bank N.A. was also raising its prime rate to 7-3/4 pct from 7-1/2 pct, effective immediately. The changes follow similar cuts yesterday by Citicorp and Chase Manhattan.