Mellon Bank Corp, looking to jump into the world of internet- based financial services, is buying discount broker Pacific Brokerage Services Inc for an undisclosed amount of cash. PBS provides services to about 100,000 individual investors across the US, but Mellon was mainly attracted to the deal by Pacific’s growing internet trading business. The 130-person strong Pacific will become part of Dreyfus, Mellon’s New York-based mutual fund company. Mellon says that virtually all of Pacific’s staff will be kept on and it will maintain its Los Angeles headquarters. The transaction is expected to be closed sometime in the fourth quarter of 1997.